Biexcitonic gain characteristics in ZnSe-based lasers with binary wells

Self Cite

The role of excitonic excitations in wide-gap II-VI and nitride semiconductors is reviewed with focus on the recent research at Bremen University. More-particle excitations such as trions and biexcitons are studied including their contributions to nonlinearities in the coherent regime and to lasing. Specific questions concerning laser processes in wide-gap materials are addressed. IntroductionWide-gap semiconductors such as most of the Zn and Cd based II-VIs or the nitride based blue to UV emitters are model materials for exciton and Coulomb correlation effects on the whole time and a large intensity scale since excitons in these compounds are much more stable as in conventional III-V semiconductors widely used for optoelectronic applications so far. This in particular holds for lower-dimensional structures exhibiting exciton binding energies exceeding those of the optical phonons. Thus, even at high electron-hole densities and rather elevated temperatures, lasing is still strongly governed by Coulomb correlation effects [1,2], what makes wide-gap materials rather different from III-Vs concerning this important aspect of application. Also with respect to very basic phenomena such as higher-order contributions to coherent multi-wave mixing, or propagation effects, the role of exciton correlation is extremely significant [3][4][5][6][7]. This paper is a short review of the recent activities of the Bremen group in the above-mentioned fields.

Section: Ii-vi Quantum Well Laserssupporting

confidence: 72%

Section: Ii-vi Quantum Well Lasersmentioning

confidence: 76%

Excitons in Wide-Gap Semiconductors: Coherence, Dynamics, and Lasing

Gutowski

Michler

Rückmann

et al. 2002

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“…Time delay a) 10 and b) 2000 ps for differently polarized pump beam and s À polarized probe beam ergy shift of the hh exciton (right-hand strong and cut-off signature in Fig. 1a), only the biexciton XX was observed as induced absorption signal identified by its polarization selection rules [7]. For delay times of 15 ps and longer, a second absorption signature grew up exactly at the X À s transition energy showing long-term persistence whereas XX vanished for delays beyond 200 ps.…”

Section: Resultsmentioning

confidence: 99%

“…In ZnSe based quantum wells, X À t state transitions have been investigated by few groups including ourselves [7,8], however, signatures of the triplet state transition X À t were only reported by [7][8][9]. An X À t term scheme based on magnetooptics has not been proposed up to now and is the object of this paper.…”

mentioning

confidence: 99%

Interplay of the Trion Singlet and Triplet State Transitions in Magnetooptical and Time-Resolved Investigation of ZnSe/Zn(S,Se) Single Quantum Wells

Gutowski

Sebald

Röder

et al. 2002

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Negatively charged trions are investigated for ZnSe single quantum wells embedded in ternary barriers. Polarization and excitation dependent photoluminescence measurements were performed in magnetic fields up to 11.8 T. From these measurements we are able to develop an energy-term scheme for the spin-singlet and the spin-triplet state of the trion.Introduction If a neutral exciton X binds a second electron (hole), a negatively (positively) charged exciton X À (X þ ) is formed which is called trion. It was first identified in CdTe/(Cd,Zn)Te quantum wells (QWs) in 1993 by Kheng et al., since then intense investigations mainly on GaAs-and CdTe-based QWs were performed (e.g. [1-3]).Since X À consists of one hole and two identical electrons Pauli's exclusion principle tells us that the total wavefunction must be antisymmetric, which can be factorized into an antisymmetrical (symmetrical) spin part and a (anti)symmetrical spatial function. The antisymmetric spin function obtained for antiparallel electron spins results, together with the hole spin, in the twofold degenerate singlet state X À s whereas parallel electron spins yield the sixfold triplet state X À t . The degeneracy can be lifted by applying a magnetic field (Zeemann splitting). While the X À s state is bound at any applied field, the X À t is unbound at low magnetic fields and becomes stabilized beyond a particular B field value [3,4].Several groups performed experiments to clarify the term scheme of X À t [4][5][6]. In ZnSe based quantum wells, X À t state transitions have been investigated by few groups including ourselves [7,8], however, signatures of the triplet state transition X À t were only reported by [7][8][9]. An X À t term scheme based on magnetooptics has not been proposed up to now and is the object of this paper.

“…Laser emission due to excitonic and biexcitonic gain mechanisms has been observed in ZnSe based structures [1,2] for temperatures up to 150 K. However, the strong interaction between the excitons and LO phonons usually results in the dissociation of the excitons at higher temperatures, reducing device efficiencies. Ionization of excitons should be prevented in systems where all the excitations of the exciton have an energy larger than that of the LO phonon, namely for E X (1s-2s) > hn LO , due to the absence of final states for the scattering process [3].…”

mentioning

confidence: 99%

Excitonic Properties of ZnS Quantum Wells in ZnMgS

Urbaszek

Townsley

Tang

et al. 2002

The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by MBE on GaP substrates has allowed the observation of heavy and light hole exciton transitions with values for the FWHM as narrow as 5 meV. The 2s state of the heavy hole exciton is identified and exciton binding energies as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings of the order of 10 meV for both the light and heavy hole exciton transitions appear in magneto-reflectivity spectra in magnetic fields up to 54 T. Large light hole exciton g-values of the order of four for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.