Near-infrared saturable and reverse saturable absorption of ion beam synthesized VO2 nanocrystals

Jan Mundry,Helmut Karl,Markus Betz,Hubert J Krenner

doi:10.1364/ome.396099

Abstract

We investigate the nonlinear optical response of a thin film of ion-implanted VO2 nanocrystals with open aperture z-scans involving femtosecond near-infrared pulses. Beyond the established nonlinearity related to the insulator-metal phase transition of VO2, the metallic state features a pronounced saturable absorption for 100 fs pulses from a modelocked Yb:fiber source at λ = 1036 nm. In contrast, we find a pronounced reverse saturable absorption for 90 fs pulses in the telecom window at λ = 1550 nm. We attribute these nonlinearities to a transient red-shift of the plasmonic resonance of the nanocrystals, in line with the temperature dependence of the linear absorption and the theoretical expectation for electronic heating. Details of the transmissivity characteristics can be tailored by the lattice temperature and/or the size of the nanocrystals. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers.

Highlights

Vanadium dioxide (VO2) has attracted the interest of researchers for decades since it exhibits an insulator-metal phase transition (IMT) near ambient temperatures
The traces become increasingly asymmetric around z = 0. This asymmetry arises from the hysteresis of the NCs, i.e. at least a part of the excitation region remains in a supercooled metallic state in the z > 0 section. (iii) On top of the IMT-related drop of the transmissivity a marked positive peak is seen which arises from a pronounced saturable absorption of the metallic phase of VO2
The nonlinear optical response of a thin film of ion beam synthesized VO2 nanocrystals is thoroughly investigated by open aperture z-scan technique involving femtosecond near-infrared pulses

Summary

Introduction

Vanadium dioxide (VO2) has attracted the interest of researchers for decades since it exhibits an insulator-metal phase transition (IMT) near ambient temperatures (transition temperature of TC ≈ 68°C for bulk crystals). We investigate the near-infrared optical nonlinearity of both the insulating and metallic state of VO2 NCs of about 100 nm diameter embedded in fused silica. To this end, we perform open aperture z-scans for a wide range of lattice temperatures and two complementary mode-locked fiber laser sources. We find a reverse saturable absorption for femtosecond pulses at a wavelength of 1550 nm, i.e., in the low-energy flank of the plasmonic resonance Both results are readily understood as arising from a transient red-shift of the plasmonic resonance during the ultrashort light matter interaction window. Such a red-shift upon heating is well known from metallic nanoparticles [8]

Sample overview and linear optical properties

Experimental approach

Results and discussion

Conclusion