OPTICS LETTERS / Vol. 30, No. 5 / March 1, 2005
Photoinduced phase transition in VO2 nanocrystals:
control of surface-plasmon resonance
Matteo Rini, Andrea Cavalleri, and Robert W. Schoenlein
Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720
René López, Leonard C. Feldman, and Richard F. Haglund, Jr.
Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson, Nashville, Tennessee 37235
Lynn A. Boatner and Tony E. Haynes
Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Received July 29, 2004
We study the ultrafast insulator-to-metal transition in nanoparticles of VO2, obtained by ion implantation and
self-assembly in silica. The nonmagnetic, strongly correlated compound VO2 undergoes a reversible phase
transition, which can be photoinduced on an ultrafast time scale.
of the metallic state results in the appearance of surface-plasmon resonance.
enhancement of optical absorption in the near-infrared spectral region that encompasses the wavelength range
for optical-fiber communications. One can further tailor the response of the nanoparticles by controlling
their shape.© 2005 Optical Society of America
320.7130, 160.4670, 320.7150, 160.4760, 300.6500.
In the nanoparticles, prompt formation
We achieve large, ultrafast
The phase transitions exhibited by strongly correlated
compounds are often associated with large changes
in the electrical, optical, and magnetic properties of
these compounds.Strong interactions among the
many degrees of freedom of the system create complex
free energy surfaces and multiple isoenergetic ground
states.One way to control competition among phases
is by light illumination, which can initiate phase tran-
sitions at moderate intensities1or drive large optical
This behavior is potentially con-
ducive to new strategies for ultrafast optical switching
However, compatibility with existing
lengths is often a critical limitation for applications
of bulk materials.The design of nanometer-scale
structures can be used to adapt desirable properties
of extended solids to specific applications.
study how dielectric confinement and particle shape
affect the change in optical properties across the
phase transition in nanoparticles of VO2embedded in
a silica matrix.We have found that confinement to
the nanometer scale greatly enhances the differential
absorption of VO2 in the optical telecommunications
VO2 is a nonmagnetic, strongly correlated com-
pound that undergoes a reversible phase transition
between a monoclinic insulator and a rutile metal
at a critical temperature Tc ? 340 K.4
transformation of the bulk oxide is accompanied by
large changes in the optical transmission properties5
and can be induced on a femtosecond time scale by
ultrashort laser pulses.6
x-ray diffraction experiments with bulk VO2 have
clarified some of the fundamental aspects of its ultra-
Prompt photoinjection of holes into
Time-resolved optical and
the correlated valence band of the room-temperature
insulator triggers nonthermal rearrangement in both
atomic and electronic structures within a time scale of
Recovery of the insulating phase takes place
within tens of nanoseconds via thermal diffusion.6
The VO2nanoparticles studied here are grown by ion
implantation and self-assembly in silica.9
and shape are controlled by variation of the anneal-
Figure 1 compares the thermally induced
changes of absorbance (Da) for a VO2 film and for a
layer of spherical nanoparticles of comparable depth
and 10% volume filling.In the thin films, absorption
in the metallic phase is enhanced by the collapse of
the 0.67-eV optical bandgap at long wavelengths and
by the appearance of the plasma edge near 800 nm.
temperature-driven phase transition for thin-film and
spherical nanoparticlesas derived
absorption measurement in the low- and high-temperature
phases. The VO2
volume filling for nanospheres is
approximately 10% in a single 100-nm layer.
Change ofabsorbance in VO2
0146-9592/05/050558-03$15.00/0© 2005 Optical Society of America
March 1, 2005 / Vol. 30, No. 5 / OPTICS LETTERS
In the nanoparticles a strong surface-plasmon reso-
nance that is absent in the bulk dominates the optical
response.This resonance is due to collective oscilla-
tions of the electron plasma, which can directly couple
to light in a dielectrically confined geometry.11
large absorption coefficient at the surface-plasmon fre-
quency can be calculated from the classical Mie formula
for the polarizability of a spherical particle, for par-
ticle sizes much smaller than the wavelength of light.
In the absence of damping, the polarizability diverges
when the real part of the dielectric constant equals
22em, where em is the dielectric constant of the sur-
rounding medium.This singularity, which is relaxed
in the presence of damping, can be observed only when
the nanoparticles are in the metallic phase, in which
the real part of the dielectric constant is negative.
The ultrafast photoinduced response in the near and
mid infrared is investigated by femtosecond pump–
The transient DT?T spectrum
measured 200 fs after 800-nm excitation is shown
in Fig. 2(a) for spherical particles.
between time-resolved data and static spectra of the
thermally induced phase transition is evidence that
the VO2nanoparticles have reached the metallic phase
immediately after excitation.
tent with previous observations in bulk samples.8
The photoinduced transmission drop appears at all
wavelengths within the experimental time resolution
(,150 fs), without significant dynamics at later
times.Figure 2(b) shows the pump–probe signal
as a function of pulse delay measured at 1.55 and
2.5 mm. Optical transparency recovers on a nanosec-
ond time scale, with the return to the insulating phase.
The ultrafast transmission changes exhibit a well-
defined fluence threshold of ?300 mJ?cm2as well as
saturation effects, as expected from a photoinduced
phase transition:Above threshold, the dielectric
properties of the nanoparticles settle to the value that
is characteristic of the new phase, independently of
the excitation fluence.
The dependence of the plasmon resonance on the
size and shape of the nanoparticles11can be exploited
to further tailor the spectral response.
transmission changes (DT?T), measured 200 fs after
excitation of the room-temperature insulator, are dis-
played in Fig. 3 for samples with mean aspect ratios
that vary from 1 (spherical particles) to 3.5.
constant amount of VO2, the transmission changes
in the longer-wavelength range scale with increasing
aspect ratio. The dependence on wavelength and
shape of the DT?T spectra is well described by Mie
scattering theory,11as shown by the solid curves
of Fig. 3.13
According to Mie theory, the resonance
condition for nonspherical particles depends on the
particles’ orientation relative to the electric field’s
polarization.For ellipsoidal nanoparticles with their
major axes parallel to the plane of incidence, Mie
theory predicts absorption features at longer wave-
lengths for polarization parallel to the major axis.
Thus the continuous angular distribution of ran-
domly oriented rods in our samples results in overall
broadening of the absorption spectrum toward longer
This result is consis-
Remarkable features of the switching behavior
investigated here include room-temperature opera-
tion, compatibility with fiber-optic environment,10and
high efficiency at telecommunication wavelengths
(1.3 1.5 mm).Moreover, the 300-mJ?cm2threshold
for the photoinduced phase transition is equivalent
to a 150-pJ pulse for a typical 50-mm2mode size in
a single-mode fiber, making such schemes attractive
for real-world applications.
highest aspect ratio, we observed a switching efficiency
of ?25% at 1.55 mm over a propagation length of less
than 100 nm and for a VO2 filling factor of less than
10%.One could straightforwardly achieve higher
switching contrasts by increasing areal filling factors
or interaction length.Clearly, the nanosecond lifetime
of the metallic state limits the suitability of VO2for all-
optical switching at high bit rates.
larly large optical nonlinearities2and optical switching
with intrinsic subpicosecond recovery times3have
been observed in bulk samples of other transition-
metal oxides, making them attractive for similar
nanoscale design strategies.
In the particles with the
Further, the capability
phase transition for the VO2 nanospheres.
of transmission in the presence of the pump; T, sample
transmission).The thermally induced curve was derived
from spectra measured at room temperature (VO2 in
the insulating phase) and at 373 K (VO2 in the metallic
phase).The photoinduced wave was measured at a delay
of 200 fs after photoexcitation.
as a function of pulse delay measured at 1.55 and 2.5 mm.
(a) Relative change of transmission across the
(b) Pump–probe signal
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OPTICS LETTERS / Vol. 30, No. 5 / March 1, 2005
wavelength for several aspect ratios (top to bottom:
2, 3.5) as obtained by varying the annealing time (3, 7, 10,
and 45 min, respectively).Squares, measured at a delay
of 200 fs after photoexcitation; solid curves, calculations
based on Mie scattering theory.
curves show the prediction of Mie theory for spherical par-
ticles (aspect ratio, 1; the same as in the topmost figure).
Representative TEM pictures of the corresponding samples
are shown at the right.
Relative change of transmission as a function of
As a comparison, dashed
of addressing the conducting states of nanoparticles,
individually or within ensembles, may be useful for
nanophotonic devices that rely on collective plasmon
modes for the transport of electromagnetic energy.14
In summary, we have shown that (1) photoexcitation
of the VO2 nanoparticles results in the formation of
the metallic phase within less than 150 fs; (2) as the
particles reach the metallic phase, the absorption
spectrum exhibits a surface-plasmon resonance near
telecommunication wavelengths; (3) the dependence
of the resonance on the shape of the nanoparticles
makes it possible to tailor the spectral responses
of the nanoparticles.Most remarkably, our results
demonstrate that nanoscale phenomena can lead to
enabling strategies for the integration of strongly
correlated solids into ultrafast optical devices.
Research at the Lawrence Berkeley National Labo-
ratory was supported by the Director, Office of Sci-
ence, Office of Basic Energy Sciences, Division of
Materials Sciences and Engineering, U.S. Depart-
ment of Energy, under contract DE-AC03-76SF00098.
Research at Vanderbilt University was supported by
the Office of Science, U.S. Department of Energy
(DE-FG02-01-ER45916), and by the National Science
Ridge National Laboratory, managed by UT-Battelle,
LLC, for the U.S. Department of Energy under
contract DE-AC05-00OR22725, was
Research at the Oak
the Laboratory Directed Research and Development
Program. M. Rini’s e-mail address is email@example.com.
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12. Theexperimental setup
Ti:sapphire laser. Pump pulses at 800 nm have
1 3-mJ energy and durations below 50 fs.
infrared pulses are obtained from an optical para-
1.6 2.6 mm).Mid-infrared
generated by difference-frequency mixing in GaSe of
signal and idler pulses.
from 100 to 150 fs, with increasing values from high
to low frequencies. As the phase transition is fully
reversible, the sample is not moved from shot to shot.
13. The particles are considered perfect ellipsoids placed in
a homogeneous electromagnetic field with their major
axes perpendicular to the direction of light propagation
and a random in-plane orientation.
effects that are due to a size and aspect ratio distribu-
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coupling mechanisms. The complex dielectric function
of the high-temperature metallic VO2is employed.8
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isbased ona 1-kHz
1.1 1.6 mm;
2.8 6 mm pulses
The time resolution varies