Transient Optical Nonlinearities Research Articles

Long-lived, pulse-induced transient absorption inLiNb1-x Ta x O3 (0≤x≤1) solid solutions: the case of three intrinsic defect sites for electron localization with strong coupling

Abstract Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in LiNb1-x Ta x O3 (0≤x≤1, LNT) solid solutions with the aim of studying transient optical nonlinearities associated with the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, a pronounced, long-lived transient absorption is uncovered for LNT which exceed lifetimes and starting amplitudes of LiNbO3 (LN) and LiTaO3 (LT) by a factor of up to 100 and 10, respectively. The transients reveal a stretched-exponential decay behavior and a thermally activated process which provide strong evidence for an underlying hopping transport mechanism of small bound polarons. All findings are discussed in comparison to the model systems LN and LT within the framework of appropriate band models and optical generation of polarons via two-photon excitation. To explain the significant differences, the simultaneous presence of Nb5+ Li , Ta5+ Li antisites, and Ta5+ V interstitial defects, i.e. a mixture of the intrinsic defects widely established for LN and LT, is assumed for LNT.

Investigation of transient optical nonlinearity in pyrene-based ferrocene and its optical limiting potential for ultrafast laser absorption

The ultrafast optical nonlinearity of a pyrene-based ferrocene derivative (Py-Fc) is systematically studied via Z-scan, transient absorption spectrum (TAS), and phase object pump probe (POPP). An ultrafast broadband absorption in visible wavelength region is observed via TAS. Based on this finding, nonlinear optical absorption of the material is investigated at various wavelengths. Results indicate that different absorption mechanisms, including reverse saturable absorption and two-photon induced excited state absorption involve in the nonlinear absorption in visible spectrum. The cooperation between the two mechanisms brings strong optical limiting properties. In addition, negative nonlinear optical refraction from the excited state is observed in POPP experiment and its dynamics is discussed. These results demonstrate that the molecular Py-Fc exhibits strong optical limiting, suggesting it is a potential material for ultrafast laser protection.

Transient optical non-linearity in p-Si induced by a few cycle extreme THz field.

We study the impact of a few cycle extreme terahertz (THz) radiation (the field strength ETHz ∼1-15 MV/cm is well above the DC-field breakdown threshold) on a p-doped Si wafer. Pump-probe measurements of the second harmonic of a weak infrared probe were done at different THz field strengths. The second harmonic yield has an unusual temporal behavior and does not follow the common instantaneous response, ∝ETHz2. These findings were attributed to: (i) the lattice strain by the ponderomotive force of the extreme THz pulse at the maximal THz field strength below 6 MV/cm and (ii) the modulation of the THz field-induced impact ionization rate at the optical probe frequency (due to the modulation of the free carriers' drift kinetic energy from the probe field) at the THz field strength above 6-8 MV/cm.

BIEXCITONIC EFFECT ON TRANSIENT NONLINEAR REFRACTION AND ABSORPTION IN A MAGNETIZED CdS QUANTUM DOTS

Transient optical nonlinearities of semiconductor quantum dots in the strongly confined regime are studied, in which the exciton, biexciton and magnetic field effects are taken into account. The photo induced band-to-band electronic transitions are considered from both the heavy-hole (hh) and light-hole (lh) valence bands to the lowest (1s) exciton state below the fundamental absorption edge. The time dependent perturbation technique for a three-level system, under the near band gap resonant excitation regime, has been employed; with the incorporation of the relaxation and dephasing mechanisms phenomenologically. The induced polarization is calculated from where the real and imaginary parts of the third-order nonlinear optical susceptibility, responsible for the occurrence of nonlinear refraction and nonlinear absorption respectively, have been derived. The temporal and detuning spectra of nonlinear refraction and absorption; and their dependence on density of biexcitons and a moderately strong magnetic field, have been analyzed. It is observed that the nonlinear refraction and nonlinear absorption enhance with increasing magnetic field in small CdS quantum dots.

Transient Reorientation of a Doped Liquid Crystal System under a Short Laser Pulse

The transient optical nonlinearity of a nematic liquid crystal doped with azo-dye DR19 is examined. The optical reorientation threshold of a 25-μm-thick planar-aligned sample of 5CB using a 50 ns pulse duration 532 nm YAG laser pulse is observed to decrease from 800 mJ/mm2 to 0.6 mJ/mm2 after the addition of 1 vol% azo dopant, a reduction of three orders of magnitude. When using a laser pulse duration of 10 ns, no such effect is observed. Experimental results indicate that the azo dopant molecules undergo photoisomerization from trans-isomer to cis-isomer under exposure to light, and this conformation change reorients the 5CB molecules via intermolecular coupling between guest and host. This guest-host coupling also affects the azo photoisomerization process.

Comparison of Exciton Optical Nonlinearities for Resonant and Non-Resonant Excitation

The saturation of exciton resonances with increasing carrier density has been measured in the case of resonant and non-resonant excitations by using a pump-probe technique. We observed that the bleaching density for resonantly excited excitons is considerably higher than that for non-resonantly excited excitons and that with increasing excitation energy, the exciton bleaching density converges to the Mott density of free carrier model. This results suggest that resonant excitons show weaker screening than an unbound electron-hole plasma mixed with excitons and that the transient optical nonlinearities associated with screening and bandgap renormalization are governed for the ratio of excitons to free carriers with dierent interactions. Also, we observed that the non-resonantly excited excitons had longer lifetimes than the resonantly excited excitons, which we attributed to the relaxation of free carriers and to the polariton bottle-neck eect.

Near-Field Spectroscopy of Disordered Nanostructures

For many years, Roland Zimmermann and his research group have devoted much of their attention to the effects of disorder and exciton localization on the optical properties of semiconductor nanostructures. With the recent development of spectroscopic techniques providing subwavelength spatial resolution, new experiments became possible to reveal some of the rich physics linked to exciton localization. This paper briefly reviews recent work of the authors' group using near-field nano-spectroscopy. Specifically, it is shown that near-field autocorrelation spectra give strong evidence for quantum mechanical level repulsion of localized exciton states and allow for a quantitative estimate of the underlying microscopic disorder features. Femtosecond non-linear near-field spectroscopy allows to probe the transient optical nonlinearity from a single localized exciton in a thin quantum film on ultrashort time scales.

Spectroscopy of four-particle correlations in semiconductor microcavities

We present a theory of ultrafast time-resolved pump and probe spectroscopy in semiconductor microcavities. The numerical solutions are in excellent agreement with experimental data. An analytical study based on the Weisskopf-Wigner approximation gives physical insight into the transient optical nonlinearities of semiconductor microcavites. It is able to explain the physical origin of the unusual transient shifts and asymmetric-excitation-induced dephasing observed experimentally. This theory demonstrates that the nonperturbative regime provides a unique tool for measuring the spectrum of four-particle correlations.

Fast recombination processes in lead chalcogenide semiconductors studied via transient optical nonlinearities

Third-order optical nonlinearities and recombination processes of photoexcited electron-hole plasma in PbSe and PbTe are directly monitored by picosecond pump-probe experiments in the wavelength range from 3 to 10 μm. After excitation, a strong blue shift of the absorption edge and large changes of the refractive index are found. Time-resolved studies of the absorption changes at lattice temperatures between 70 and 160 K reveal a fast partial decrease of the carrier density within 100 ps at excitation densities exceeding 7×1017 cm−3. Comparative emission spectra demonstrate that this behavior is due to recombination by stimulated emission. At room temperature the nonradiative Auger recombination dominates on a 0.5–2 ns time scale below the threshold of stimulated emission. The overall Auger coefficients for carrier densities of about 1018 cm−3 are derived from the data and have values of cAPbSe=1.1±0.3×10−28 cm6/s and cAPbTe=2.5±1.3×10−28 cm6/s.

Spectral hole burning and carrier-heating effect on the transient optical nonlinearity of highly carrier-injected semiconductors

An improved density-matrix theory is developed that can treat both spectral hole burning and carrier heating self consistently. Various intraband and interband relaxation terms characterized by different relaxation times and quasi-equilibrium distributions are introduced into the density-matrix equations within a relaxation-time approximation. Conservation of total number and energy densities of carrier systems in each band is considered to determine the quasi-equilibrium distributions. Formalism is applied to the calculation of the transient optical nonlinearity of highly carrier-injected semiconductors. Spectral hole burning and carrier-heating effects on the spectral and temporal characteristics are then clarified. In particular, the significant four-wave-mixing effect due to carrier heating is pointed out. An experiment that can be used to directly prove the existence of the carrier-heating effects on gain nonlinearity is also proposed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transient grating optical nonlinearities of dye-doped cholesteric liquid crystals

Abstract Transient optical nonlinearities in dye-doped cholesteric liquid crystals are investigated with laser-induced dynamic grating experiments using nanosecond and picosecond excitation pulses. Selfdiffraction efficiencies of the induced optical gratings up to more than 10% have been observed. Grating buildup and decay are investigated by diffraction of a cw laser beam. The observed nonlinearities are explained in terms of fast electronic effects occuring in the dye-molecules as a primary process, and slower secondary processes connected with thermal gratings arising from radiationless recombination of excited molecules, which change the optical properties of the liquid crystalline host.

Transient optical nonlinearities in CdS studied by laser-induced grating spectroscopy at room temperature

We report laser-induced transient grating experiments performed on a 4-μm-thick CdS platelet near its band edge at room temperature. With this technique the lifetime and the diffusion of the plasma, generating the optical nonlinearities, could be studied with a picosecond time resolution. The diffusion of the electrons and the holes constituting the generated plasma has been found to be ambipolar at low carrier densities. A diffusion constant D = 1.6 cm2/s and a lifetime T1 = 3 ns have been determined. Under high-excitation conditions the gratings show deviations from simple exponential decays for only their smaller spacings, when their smearing out is mainly governed by the diffusion. This behavior cannot be explained by higher recombination rates like radiative and Auger ones. To describe our experimental results, we consider a phenomenological model in which the diffusion is no longer ambipolar.

Instability of almost opposite mutually noncoherent waves in nonlinear media with a local transient response

The instability thresholds were found for almost opposite waves whose interference pattern could not be recorded by a medium utilizing a transient optical nonlinearity of the local type. Pairs of waves with an angular shift relative to the direction opposite to the pump radiation were generated.

Dynamics and spectral characteristics of exciton bleaching in type-II Al xGa 1−xAs/AlAs superlattices

We present experimental investigations of the transient optical nonlinearities near the direct band-gap of type-II Al xGa 1−xAs/AlAs superlattices. The picosecond or subpicosecond response of the optical nonlinearity is determined by the spatial Γ−X transfer of electrons and critically depends on the position of the laser photon energy with respect to the lowest exciton resonance. We find that the initially bleached absorption changes to induced absorption in the low-energy part of the exciton resonance after the Γ−X transfer of the electrons. We conclude that an enhanced collisional broadening of the exciton resonance by heavy-holes is responsible for the observed energy dependence of the optical nonlinearity.

Ultrafast optical nonlinearities of type II AlxGa1−xAs/AlAs multiple quantum wells

We present experimental studies of the transient optical nonlinearities near the direct band gap of type II AlxGa1−xAs/AlAs multiple quantum wells. The response of the optical nonlinearity at the band-gap excitonic transition is critically dependent on laser photon energy. A complete recovery of the bleached absorption in the lower part of the excitonic resonance with a picosecond time constant at room temperature occurs due to an enhanced collision broadening of the excitonic resonance after the fast spatial Γ–X transfer. Our results demonstrate that the intrinsic response of optical devices based on exciton nonlinearities can be much faster in type II structures as compared to type I structures.&amp;lt;squeeze;2p&amp;gt;