This study explores the potential for coherent control of the quantum state of quantum confined InGaN nanostructures, known for large exciton binding energies, using optical fields for future applications such as quantum information processing up to room temperature. Here we specifically report on the coherent nonlinear optical spectrum of InGaN disks grown in GaN nanowires at both cryogenic temperatures and room temperature. Two different samples are featured in this study: a self-assembled nanowire sample grown on a silicon substrate and a selective area sample grown on GaN/sapphire. Self-assembled DINWs grown on silicon are important for on-chip silicon photonics applications, however the morphological properties of the nanowires are much more difficult to control compared to selective area samples. Using frequency domain nonlinear spectroscopy methods, we find resonances in an ensemble of self-assembled InGaN disks in GaN nanowires that persist without significant broadening up to room temperature that we assign to quantum-confined excitons. Furthermore, the nonlinear optical spectrum as a function of excitation frequency depends only on the disorder state dominated photoluminescence excitation spectrum. Under these growth conditions, we find that the kinetics are dominated by metastable traps with strongly temperature dependent lifetimes corresponding to an activation energy between 5 and 12 meV. The metastable traps are mostly likely formed due to the coalescence of multiple nanowires during growth. The selective area sample is grown free of nanowire coalescence; therefore we do not observe effects from metastable states in the system. The nonlinear optical spectrum in the selective area shows some evidence of spectral hole burning from exciton states without significant additional broadening at room temperature, where the spectral holes have homogeneous linewidths ~20-30 meV at both low temperature and room temperature corresponding to ultrafast decoherence processes. The ultrafast decoherence could be associated with fast electron-hole transfer within a large density of background disorder states that is observed in the nondegenerate nonlinear absorption spectrum. In summary, although the samples show evidence of excitons with high temperature stability, the presence of material disorder and metastable trap states severely limits the use of InGaN disks-in-nanowires for coherent control applications without further material engineering.
This study explores the potential for coherent control of the quantum state of quantum confined InGaN nanostructures, known for large exciton binding energies, using optical fields for future applications such as quantum information processing up to room temperature. Here we specifically report on the coherent nonlinear optical spectrum of InGaN disks grown in GaN nanowires at both cryogenic temperatures and room temperature. Two different samples are featured in this study: a self-assembled nanowire sample grown on a silicon substrate and a selective area sample grown on GaN/sapphire. Self-assembled DINWs grown on silicon are important for on-chip silicon photonics applications, however the morphological properties of the nanowires are much more difficult to control compared to selective area samples. Using frequency domain nonlinear spectroscopy methods, we find resonances in an ensemble of self-assembled InGaN disks in GaN nanowires that persist without significant broadening up to room temperature that we assign to quantum-confined excitons. Furthermore, the nonlinear optical spectrum as a function of excitation frequency depends only on the disorder state dominated photoluminescence excitation spectrum. Under these growth conditions, we find that the kinetics are dominated by metastable traps with strongly temperature dependent lifetimes corresponding to an activation energy between 5 and 12 meV. The metastable traps are mostly likely formed due to the coalescence of multiple nanowires during growth. The selective area sample is grown free of nanowire coalescence; therefore we do not observe effects from metastable states in the system. The nonlinear optical spectrum in the selective area shows some evidence of spectral hole burning from exciton states without significant additional broadening at room temperature, where the spectral holes have homogeneous linewidths ~20-30 meV at both low temperature and room temperature corresponding to ultrafast decoherence processes. The ultrafast decoherence could be associated with fast electron-hole transfer within a large density of background disorder states that is observed in the nondegenerate nonlinear absorption spectrum. In summary, although the samples show evidence of excitons with high temperature stability, the presence of material disorder and metastable trap states severely limits the use of InGaN disks-in-nanowires for coherent control applications without further material engineering.
We report on high frequency resolution coherent nonlinear optical spectroscopy on an ensemble of InGaN disks in GaN nanowires at room temperature. Sub-$\ensuremath{\mu}\mathrm{eV}$ resonances in the inhomogeneously broadened third order $({\ensuremath{\chi}}^{(3)})$ absorption spectrum show asymmetric line shapes, where the degree of asymmetry depends on the wavelength of the excitation beams. Theory based on the optical Bloch equations (OBE) indicates that the line shape asymmetry is a result of fast decoherence in the system and the narrow resonances originate from coherent population pulsations that are induced by decoherence in the system. Using the OBE, we estimate that the decoherence time of the optically induced dipole (formed between the unexcited ground state and the excited electron-hole pair) at room temperature is 125 fs, corresponding to a linewidth of $\ensuremath{\sim}10$ meV. The decay time of the excitation is $\ensuremath{\sim}5--10$ ns, depending on the excitation energy. The line shapes are well fit with the OBE indicating that the resonances are characterized by discrete levels with no evidence of many body physics.
GaN/AlN quantum dots are semiconductor nanostructures where the huge conduction band offset of 1.75 eV leads to intraband transitions at exceptionnaly short wavelengths compared to III-As semiconductor materials. For specific quantum dot dimensions, it was shown that the absorption spectrum covers the optical communication spectral range. The ultrafast relaxation dynamics observed for these materials motivates the development of opto-electronic devices, and especially ultrafast switches. However, a detailed insight into the microscopic processes governing the relaxation dynamics is still missing although of fundamental importance for the understanding and the optimization of the devices operation. In particular, there is no study of the homogeneous broadening which drives the saturation properties of GaN/AlN quantum dots-devices. In this paper, we present homogeneous linewidth measurements by means of the nonlinear optical technique of spectral hole-burning. Spectrally-resolved pump-probe experiments with cw-lasers allow us to characterize the coherence relaxation dynamics down to 5K by resolving the homogeneous linewidth in an ensemble of GaN/AlN quantum dots. The differential transmission measured at 5K for a TM-polarized pump shows a pronounced bleaching with an amplitude of 2x10-4 on a background signal of 5x10-5. According to the selection rules of the s-pz intraband transition in GaN/AlN QDs, we attribute the photoinduced increase of the probe transmission to the partial saturation of the quantum dots by the pump laser, and we deduce a homogeneous linewidth of 15 meV at 5K. We find that the differential transmission signal scales like the square-root of the incident pump power, thus revealing the importance of electron-electron scattering in the population relaxation dynamics. As a matter of fact, we find that the population relaxation from the pz excited state is predominantly governed by Auger-type processes whereas optical phonon emission only plays a minor role, even if it may contribute to secondary processes during the carrier cascade to the fundamental s state via intermediate confined states.
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.
Ultrafast nonlinear optical spectroscopy is used to study the coherent dynamics of optically excited electron-hole pairs in semiconductors. Coulomb interaction implies that the optical inter-band transitions are dominated, at least at low temperatures, by excitonic effects. They are further enhanced in quantum confined lower-dimensional systems, where exciton and biexciton effects dominate the spectra even at room temperature. The coherent dynamics of excitons are at modest densities well described by the optical Bloch equations and a number of the dynamical effects known from atomic and molecular systems are found and studied in the exciton-biexciton system of semiconductors. At densities where strong exciton interactions, or many-body effects, become dominant, the semiconductor Bloch equations present a more rigorous treatment of the phenomena. Ultrafast degenerate four-wave mixing is used as a tool to study the coherent exciton dynamics, and the importance of performing transform limited spectroscopy is demonstrated throughout.
Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS).\nThe effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [1-10] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence.\nSpectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures >20 K.\nExciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing rate in GaAs quantum wells is due to the bosonic character of excitons. We fit slices from simulated spectra to those from the experimentally measured spectra and show that interference between two different quantum mechanical pathways results in a slower dephasing rate for co-circular and co-linear polarization of optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a faster dephasing rate. Additionally, we were able to separately quantify inter- and intra-mode interactions between excitons through exciton-density-dependent measurements.
We review the nonlinear-optical effects observed at room temperature in semiconductor quantum-well structures photoexcited near the band gap. A comprehensive discussion of optical transitions in these microstructures is given, including excitonic effects and the specific features of room-temperature exciton resonances. Experimental investigations using continuous-wave, picosecond-, and femtosecond-laser sources are presented. They show extremely efficient nonlinear processes. In the case of excitations that are long compared with the exciton-ionization time, the induced changes in absorption and refraction do not depend on the wavelength or on the duration of excitation. These changes depend only on the density of absorbed photons and are interpreted in terms of electron–hole plasma screening and band filling. In contrast, for ultrashort excitation, nonlinear processes depend critically on the excitation wavelength. The selective generation of excitons is found to produce effects larger than a plasma of the same density. This unexpected result is shown to arise from the low temperature of the exciton gas before it interacts with the lattice and from the decrease of screening that is the reduced dimensionality of quantum-well structures.