Physics of Stimulated Emission in Blue Semiconductor Lasers

Arto V. Nurmikko

Division of Engineering and Department of Physics,
Brown University, Providence RI 02912 USA

The past few years have brought to life a new class of semiconductor diode lasers, spanning the spectral range from the violet to the green, based on wide bandgap semiconductors. The first laser demonstrations in the blue/green were made in ZnSe-based quantum well (QW) heterostructures [1]. This has been followed by the rapid recent developments in the GaN-compounds that have brought the latter devices to the threshold of technological viability [2].

In this presentation we focus on the rich realm of optical physics that the new lasers offer, especially in terms of light-matter coupling in the regime of stimulated emission within a high density electron-hole two-dimensional system in a ZnCdSe or InGaN quantum well. Although these active optical media appear at first sight rather similar in terms of (effective mass theory) expectations that strong Coulomb (excitonic) correlations be much more dominant that in conventional semiconductor lasers, we find considerable contrasts between the widegap II-VI and III-V systems at present.

The ZnSe-based QWs enjoy the benefit of very high quality heteroepitaxy and small crystalline disorder, as a consequence of which excitonic contributions can be readily observed to make a profound enhanced contributions to optical gain, with characteristic spectral fingerprints. At cryogenic tempetures, for example, excitonic molecules dominate optical gain which can reaching peak values of the gain coefficient in excess of g> 105 cm-1. In the room temperature continuous-wave diode lasers, the excitonic enhancements remain very potent and aid in reducing the threshold current densities to values below 200 A/cm2. The importance of pairwise Coulomb correlation of the electron-holes can be directly seen, for example, by studying the emission spectra of the diode lasers in an external magnetic field, which remain diamagnetic up to 30 Tesla in our experiments. In another example of the excitonic aspect of optical gain in a ZnCdSe QW, we describe experiments in vertical cavity microresonators in which the light-matter interaction is enhanced by the so-called normal mode coupling between the exciton and photon oscillators (Rabi splitting analog in atomic systems), leading to vertical cavity lasing in a regime not accessible by conventional perturbation treatment of a laser.

In contrast, the many-electron contributions to the physics of optical gain and stimulated emission in the nitride diode lasers is considerably complicated, and largely masked by the presence of a significant amount of compositional disorder in the InGaN QW material. The disorder arises from the non-random nature of the ternary semiconductor, leading to partial clustering of In-rich regions whose size spans a wide distribution range from atomic to mesoscopic (>100 nm) scale. For example, we have observed such optically inhomogeneous properties at high injection levels by high resolution near-field optical microscopy. In terms of a density of states picture for interband transitions, the striking compositional anomalies contribute to a relatively high density of localized states even at a very low average In-concentration (<0.05), that reach deeply into the bandgap of InGaN, usually well in excess of 100 meV. As one consequence, direct measurements of optical gain in laser devices show the nececessity of filling the localized states before stimulated emission commences from extended states that correspond to spatial interconnects between the In-rich clusters. From a device point of view, such circumstances lead to high threshold current densities, with corresponding electron-hole pair densities at threshold typically in excess of 1019 cm-3 . A major contemporary, and yet unresolved question is how the many-body Coulomb interactions impact such a disordered optical gain medium which is quite unusual in the history of semiconductor diode lasers to date.

[1] M. Haase et al Appl. Phys. Lett. 59, pp.1272-1274 (1991); H. Jeon et al, ibid 59, pp. 3619-3621 (1991)
[2] see the paper by S. Nakamura in this conference