APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 15
13 OCTOBER 2003
Terahertz imaging with nanometer resolution
Hou-Tong Chen and Roland Kerstinga)
Department of Physics, Rensselaer Polytechnic Institute, Troy, New York 12180
Gyu Cheon Cho
IMRA America, 1044 Woodridge Avenue, Ann Arbor, Michigan 48105
共Received 28 May 2003; accepted 15 August 2003兲
We report on the application of scanning near-field optical microscopy for terahertz imaging. We
demonstrate a spatial resolution of 150 nm for 2.0 THz pulses. Our experiments show the feasibility
of submicron THz microscopy for imaging of biologic tissues on the cell level or for the
investigation of individual submicron semiconductor devices. © 2003 American Institute of
Physics. 关DOI: 10.1063/1.1616668兴
The first demonstrations of terahertz 共THz兲 imaging
have stimulated many suggestions for applications, ranging
from biomedical imaging to the inspection of semiconductor
devices.1,2 It soon became clear that for many applications a
microscopic resolution would be required. Biomedical THz
imaging of the inner cell will demand submicron resolution
since the cell itself can be smaller than 10 ␮m. Similarly, the
inspection of individual semiconductor devices would require nanometer spatial resolution. Such resolution has been
out of range for a long time since Rayleigh’s criterion restricts resolutions in far-field microscopy to the wavelength’s
scale, which can be several hundred micrometers for THz
radiation. Far-field imaging techniques with resolutions close
to the diffraction limit have been demonstrated on biological
tissues and semiconductors.1,3 However, subwavelength
resolutions require near-field techniques. Resolutions down
to 7 ␮m were achieved using metallic subwavelength
apertures4,5 or optically gated apertures.6,7 Further improvements are mostly restricted by the strong signal attenuation
due to the apertures, which leads to a decrease of the field
strength with the third power of the aperture’s diameter.8
An alternative concept can be adapted from scanning
near-field optical microscopy 共SNOM兲 and was recently
demonstrated for the mid-infrared9 and the microwave
regions,10 where resolutions as small as 10⫺6 ␭ were
achieved. In these works, the resolving aperture was substituted with a sharp tip and scattering of the incident radiation
was measured. In this letter, we describe the application of
this apertureless SNOM technique for THz imaging. We
achieved a spatial resolution of 150 nm.
Few-cycle THz pulses were generated by femtosecond
laser excitation of n-doped InAs with 70 fs laser pulses of
780 nm wavelength and 8 nJ pulse energy. Their center frequency is 2.0 THz, which corresponds to a wavelength of
150 ␮m. A more detailed description of the spectroscopic
configuration, as well as typical THz pulse shapes, can be
found in Refs. 11 and 12. As shown in Fig. 1, the THz pulses
are focused onto the structure under investigation to a spot
size of about 500 ␮m. The angle of incidence is 70° and the
electric field is p-polarized. Spatial resolution is achieved by
a sharp metallic tip, which is placed above the structure. The
a兲
Electronic mail: [email protected]
incident THz pulses induce a dipole moment in the tip–
surface system, leading to scattering and absorption of part
of the THz radiation. The remaining transmitted signal is
detected either by electro-optic sampling13 or by a 4 K bolometer. Electro-optic sampling allows us to map the electric
field of the reflected THz pulse in time-domain with a bandwidth of 2.5 THz. Alternatively, we used for detection the
bolometer, which has a bandwidth of 10 THz. However, in
this case, only the power of the transmitted radiation can be
detected. In order to determine the spatial resolution, we fabricated metallic stripes on semi-insulating silicon. These gold
structures have periodicities between 10 and 40 ␮m and a
thickness of 2 ␮m. The spacings between the metallic lines
are nonconductive, which leads to transmission properties as
described in Ref. 11. Images of the stripes were recorded by
moving the structures with respect to needle and the focus of
the THz pulses.
In our SNOM technique, the spatial resolution is
achieved by the dipole moment that is induced by the THz
radiation in the tip and in the underneath region below the
sample surface. The most commonly used model for the tip–
surface polarizability approximates the imaging tip as a
sphere of radius R, which has a distance z to the surface. In
this model, the polarizability of the tip and the effective po-
FIG. 1. Schematic of the experimental technique. The THz beam is focused
to the diffraction limit onto the surface of the sample. Part of the incident
radiation is scattered and absorbed by the tip-surface system. The transmitted THz radiation is detected either by electro-optic sampling or with a
bolometer.
0003-6951/2003/83(15)/3009/3/$20.00
3009
© 2003 American Institute of Physics
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3010
Appl. Phys. Lett., Vol. 83, No. 15, 13 October 2003
Chen, Kersting, and Cho
FIG. 2. Line scan across metallic stripes, which have a spacing of 10 ␮m.
The profile is reproduced by THz electro-optic imaging.
FIG. 3. Terahertz image of semi-insulating silicon with metallic stripes of
nominally 10 ␮m width.
larizability of the tip–surface system are given by:9,14
␣ tip⫽4 ␲ R 3
⑀ t ⫺1
,
⑀ t ⫹2
␣ eff⫽
␣ tip关 1⫹ 共 ⑀ s ⫺1 兲 / 共 ⑀ s ⫹1 兲兴
1⫺
␣ tip 关共 ⑀ s ⫺1 兲 / ⑀ s ⫹1 兲 ]
16␲ 共 R⫹z 兲
,
3
共1兲
where ⑀ t and ⑀ s are the dielectric constants of the tip and of
the surface, respectively. Applying Mie theory leads to scattering and absorption cross sections of
C sca⫽
8␲3
3␭
4
兩 ␣ eff兩 2 ,
C abs⫽
2␲
Im兵 ␣ eff其 .
␭
共2兲
As pointed out in Ref. 14, the absorption cross section exceeds the scattering cross section by orders of magnitude.
Furthermore, the scattering efficiency strongly depends on
the radius of the tip used. We expect a dependence on R 3 for
the reduction of the THz field strength due to absorption and
scattering using material parameters of the tip–surface system for 2.0 THz.15 Even more important is that the scattering
efficiencies are extremely small. Using a tip radius of R⫽1
␮m, we calculated relative changes of the THz field, which
are smaller than 10⫺4 .
Figure 2 shows a THz line scan across metallic stripes
with a nominal linewidth of 10 ␮m. The data were recorded
using the electro-optic sampling technique and a tungsten
needle with 1 ␮m tip radius. For reference, we first recorded
the profile of the gold lines by measuring a small probecurrent through the tungsten tip in contact mode 共upper part
of Fig. 2兲. During the following THz measurement the tip is
held at a constant height of about 200 nm above the sample.
The lower part of the figure shows that the periodic profile is
reproduced by the THz measurements. The amplitude of the
modulation signal was 0.25 nA, which corresponds to 0.2%
of the incident THz signal. Higher signal intensities of up to
0.6% can be achieved when scanning closer to the surface.
We deduced spatial resolutions 共10% to 90%兲 as small as 4
␮m, of which a significant part results from the tip radius of
1 ␮m. Measurements of the scattered signal were performed
in time-domain for tip positions above the gold lines and
between them. Except for a difference in the overall ampli-
tude, no significant dispersive differences were observed.
This is reasonable because neither gold nor silicon have
strong dielectric features in the spectral range covered by our
THz pulses. We therefore limit the following discussion to
the spatial resolution, which can be achieved with the
scanning-tip method.
Figure 3 shows a two-dimensional scanning image of
metallic stripes having a width of 10 ␮m. The data were
recorded by electro-optic sampling of the THz pulse. The
image consists of 60⫻20 data points and was measured with
an integration time-constant of 300 ms. This allows for an
acquisition time of less than 10 min. The metallic stripes can
be clearly resolved. The deduced resolution is 3 ␮m, which
corresponds to about ␭/50.
In fact, submicron resolutions can be achieved with
smaller tip sizes. Figure 4 shows a line scan across the edge
of a grating line, which was recorded with a tip of about 100
nm radius. The upper part illustrates the profile of the step
FIG. 4. Line scan across the edge of a stripe. A 10% to 90% resolution of
150 nm was achieved.
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Appl. Phys. Lett., Vol. 83, No. 15, 13 October 2003
and the lower part shows the detected THz signal. The profile
of the step as given by the current through the needle is
reproduced by the THz data with a 10% to 90% resolution of
150 nm. Again the signal intensity is about 0.4% of the incident THz signal. We observed a similar relative signal intensity of about 0.3% in experiments where a bolometer was
used for time-integrated detection.
Remarkably, we did not observe a reduction of the signal
intensity by a factor of 10⫺3 when reducing the tip size from
1 ␮m to 100 nm, as it would be expected for a spherical
scatterer. Furthermore, we achieved signal intensities of
about 0.5%, either for the modulated field amplitude or for
the power of the modulated THz signal,16 which exceed the
theoretical values by two orders of magnitude. These results
indicate that the model of a spherical scatterer needs to be
refined.14 In fact, the scatterer does not only consist of the
spherical tip, but also of the upper part of the needle, which
has antenna-like properties that lead to a highly confined
field below the tip.17,18 Our preliminary model calculations
have shown field enhancements of up to three orders of magnitude and a field energy below the needle, that is nearly
independent on the tip diameter. These findings may explain
the strong signal intensities that we observed, which will be
discussed in a forthcoming work. The implication of our experimental findings is that even higher spatial resolutions can
be realized by using nanometer-sized tips without suffering
extreme signal losses.
In conclusion, we have demonstrated the application of
scanning near-field microscopy techniques for THz imaging.
A spatial resolution of 150 nm has been achieved, which
corresponds to about ␭/1000. These submicron resolutions
may allow THz imaging of the interior of biologic cells.
Chen, Kersting, and Cho
3011
Other applications may arise in the field of submicron semiconductor device inspection and far-infrared quantum optics
on individual quantum dots.
This research was supported by an award from Research
Corporation and by Semiconductor Research Corporation.
The authors thank I. Wilke and X.-C. Zhang for stimulating
discussions and S. Kraatz for technical assistance.
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