The highly aligned ZnO whiskers, which are grown by hydrothermal oxidation of metallic zinc plate in the presence of ethylenediamine molecules, exhibit single and/or a few modes of supernarrow spectral emissions (∼0.7 nm) at near 378 nm and negligible deep level defects emissions. Time-integrated and time-resolved photoluminescence show that the supernarrow spectral peaks are due to the laser action with an excitation threshold about 70 μJ/cm2 and emission lifetime of <30 ps. The lasing emission is highly polarized along the excitation laser polarization direction. The low lasing threshold, stable and regular supernarrow longitudinal modes, and strong lasing polarization effects can be well explained by the model of microcavity laser where the two end facets of whisker form the microcavity.

Microcavity lasing behavior of oriented hexagonal ZnO nanowhiskers grown by hydrothermal oxidation

A light source in a three-dimensional photonic crystal has been realized by depositing CdTe nanocrystals on the inner surface of TiO2 inverted opal films. Two effects, the photonic band gap of inverted opal and the energy exchange between CdTe nanoparticles and the TiO2 framework, have been identified as major sources of emission spectrum modification. An analysis of input-output characteristics of the spontaneous emission demonstrates the saturation of the excitation-to-emission conversion in the emission band and the conversion efficiency improvement associated with the presence of the photonic band gap.

Modification of the spontaneous emission of CdTe nanocrystals in TiO2 inverted opals

Characteristics of a planar microcavity laser using rhodamine 6G with pulsed excitation is studied. Theoretical aspects of controlled spontaneous emission and oscillation in a planar microcavity laser are discussed. The measured spectrum and the angular divergence of spontaneous emission below threshold are in good agreement with theory. The angular divergence yields the radius of the cavity quasimode. The spontaneous emission coupling ratio obtained from the measured input‐output characteristics is in reasonable agreement with the theoretical value. The expression for the coupling ratio derived for a Fabry–Perot‐type microcavity is shown to be essentially equal to that of a closed cavity or guided mode cavity derived by Yamamoto, Machida, and Bjork [Phys. Rev. A 44, 657 (1991)]. The observed spectral behavior near the threshold approximately follows the Schawlow–Townes formula, but for a limited range of output power. The observed spectrum and divergence above threshold indicate incoherence much worse than the Schawlow–Townes limit.

Spontaneous emission and oscillation in a planar microcavity dye laser

The polarization states of lasers are crucial issues both for practical applications and fundamental research. In general, they depend in a combined manner on the properties of the gain material and on the structure of the electromagnetic modes. In this paper, we address this issue in the case of solid-state organic lasers, a technology which enables one to vary independently gain and mode properties. Different kinds of resonators are investigated: in-plane microresonators with Fabry-Perot, square, pentagon, stadium, disk, and kite shapes, and external vertical resonators. The degree of polarization $P$ is measured in each case. It is shown that although transverse electric modes prevail generally ($P$ g 0), the kite-shaped microlaser generates negative values for $P$ (i.e., a flip of the dominant polarization which becomes mostly transverse magnetic polarized). In general, we demonstrate that both the pump polarization and the resonator geometry can be used to tailor the polarization of organic lasers. With this aim in view, we, at last, investigate two other degrees of freedom, namely upon using resonant energy transfer and upon pumping the laser dye to a higher excited state. We then demonstrate that significantly lower $P$ factors can be obtained.

Polarization properties of solid-state organic lasers

The polarization of lasing, as a fundamental property related to the emission coherence of microlasers, is one of the criteria for a high quality laser beam but has been rarely investigated. Unlike conventional lasers which can be induced by highly polarized seed light or by using a polarizing film, the microlaser for on-chip integrated photonic circuits generally possesses a low degree of polarization due to unpolarized spontaneous emission. Here, we firstly demonstrate that the Vernier effect can be used to improve the degree of polarization from ∼0.2 to 0.78 based on metal halide perovskite microspheres. After coupling, linearly polarized single-mode lasing with a low threshold and high quality can be achieved. In addition, by using the finite element method, the mode distributions of CsPbBr3 microspheres before and after coupling are analyzed systematically and a clear physical diagram of coupled resonances is obtained. Our work clearly suggests that a coupled linearly polarized single-mode microlaser by the Vernier effect would offer a real step closer to the platform for on-chip integrated photonics.

Linearly polarized lasing based on coupled perovskite microspheres.

We propose and demonstrate the use of polarization to determine the threshold of an optical microcavity when the output power versus pump power does not show a sharp threshold. A lasing threshold is observed via a sharp transition in the polarization and a simultaneous reduction in emission linewidth. We find that dipole angular diffusion in a dye gain medium has a significant effect on the polarization of the emission. A theoretical model including the diffusion process is presented. Quantitative agreement with the measurements is obtained.

H. Yokoyama

Papers

Lasing threshold behavior of microcavities: Observation by polarization and spectroscopic measurements.