Jinendra K. Ranka

Autocorrelation measurement of 6-fs pulses based on the two-photon-induced photocurrent in a GaAsP photodiode

We experimentally characterize the two-photon response of a GaAsP photodiode by use of a femtosecond Ti:sapphire laser tuned below the diode bandgap. The photodiode is shown to be highly suitable for real-time second-order autocorrelation measurements of pulses as short as 6fs in duration and with energies as small as a few picojoules.

Breakdown of the slowly varying envelope approximation in the self-focusing of ultrashort pulses

We show theoretically and experimentally that with ultrashort pulses much longer than a single optical cycle, the effects of self-steepening and space–time focusing are important for describing the nonlinear dynamics of self-focusing. Asymmetric temporal splitting of the pulse envelope is observed in which the relative magnitudes of the peaks are reversed as the input power is increased.

Femtosecond ultraviolet autocorrelation measurements based on two-photon conductivity in fused silica

We utilize the two-photon conductivity of a fused-silica substrate to produce a photoconductive switch for use in an intensity autocorrelator for ultraviolet ultrashort pulses. We perform measurements at 267 nm with pulse durations in the range of 110-330 fs and with energies as weak as 10 nJ. Based on the bandgap of fused silica, this device can potentially operate in the wavelength range of 140-280 nm.

Measurements of the tensor properties of third-order nonlinearities in wide-gap semiconductors

We extend the z-scan technique to provide for measurements of the sign and the magnitude of all the independent components of chi((3)) for isotropic and cubic-symmetry materials. This technique is used to measure the dispersion of the tensor components of the real and the imaginary parts of chi((3)) for various wide-gap semiconductor materials by use of femtosecond laser pulses. Our measurements of the polarization dichroism of the nonlinear-index and two-photon absorption coefficients are in fair agreement with recent theoretical calculations; however, substantial discrepancies exist between the measured and predicted values for the corresponding anisotropy parameters.

Autocorrelation measurements of femtosecond ultraviolet pulses using two-photon conductivity in wide-gap dielectrics

We report on the application of planar photoconductive switches on wide-gap dielectrics to measurements of the intensity autocorrelation of femtosecond pulses at 270 nm. In our scheme, metal layers were deposited on the wafers of fused silica and magnesium fluoride (bandgaps of approximately 8 eV and 11 eV, respectively). These large bandgaps allow for the observation of two-photon conductivity for pulses with wavelengths below 300 nm. The third harmonic of the output of a Ti:sapphire regenerative laser system was injected into a Michelson-type autocorrelator, and the output was focused by a mirror onto the structure. The interferometric single-scan and accumulated (averaged) autocorrelation traces are presented for the SiO/sub 2/ substrate. The averaged trace fitted by a Gaussian function corresponds to pulses of 380-fs duration. The photocurrent became saturated for incident pulse energies above 400-600 pJ for the SiO/sub 2/ wafer. We have also made measurements using MgF/sub 2/ substrates and observed nonlinear response in the photoconductivity, but the signal and saturation current were found to be lower, which resulted in noisier autocorrelation traces with reduced contrast ratio. The simplicity of the device and its sensitivity are promising for accurate ultrashort pulse duration measurements in the UV and should allow for pulse characterization at wavelengths below 200 nm.

Spectral Modification of Ultrashort Pulses Propagating through an Atomic Vapor

Extensive research has been performed on the temporal behavior of resonant short pulses propagating through an atomic vapor.1 In our studies, we have investigated experimentally and theoretically the modification of the spectrum of pulses propagating through an atomic vapor under conditions in which the pulse duration is much shorter than any of the relaxation times of the atomic system. We observe novel features in the spectrum of the transmitted pulse under conditions in which the area of the incident pulse is of the order of unity or greater.