Ultrafast carrier dynamics in a monolayer MoS2at carrier densities well above Mott density.

Due to the growing interest in monolayer (ML) molybdenum disulfide (MoS2) in several optoelectronic applications like lasers, detectors, sensors, it is important to understand the ultrafast behavior of the excited carriers in this material. In this article, a comprehensive study of the charge carrier dynamics of a monolayer MoS2flake has been studied using transient transmission technique near A-exciton under high excitation densities well above the Mott density. Fluence dependent studies has been carried out to understand the origin of the processes which modifies its optical response under excitation. The dissociation of excitons leads to an observed fast bandgap renormalization. At later times when large number of carriers relax the remaining carriers forms excitons leading to a bleaching effect.

Ultrafast carrier dynamics in Ag-CdTe hybrid nanostructure: non-radiative and radiative relaxations.

In this article, we study non-radiative and radiative relaxation processes in a hybrid formed by combining Ag nanoparticle (NP) and CdTe quantum dots (QD) using transient transmission spectroscopy. The ultrafast transient transmission of hybrid, when excited at 400 nm, shows a faster recovery of hot electrons at a shorter time scale (few picoseconds) while it shows a slower recovery at longer time scale (few tens of picoseconds). Further it is found that the contribution of CdTe QD to the transient transmission is increased in the presence of Ag NP. However, the radiative relaxation in CdTe QDs get quenched in the presence of Ag NP. This work provides significant insight into the various relaxation processes that leads to the charge transport and PL quenching mechanisms in metal-semiconductor hybrids.

Filtering noise in time and frequency domain for ultrafast pump-probe performed using low repetition rate lasers.

Optical pump-probe spectroscopy is a powerful tool to directly probe the carrier dynamics in materials down to sub-femtosecond resolution. To perform such measurements, while keeping the pump induced perturbation to the sample as small as possible, it is essential to have a detection scheme with a high signal to noise ratio. Achieving such a high signal to noise ratio is easy with phase sensitive detection based on a lock-in-amplifier when a high repetition rate laser is used as the optical pulse source. However, such a lock-in-amplifier based method does not work well when a low repetition rate laser is used for the measurement. In this article, a sensitive detection scheme, which combines the advantages of a boxcar that rejects noise in time domain and a lock-in-amplifier that isolates the signal in the frequency domain for performing pump-probe measurements using a low-repetition rate laser system, is proposed and experimentally demonstrated. A theoretical model to explain the process of signal detection and a method to reduce the pulse to pulse energy fluctuation in probe pulses is presented. By performing pump-probe measurements at various detection conditions, the optimum condition required for obtaining the transient absorption signal with low noise is presented. The reported technique is not limited to pump-probe measurements and can be easily modified to suit for other sensitive measurements at low repetition rates.

Single-shot measurement of nonlinear absorption and nonlinear refraction.

A single-shot method for measurement of nonlinear optical absorption and refraction is described and analyzed. A spatial intensity variation of an elliptical Gaussian beam in conjugation with an array detector is the key element of this method. The advantages of this single-shot technique were demonstrated by measuring the two-photon absorption and free-carrier absorption in GaAs as well as the nonlinear refractive index of CS2 using a modified optical Kerr setup.