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Europhysics Letters (EPL)

Resonant creation of indirect excitons in diamond at the phonon-assisted absorption edge

Yuji Hazama, N. Naka, Makoto Kuwata‐Gonokami, Kōichiro Tanaka

Published Nov 1, 2013• 11 citations
Publisher

Abstract

We have developed a new scheme for generating cold and high-density excitons in diamond by resonant excitation at the phonon-assisted absorption edge. Both the thermalization of excitons between fine-structure states and the effective temperature are examined with the tuning of the photon energy. The down-conversion rate for the excitons in the fine-structure states is found to be 3.5 ns independent of the photon energy, which means that the lowest-energy dark exciton can be created within a time scale comparable to its lifetime via the generation of a higher-energy bright exciton. With careful choice of excitation photon energy, we have achieved an electron-hole pair density of while maintaining the effective excitonic temperature below 15 K.

DOI
10.1209/0295-5075/104/47012

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We have developed a new scheme for generating cold and high-density excitons in diamond by resonant excitation at the phonon-assisted absorption edge. Both the thermalization of excitons between fine-structure states and the effective temperature are examined with the tuning of the photon energy. The down-conversion rate for the excitons in the fine-structure states is found to be 3.5 ns independent of the photon energy, which means that the lowest-energy dark exciton can be created within a time scale comparable to its lifetime via the generation of a higher-energy bright exciton. With careful choice of excitation photon energy, we have achieved an electron-hole pair density of while maintaining the effective excitonic temperature below 15 K.
Abstract We have observed edge emission due to free excitons and electron–hole droplets in diamond by two‐photon excitation using subpicosecond laser pulses at 257 nm. We find that excitonic temperatures under two‐photon excitation are much lower than the case of one‐photon excitation. A minimum excitonic temperature of 3.4 K is extracted by spectral analysis. In order to understand the difference of the excitonic temperatures under two‐photon and one‐photon excitations, we discuss a heating mechanism which depends on the density distribution of the electron–hole pairs in the crystal along the laser beam. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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We study low-strain synthetic high pressure, high temperature diamonds by cathodoluminescence and observe novel fine structure in the free exciton and the boron-bound exciton emission. The basic spectral structure is a doublet with DeltaE approximately 11 meV common to both exciton spectra. This resolves the previously found inequivalence of free exciton ( approximately 7 meV) and bound exciton ( approximately 12 meV) fine splitting. It is argued that for a spin-orbit interaction Delta(0) much smaller than the excitonic binding ( E(X) approximately 80 meV) and the excitonic localization ( E(loc) approximately 51 meV) at the boron acceptor, the orbital momentum and the spin of the particles constituting the electron-hole pair are recoupled to form spin singlet and triplet exciton states as the elementary excitations.
We report on time-resolved photoluminescence of a free-exciton in IIa chemical vapor deposition diamond crystal. Large difference between decay times for one- and two-photon excitation processes was observed. The longest room-temperature exciton photoluminescence lifetime τ(FE)=220 ns was obtained under two-photon excitation with a photon energy of 4.7 eV. The role of diffusion and surface recombination velocity in exciton photoluminescence dynamics was studied using a new optical method based on two-photon excited time-resolved photoluminescence. The measured room-temperature value of diffusion coefficient in diamond was D=40 cm(2)/s.
We observe exceptionally high diffusion of excitons in diamond using time-resolved photoluminescence imaging. The diffusivity is found to increase with decreasing temperature more rapidly than well-established predictions. The highest diffusion constant, $9.2\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\text{s}$, measured for thermalized excitons is 150 times that recorded previously in diamond. We elucidate the momentum relaxation mechanisms determining transport: intraband and interband scattering by acoustic phonons in the exciton fine-structure levels. The enhanced diffusivity above 100 K is explained by a free-carrier contribution.