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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Double resonance and four wave mixing FEL studies of InAs/GaAs quantum dots and local modes in ionic crystals

C. R. Pidgeon, J.‐P. R. Wells, Ian V. Bradley, Benedict N Murdin

Published Nov 7, 2002• 0 citations
Publisher

Abstract

A brief review is given of lifetime and line broadening studies with the free electron laser at FOM-Rijnhuizen (FELIX), emphasising work on far infrared (FIR) modulated photoluminescence (PL) of bound-to-bound transitions in InAs/GaAs quantum dots and mid-infrared (MIR) four wave mixing experiments on localised modes of H/sup -/ ions in calcium fluoride crystals. We have made new far/near infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The FIR resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots, and analysis of the FIR double resonance linewidth enables us to show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation. In addition we have made time resolved studies of local modes in ionic crystals, where nonradiative decay plays an important role in the optical pumping cycle of laser gain media. We have made pump-probe studies of the local modes created upon the introduction of a light impurity, in particular the H/sup -/ ion, in CaF/sub 2/ in the spectral region 700 to 1200 cm/sup -1/. We have also measured the free induction decay of this mode using a noncollinear degenerate four wave mixing geometry. The observed coherent transient is striking in having large quantum beat modulations at negative time which are associated with vibrational ladder climbing.

DOI
10.1109/icimw.2000.892982

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A brief review is given of lifetime and line broadening studies with the free electron laser at FOM-Rijnhuizen (FELIX), emphasising work on far infrared (FIR) modulated photoluminescence (PL) of bound-to-bound transitions in InAs/GaAs quantum dots and mid-infrared (MIR) four wave mixing experiments on localised modes of H/sup -/ ions in calcium fluoride crystals. We have made new far/near infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The FIR resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots, and analysis of the FIR double resonance linewidth enables us to show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation. In addition we have made time resolved studies of local modes in ionic crystals, where nonradiative decay plays an important role in the optical pumping cycle of laser gain media. We have made pump-probe studies of the local modes created upon the introduction of a light impurity, in particular the H/sup -/ ion, in CaF/sub 2/ in the spectral region 700 to 1200 cm/sup -1/. We have also measured the free induction decay of this mode using a noncollinear degenerate four wave mixing geometry. The observed coherent transient is striking in having large quantum beat modulations at negative time which are associated with vibrational ladder climbing.
Time-resolved four-wave mixing in an InAs/InGaAs/GaAs electrically pumped quantum-dot amplifier is measured at room temperature for different applied bias currents going from optical absorption to gain of the device. The four-wave mixing signal from 140 fs pulses shows a transition from a delayed photon-echo response in the absorption regime to a prompt free polarization decay in the gain regime. This corresponds to a pronounced reduction of the dephasing time from 250 fs at zero bias to less than 50 fs at the maximum applied current. The four-wave mixing response at transparency of the device shows a composite structure with both photon echo and free-polarization decay. This is a signature of the digital occupation number in quantum dots, resulting at transparency in a signal from dots occupied with either zero or two excitons corresponding to absorption or gain of the dot ground state.
The three-dimensional confinement created by the ultrasmall semiconductor structures known as quantum dots greatly modifies the optical properties of the spatially localized carriers. These systems are presently of great interest to the research community, both for the improved understanding of the physics of confined structures, and for the potential applications of these systems to areas such as optoelectronics and quantum computing. Here, the techniques of transient coherent nonlinear optical spectroscopy are applied to the investigation of excitons confined within InAs/GaAs self-assembled quantum dots. These methods provide a powerful tool for the study of dynamic processes in these novel materials. The nonlinear measurements are based on the differential transmission experimental geometry, and utilize ultrafast pulsed lasers in order to temporally resolve the carrier dynamics after excitation. The experiments performed here are resonant with the ground state absorption of the quantum dots, as evidenced by comparison with the photoluminescence measurements which are used to characterize the sample. The incoherent relaxation dynamics of the population of photoexcited excitons are examined, allowing for a determination of both the lifetime of the excitons and the time scale for relaxation between the two polarization eigenstates of the system. The three-dimensional quantum confinement results in the suppression of these relaxation processes, as predicted. The use of coherent spectroscopic techniques allows for the first unambiguous demonstration of an optically-induced nonradiative Raman coherence in these quantum dots. Simultaneous excitation of both polarization states of the exciton results in a coherent superposition state, whose temporal evolution is observed through the homodyne-detection scheme. In these transient experiments, this is manifested as a beating in the nonlinear response, which allows us to directly measure the fine-structure splitting between the polarization states. Utilizing optical pulse shaping techniques, transient spectral hole burning is performed, which allows for the nonlinear response to be spectrally resolved. As expected for the strongly confined quantum dot excitons, the results show that diffusion among the localized states is not a significant effect. Temperature dependent measurements show that at elevated temperatures, extra dephasing processes, potentially related to acoustic phonon scattering, become important. Finally, this technique allows for the first direct observation of the induced absorption from the exciton to biexciton transition in these InAs quantum dots, and a measurement of the associated binding energy. These results lay the foundation for future studies in this material system, and for the possible development of quantum logic devices utilizing quantum dots.
We have performed time-integrated degenerate four-wave mixing experiments in InAs/GaAs self-assembled quantum dots. The quantum dots are excited in resonance with an intersublevel transition between the valence states of the dots. The dephasing time T2{T}_{2}T2​ is deduced from the time dependence of the photon echo amplitude as a function of the delay between the two incident pulses. At low temperature, a dephasing time T2{T}_{2}T2​ of 15 ps is measured for a resonant excitation at 7.4 \ensuremath{\mu}m wavelength. This dephasing corresponds to an homogeneous broadening of the intersublevel transition of 87 \ensuremath{\mu}eV. The polarization decay time is much longer than the one observed for resonant intersubband excitations in quantum wells. The dephasing time of the intersublevel polarization exhibits only a weak temperature dependence, indicating that the dephasing is not dominated by phonon-assisted scattering mechanisms.
Current issues in the study of light-matter interactions are coherence and quantum interference. With ultrafast nonlinear spectroscopy such as transient four-wave mixing (TFWM) the imposed coherence on the optical excitations in e.g. GaAs multiple quantum wells has been studied. In this contribution, we study the influence of inhomogeneous broadening on quantum beats observed by spectrally resolved TFWM. In artificially grown semiconductor structures as e.g. quantum wells, inhomogeneities due to interface-disorder has, up till now, been inevitable. In the experiments, the light-hole exciton (lhx) and the heavy-hole exciton (hhx) in a 100 Å GaAs multiple quantum well are excited simultaneously with 150 fs pulses from a self mode-locked Ti:sapphire laser. The spectral profile is shown in the inset of Fig. 1.
We report degenerate four-wave mixing experiments at room temperature, using a three-beam geometry and sub-picosecond laser to study GaAs/GaAlAs multiple quantum well material. Diffraction efficiencies and grating decay times for two samples of different carrier lifetimes have been measured in the spectral region corresponding to excitonic resonances. The measurements provide values of the nonlinear refractive index per carrier pair (n eh) deriving from the excitonic saturation, and also the recombination times and diffusion coefficients for the samples.
We report the observation of giant quantum coherence effects in the localized modes of ionized hydrogen in synthetic fluorite. Infrared free induction decay experiments on the substitutional H- center show dramatic modulations at negative delay times due to interference between multiple vibrational levels. Spectrally resolving the degenerate four wave mixing signal allows unambiguous assignments of the participating vibrational states. The dependence of the signal intensity upon the delay path between the exciting free electron laser pulses can be accounted for in terms of the resonant third order polarization with a common dephasing time for the excited states.