J. Jayabalan

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.

Quantum beats from the coherent interaction of hole states with surface state in near-surface quantum well

We report tunneling assisted beating of carriers in a near-surface single GaAsP/AlGaAs quantum well using transient reflectivity measurement. The observed damped oscillating signal has a period of 120 ± 6 fs which corresponds to the energy difference between lh1 and hh2 hole states in the quantum well. Comparing the transient reflectivity signal at different photon energies and with a buried quantum well sample, we show that the beating is caused by the coherent coupling between surface state and the hole states (lh1 and hh2) in the near-surface quantum well. The dependence of decay of coherence of these tunneling carriers on the excitation fluence is also reported. This observation on the coherent tunneling of carrier is important for future quantum device applications.

Effect of light-hole tunnelling on the excitonic properties of GaAsP/AlGaAs near-surface quantum wells

Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spectroscopy and transient reflectivity measurements in the tensile-strained GaAsP/AlGaAs near-surface quantum well (NSQW) samples by reducing the top barrier layer thickness from 275 to 5 nm. The ground state transition (e1–lh1) remains excitonic even at room temperature (RT) for a buried quantum well sample with 275 nm thick top barrier. When the top barrier thickness is reduced to 50 nm the same transition is found to be excitonic only at low temperatures but changes to free-carrier recombination at higher temperatures. When the top barrier layer thickness is further reduced to 5 nm, the ground state transition is no longer excitonic in nature, where it shows free-carrier behaviour even at 10 K. We therefore find a clear relationship between the character of the ground state transition and the top barrier layer thickness. Light-hole excitons cannot be formed in NSQW samples when the top barrier layer thickness is kept reasonably low. This is attributed to the quantum mechanical tunnelling of free light holes to the surface states, which is found to be faster than the exciton formation process. A tunnelling time of ~500 fs for light holes is measured by the transient reflectivity measurements for the NSQW sample with a 5 nm top barrier. On the other hand, heavy-hole-related transitions in NSQW samples are found to be of excitonic nature even at RT because of the relatively large tunnelling time. It supports the dominance of excited state feature over the ground state transition in PL measurements at temperatures higher than 150 K.

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.

A parametric study of negative feedback Nd:YAG laser

The output pulse parameters of a mode-locked Nd:YAG laser with a passive negative feedback element were studied experimentally. The pulse evolution within the train was experimentally recorded using a modified second harmonic generation autocorrelator. By comparing the laser operation with and without an acousto-optic modulator, we found that with the later there is a significant increase in the mode locking probability and the pulse acquires a temporally Gaussian symmetric pulse shape. Further with the active modulator there is a relaxation in alignment tolerances and increase in the range of permissible dye concentrations for stable mode locking. It was also observed that the pulse width of the negative feedback laser depends on the saturation intensity of the mode locking dye and reduces for a dye with higher saturation intensity. The pulse width was also found to reduce linearly as the initial transmission of the dye is reduced.