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arXiv:1705.10782 · Physical review. B./Physical review. B

Sub- μeV decoherence-induced population pulsation resonances in an InGaN system

Cameron Nelson, Yong‐Ho Ra, Zetian Mi, P. R. Berman, Duncan G. Steel

Published Sep 5, 2017• 2 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.96.115302

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We report on high frequency resolution coherent nonlinear optical spectroscopy on an ensemble of InGaN disks in GaN nanowires at room temperature. Sub-\ensuremathμeV\ensuremath{\mu}\mathrm{eV}\ensuremathμeV resonances in the inhomogeneously broadened third order (\ensuremathχ(3))({\ensuremath{\chi}}^{(3)})(\ensuremathχ(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∼10\ensuremath{\sim}10\ensuremath∼10 meV. The decay time of the excitation is \ensuremath∼5−−10\ensuremath{\sim}5--10\ensuremath∼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.
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.
The optical Bloch equations which incorporate the phenomenological population (T1)({T}_{1})(T1​) and dipole dephasing (T2)({T}_{2})(T2​) times have been tested recently by optical free-induction-decay (FID) measurements on an impurity-ion crystal Pr3+{\mathrm{Pr}}^{3+}Pr3+:La3{\mathrm{La}}_{3}La3​ at 1.6 K. At low optical fields, the observed Pr3+{\mathrm{Pr}}^{3+}Pr3+ optical linewidth is dominated by magnetic fluctuations arising from pairs of fluorine nuclear flip-flops where the condition ${T}_{1}\ensuremath{\gg}{T}_{2}$ prevails. At elevated fields, this nuclear broadening mechanism is quenched and the Bloch equations are violated with ${T}_{2}\ensuremath{\rightarrow}{T}_{1}$. In this paper, a microscopic theory appropriate for a low-temperature impurity solid is presented which reveals the above features both for optical and radio frequencies, and a simple physical interpretation of this line narrowing phenomenon is given. Modified Bloch equations of a novel form are derived to second order and yield analytic FID solutions over the entire range of optical-field strength. A discussion of the earlier NMR theories is given, pointing out similarities and differences.
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The purpose of this research is to study the relaxation of optically generated excitons and carriers in semiconductor microstructures using four wave mixing (FWM) spectroscopy. The systems studied include CdSSe microcrystallite doped glasses and GaAs/AlGaAs multiple quantum well structures (MQWS). Exposing a semiconductor to optical radiation near the fundamental band gap results in the creation of populations of elementary excitations including electrons, holes, and excitons, and also results in the creation of a superposition state between the ground and excited state of the solid. The work in this dissertation examines the relaxation of the populations of elementary excitations and the relaxation of the coherence between the ground and excited state using four wave mixing spectroscopy. First, the nonlinear optical response of simple two level systems is examined in order to provide insight into the types of line shapes expected from semiconductors. It is shown that the line shape is strongly dependent on how the system is coupled to the reservoir and the consequences of coupling to a reservoir are examined in a FWM measurement made in atomic sodium. The first semiconductor system studied is CdSSe microcrystallite doped glass. This system is shown to have a very slow component to the nonlinear response which has a optical intensity dependence and temperature dependence which suggests that the FWM response in these materials is trap mediated. Room temperature FWM measurements in GaAs MQWS enables the measurement of the carrier recombination time and the ambipolar diffusion coefficient. Using the technique of correlated optical fields, a slow component to the nonlinear response was measured showing an interference profile which suggests a possible shift of the exciton resonance due to the optically generated carriers. At low temperatures (2-20 K), measurements of the exciton line shape and relaxation time were made and evidence for exciton spectral diffusion was found. The low temperature line shapes can be qualitatively reproduced using Modified Optical Bloch equations which include the effects of spectral diffusion.
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