We discuss in this paper the feasibility of dynamically modulating both resonance wavelength and spectral width
of single nanostructures exhibiting plasmonic effects by cycling through a metal-insulator transition (MIT) in
vanadium dioxide (VO2). Using full-field 3D finite-difference time domain (FDTD) simulation method with
nonuniform mesh techniques, we study the effects of this modulation by varying the lateral dimensions of these
nanostructures from 40 nm to 120 nm radially and changing its configuration as well, that is VO2 nanodisk on
gold one and vice-versa. As an initial step towards fabricating those single composite nanostructures showing the
greatest modulating effect, we start by making single NPs of VO2 and single gold NPs embedded between two
60 nm layers of VO2. The samples are fabricated on 130 μm thin glass substrates by electron-beam lithography,
pulsed laser deposition of VO2 and electron-beam evaporation of gold. Using confocal extinction spectroscopy,
we hereafter provide for the first time experimental observations of spectral tuning in these lithographically
prepared single nanostructures. However, we discussed the variability in spectra obtained. Indeed, as the gold
NP size decreases, it becomes comparable to the domain sizes of the embedding VO2 and this prevent the correct
acquisition of the flat field. Hence the study of the tunability of gold particle plasmon resonance is imparted.
However, we conclude that this study will be feasible for truly hybridized NP, that is gold nanodisk stacked on
VO2 nanodisk and vice-versa. As hinted by our simulation studies and preliminary experimental results, these
hybridized composite NPs could potentially be used in the dynamic spectral tuning of plasmonic waveguides.
Vanadium dioxide (VO2) is a strongly-correlated electron material with a well-known semiconducting to metallic
phase transition that can be induced thermally, optically, or electrically. When switched to the high-temperature
(T > 68°C) metallic phase, the greatest contrast in the optical properties occurs at wavelengths in the near-to-mid-infrared and beyond. In the visible to near-infrared, however, upon switching for wavelengths between
~500-1000 nm, VO2 transmits more light in the metallic phase. In this paper, we report studies of the effect
of near-IR irradiation (785 nm) on lithographically prepared arrays of gold nanoparticles (NPs) covered with a
thin film of VO2 and find that the presence of the NPs substantially lowers the laser threshold for low-power
induction of the phase transition. Hybrid Au::VO2 structures were created by coating lithographically prepared
arrays of gold nanoparticles (NPs) (diameters 140 and 200 nm, array spacing 450 nm) with 60 nm thick films
of VO2 by pulsed laser deposition. Due to resonant absorption of the Au particle-plasmon resonance (PPR) at
785 nm, a temperature-dependent shift in the PPR can be generated by switching the VO2 from one phase to
another. We have measured the switching behavior of VO2 and Au::VO2 structures using shuttered CW laser
irradiation in order to study both optical and thermal mechanisms of the phase transition. Transient absorption
measurements using a shuttered 785 nm pump laser corresponding to the PPR resonance of the Au NPs and
1550 nm CW probe show that the presence of the Au NPs lowers the threshold laser power required to induce
the phase transition.
Previous observations on arrays of single nanoparticles (NPs) have shown that particle separation and grating
constant determine the peak extinction wavelength of the local surface plasmon resonance (LSPR). Recently, it
has been predicted that the LSPR peak extinction wavelength in arrays of nanodimers (NDs) exhibit enhanced
sensitivity to changes in the local dielectric function compared to single NPs. In order to test this prediction,
arrays of NPs, NDs and heterodimers comprising three different NP sizes were fabricated by electron-beam
lithography with various grating constants, particle diameters, and interparticle separations. Another set of
arrays were also fabricated and coated with approximately 60-nm of vanadium dioxide, which undergoes an
insulator to metal phase transition at a critical temperature near 68.C. By tuning the temperature of the
samples through the strong-correlation region around the critical temperature, we varied the effective dielectric
constant surrounding the NP arrays over a significant range. Linear extinction measurements on the arrays were
made at temperatures above and below the critical temperature, with linear polarizers placed in the incident
beam in order to distinguish between LSPR modes. Measurements show a clear dependence of LSPR sensitivity
to interparticle separation as well as the dielectric function of the surrounding medium. Finally, finite-difference
time-domain (FDTD) simulations were carried out for comparison with the experimental results.
We have studied the effects of planar inversion symmetry and particle-coupling of gold nanoparticle (NP) arrays
by angle dependent second-harmonic generation (SHG). Time- and angle- resolved measurements were made
using a mode-locked Ti:sapphire 800 nm laser onto gold NP arrays with plasmon resonance tuned to match the
laser wavelength in order to produce maximum SHG signal. Finite-difference time domain simulations are used
to model the near-field distributions for the various geometries and compared to experiment. The arrays were
fabricated by focused ion-beam lithography and metal vapor deposition followed by standard lift-off protocols,
producing NPs approximately 20nm high with various in-plane dimensions and interparticle gaps. Above a
threshold fluence of ~ 7.3 × 10-5 mJ/cm2 we find that the SHG scales with the third power of intensity, rather
than the second, and atomic-force microscopy shows that the NPs have undergone a reshaping process leading
to more nearly spherical shapes.
We describe experiments aimed at distinguishing possible mechanisms of second-harmonic generation (SHG) in
lithographically prepared arrays of metal nanoparticles. It is well-known that even-order harmonics cannot be
generated by electric dipole-dipole interactions in centrosymmetric systems. The experiment employs two basic
sample geometries. In our first geometry, as in our previous work, the NPs are left exposed to air, producing
an asymmetric local dielectric environment with ITO on one side and air on the other. In the second geometry,
we propose coating the arrays with the same material as they are created on, thus producing a centrosymmetric
environment in which any SHG observed can not be due to asymmetry in the medium, but to nonlocal or
retardation mechanisms in the particles. The arrays are fabricated using focused ion-beam lithography and vapor
deposition of the metal, followed by standard lift-off protocols. This procedure yields typical NP dimensions
between 60 nm and 200 nm in diameter, and between 15 nm and 30 nm in height, as characterized by scanning
electron and atomic-force microscopy. By tuning the NP resonances to the excitation wavelength the SHG signal
can be substantially enhanced. Surface melting effects are minimized by the use of ultra-short (50-fs) pulses
which give high intensity while allowing us to work at relatively low fluence.
Closely spaced pairs or "dimers" of elongated gold nanoparticles may be expected to exhibit electric field hotspots. We investigate the possible influence of hotspots on second harmonic generation. Preliminary results show that arrays of nanoparticle dimers exhibit reduced second-harmonic generation compared with arrays of single nanoparticles having similar extinction spectra, contradicting a simple model of second-harmonic generation (varying as the fourth power of the local fundamental field) if hotspots can be shown to exist in such gaps.
KEYWORDS: Brain, Blood, Absorption, Near infrared spectroscopy, Spectroscopy, Oxygen, Near infrared, Signal attenuation, Animal model studies, Magnetic resonance imaging
Transient global cerebral ischemia accompanying cardiac arrest (CA) often leads to permanent brain damage with poor neurological outcome. The precise chain of events underlying the cerebral damage after CA is still not fully understood. Progress in this area may profit from the development of new non-invasive tools that provide real-time information on the vascular and cellular processes preceding the damage. One way to assess these processes is through near-IR spectroscopy, which has demonstrated the ability to quantify changes in blood volume, hemoglobin oxygenation, cytochrome oxidase redox state, and tissue water content. Here we report on the successful implementation of this form of spectroscopy in a rat model of asphyxial CA and resuscitation, under hypothermic and normothermic conditions. Preliminary results are shown that provide a new temporal insight into the cerebral circulation during CA and post-resuscitation.
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