Experimental And Theoretical Approaches To Characterization Of Electronic Nonlinearities In Direct-gap Semiconductors

The general goal of this dissertation is to provide a comprehensive description of the limitations of established theories on bound electronic nonlinearities in direct-gap semiconductors by performing various experiments on wide and narrow bandgap semiconductors along with developing theoretical models. Nondegenerate two-photon absorption (2PA) is studied in several semiconductors showing orders of magnitude enhancement over the degenerate counterpart. In addition, three-photon absorption (3PA) is studied in ZnSe and other semiconductors and a new theory using a Kane 4-band model is developed which fits new data well. Finally, the narrow gap semiconductor InSb is studied with regard to multiphoton absorption, free-carrier nonlinearities and decay mechanisms. The non-degenerate two-photon absorption was investigated in several direct-gap semiconductors with picosecond and femtosecond pulses. Large enhancements in 2PA were demonstrated when employing highly non-degenerate photon pairs and the results were shown to be consistent to a simple 2-parabolic band theory based on a dressed state approach. The nonlinear refractive index induced in such configurations was also calculated and possible implications of such extreme behavior are discussed. A large number of measurements of 3PA were taken at multiple wavelengths and in several semiconductors. The subsequent analysis has shown that simple 2-band model calculations (based on either perturbative or tunneling approaches) do not adequately describe the experimental trends. A more comprehensive model, based on Kane’s 4-band theory was developed and we calculate three-photon spectra for zincblende structures within the perturbative iv framework. We have confirmed the results of our calculations performing a series of Z-scans in semiconductors ZnSe and ZnS, yielding complete experimental three-photon spectra. A systematic approach based on using a large variety of pulse durations was needed to quantify the wealth of nonlinear optical processes in InSb, accessible in the mid-infrared range. Femtosecond pulses provided a lower limit to measurements of the instantaneous effects (absorptive and refractive), while picosecond pulses allowed further characterization of the freecarrier effects, including population dynamics in the high density regime (Auger effects). The model developed permitted us to verify the temperature dependence of free-carrier absorption recently predicted, and to successfully model optical limiting data with longer, nanosecond pulses

Enhanced carrier multiplication in engineered quasi-type-II quantum dots

Sem informaçãoOne process limiting the performance of solar cells is rapid cooling (thermalization) of hot carriers generated by higher-energy solar photons. In principle, the thermalization losses can be reduced by converting the kinetic energy of energetic carriers into additional electron-hole pairs via carrier multiplication (CM). While being inefficient in bulk semiconductors this process is enhanced in quantum dots, although not sufficiently high to considerably boost the power output of practical devices. Here we demonstrate that thick-shell PbSe/CdSe nanostructures can show almost a fourfold increase in the CM yield over conventional PbSe quantum dots, accompanied by a considerable reduction of the CM threshold. These structures enhance a valence-band CM channel due to effective capture of energetic holes into long-lived shell-localized states. The attainment of the regime of slowed cooling responsible for CM enhancement is indicated by the development of shell-related emission in the visible observed simultaneously with infrared emission from the core.518Sem informaçãoSem informaçãoSem informaçãoC. M. C., L. A. P., K. A. V., I. R., J.M.P. and V. I. K acknowledge support of the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center (EFRC) funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES). N.S.M. is a CASP member supported by LANL Director's Postdoctoral Fellowship. Q. L. and H. L. are CASP affiliates supported by the New Mexico Consortium and Los Alamos National Laboratory

Three-Photon Absorption Spectra and Bandgap Scaling In Direct-Gap Semiconductors

This paper presents three-photon absorption (3PA) measurement results for nine direct-gap semiconductors, including full 3PA spectra for ZnSe, ZnS, and GaAs. These results, along with our theory of 3PA using an eight-band Kane model (four bands with double spin degeneracy), help to explain the significant disagreements between experiments and theory in the literature to date. 3PA in the eight-band model exhibits quantum interference between the various possible pathways that is not observed in previous two-band theories. We present measurements of degenerate 3PA coefficients in InSb, GaAs, CdTe, CdSe, ZnTe, CdS, ZnSe, ZnO, and ZnS. We examine bandgap, Eg, scaling using -band tunneling and perturbation theories that show agreement with the predicted Eg−7 dependence; however, for those semiconductors for which we measured full 3PA spectra, we observe significant discrepancies with both two-band theories. On the other hand, our eight-band model shows excellent agreement with the spectral data. We then use our eight-band theory to predict the 3PA spectra for 15 different semiconductors in their zinc-blende form. These results allow prediction and interpretation of the 3PA coefficients for various narrow to wide bandgap semiconductors

Spectral and temperature dependence of two-photon and free-carrier absorption in InSb

The nonlinear absorption spectrum of InSb was measured using a combination of tunable similar to 160 fs, similar to 10 ps, and similar to 150 ns IR sources along with a cryostat for controlling the sample temperature to vary the band gap energy from 0.17 to 0.23 eV. The measured nonlinear optical properties in InSb are consistent with those predicted by the models which include two-and three-photon absorption (2PA and 3PA), multiphoton generated free-carrier absorption (FCA) and various recombination mechanisms. Temperature-dependent Z-scan and nonlinear transmission measurements yield information on the temperature and spectral dependence of 2PA, FCA, and carrier recombination processes of Shockley-Read-Hall, and Auger mechanisms. We find good agreement between the measured and the modeled nonlinear properties is possible only when the recently predicted temperature dependence of the FCA is considered. The wavelength-and temperature-dependent 2PA and 3PA coefficients in InSb were experimentally obtained. The inferred values of the 2PA and 3PA are consistent with the scaling rules of a simple two-parabolic band model. We further determine recombination rates from nonlinear transmittance of nanosecond pulses of CO2 laser

High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge

A new route to efficient generation of THz pulses with high-energy was demonstrated using semiconductor materials pumped at an infrared wavelength sufficiently long to suppress both two- and three-photon absorption and associated free-carrier absorption at THz frequencies. For pumping beyond the three-photon absorption edge, the THz generation efficiency for optical rectification of femtosecond laser pulses with tilted intensity front in ZnTe was shown to increase 3.5 times, as compared to pumping below the absorption edge. The four-photon absorption coefficient of ZnTe was estimated to be β₄=(4±1)×10⁻⁵ cm⁵/GW³. THz pulses with 14 μJ energy were generated with as high as 0.7% efficiency in ZnTe pumped at 1.7 µm. It is shown that scaling the THz pulse energy to the mJ level by increasing the pump spot size and pump pulse energy is feasible

Femtosecond-To-Nanosecond Nonlinear Spectroscopy Of Polymethine Molecules

The linear and nonlinear optical properties of a series of polymethine molecules are investigated to study the effects of molecular structure and the host environment on overall nonlinear absorption performance. The linear characterization includes measuring the solvatochromic shifts between absorption and fluorescence peaks and studying the excited-state orientational diffusion kinetics. The nonlinear characterization involves measuring the excited-state absorption spectra with a femtosecond white-light-continuum pump-probe technique and performing Z scans and nonlinear transmission measurements from the picosecond to the nanosecond time regimes. The results of these experiments allow us to develop an energy-level structure for the polymethines, which accurately predicts nonlinear absorption properties from the picosecond to the nanosecond time regimes. From this model we are able to identify the key molecular parameters for improved nonlinear absorption. © 2005 Optical Society of America

Beam Deflection Measurements of Nondegenerate Nonlinear Refractive Indices in Direct-gap Semiconductors

We use the beam-deflection method to measure nondegenerate nonlinear refractive indices of ZnO and ZnSe and show, in agreement with theory, extremely nondegenerate nonlinear refraction is significantly larger than in the degenerate or near-degenerate case. © OSA 2015

Study Of The Dispersion Of Nonlinear Refraction In Insb

We studied the nonlinear refraction due to bound electrons in InSb using femtosecond pulses in the range of 8-13 μm and show that the nonlinear refraction coefficient changes sign at a wavelength around 10 μm. © 2006 Optical Society of America

Nondegenerate Two-Photon Gain in GaAs

We present data indicating two-photon gain using extremely nondegenerate (END) photons in optically excited GaAs. These results are consistent with our demonstration of END twophoton absorption enhancement and points a possible way to two-photon lasing. © OSA 2015