Microcavity physics and design will be reviewed. Following an overview of applications in quantum optics, communications and biosensing, recent advances in ultra-high-Q research will be presented.

Optical microcavities

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Cavity quantum electrodynamics (QED) systems allow the study of a variety of fundamental quantum-optics phenomena, such as entanglement, quantum decoherence and the quantum–classical boundary. Such systems also provide test beds for quantum information science. Nearly all strongly coupled cavity QED experiments have used a single atom in a high-quality-factor (high-Q) cavity. Here we report the experimental realization of a strongly coupled system in the solid state: a single quantum dot embedded in the spacer of a nanocavity, showing vacuum-field Rabi splitting exceeding the decoherence linewidths of both the nanocavity and the quantum dot. This requires a small-volume cavity and an atomic-like two-level system. The photonic crystal slab nanocavity—which traps photons when a defect is introduced inside the two-dimensional photonic bandgap by leaving out one or more holes—has both high Q and small modal volume V, as required for strong light–matter interactions. The quantum dot has two discrete energy levels with a transition dipole moment much larger than that of an atom, and it is fixed in the nanocavity during growth.

Vacuum Rabi splitting with a single quantum dot in a photonic crystal nanocavity

We describe the recent experimental progress in the control of spontaneous emission by manipulating optical modes with photonic crystals. It has been clearly demonstrated that the spontaneous emission from light emitters embedded in photonic crystals can be suppressed by the so-called photonic bandgap, whereas the emission efficiency in the direction where optical modes exist can be enhanced. Also, when an artificial defect is introduced into the photonic crystal, a photonic nanocavity is produced that can interact with light emitters. Cavity quality factors, or Q factors, of up to 2 million have been realized while maintaining very small mode volumes, and both spontaneous-emission modification (the Purcell effect) and strong-coupling phenomena have been demonstrated. The use of photonic crystals and nanocavities to manipulate spontaneous emission will contribute to the evolution of a variety of applications, including illumination, display, optical communication, solar energy and even quantum-information systems.

Spontaneous-emission control by photonic crystals and nanocavities

We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-$Q$ single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

Controlling the spontaneous emission rate of single quantum dots in a two-dimensional photonic crystal.

We demonstrate a two-dimensional photonic crystal defect laser diode based on a coupled cavity waveguide. The laser cavity is formed by 40 coupled hexagonal defect microcavities in a triangular lattice of air cylinders, which are etched into an InGaAsP/InP laser structure. The coupling of the individual cavity modes creates minibands within the photonic band gap. Stable single-mode lasing occurs on the first miniband mode with the lowest group velocity with side mode suppression greater than 40 dB. The lasers emit up to 2.6 mW at a 1.53 μm wavelength under continuous-wave operation at room temperature.

Two-dimensional photonic crystal coupled-defect laser diode

We have studied the coupling of a classic ridge waveguide with a two-dimensional photonic crystal (PC) waveguide, using finite-difference time-domain calculations. The ridge waveguide exhibits only a weak refractive-index confinement of light, as it is commonly used in buried-heterostructure or ridge-waveguide lasers. The light is coupled to a PC waveguide that consists of one missing row along the ΓK direction in a triangular lattice of air cylinders in AlGaAs. We compare various designs for PC tapers with that of a classic taper and for butt coupling. The calculation yields high coupling efficiency that exceeds 80% for a 2.5‐μm-long PC taper. In addition, the dependence of the efficiency on the PC air-fill factor and on alignment tolerances is analyzed.

Photonic crystal tapers for ultracompact mode conversion.

The interaction of self-assembled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dots with nondegenerate modes of a two-dimensional photonic-crystal defect microcavity has been investigated in the weak-coupling regime. Photoluminescence intensity measurements show a transition from a pump rate limited regime at low excitation to cavity-controlled modes at high excitation. We observe considerably different saturation levels for quantum dots on and off the defect resonance. An enhancement of light emission by a factor of 9 due to the Purcell effect is obtained from this saturation behavior. Using the different temperature shifts of the quantum dot emission lines and the cavity mode, individual dots are tuned in and out of resonance, thus controlling their spontaneous emission rate.

Enhanced light emission of In x Ga 1 − x As quantum dots in a two-dimensional photonic-crystal defect microcavity

We have developed a fabrication scheme for two-dimensional (2D) triangular photonic crystals (PCs) on InP-based material systems involving high resolution electron beam lithography, pattern transfer to a SiO2 etch mask, and a Cl2/Ar electron cyclotron resonance reactive ion etch step yielding PCs with periods of a=300–450 nm and air fill factors of f=18%–63%. These PCs are employed as high reflectivity mirrors for 1.5 μm short cavity lasers, which are key components in future highly integrated PC based photonic circuits. We have fabricated lasers with 2 PC mirrors and cavity lengths down to 100 μm. Threshold currents as low as 7.6 mA were achieved for the shortest lasers with 2 PC mirrors. The short laser cavity results in a large Fabry–Perot mode spacing and mode competition leads to single mode lasing over a reasonably large current range up to 4.5× threshold. Lasers with one PC back mirror and a cleaved output facet show a higher threshold current of 13 mA and a maximum output power of more than 4 mW. ...

Nanofabrication of two-dimensional photonic crystal mirrors for 1.5 μm short cavity lasers

We report the fabrication of short-cavity lasers with highly reflective two-dimensional photonic crystal mirrors on an InGaAsP/InP laser structure emitting at 1.57 μm. An intracavity photonic crystal mirror creates two coupled cavities, which provide additional longitudinal mode selection for stable single-mode operation with side-mode suppression ratios exceeding 35 dB. The shortest lasers with l=100 μm overall length have a threshold current of 13 mA and provide more than 4 mW power under continuous wave operation. Longer devices with l=200 μm deliver up to 9 mW. A maximum modulation bandwidth of 7.9 GHz was determined by relative intensity noise measurements.

T.D. Happ

Papers

Two-dimensional photonic crystal coupled-defect laser diode

Photonic crystal tapers for ultracompact mode conversion.

Enhanced light emission of In x Ga 1 − x As quantum dots in a two-dimensional photonic-crystal defect microcavity

Nanofabrication of two-dimensional photonic crystal mirrors for 1.5 μm short cavity lasers

Single-mode operation of coupled-cavity lasers based on two-dimensional photonic crystals