DOI 10.1515/nanoph-2014-0007 Nanophotonics 2014; 3(1-2): 3–18
Overview
Aaron Lewis*, Dmitry Lev, Daniel Sebag, Patricia Hamra, Hadas Levy, Yirmi Bernstein,
Aaron Brahami, Nataly Tal, Omri Goldstein and Talia Yeshua
The optical near-field: super-resolution imaging
with structural and phase correlation
Abstract: An overview of near-field optics is presented
with a focus on the fundamental advances that have been
made in the field since its inception 30 years ago. A focus
is placed on the advancements that have been achieved in
instrumentation. These advances have led to a greater generality of use with ultra-low mechanical and optical noise
and the ultimate in force sensitivity with near-field optical probes. An emphasis is placed on the importance of
fully integrating near-field optics with other imaging and
spectroscopic modalities including Raman spectroscopy
and electron/ion beam imaging. Important directions in
probe design, force feedback methods and scanner flexibility are described. These developing avenues provide
considerable optimism for an ever increasing incorporation of near-field optics to help resolve critical problems
in fundamental and applied science.
Keywords: near-field optics; nanophotonics; multiprobe;
PALM; STORM; STED; SNOM; sSNOM; Raman; SEM; FIB.
*Corresponding author: Aaron Lewis, Department of Applied
Physics, Selim and Rachel Benin School of Engineering and
Computer Science, The Hebrew University of Jerusalem, Jerusalem,
Israel, e-mail: [email protected]
Dmitry Lev, Daniel Sebag, Patricia Hamra, Hadas Levy, Yirmi
Bernstein, Aaron Brahami, Nataly Tal, Omri Goldstein and
Talia Yeshua: Department of Applied Physics, Selim and Rachel
Benin School of Engineering and Computer Science, The Hebrew
University of Jerusalem, Jerusalem, Israel
1 The origin of nanophotonics
Nanophotonics or for that matter nano optics was certainly not a field in the early 1980s when the first experiments were done that led to what is now called near-field
optics. This issue in the journal Nanophotonics focuses on
progress in various areas of near-field optics demonstrating its generality of application. The object of this paper is
to act both as an overview of the field and an introduction
to the articles in this issue.
In addition to super-resolution near-field optics
addresses the vexing problems in optics of confinement of
light in X, Y, and Z, the structural correlation of the light distribution in an image and determining the amplitude and the
phase of the electromagnetic field at each point in the image.
Within the context of the current issue of Nanophotonics, this
paper aims at giving the reader a perspective of the breadth
and importance of this field on the 30th anniversary of the
first experimental reports that founded what has become a
critical field in modern optics with great future potential.
The field of near-field optics has been instrumentation limited since its introduction in the 1980s. Thus, in
trying to address the potential and limitations of the field
and to present as broad a view as possible we focus, in
this paper, on the instrumental developments that have
advanced the field in the past and those that have the
potential to have a tremendous impact on the use of the
technique in the future. What is clear today is that nearfield optics has the potential to touch every area of fundamental and applied optics. We can already see that it will
impact many areas of science and technology from device
physics to chemistry and even to biology where the impact
of near-field optics has thus far been limited but already
its potential significance is clear.
2 The near-field: a crucial step
To understand the basis of these instrumental developments it is important to emphasize that a key component
in the development of this new field in optics was obviously the concept of the near-field.
In terms of physics it had been well known from the
time of Sommerfeld that a radiating dipole had a nearfield. However, in all of the decades since that knowledge
was clearly in the literature the challenge of using the
near-field for super-resolution imaging was not considered. Thus, from a fundamental point of view a key first
step was the stated realization that the near-field could
indeed be used for imaging.
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4 A. Lewis et al.: The optical near-field: super-resolution imaging
Once that realization was appreciated it was necessary to:
–– Fabricate an element that could be analyzed by known
means that would emulate a radiating dipole.
–– Reproducibly make such an element so that it could
be investigated, in a quantifiable way, for light
throughput. This was necessary in order to make
a judgment as to the ability to use such elements
for either point interrogation or spectroscopy and
ultimately optical microscopy in the near-field.
–– Produce such an element in a way that would allow
tracking of even rough surfaces so that the radiating
dipole could be kept within the near-field of even
rough samples.
A key paradigm shift in appreciating the criticality of the
near-field appeared in the literature in an abstract that
was published in 1983 in the Biophysical Society Meeting
[1]. In this abstract it was for the first time indicated that
there may be a method to obtain the non-radiating components in the near field that are associated with obtaining super resolution optically. This paper was the first
published in the public record that fully appreciated that
the near-field was the essential element that was going to
achieve a new direction in optics.
The first paper published based on this abstract, highlighted the importance of the near-field for such imaging
and was the first paper that also quantified such a potential radiating dipole with standard methods. This was the
paper published from our laboratory in 1984 [2]. In this
paper the words and the importance of the near-field was
mentioned as in the 1983 abstract in connection with such
imaging. Also the experiments that were done measured
light by known methods and correlated the light from such
a quantifiable man-made radiating dipole with electron
microscopic imaging that accurately imaged the dimension
of the radiating aperture. Thus, the first and second criteria
mentioned above were already met in 1983 and 1984.
Specifically, to obtain such results a well formed aperture, with a known geometry, was produced by the then
incipient technique of electron beam lithography. This
was done using polymethyl methacrylate which was just
then being realized as an important resist for such electron beam lithography. Of course subsequent developing,
etching and coating was required to produce the apertures
in a gold palladium film. This produced the first manmade radiating dipole in the optical regime with an effective near-field that could be investigated.
What was particularly exciting about this advance
was that, as a result of the fact that the size of the aperture was clearly determined, the amount of light from
such an aperture could be related exactly to the aperture
size. What was surprising was that these apertures made
in Au/Pd showed a throughput that was clearly larger
than what would be expected from either electromagnetic
calculations or simple illumination area versus transmission estimation. This led to the conservative statement in
this paper “Although the transmission through the 1 μm
apertures appears consistent with exact electromagnetic
field calculations, there appears to be more transmission
through the smallest apertures”.
Being conservative in those days was prudent since to
make such apertures was extremely challenging. Therefore, to make a definitive statement about having more light
transmitted than electromagnetic calculations one needed
apertures with different spacings in numerous materials
and this was experimentally inaccessible. Nonetheless,
having the thickness of the metal film, the exact aperture
dimensions of arrays of apertures from 1 μm to 15 nm in
the same film as verified with electron microscopy and
making accurate throughput measurements on many of
the apertures in the array allowed us to accurately predict
the ultimate resolution of near-field optical microscopy
with apertures to be 50 nm. No verifiable example of higher
resolution has been reported. Furthermore, our assertion
of higher light throughput than predicted by electromagnetic calculations based on Maxwell’s equations were verified by the elegant results of Ebbesen published 14 years
later [3]. These later measurements used fully the power of
the availability in the late 1990s of ion beam lithography
that opened the horizons of the nanofabrication needed to
definitely prove without a doubt such an assertion.
3 Early concepts
Up until this point no work emphasized in any publication
the near-field for imaging. This was in spite of the fact that
there were four periods where related research appeared
without any enunciation of the concept of the near-field
originally associated with the electromagnetic radiation
pattern of a radiating dipole.
The first was work done by Synge in 1928 when three
things were highlighted. The first was a possible use of an
aperture as an element for super-resolution. The second
was placing the aperture at a tip and the third was possibly producing some sort of aperture by crushing such
a fully coated protruding tip of a glass shard against a
surface. Synge made no attempts to experimentally verify
or theoretically quantify any of these concepts [4–6].
In 1956 another paper appeared by O’Keefe [7] who
did not know of the work of Synge. This paper used as
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A. Lewis et al.: The optical near-field: super-resolution imaging 5
a paradigm for super-resolution what could be achieved
by the “occultation of a star by the moon (which) yields
(as noted by O’Keefe) a somewhat analogous situation. A
study of the light variation with time during the occultation allows a fellow with a small telescope to measure one
component of the separation of a double-star pair which
cannot be resolved with the biggest optical telescope.
Occultation by an asteroid gives another large factor in
resolution; that is how the rings of Uranus were discovered” [8]. Obviously once again in this paradigm there was
little connection to the optical near-field.
Then in 1972 a paper appeared by Ash and Nichols [9]
who did not know of the work of either O’Keefe or Synge.
They investigated the concept of a sample being scanned
opposed to a closely spaced aperture in the microwave
regime and obtained λ/60 resolution. In spite of this work
the optical near-field is a very different problem since
as was mentioned in an early paper [10, 11] metal films
around apertures have infinite conductivity in the microwave region of the spectrum and so the contrast between
the aperture and the film is large. In the optical regime
the metal film around the aperture has finite conductivity
and this limits the contrast and the possible effectiveness
of such a concept in the optical regions of the spectrum.
Finally, in 1984 the first papers appeared in the
optical regime. We were not aware in our paper of Synge
and O’Keefe but a reviewer of a grant proposal alerted us
about Ash and Nichols.
In concert with our paper a report by Pohl et al.
appeared which emphasized Optical Stethoscoopy rather
than the near-field [12]. It was noted in this paper that a
stethoscope using sound waves also obtains super-resolution as an aperture scanning over a surface. As was
noted [11] however, acoustic waves always exhibit a propagating mode even in a sub-wavelength aperture whereas
electromagnetic waves are evanescent in subwavelength
apertures. Nonetheless, Pohl, also unaware of Synge and
O’Keefe used the concept of Synge of crushing a coated
glass tip against a surface to obtain an aperture and scanning a sample under such a glass tip. The aperture in this
study could not be quantified and thus, the origins of
25 nm resolution reported in this paper are not clear.
4 Some next steps
Within the context of the above it is worth noting that our
emphasis of the near-field led to the first calculations also
from our laboratory [10] that showed the near-field extent
as applicable to near-field optical microscopy. And finally
the acronym near-field scanning optical microscopy or
NSOM which has become widely used was introduced by
our laboratory in a paper in the Biophysical Journal in 1986
[11]. This paper also detected for the first time fluorescence
through such apertures a concept recently expanded by
Ebbesen et al. [13].
Such point fluorescence through subwavelength
apertures in the near-field has also taken on several other
names including “zero mode waveguide” [14] for the
detection of point fluorescence with small illumination
volumes as in our earlier paper. Such concepts have been
applied to DNA sequencing [15] and fluorescence correlation spectroscopy which has the potential for great import
in biology with the resolution of instrumental problems
of NSOM as applied to soft samples as noted below [16,
17]. With such advances in the near-term there should
be exciting applications in biology (see for example the
paper in this issue by Bhattacharya and Mukhopadhyay
on the fluorescence of amyloid fibers on page 51).
5 N
ear-field optical approaches
within the context of other
more recently developed super-­
resolution methodologies
So where does near-field optics stand today? Obviously
it is today recognized as a critical and important field of
optics that has made numerous contributions and is set to
make even more. Some of these advances are described in
the papers included in this issue. As noted above however,
in this report we would like to place these papers within
the context of the numerous other areas of application
that could obviously not be covered in one issue of a
journal. Our goal is to place near-field optics within the
wider field of super-resolution optics or attempts to break
the Rayleigh diffraction limit. Furthermore, it is an objective of this report to indicate the areas of application and
developing application that have led the optical near-field
to have a pervasive influence in nearly every area of optics.
From a subjective perspective we can say near-field
optics not only has made clearly critical contributions
since its introduction in 1984 but is in an exponentially
rising phase where its influence is expected to be ever
more pervasive in the future than in the past.
What are the highlights of today that can lead to this
future?
First, it should be noted that near-field optics has
already demonstrated the highest resolution of any
method of optics in the most general of circumstances.
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6 A. Lewis et al.: The optical near-field: super-resolution imaging
Over the years since its introduction the concept of
breaking the diffraction limit of Rayleigh however has
become a central focus of optical imaging. This has led to
many new concepts.
An example is stimulated emission depletion or
STED microscopy [18] where depending on the intensity of light that the sample can withstand fluorescence
super-resolution can be obtained in the far-field below
100 nm resolution. The technique uses ultrafast lasers to
stimulate the emission of dye molecules thus achieving a
narrower point spread function than would be obtained
normally. This technique is limited to certain types of dye
fluorescence.
To this can be added the exciting recent advances
of PALM [19] and STORM [20]. Both of these techniques
are once again limited to fluorescence and depend on a
specific sub-class of dyes that can undergo reversible
photobleaching. As a result of such bleaching closely
spaced dye molecules can be interrogated individually
and placed accurately as a result of an analysis of their
far-field radiation pattern. This subclass of techniques
also obtain super-resolution in far-field optics.
All of these far-field techniques, in addition to their
intrinsic difficulties and limitations of being limited to a
specific class of fluorophores have important limitations
that have not been solved by any technique other than
near-field optics. These are out-of-focus light contributions, the inability to be correlated with 3D structure of
the object under investigation and the inability to obtain
phase information on the electromagnetic field distribution of the object under study. Thus, when one wants to
determine accurately the point spread function of the
beam in STED a near-field optical element has to be used
due to its generality of application [21].
Within this concept of super-resolution in far-field
imaging an exciting new development is the use of the
near-field optical point source radiating dipole as a reference source for a far-field image obtained with a CCD
camera in order to obtain without iteration the exact
phase and 3D structure of an object [22]. Added to this is
the connection of the placement of this radiating dipole
reference source with the precision of atomic force
microscopy which has the potential to offer super-resolution imaging even in non-fluorescent mode by using
the instrumental implementation of a non-obstructing
scanned probe microscope/NSOM system. Furthermore, such imaging implementations based on nearfield optical developments has already the potential
to be combined with such modalities as STED in order
to achieve 3D structure and phase together with STED
super-resolution. These concepts of a reference source
should also be very effectively combined with other
modalities of far-field imaging such as structured illumination [23] which also like STED and PALM/STORM can
achieve far-field super-resolution.
In spite of the generality of the imaging and superresolution modalities available with near-field optics as
shown in this issue, the optical near-field does have of
course inherent depth limitations not shared by far-field
techniques noted above. Nonetheless, this disadvantage
has to be viewed within the ability of near-field imaging to
provide phase information without any out-of-focus light
contribution. Such information when combined with the
amplitude that is known at each point in the image allows
for back propagating the wave front which could in principle define the nature of the wave front at any depth of the
object being imaged.
The power of this combination was clearly enunciated early on in the application of near-field imaging
to photonic band gap materials [24]. This has applicability in silicon photonics [see cover page of this issue
where the propagation of the amplitude and phase of a
wave in a silicon waveguide in 3D was readily obtained
using the unique feedback features offered by normal
force tuning fork feedback discussed below]. Obviously
such information is also of importance in plasmonics
and a report on plasmonic applications is included in
this issue on page XX. Other aspects of the importance
of phase will be discussed below within the context of
advancements of different available probes in nearfield optics.
6 A
n instrumental perspective of
present and future advances
In order to achieve the bright future for near-field optics
predicted in this article new concepts in optically integrated scanned probe microscopy have to be emphasized. Within this goal it is important to note that the
1984 introduction of near-field optics occurred 2 years
before the introduction of atomic force microscopy
(AFM) [25]. And, even after this critical development of
AFM which near-field optics depends so integrally on,
the probes used, the scanning mechanisms employed
and the feedback implemented all had serious limitations as far the development of the optical near-field.
Thus, in this section we will deal with each of these elements of an NSOM system since, as noted, instrumental limitations have been the main hurdle in its full and
general applicability.
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7 The probe
7.1 First steps
The most successful idea to develop a near-field optical
probe was introduced by Harootunian et al. in 1986 [26].
This method realized that glass pulling technology which
was so prevalent in biology could be applied effectively
to readily form a near-field optical aperture at the tip of a
tapered glass structure. When introduced by Harootunian
et al. the available glass pullers had nichrome wires as
heaters which could only pull borosilicate glass. Subsequently, the company, who has practical monopoly on such
pullers, Sutter Inc., in Novato, California, produced a puller
based on a carbon dioxide laser and this permitted fibers
to be pulled by the same technology. By then Eric Betzig
who was in the second wave of students at Cornell working
on near-field optics had moved to Bell Labs and published
the first paper with pulled fibers using the technique of
Harootunian. Fibers showed of course greater efficiency of
t­ransmission and so larger signal and in a prelude to nearfield microscopy Massey had already suggested this [27].
Already in the 1986 Harootunian paper fluorescence
imaging with near-field optics was reported and this was
done with a straight pipette probe into which a fiber was
inserted. The index of refraction mismatch between the
fiber and the air in the pipette and the subsequent region of
evanescence in the subwavelength aperture of the coated
pipette made for low efficiency. Nonetheless, even with
such low efficiency fluorescence imaging was possible.
In this issue, in addition to the work of Bhattacharya and
Mukhopadhyay we see a highly advanced version of NSOM
fluorescence in the paper of Berezin et al. on “Multiprobe
NSOM Fluorescence” that has many future possibilities.
The early work in near-field optics all used straight nearfield optical elements which worked without any feedback.
The groups of Toledo Crow [28] in Rochester and Eric Betzig
in Bell Labs [29] first introduced feedback for straight probes.
These shear force methods were rapidly replaced by the much
more successful method based on straight NSOM probes held
with a parallel tuning fork along its axis also using shear
force. This latter method was introduced by Karrai [30]. An
advanced version of a tuning fork method using normal force
tuning fork methodology was used with a cantilevered glass
NSOM fiber probe in the paper by Mrejen et al. [31].
7.2 Cantilevered NSOM probes for reflection
Cantilevered glass fiber probes were introduced in 1995 [32]
together with the first open axis SPM system such probes
resolved the problem of reflection NSOM since such cantilevered probes, unlike 99% of AFM probes have the probe
tip exposed. In addition, the angle of the bend is made
such that the angle of a light source through a lens from
above follows the outer periphery of the tapered glass tip
thus not obscuring any of the light from the surface from
being illuminated through the probe and collected by the
lens.
Cantilevered glass probes made reflection NSOM
imaging routine and also permitted illumination from the
top with a lens and collection with the NSOM probe. Such
geometries also allowed for Raman scattering in standard
upright geometries. As indicated in an early publication
on the utility of such probes for tip enhanced Raman scattering “the tip apex is not shadowed, as the laser beam is a
focused beam with a converging (or for that matter collecting) angle of 26.7°, while the half angle of the tip is only
about 4°. In addition, the tip is not perpendicular to the
sample surface where the angle between the tip axis and
sample surface is about 60°” [33].
Reflection NSOM has growing import in many applications including those in the semiconductor industry as
was noted by Lewis et al. [34]. Although these semiconductor applications did not generate much interest when
originally reported, today with device structures becoming
smaller and the near-field technique of a solid immersion
lens introduced by Mansfield and Kino in 1990 [35] reaching its limit of 160 nm apertured based NSOM has become
quite attractive. Of additional importance is the fact that
aperture near-field optics is a near-surface technique that
clearly allows the images of devices under surfaces of polished silicon as seen in the images reported by Lewis et al.
[34]. Therefore, working devices are clearly amenable to
imaging with < 100 nm optical resolution since they could
be covered by chemically mechanically polished surfaces
as was reported earlier [34].
Furthermore, the reflection geometry has proved very
interesting for possible high resolution refractive index
monitoring. In a most interesting paper Aristide et al. [36]
have reported that as one moves a near-field aperture from
a surface that it is illuminating a standing wave interference is detected between the aperture and the surface. This
allows for an analysis of the index of refraction of the solid
surface since the probe position is very well determined
at each Z point by the on-line AFM capability, standard in
NSOM of today. In principle the reflection off a surface by
light should be indicative of the index of refraction of the
surface. However, in near-field optics, the surface changes
the boundary conditions of the probe. Thus the reflection
signal with the probe in contact or very near contact with
the surface is a complex function of the surface and the
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8 A. Lewis et al.: The optical near-field: super-resolution imaging
new boundary conditions imposed on the probe by the
surface. The method of Aristide addresses this problem
by the detection of reflection as a function of controlled
and known distance of the probe from the surface. Thus,
reflection near-field optics could have significant importance for the accurate determination of index of refraction
with high spatial and refractive index resolution. This is of
growing importance for devices such as silicon and other
solid state waveguides that are of increasing importance
in areas of optical interconnects in the semiconductor
industry and in other areas of integrated optics.
Also within the realm of near-field reflection is the use
of a super-continuum laser for spectrally resolved nearfield imaging which has been done already quite a few
years ago with these glass probes. It has been shown that
such super-continuum sources can be coupled readily to
near-field optical fiber probes and this opens numerous
possibilities for white light imaging with near-field optical
spatial resolution [37].
It should be noted within the context of the above
even though pulled glass technology probes as discussed
above today account for the largest group of applications
of NSOM. There are other probe concepts that have to be
noted. First, was the introduction by Oesterschulze et al.
[38] of the method of a silicon probe with an aperture in
the silicon cantilever. Such a probe has to be illuminated
with a lens from above and thus cannot be scanned. Also
such probes essentially preclude reflection NSOM since
the aperture needs to be under a silicon cantilever that
effectively blocks collecting the reflected signal from the
surface.
The glass optical probes, in addition to having a
probe tip exposed to the optical axis and not obscured by
the cantilever, have waveguiding properties and thus can
be scanned relative to the sample. This is of importance
when one wants to inject light at one position in a photonic device and image the light distribution as a function
of distance from this injection point. In such a case the
sample should be fixed. It also has importance when one
wants to inject light at one position in the sample with an
NSOM probe and then collect light from above or below
with a lens at another point in the sample.
This geometry was used in what is one of the most
cited applications of near-field optics and was one of the
papers that stimulated a lot of the interest in plasmonics
that developed. The paper by Maier et al. [39] used the
platform described in our 1995 paper [32] to illuminate
a plasmonic waveguide of silver nanoparticles spaced
approximately 50 nm apart. This waveguide at its end was
placed in close proximity to a fluorescent bead. The nearfield optical probe acted as a fixed illumination point and
the fluorescence bead acted as a fluorescent detector of
the plasmonic propagation. The fluorescence was then
detected with a lens of a standard upright microscope
directly above the bead since the surface was opaque.
The geometry of the platform allowed for a completely
free axis from above. This paper has been cited 1621 times
as of March 2014. It has spawned many important investigations with NSOM including those described in this
issue by MacNaughton et al. on page 33. Today the field
of NSOM investigation of plasmonic structures is ripe with
excitement with many new and critical experiments constantly appearing [40].
7.3 Coax probe geometries
In addition to the above developments of near-field
optical probes there has been interest in effectively producing an optical coax. A proponent of such structures
has been Fischer who was part of the pioneering efforts
in near-field optics [41]. This is a technique that uses plasmonic effects to increase the propagation of light at the
tip of a subwavelength structure. A recent emulation of
this concept is far-field illumination of a gold wire that
guides plasmons along the wire to its tapered tip where it
emits photons. Consistent with what was determined for
conventional apertured glass probes earlier [42] femtosecond pulses propagate while maintaining their temporal profile and pulse characteristics [43]. Many concepts
in this later area of metallic nanofocusing have to be credited to the theoretical work of Stockman [44]. Materny in
this issue on page 61 focuses on ultrafast imaging applications with NSOM.
More recently there is the very interesting development of trying to achieve a near-field optical aperture with
higher efficiency by extending such coax concepts. Basically this work of Bao et al. [45] has sculpted an aperture
based probe at the tip of a fiber with more efficient plasmonic propagation and thus more efficiency. Photoluminescence of InP nanowires is investigated with resolutions
using 40 nm. The AFM of such probes is compromised
and the resolution sits close to but a little bit lower than
conventional NSOM fiber probes (see Multiprobe NSOM
Fluorescence in this issue). The probes as they presently
are conceived would, from a general perspective, have
difficulty in integrating in several applications, such as
multiprobe pump/probe NSOM to be discussed below.
Nonetheless the paper of Bao is exhilarating since it shows
both the excitement in near-field optics and the evolution
of computation since the first near-field computation as
applied to microscopy [10].
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A. Lewis et al.: The optical near-field: super-resolution imaging 9
7.4 F ar-field illumination and apertureless
probes
An early example of light at the tip of a probe that could
be used for near-field imaging was the production of fluorescent material at the tip of a probe [46]. This was later
extended in a beautiful piece of work to the single molecule limit at the tip of a probe at 1.4°K [47]. This approach
has great potential either as a far-field optically excited
point of light in the near-field of a sample or as a fluorescent detector of a near-field effect as illustrated by Maier
et al. [39].
Sensing ionic changes with such a fluorescent NSOM
far-field excited probe has also been demonstrated [48].
Along this same line is an electrically excited light source
in the near-field that was originally reported by Kuck et al.
[49]. In this same vein and of considerable interest is an
actively (electrically) excited near-field heat source on a
cantilever that is exposed to the optical axis [50]. This has
potential as a broad band infrared source. An FTIR spectrum of such a black body source is shown in Figure 1.
Such probes have considerable potential in ultra-high
resolution infrared microscopy. Initial work in this direction was done by Dekhter [51]. All of these approaches
have, for generality of application, to be coupled with an
appropriate general feedback mechanism that would work
even with the softest samples. This issue will be dealt with
in more detail when feedback in NSOM is considered.
In this vein is also the concept of scattering or aNSOM
which is an apertureless NSOM technique with a passive
rather than active probe tips as described above. This
concept has evolved into a laser focused in the far-field
onto a tip of an AFM probe. The far-field focus is of course
much larger than the size of the tip and so there is a lot
of unwanted scatter. Modulation is used in concert with
interferometric techniques to try and extract the near-field
of the tip. This has been recently described with great
clarity by Scott et al. [52].
The technique has generated most interesting results
for elastic scattering of photons. However, as noted in
the literature “the technique consists of the presence of
an intense background light reflected from outside the
interaction area between the probe and the sample, which
makes the detection of the near-field scattered light very
difficult” [53]. Also reliability and consistency problems in
comparing the same or different materials have appeared
[54] and these probably arise from the fact that the technique depends a great deal on instrumental parameters.
These include the oscillation amplitude of reference
mirrors in the interferometric techniques used to try
and isolate the signal of the near-field. A variety of other
factors are also shown to have an influence including any
slight change in the tip sample distance, tip roughness
and shape etc and as the effect is further modeled new
influences are uncovered. As a result, the literature is not
without its artifacts.
In addition to the fact that there is a lack of generality since inelastic photons cannot be interrogated with
such techniques there is also the issue of background
free imaging which is inherent in many near-field optical
techniques and is required in many areas. An example is
fluorescence where especially organic materials are very
susceptible to bleaching of the surrounding by the farfield illumination.
2.50E-03
Intensity 0 mA
2.00E-03
Intensity 50 mA
Intensity
Intensity 70 mA
Pt-wire
Intensity 90 mA
1.50E-03
Intensity 110 mA
1.00E-03
5.00E-04
Glass
0.00E+00
0
500
1000
1500
2000
2500
3000
Wavenumber (cm-1)
Figure 1 A glass encased dual wire thermal source AFM sensing probe with its associated IR spectrum for a variety of currents traversing
through the junction. One readily sees in air the spectrum of water vapor and carbon dioxide in the surrounding excited by the probe.
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10 A. Lewis et al.: The optical near-field: super-resolution imaging
Additional examples of the need for background
free super-resolution illumination not provided by other
methods available today is in the area of photoconductivity described by Narayan in this issue on page 19.
Furthermore, in addition to such needs for background free illumination there is the need in photonics to
be able to collect a light field attached to the near-field of
a surface or for that matter profiling in free space a propagating fields from say a point source without out-of-focus
contributions.
In a number of spectroscopies such as linear and nonlinear Raman spectroscopy or second harmonic generation apertureless methods are certainly applicable since
there is no issue of bleaching which requires light confinement and the scattered radiation is at another wavelength.
Thus, far-field illumination can be used to excite the tip of
a probe and such approaches have been used in Raman
spectroscopy (see article by Zachary Schultz in this issue)
or such approaches have been employed in second harmonic generation [55].
In the vein of vibrational spectroscopy infrared interferometric methods are applicable if infrared lasers are
employed. However, there are competing methods both
with active NSOM sources as noted above [50, 51] and new
modes of detection that could have significant import
in this area also. This is especially the case in view of
the relatively low signal to noise that results from infrared scattering NSOM. These new methods involve either
nanometric temperature sensing which is appearing to be
ultrasensitive in the milli degree centigrade range [56] or
employ ultra sensitive capabilities of normal force sensing
of the photon force [57].
In summary, apertureless NSOM within its limitations of applicability is making important contributions.
Current instrumental advances to be discussed below have
the potential to make this an even more fruitful source of
research in the future. This is especially the case when
one considers multiprobe SPM/NSOM described at the
end of this article and illustrated in the paper by Berezin
et al. in this issue. Such methodologies have the potential
to reduce the extent of the scattered light component in
order to improve the signal to noise. An example of this
is readily seen as a gold nanoparticle probe comes within
the near-field of an apertured probe. This is the case even
when a slightly larger aperture where trillions of photons
are visible in the link of the video included [58]. Also the
increase in the signal to noise offered by such apertureless
protocols has the potential to replace the scattering probe
with a low dielectric glass probe which would have a
highly reduced perturbation on the investigated structure
especially those where plasmonic imaging is important.
8 The feedback mechanism
A repeating theme in this overview is the quest for generality of near-field optical application. Within this stricture
another repeating theme is to achieve low background
that reduces noise. In essence, as all experimentalists
know the way to better characterization and imaging
methodology is certainly to increase the signal but of even
greater importance is to reduce the background and any
noise sources.
This section deals with Feedback Mechanisms that
can be used with near-field optical probes to achieve both
these aims: Generality and lower noise.
In addition, this section deals with a great attribute of
near-field optics the ability to provide the optical distribution within the three dimensional structure of the sample
being studied. In order to fully achieve this, one needs to
have the ultimate in AFM both in terms of resolution and
in terms of force sensitivity and positional sensitivity relative to the sample.
What can provide such sensitivity?
To understand the answer to this question within
what is understood about force microscopy and NSOM
fiber probes which are the most generally applicable
optical element in near-field optics let us consider the
alternatives within the context of these probes.
The standard method that is used with atomic force
microscopy to monitor mechanical properties of materials
such as elasticity and adhesion is based on beam bounce
technology (see Figure 2). In this technology a laser beam
bounces off an appropriately chosen cantilever that is soft
enough for the particular material that is being investigated. The laser beam then tracks on a position sensitive
detector the bending of the cantilever or its amplitude as a
surface is being approached and this is analyzed for trying
to understand the mechanical properties of the material
of interest.
With such an approach there are two major mechanical problems. The first is jump to contact. This occurs on
the approach of an AFM cantilever to a surface. As the tip
of the cantilever approaches within ∼10 nm of the surface
Figure 2 Beam bounce AFM feedback.
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A. Lewis et al.: The optical near-field: super-resolution imaging 11
there is jump to contact since the cantilever is bent by the
surface forces. This effectively negates analyzing forces or
other physical parameters such as the intensity of light
at such ultra close distances where extremely interesting
phenomena are occurring.
In addition, on retraction from the surface there is an
adhesion force that keeps the probe stuck to the surface.
This causes a second instability in monitoring the force as
a function of the distance of the probe from the surface.
During this entire process an oscillation of the probe
is induced to try and understand the change in amplitude
of the probe that results from the elasticity of the material under investigation. Thus, these instabilities mask
the true interaction of the tip with the surface and complicated methodologies both mathematical and instrumental
are needed to try and unmask the behavior of the interaction. However, in spite of such valiant attempts even the
critical exact point of touching the surface is masked and
estimated.
What one would want is a smooth approach and
retract curve from the surface without such discontinuities. In essence what is really needed is that the
tip would smoothly approach the surface monitoring
the force with ultra sensitivity even at ultra short distances while monitoring optical phenomena and force
simultaneously.
In a pioneering series of papers it has been realized
that the best way in which to avoid these and other problems is to use the frequency of the oscillation of a cantilever rather than the amplitude. However, soft cantilevers
even those like near-field optical probes that are relatively
stiff (1–10 N/m) oscillate at many eigenfrequencies and
have a broad spectrum. Such a broad spectrum is very
insensitive to the small changes in frequency that ultra
weak forces impose.
An alternate to very soft cantilevers is the other end of
the spectrum and that is the use of very very stiff cantilevers that have a very sharp frequency of oscillation since
the stiffness prevents other frequencies of motion. When
there is such a sharp frequency spectrum there is incredible sensitivity to small changes in ultra-small forces of a
surface interacting with a tip. This method of feedback is
called Frequency Modulation.
An ideal example of a stiff device that is even selfoscillating (since it is formed of a piezo material) is a
tuning fork which has very sharp resonances. The sharper
the resonance, the sharper the Q or the quality of the oscillator or Q factor. Correspondingly, the higher the Q the
higher the sensitivity to any external perturbation such as
surface forces. Thus, Frequency Modulation with tuning
forks is ideal for force spectroscopy.
This has resulted in some spectacular successes ­[59–
61] and have validated the fundamental force limits of a
tuning fork that were estimated to be less than a pN over
a decade ago [62].
The utility of tuning forks in near-field optics was due
to the pioneering efforts Karrai [30]. These efforts focused
on how one may apply tuning forks to straight near-field
optical glass probes. For such probes the tuning fork was
placed with its axis normal to the surface under investigation (see Figure 3). Such a configuration was used with
a driving oscillation that monitors the shear force or frictional force of the sample.
Many instruments were based on this elegant work
but in terms of both force sensing and generality of application this approach was not ideal. In terms of generality of application this geometry perturbs the optical
axis from above and makes it quite difficult to work in
liquid. In terms of force sensing shear forces are perturbing to surfaces and are less sensitive and more difficult to
account for theoretically. On the other hand cantilevered
near-field optical probes can be combined with the ultra
high sensitivity using a normal force geometry as shown
in Figure 4.
Excitation source
Mounting
with electrical
connections
Detector
Driving
oscillation
Figure 3 Shear force tuning fork/fiber probe geometry [63].
Figure 4 A cantilevered NSOM fiber probe mounted in a normal
force geometry for ultra sensitivity of force even with stiff probes.
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12 A. Lewis et al.: The optical near-field: super-resolution imaging
This geometry allows for the ultimate in force sensing
and of greater importance can fully implement the pioneering theory of Sader and Jarvis that has shown theoretically that it is possible to derive accurate formulas for
the force versus frequency in such Frequency Modulation
feedback methods [64]. These formulas relate the change
in Q which is related to the frequency change to the force.
Recently, Kohlgraf-Owens et al. has shown that
the force of a photon can be measured with such glass
probes and normal force tuning fork. A force of 1.6 pN was
detected for the force of a photon. This is close to the ultimate in force sensitivity predicted by Grober et al. [64].
This latter work has been extended to use force for imaging
the photon distribution in the near-field [57] and also see
article by Dogariu in this issue. Such an elegant approach
that results from the ultimate sensitivity of the tuning fork
has many applications as noted by the author especially
in the area of a new detection methods for imaging vibrational absorption in the infrared in an apertureless fashion.
In addition to the tuning fork attributes noted above
there is an additional characteristic of tuning forks that
is critical for achieving the ultimate in force spectroscopy.
To appreciate this attribute consider, once again, that the
Q and the associated frequency is used as the feedback.
Thus, as the tip attached to the tuning fork approaches the
surface the Q changes and this is monitored as a smooth
force distance curve where there is no jump to contact
or adhesion ringing. Along with the frequency change
the tuning fork is of course being modulated in this Frequency Modulation scheme. As a result of this, the tuning
fork amplitude of course can be monitored in a separate
channel unrelated to feedback. At the very point at which
the tip touches the surface one experimentally sees the
amplitude change as diagrammatically illustrated in
Figure 5.
This diagram shows an Amplitude vs. Distance graph.
As the probe approaches the surface from say the right on
F
FT
T
FR
Amplitude
R
I
FM
M
Retract
0
Approach
Distance (d)
Figure 5 An amplitude versus distance dependence seen simultaneously with the frequency alterations of the tuning fork as it
approaches and retracts from the surface.
the graph, from I, the point at which the slope abruptly
changes, T, is the point of contact without jump to contact.
This point can readily be verified by taking the intersection of straight line I T and the straight line TM. At M one
has the point of greatest penetration. Similarly the point
of retraction from the surface without ringing is readily
and smoothly determined by the intersections of MR with
the straight line RF.
Obviously the forces are related by Sader and Jarvis
[63] to the monitored Q at T, M and R which are FT, FM and
FR. The change in the slope of the amplitude distance
curve gives the point of contact and release of the probe
tip to and from the surface. Thus, one knows experimentally the point of contact and this gives all of the parameters for accurately determining the forces even 1 nm from a
surface and for calculating with experimental parameters
Young’s modulus. Of course one also gets energy dissipation of the probe on-line from the change in amplitude at
each pixel without any need for calculation.
The above is opposed to beam bounce feedback where
the amplitude is integrally tied in with the feedback and
there are additional problems of jump to contact and adhesion ringing instabilities. This combination of interrelated
effects require on-line digitization of the force curve and
subsequent deconvolution from the above complications
and even then one gets only an estimate of the amplitude
and the point of contact.
Of critical importance to NSOM is that the NSOM
probes have relatively large force constants (1–10 N/m)
which precludes applications with beam bounce amplitude feedback methods. Tuning forks with their close to
19,000 N/m force constants readily accommodate such
force constants while permitting the ultimate in force
sensitivity and permitting ultraclose approach without
jump to contact. This readily permits such applications
as energy transfer between a tip and a surface that was
initially attempted using tunneling NSOM feedback [65].
With no jump to contact tuning fork feedback one
can now switch on-line using an NSOM probe or for that
matter any probe from force feedback to tunneling feedback. Shown in Figure 6 is the same atomic step on a
highly oriented pyrolytic graphite (HOPG) surface done
sequentially with force and tunneling using an etched
gold wire probe similar to the one described above within
the context of fsec pulse propagation. Why is this important for NSOM? First tunneling to a gold surface has been
shown to be an effective method to enhance Raman of
molecular species placed on gold [66]. Second tunneling
is a known method to investigate plasmonic optical effects
[67]. Third tunneling is a good method to induce carriers
in semiconductors. Thus the ability to switch between
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A. Lewis et al.: The optical near-field: super-resolution imaging 13
Single HOPG atomic step
3
1
Z (nm)
Normal force tuning fork
based AFM has no jump
to contact thus tuning
forks can allow for AFM
and tunneling with the
same Au probe
Z (nm)
2
0
-1
AFM
-2
0
0.5
1.0
1.5
2.0
2.5
3.0
3
2
1
0
-1
-2
-3
Tunneling
0
X (µm)
0.5
1.0 1.5
X (µm)
2.0
Figure 6 Monitoring the same atomic step in highly oriented pyrolytic graphite alternately with AFM and Tunneling.
AFM and tunneling in the same probe is most significant
especially within the context of the discussion of multi
probes in the next section of this article.
The great utility of tuning forks has even been realized by the biggest AFM silicon probe manufacturer,
Nanosensors (see link: http://www.nanosensors.com/
news_10112008.html). In its website Nanosensors rightfully says that these new tuning fork probes are “ > > for
creating a new generation of scanning probe microscopy
(SPM) instruments.”
Nonetheless, commercial manufacturers have been
reticent to embrace tuning forks since a way had not been
found for making them multifunctional and using tuning
forks in aqueous conducting media. Over the years this
problem has been solved by coating the tuning fork with
a mono molecular layer of pyrelene. This has opened the
great utility of tuning forks generally to a variety of problems in biology where the ultimate in force sensitivity is
required.
In summary, normal force tuning fork feedback is the
ideal feedback for NSOM for numerous reasons. These
include:
–– The ultimate in force sensitivity allowing either single
atom manipulation [59] or single photon detection
[60] or apertureless force based detection for imaging
in a broad spectrum of wavelengths including the mid
infrared [64].
–– No jump to contact instability giving the ability to
investigate optical effects even within one nanometer
of a surface.
–– No adhesion ringing.
–– Direct experimental determination of energy
dissipation in the sample by monitoring the
amplitude of the tuning fork which is independent of
the feedback based on frequency and thus requires
no computational algorithms to image energy
dissipation.
–– The only feedback method that experimentally and
instrumentally without computational estimation
gives the point of contact with the surface.
–– Obtaining all parameters experimentally for providing
Young’s modulus using presently available equations
such as those of Derjaguin et al. [68].
–– Allowing for on-line switching between force and STM
feedback.
–– Overcoming the problem of the high force constants
of NSOM probes.
–– Having higher Q factors and correlated force
sensitivity in liquids by several orders of magnitude
over those achieved in beam bounce feedback even
with NS0M probes.
–– Having ultra sharp frequency spectra for lockin, homodyne and heterodyne operation with no
positional jitter in the Z axis.
–– Allowing for exceptional interferometeric operation.
–– Having no background light from optical feedback.
Thus, the turning fork fulfills fully the mandates noted
in the beginning of this section of generality, low noise
both optical and mechanical and the ultimate in force
sensitivity. Furthermore, as will be discussed in the next
section it also fulfills other required characteristics of
simple optical integration and makes multiprobe operation readily possible.
8.1 The scanners
Thus, far in this instrumental perspective we have concentrated on probes and feedback as would be most appropriate for NSOM. We now speak about scanning mechanisms.
Most cylindrical
scanner geometries
are obscuring from
above and have
limited Z ranges
Frequency
generator
Lock-in
amplifier
X Y Range 100 µ
and Z range 6 µ
Figure 7 Cylindrical piezo scanners in a generally applied standing
geometry [69].
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14 A. Lewis et al.: The optical near-field: super-resolution imaging
An oft employed scanner solution for fiber probes
and for that matter all AFM systems are cylindrical piezo
devices which are standing upright. These block lenses
from above for reflection NSOM which in any event is
complicated with either straight fiber probe solutions or
silicon cantilevers with holes.
To overcome these problems and to deal with the
issue of extent of Z scanning while having a geometric profile that is amenable for optical and electron/ion
optical integration, our laboratory invented one of the
first reported flexure scanning mechanisms [32]. This is
shown in Figure 8. Specifically these scanners use 4 such
cylindrical piezo elements that are lying flat. This resolved
the problem of having a large Z range which is critical in
optics without affecting the X Y range of the scanner. This
occurs since the motion of such piezos against the piezo
axis is large while the axial extension of the scanner is
very small. It also accomplished this in a very small form
factor and a free optical axis. Thus, these ultrathin < 7 mm
scanners allow for a form factor that permits unique
compact SPM geometries. An example shown in Figure 8
was the first probe and sample scanning SPM and had
such an ultra compact geometry due to the form factor
of these scanners. The importance of probe and sample
scanning is seen in several papers in this issue. A particular interesting example is the use of such a system
in an electron beam (see paper by Haegel on page 75). As
noted in original work by Haegel with this geometry the
platform “provides capability for independent scanning
of both sample and tip. The electron beam is incident in
a fixed location and the light is collected through a fiber
probe, operating simultaneously as the tip for atomic
force microscopy and near-field optical collection. The
unique open architecture is critical to the ability to scan
the collecting probe, while keeping an independent generation source fixed at a point of interest on the sample”
Non-obscuring
UltraFlat Scanners
with an
X Y and Z Range 85 µ
Figure 8 Ultra flat scanning mechanisms with large Z and XY range with free optical axes together with a probe design that permits a free
optical and electron/optical axis from above or below using tip and sample scanning on-line.
Figure 9 Examples of integration of these platforms in diverse imaging tools including Raman (left) and SEM (right).
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A. Lewis et al.: The optical near-field: super-resolution imaging 15
Figure 10 The combination of cantilevered glass based probes, normal force tuning forks and unique scanning mechanisms allow for
multiprobe operation with a completely free optical axis for integration of all such platforms in many imaging and spectroscopic imaging
modalities. Shown in this figure above is an ambient multiprobe design (left) and a cryogenic version (right) of these AFM/NSOM based
multiprobe stations.
[70]. Using these attributes of this combination light generated by carriers in this semiconducting material by the
electron beam could be imaged with ultra high spatial resolution and full correlation with topography of even low
profile structures difficult to profile in the SEM. In essence
this provided spatially correlated ultra high resolution
cathodoluminescence of the structures discussed.
Shown in Figure 8 the generality of the solution
allows for even dual optical microscope operation and
this is important for permitting both transmission, reflection, collection and illumination mode NSOM in the same
platform.
As noted together with the cantilevered nature of
the NSOM probe the combination has a free optical or
electron/ion beam axis and readily permits insertion
into many different types of spectroscopy and imaging
systems. This includes standard Raman systems and SEMs
as seen in Figure 9 and also permits (see Figure 10) multiprobe geometries in ambient and ultra low temperature
(10°K) multprobe AFM/NSOM based probe stations. The
latter have applications in many areas including studies
in plasmonics at low temperature, photoconductivity at
low temperature of 2D crystals such as graphene, MoS2,
WS2 etc. Even these low temperature platforms have
unencoumbered optical axes for transparent integration
with other imaging modalities and can incorporate high
field super-conducting magnets.
The multiprobe platforms described in this article and
developed as part of the research efforts of Aaron Lewis
have been used in numerous experiments. Examples of
such experiments are described in references [70–72] and
are also described in this issue in the paper by Berezin
et al. The goal of this article was to introduce the papers in
this issue of Nanophotonics and to place them within the
breadth of investigations in near-field optics. We end this
overview with an example of a measurement sequence
completed in our laboratories with the platforms that
are based on the principles indicated in this paper. The
example (in Figure 11) is chosen to highlight a super-resolution imaging task that near-field optics, with its generality, readily and uniquely achieves. In this task a v-grooved
A
C D
QWR293 -NSOM
QWR294 -Temperature
E
F
A
B
1 µm
B
1 µm
Figure 11 Multidimensional nanoptical, nanostructural and nanothermal characterization of a v-grooved quantum wire laser.
(A) Collection mode NSOM. (B) AFM structure of the polished surface
of the laser facet. (C) and (D) The mode structure of the laser at
805 nm and 805.8 nm respectively. (E) and (F) Comparison of the
optical and thermal distribution of the laser facet indicating that the
heating arises from phenomena other than the optical distribution.
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16 A. Lewis et al.: The optical near-field: super-resolution imaging
quantum wire laser emission is imaged by collection mode
NSOM relative to the on-line AFM [73]. The correlated
images accurately place the optical emission relative to
the structure at the apex of the v-groove. They allow delineation of the offset of the optical distribution (Figure 11A)
with the structure that is accurately positioned by the
AFM on the flat facet of this laser. These measurement
tools also permit full spectral correlation of the imaging
to give the mode structure of the laser from the light collected and displayed in Figure 11C and D at 805 nm and
805.8 nm respectively. No other technique could detect
this mode structure alteration with wavelength. The resolution clearly approaches 50 nm. A 0.5 mm bar is included
for comparison since it is the resolution normally achievable in a standard optical microscope. Note that at 0.5 mm
standard resolution only a very few points over the optical
distribution would have been distinguished. Finally, the
multi dimensional nature of these probe stations allow for
comparison of the optical and the temperature distribution. The results of the two images show that the temperature profile is less connected with the optical distribution
but rather is associated with bowing toward the p contact
in this quantum semiconductor laser structure.
In summary, as can be seen the advances in NSOM
both in its instrumentation and its penetration into
numerous areas of science and technology bode well for a
continual series of important contribution in many of the
areas of future scientific achievement.
Acknowledgments: The authors would like to thank the
Israel Ministry of Science and the NanoSci-E+ program
for grant support. Discussions with Rimma Dekhter of
Nanonics Imaging Ltd., Jerusalem, Israel on nanoIR imaging are also acknowledged.
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