Asha Singh

Two-photon absorption and nonlinear refraction in commercial colour glass filters

We report the measurement of two-photon absorption coefficients , free-carrier absorption cross sections and nonlinear refraction coefficients in the transparency region of a large number of commercial colour glass filters of different manufacturers. The measurements were performed by z-scan and by monitoring the energy transmission of a 6 ps Nd:glass laser pulse at a wavelength of 1.054 m. The nonlinear refraction is primarily due to the two-photon-generated free carriers and has a negative sign. For Schott filters the magnitudes of nonlinear refraction per electron-hole pair are in agreement with those reported in the literature. The magnitude of changes by an order of magnitude for different filters, however, at a given ratio of photon energy/bandgap, the change in is less than a factor of two in filters of different manufacturers.

Ultrafast third-order nonlinearity of silver nanospheres and nanodiscs

We have measured and compared the absolute values of nonlinear susceptibility of colloidal solutions containing silver nanospheres and nanodiscs at their respective plasmon peaks using a femtosecond laser. The nonlinear process responsible for the laser-induced grating formation in the sample is determined to be of third order. The ratio between the third-order susceptibility (|?(3)|) and the linear absorption coefficient (?) of the nanodiscs at 590?nm is three times than that of the similar ratio for nanospheres at 398?nm. Using a randomly oriented ellipsoidal model, we have shown that the increase in |?(3)|/? for a nanodisc at 590?nm can be attributed to the change in the field enhancement factor with shape.

Transient absorption and higher-order nonlinearities in silver nanoplatelets

We show that the imaginary parts of higher-order optical nonlinearities and their decay times can be determined by a time-intensity domain analysis of the conventional transient absorption data. Using this method we have measured the values and decay times of third, fifth and seventh-order nonlinear susceptibilities of silver nanoplatelets in water. The origin of these higher-order nonlinearities is explained using a two-temperature model.

Tuning the localized surface plasmon resonance of silver nanoplatelet colloids

The effect of femtosecond laser irradiation on silver nanoplatelet colloids is described. It is shown that irradiation with a femtosecond laser of appropriate fluence can be used to tune the localized surface plasmon resonances of triangular silver nanoplatelets by a few tens of nanometres. This peak shift is shown to be caused by the structural modifications of the particle tips. We have also shown that post-preparation addition of poly-vinyl pyrrolidone to the nanocolloid arrests the peak shift.

Effect of edge smoothening on the extinction spectra of metal nanoparticles

To calculate the extinction spectrum of a metal nanoparticle, it is common to use a regular shape which is close to the experimental one. We show that, to model a real metal particle, it is essential to remove sharp corners and tips and smoothen the bounding surface. An efficient and simple method to smoothen the tips and corners of the model shape of the particle is presented. The extinction calculated using smoothed particles predicts more accurately the extinction spectrum of as grown particles as well as the changes in the extinction spectrum during melting and reshaping of the particle.

Third-order nonlinearity of metal nanoparticles: Isolation of instantaneous and delayed contributions

Measurement of transient absorption of a sample allows estimation of the evolution of the nonlinear response of materials after excitation by a short pump pulse. In this work, the time dependent nonlinear response of silver nanoplatelets in water was measured using transient absorption technique for various volume fractions of silver in water. These measurements were carried out in subpicosecond time scales and a suitable theoretical model was developed. It is been shown that the hot-electron contribution to the third-order nonlinearity of metal colloids is much higher than its instantaneous third-order nonlinearity. At low volume fractions, the delayed hot-electron contribution to third-order nonlinearity increases linearly with the volume fraction.

Aggregated nanoplatelets: optical properties and optically induced deaggregation

A study of aggregation and laser-induced deaggregation of silver nanospheres and nanoplatelets in colloidal form is presented. Changes in the extinction spectrum caused by aggregation are explained using a two-particle approximation. In the case of platelets, controlled laser irradiation is shown to reverse the aggregation process.

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.

Experimental Study on Soliton Rain Patterns in Yb-Doped All-Fiber Standing Wave Cavity Configuration

We demonstrate for the first time, experimental observation of soliton rain (SR) state at relatively low pumping power in Yb-doped fiber laser in all-fiber standing wave cavity configuration. SR is shown to be controllable in terms of speed, density, and direction of drifting. Other SR states, such as stationary SR, cyclic SR, and harmonic SR, were also observed by adjusting polarization controller settings. We carefully identified the portion of the spectra contributing to the SR state and correlate its influence on the speed and direction of rain drift relative to the condensed phase. Observation of SR in our cavity can help in enhancing the understanding of dissipative soliton dynamics in all normal dispersion fiber lasers.

Volume fraction dependence of transient absorption signal and nonlinearities in metal nanocolloids

Electron?lattice thermalization dynamics in metal nanoparticles or in bulk metal is usually estimated by measuring the decay time of the change in transmission following an optical excitation. Such measurements can be performed in transient absorption geometry using a femtosecond laser. We find that for silver nanoplatelet/water colloids, the decay time of the transient absorption depends on the volume fraction of silver in water. By estimating the volume fraction dependence of nonlinearities in the same samples, we show that the variation in the measured decay time is due to pump-depletion effects present in the sample. The correct correction factor for taking into account pump-depletion effects in fifth- and higher-order nonlinearities is also presented.