InGaN Cavity Polaritons

Takehiko Tawara1, Hideki Gotoh1, Tetsuya Akasaka2 and Toshiki Makimoto2
1Optical Science Laboratory, 2Materials Science Laboratory

 Cavity polaritons are quasi-particles that are created by strong exciton-photon coupling in a semiconductor quantum well (QW) microcavity. In recent years, much attention has been directed to the behavior of these cavity polaritons as composite bosons at sufficiently small densities. One area of interest relates to polariton devices that employ bosonic behavior, such as the threshold-less “polariton” laser. To achieve these devices and room temperature operation, we have to choose materials that have large oscillator strength and a large exciton binding energy.
 GaN-based semiconductors have a large exciton binding energy, and this energy allows excitons to exist even at room temperature. Moreover, the oscillator strength of this system will be larger than that of typical III-V semiconductors owing to the large effective mass of nitrides. Therefore, we can expect very strong exciton-photon coupling in GaN-based QW microcavities that will enable us to achieve polariton devices that operate above room temperature.
 We used smooth and crack-free InGaN QW microcavities to realize strong coupling. The microcavities were fabricated using a wafer bonding technique with an InGaN/AlGaN QW layer and dielectric DBRs (Fig. 1). We observed a lasing action in the fabricated microcavity as shown in Fig. 2 [1]. Figure 3 shows the reflection spectra of the QW microcavities for various degrees of cavity detuning (δ). We observed the appearance and disappearance of splitting and these positions varied with δ. This behavior is evidence of cavity polariton formation, and the vacuum-field Rabi splitting which reflects the strength of the exciton-photon coupling is about 6 meV. We deduced from these results that the oscillator strength of InGaN QW excitons is one order of magnitude larger than that of GaAs QW excitons. These results indicate that GaN-based semiconductors are advantageous for studying the polaritonic effect and its device applications.

[1] T. Tawara, et al., Appl. Phys. Lett. 83 (2003) 830.
[2] T. Tawara, et al., Phys. Rev. Lett. 92 (2004) 256402.


Fig. 1. Fabrication. 
Fig. 2. Lasing by optical pumping.
Fig. 3. Formation of
cavity polariton.

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