Whispering-gallery mode resonators for highly unidirectional laser action.

Optical microcavities can be designed to take advantage of total internal reflection, which results in resonators supporting whispering-gallery modes (WGMs) with a high-quality factor (Q factor). One of the crucial problems of these devices for practical applications such as designing microcavity lasers, however, is that their emission is nondirectional due to their radial symmetry, in addition to their inefficient power output coupling. Here we report the design of elliptical resonators with a wavelength-size notch at the boundary, which support in-plane highly unidirectional laser emission from WGMs. The notch acts as a small scatterer such that the Q factor of the WGMs is still very high. Using midinfrared (λ ∼ 10 µm) injection quantum cascade lasers as a model system, an in-plane beam divergence as small as 6 deg with a peak optical power of ∼5 mW at room temperature has been demonstrated. The beam divergence is insensitive to the pumping current and to the notch geometry, demonstrating the robustness of this resonator design. The latter is scalable to the visible and the near infrared, thus opening the door to very low-threshold, highly unidirectional microcavity diode lasers.

Interplay of Goos-Hänchen shift and boundary curvature in deformed microdisks.

As the fabrication of wavelength-scale optical microcavities is becoming feasible, extended ray models which include first-order wave corrections have attracted considerable interest. By using such a model, we find an unexpected shift of phase-space structures in momentum direction which can be attributed to the Goos-Hänchen shift in position direction and the boundary curvature ("periodic orbit shift," POS); this shift is calculated analytically for a general cavity shape. By comparing it to wave calculations in the special case of a limaçon-shaped microcavity, it is shown that mode localization occurs on the shifted, rather than the original, phase-space structures. Comparing of our analytical result to literature data, we find good agreement, which suggests that the POS may be responsible for many cases of previously reported, but unexplained, mismatches between Husimi functions and the ray-dynamical phase space.

Goos-Hänchen shift and localization of optical modes in deformed microcavities.

Recently, an interesting phenomenon of spatial localization of optical modes along periodic ray trajectories near avoided resonance crossings has been observed [Wiersig, Phys. Rev. Lett. 97, 253901 (2006)]. For the case of a microdisk cavity with elliptical cross section, we use the Husimi function to analyze this localization in phase space. Moreover, we present a semiclassical explanation of this phenomenon in terms of the Goos-Hänchen shift, which works very well even deep in the wave regime. This semiclassical correction to the ray dynamics modifies the phase-space structure such that modes can localize either on stable islands or along unstable periodic ray trajectories.