Sayantan Bhattacharya

Second harmonic generation of femtosecond pulses using ZnO nanorods grown by chemical bath deposition with drop casted seed layer

We report efficient second harmonic generation (SHG) of femtosecond (fs) pulses using ZnO nanorods grown by chemical bath deposition (CBD) method with drop-casted seed layer. The SHG behavior of the nanorods are tested using an amplified Ti:sapphire fs laser of pulse duration of 100fs at 800nm. The SHG signal from the ZnO nanorods is found to be of very high intensity as detected by a low cost, compact spectrometer. In a comparative study, the SHG signal from ZnO nanorods grown over seed layer is found to be 12 times higher than the SHG signal observed from the ZnO nanorods grown on substrate without any seed layer. The efficient SHG in former case is due to the growth of high density, well oriented nanorods whereas the lower signal in the later case is due to growth of low density, randomly oriented nanorods. The polarization dependence behavior of the SHG signal is studied both experimentally and theoretically.

Dynamic gain aperture modelocking in picosecond regime based on cascaded second-order nonlinearity

The operation of a cascaded second-order mode-locked Nd:YVO4 laser has been investigated considering it as a soft-aperture Kerr lens type and using complex beam parameters. A self consistent complex beam propagation method is used to incorporate the effect of cascaded Kerr nonlinearity on radially varying gain aperturing. The analysis deduces a stable pulsewidth of ~9.5 ps which agrees well with the experimental value of 10.3 ps.

Exotic ultrafast optical nonlinearity in reduced graphene oxide via comprehensive dual beam approach

Using an optical pump and a time delayed white light super continuum probe, delay dependent switching is achieved between saturation absorption (SA) and reverse saturation absorption (RSA) above a threshold pump intensity in reduced graphene oxide (RGO2). RGO2 is obtained using photo-thermal reduction and chemical reduction respectively. The wavelength regime which experience switching can be varied by changing the degree of reduction. At 415 nm pump, the threshold intensity to obtain switching property decreases to 9 GW/cm2 for RGO2 from 18 GW/cm2 in graphene oxide(GO) and the tunability range shifts from 471-526 nm for as grown GO to 519-623 nm in maximally reduced RGO2.Though the saturation intensity of intrinsic non-degenerate two photon absorption (nd-TPA) is found to be lower in GO (4.3 GW/cm2) than RGO2 (18.2 GW/cm2), nd-TPA coefficient increases from 0.0015 cm/GW (GO) to 0.0026 cm/GW (RGO2) with increasing reduction. Decay dynamics of scattering processes show faster relaxation of electron in RGO than in GO. Results are accounted using a model band diagram based on amorphous-carbon model.

Effect of size confinement in C exciton dynamics of few-layered MoS 2 nano-sheets

We study transient absorption response of few-layered MoS2 nano-flakes in dispersion, mainly focusing on its high energy exciton (commonly known as C exciton). We use a simple sono-chemical exfoliation technique to obtain confined nano-crystals of MoS2 of average diameter 2 nm, inter-dispersed in the flakes and study the effect of quantum confinement on this layered semiconductor. We emphasize on the interplay between exciton bleaching and excited state absorption upon a blue-detuned pumping. The relaxation times for the exciton are found and for the nano-crystals the radiative relaxation process is found to be slower as compared to that of the nano-flakes.

Anomalous Dynamics in tert-Butyl Alcohol–Water and Trimethylamine N-Oxide–Water Binary Mixtures: A Femtosecond Transient Absorption Study

In this article, we have investigated the unusual dynamics of tert-butyl alcohol (TBA)–water and trimethylamine N-oxide (TMAO)–water binary mixtures using solvation dynamics as a tool. For this purpose, femtosecond transient absorption spectroscopy has been employed. Although these two molecules are isosteres to each other, a significant difference in water dynamics has been observed. The solvation times in TBA–water binary mixtures are found to be between 1.5 and 15.5 ps. On the contrary, we have observed very fast dynamics in TMAO–water binary mixtures (between 210 and 600 fs). Interestingly, unusual retardation in dynamics is observed at 0.10 mole fraction of TBA and TMAO in both the binary mixtures.

Ultrafast time-resolved investigations of excitons and biexcitons at room temperature in layered WS 2

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel exciton-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for real device applications. We explore the ultrafast excitonic processes in WS2 dispersion using broadband femtosecond pump-probe spectroscopy at room temperature. We detect the biexcitons experimentally and calculate their binding energies, in excellent agreement with earlier theoretical predictions. Using many-body physics, we show that the excitons act like Weiner-Mott excitons and explain the origin of excitons via firstprinciples calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of excitons and biexcitons in WS2. Indeed, our results demonstrate the potential for excitonic quasiparticle-controlled TMDs-based devices operating at room temperature.

Dielectric response of pure and doped-GaSe crystals studied by an indigenously developed broadband THz-TDS system

Publisher’s Note: This paper, originally published on 12 July 2016, was replaced with a corrected/revised version on 26 July 2016. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance. We have developed a terahertz time domain spectroscopy system (THz TDS). For THz generation, optical rectification process and for detection, electro-optic sampling processes are used. Identical < 110 > cut ZnTe crystals are used for both generation and detection of THz radiation.This spectroscopy system can be used for the noninvasive and contactless electrical and optical characterizations of various samples. In this work spectroscopic measurements of pure, Chromium and Indium doped GaSe crystals within 0.4 THz to 3 THz range are taken by the developed set-up to study the dielectric response of the samples.

Infrared reduction, an efficient method to control the non-linear optical property of graphene oxide in femtosecond regime

Graphene Oxide (GO) has been prepared by modified Hummers method and it has been reduced using an IR bulb (800-2000 nm). Both as grown GO and reduced graphene oxide (RGO) have been characterized using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectra shows well documented Dband and G-band for both the samples while blue shift of G-band confirms chemical functionalization of graphene with different oxygen functional group. The XPS result shows that the as-prepared GO contains 52% of sp2 hybridized carbon due to the C=C bonds and 33% of carbon atoms due to the C-O bonds. As for RGO, increment of the atomic % of the sp2 hybridized carbon atom to 83% and rapid decrease in atomic % of C=O bonds confirm an efficient reduction with infrared radiation. UV-Visible absorption spectrum also confirms increment of conjugation with increased reduction. Non-linear optical properties of both GO and RGO are measured using single beam open aperture Z-Scan technique in femtosecond regime. Intensity dependent nonlinear phenomena are observed. Depending upon the intensity, both saturable absorption and two photon absorption contribute to the non-linearity of both the samples. Saturation dominates at low intensity (~ 127 GW/cm2) while two photon absorption become prominent at higher intensities (from 217 GW/cm2 to 302 GW/cm2). We have calculated the two-photon absorption co-efficient and saturation intensity for both the samples. The value of two photon absorption co-efficient (for GO~ 0.0022-0.0037 cm/GW and for RGO~ 0.0128-0.0143 cm/GW) and the saturation intensity (for GO~57 GW/cm2 and for RGO~ 194GW/cm2) is increased with reduction. Increase in two photon absorption coefficient with increasing intensity can also suggest that there may be multi-photon absorption is taking place.