Optical nonlinearities in VO2 nanoparticles and thin films

Abstract

Z -scan and pump-probe measurements with ultrafast, 800nm laser pulses were used to compare the ultrafast optical nonlinearities of VO2 nanoparticles and thin films in both semiconducting and metallic states. In the metallic state, both the nanocrystals and thin films exhibit a positive, intensity-dependent nonlinear index of refraction. However, the nonlinear effects are relatively larger in the VO2 nanocrystals, which also reveal a saturable nonlinear absorption. When the semiconductor-to-metal phase transition is induced by the laser pulse, VO2 thin films exhibit a negative equivalent nonlinear index of refraction while the nanocrystals exhibit a smaller but still positive index. Both the VO2 nanocrystals and thin films undergo the phase transition within 120fs.

Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation

“…The third problem has to do with the fact that the thermochromic reflectance modulation in VO 2 films is strong only in the near-infrared and at wavelengths for which the solar radiation is weak, which means that the modulation of the solar energy throughput between a low-temperature and a high-temperature state is modest; typically it is ϳ5% for a VO 2 film though, exceptionally, it can run up to 10%-15% in a well designed TiO 2 / VO 2 / TiO 2 / VO 2 / TiO 2 structure. 22,23 VO 2 -based nanoparticles of various types have received much interest in recent work, as discussed at length below, and there are a number of demonstrations that clear-cut thermochromic effects can be observed; data have been obtained for nanostructures made by ion implantation followed by annealing, [27][28][29] ion beam lithography and pulsed laser deposition, 30,31 ultrasonic spray and laser pyrolysis, 32 molten salt synthesis, 33 and aqueous chemistry. 34 Our present study demonstrates that nanoparticles dispersed in a dielectric host have advantages over what a thin solid film can offer.…”

Structure and Properties of Nanoparticles Formed by Ion Implantation

“…The threshold for the photoinduced phase transition is 300 mJ cm −2 , which is equivalent to a 150-pJ pulse for a typical 50-μm 2 size single-mode fiber, making possible the use of nanoscale VO 2 in all-optical switching schemes. In the particles with the highest aspect ratio, a switching efficiency of ∼25% was observed at 1.55 μm over a propagation length of less than 100 nm and for a VO 2 filling factor of less than 10% [89]. One might achieve higher switching contrasts by increasing aerial filling factors or interaction length, although the nanosecond lifetime of the metallic state may limit the suitability of VO 2 for all-optical switching at high bit rates.…”

“…In particular, small VO 2 particles in the nanoscale regime have been produced in SiO 2 and Al 2 O 3 substrates by coimplanting vanadium and oxygen ions at room temperature in a stoichiometric proportion. The implantation energies were 150 keV for vanadium and 55 keV for After [89] oxygen ions, to ensure overlapping concentration profiles within 200 nm from the specimen surface. Thermal processing at 1000…”

Electrical Conducting Behavior of W- Doped VO2 Thin Films / La0.5Sr0.5CoO3Heterostructure

“…For the more practical applications, there have been many approaches to reduce the transition temperature of VO 2 such as doping element with higher valence, different substrate type and various fabrication techniques [8]- [11]. In this study, we prepare VO2 thin films by sol-gel method with W element doping of different doping concentrations.…”