Metal Nanoparticle Composites Fabricated by Negative Ion
Implantation for Optical Applications
Yoshihiko Takeda, Jing Lu and Naoki Kishimoto
Nanomaterials Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, 305-0003, Japan
Abstract. Steady-state and laser-induced transient absorption of metal nanoparticle composites were studied for optical
measurements in the visible range. Negative ion implantation at 60 keV was applied to fabricate the metal
manoparticles in insulating substrates with various refractive indices. The steady-state absorption of implanted samples
showed a clear surface plasmon peak, which resulted from fabrication of nanoparticles. The combination of ion species
and matrices tuned the surface plasmon resonance to wavelengths demanded for optical applications. Transient
nonlinear absorption was measured by the pump-probe method with a femtosecond laser system. The metal nanoparticle
composites showed a large optical nonlinearity around the surface plasmon resonance. The values of the bleached
absorption, being pumped at 5 GW/cm2 of a 200 fs pulse, attained about -3%. The nonlinearities recovered in several
picoseconds via the electron-phonon interaction. An all-optical switching with a nanoparticle composite was proposed.
INTRODUCTION
composites, fabricated in various transparent substrates
by negative ion implantation. We focus on a surface
plasmon resonance and an optical switching where a
thin nanoparticle composite is built in.
Optical communications have been already
established in trunk networks and have been rapidly
developed as FTTH (Fiber-To-The-Home) in city
networks [1,2]. In the near future, all-optical devices
are inevitable for ultrafast all-optical circuits ahead of
semiconductor devices.
The all-optical devices
directly operate optical data signals with optical gate
signals and work at THz or over.
The metal
nanoparticle composites consist of metal nanoparticles
dispersed in a transparent insulator and are of great
interests in the optical applications because of a large
photo-induced optical nonlinearity with picosecond
response [3-5]. Ion implantation is one of the most
powerful techniques to fabricate metal nanoparticles in
insulating substrates and has advantage of applicability
to various ion species and insulating substrates. The
negative ion implantation has enabled us to form selfassembled metal nanoparticles in silica glass and other
insulators. The nanoparticles are located around a
projectile range near the surface with a narrow depth
distribution [6]. The thin composites with twodimensional structure are also promising as integrated
optical device applications in the future.
In this paper, we present steady-state and
nonlinear transient absorption of metal nanoparticle
EXPERIMENTAL
Negative metal ions of 60 keV were produced by a
Cs-assisted plasma-sputter-type ion source with a cusp
magnetic field. The details of the techniques have
already been described elsewhere [6]. The typical dose
rate and total dose were 0.3 to 10 mA/cm2 and 3 ¥ 1016
ions/cm2, respectively. Insulating substrates used were
amorphous (a-)SiO2 and single crystals of magnesium
aluminate spinel MgO2.4(Al2O3), SrTiO3, LiNbO3 and
TiO2. Steady-state absorption spectra were measured
using a dual beam spectrometer. Transient absorption
experiments were carried out using the pump-probe
method with a femtosecond-laser system. We used an
output pulse from an optical parametric amplifier
(OPA) with sum-frequency generation as a pump pulse.
The OPA was pumped by a regenerative amplified
Ti:sapphire laser system. The wavelength of the pump
pulse was tuned near the surface-plasmon resonance,
574 nm (2.16 eV). The typical output energy of the
OPA was 16 mJ at 1 kHz-repetition rate and the pulse
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
597
0.80
0.40
Cu:TiO
0.35
2
0.60
Cu:LiNbO
0.50
Absorbance
Absorption (arb. unit)
0.70
3
0.40
Cu:SrTiO3
0.30
Cu:MgO2.4(Al2O3)
0.20
Cu:SiO2
0.10
0.00
0.5
1.0
1.5
2.0
2.5
Ta:a-SiO2
0.25
0.20
0.15
W:a-SiO
2
0.10
0.05
0.00
3.0
0.0
Photon energy (eV)
1.0
2.0
3.0
4.0
5.0
6.0
Photon energy (eV)
FIGURE 1. Steady-state absorption spectra of Cu
nanoparticle composites in various insulators, implanted
at 10 mA/cm2.
FIGURE 2. Steady-state absorption spectra of Ta and
W nanoparticle composites in amorphous SiO2.
duration was a few hundreds of femtoseconds. To
probe the nonlinear change on the absorption caused
by the pump pulse, we used the white-light probe pulse
generated through the self-phase modulation process in
a water flow cell by the regenerative amplified pulse.
The pump beam was overlapped with the probe beam
in the sample after traveling through a variable
mechanical delay line. The diameters of the pump and
probe beams were both about 0.3 mm. The typical
energy density of the pumped pulse on a sample was 1
mJ/cm2 corresponding to the power density of 5
GW/cm2. The transient absorption spectrum was
obtained by dividing the absorption spectrum with the
laser-pumping by that without the pumping. All the
optical experiments were carried out at ambient
temperature.
Numerical calculation of optical
waveguides was estimated with OptiFDTD (Optiwave
Corp.).
where p denotes the metal volume function, em (= e’m
+ ie’’m = n2) the dielectric constant of the metal
particle and ed is that of the surrounding medium. The
resonance condition is basically given by the following
equation:
em + 2ed = 0.
(2)
The origin of the resonance is attributed to the
collective oscillation of free electrons at the particle
surface faced with the matrix. The resonance depends
on both metal particles and the matrix. According to
equation (2), the surface plasmon peak is enhanced
and shifts to red with increasing refractive index of the
substrate [7]. A selection of ions, not only applying to
noble metals, is also vital to tune the plasmon peak.
Figure 2 shows steady-state absorption spectra of Ta
and W nanoparticle composites in an amorphous-SiO2.
The plasmon resonance is also located in the infrared
and the visible range and the W nanoparticle
composite especially matches around a wavelength of
1.5 mm corresponding to trunk optical networks. The
arbitrary selection of ion species and substrates at ion
implantation enables us to tune the resonance band to
the demanded wavelengths for optical applications.
Laser irradiation excites electrons in nanoparticles
and induces a large optical nonlinearity. Figure 3
shows transient absorption, immediately after laser
pumping, of various Cu nanoparticle composites [7].
The laser pumping causes the bleaching of absorption
around the surface plasmon resonance. The transient
optical response recovers in several picoseconds [7].
The recovery is attributed to energy transfer from
excited electrons to phonons in a nanoparticle via the
electron-phonon coupling, which depends on the metal
element of nanoparticles [9]. The values of the
bleached absorption, Da, pumped at 5 GW/cm2 of a
RESULTS AND DISCUSSION
Metal nanoparticle composites consist of
nanoparticles dispersed and the matrix and have a
characteristic peak structure in an absorption spectrum.
Figure 1 shows steady-state absorption spectra of Cu
nanoparticle composites fabricated in various
substrates by negative ion implantation [7]. A plasmon
peak clearly appears around 2eV and shows formation
of nanoparticles distributed in the substrates.
According to the Maxwell-Garnett approximation [8],
an effective dielectric constant of the nanoparticle
composite is given by
eeff = ed
0.30
1+ 2 p(em - ed ) /(em + 2ed )
, (1)
1- p(em - ed ) /(em + 2ed )
†
598
Transient Absorption (arb. unit)
0.10
0.05
L0 = l 0/2neff
pumping
Cu:MgO2.4(Al2O3)
Nanoparticle composite
Grating
l0,l1
l0
l1
Waveguide
Substrate
0.00
L 0 = l1/2(neff + Dneff)
-0.05
l0,l 1
Cu:TiO2
Cu:LiNbO3
1.7
l0
Cu:SiO2
-0.10
-0.15
1.6
l1
Laser pumping
1.8
1.9
2.0
2.1
2.2
2.3
FIGURE 4. An all-optical switching composing of a
waveguide and a nanoparticle composite grating.
2.4
Photon Energy (eV)
FIGURE 3. Transient absorption spectra, immediately
after pumped by a femtosecond laser, of Cu nanoparticle
composites in various insulators
changes the Bragg wavelength.
The nonlinear
refractive index of the Cu nanoparticle composite,
excited by a pulse power of 5 GW/cm2, may cause a
shift of 5-10nm on the Bragg wavelength as roughly
estimated.
The operation works as an optical
switching, and the thin nanoparticle composite have
enough potential as an optical switching component.
A further optimization is necessary to make all-optical
switching operative at THz.
pulse, attain about -3% (Transient transmittance, DT ~
+7 %). Nonlinear refractive index and extinction
coefficient have been reported by several researchers
[3,10] and have exhibited values of 1 ¥ 10-9 to 2 ¥ 1012
cm2/W and -5 ¥ 10-11 cm2/W to -2 ¥ 10-12 cm2/W,
respectively. The nonlinear constants depend on a
pulse-width of pumping laser, volume fraction of
nanoparticles and so on. From our pump-probe
measurements, we can roughly estimate nonlinear
refractive index and extinction coefficient to be 2 ¥ 1011
cm2/W and –2 ¥ 10-11 cm2/W, respectively.
Several types of all-optical switching, where metal
nanoparticle composite is applied to, have been
proposed [3]. The optical switching is composed of a
Fabry-Pérot interferometer or a Mach-Zehnder
interferometer, which consists of waveguides and a
nonlinear medium. However the metal nanoparticle
composites, fabricated by keV ion implantation, have a
thin layer structure with about 50 nm thick and do not
have thickness enough for a nonlinear medium where
the light travels through. Figure 4 illustrates one of
other basic structures of optical switching.
The
switching is composed of a waveguide with grating of
a thin nanoparticle composite. The grating diffracts
optical traveling waves in the waveguide. The period
of the grating, L0, is given by
L0 = l0/2neff,
(3)
SUMMARY
Metal nanoparticle composites, fabricated by
negative ion implantation in insulators with various
refractive indices, have a characteristic peak structure
in an absorption spectrum. The surface plasmon
resonance in steady-state absorption can be tuned with
flexible selection of ion species and the matrix. The
laser-induced transient absorption bands also shift with
the steady-state plasmon band. The values of the
bleached absorption, being pumped at 5 GW/cm2 of a
200 fs pulse, attained about -3%. The nonlinear
optical response recovers in several picoseconds and
has a sufficient potential for an all-optical switching at
THz. We propose an all-optical switching consisting
of a waveguide and a nonlinear grating, which consists
of a metal nanoparticle composite with thin layer
structure.
where neff is an effective refractive index including
grating and l0 is a demanded wavelength set to the
Bragg wavelength. The period of the grating denotes
about 0.2 mm for l0 = 575 nm. The waveguide with
grating usually works as a band-pass filter. Laser
pumping modulates the effective refractive index and
ACKNOWLEDGMENTS
A part of this study was financially supported by
the Budget for Nuclear Research of the MEXT, based
on the screening and counseling by the Atomic Energy
Commission. The authors are grateful to Dr. H.
599
Amekura, Mr. K. Kono, Dr. O. Plaxine, Dr. T. Suga,
Mr. N. Umeda and Mr. N. Ookubo for their assistance
in the experiments.
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