Recently it has been shown that it is possible to achieve directional emission out of a subwavelength aperture in a periodically corrugated metallic thin film. We report on theoretical and experimental studies of a related phenomenon concerning light emitted from photonic crystal waveguides that are less than a wavelength wide. We find that the termination of the photonic crystal end facets and an appropriate choice of the wavelength are instrumental in achieving very low numerical apertures. Our results hold promise for the combination of photonic crystal waveguides with conventional optical systems such as fibers, waveguides, and freely propagating light beams.

We report on a series of experiments on the second-order interference of two single photons emitted sequentially by a single quantum dot. The visibility of this interference probes the indistinguishability of the emitted photons; visibilities as high as 0.75 at 4 K have been achieved. At higher temperatures, dephasing of the quantum dot exciton degrades the indistinguishability of the emitted photons and the visibility of the interference. However, we demonstrate that engineering of the radiative lifetime of the quantum dot by the implementation of the Purcell effect in a microcavity, can restore indistinguishability and improve the visibility of second-order interference. At the same time, we demonstrate the resonant character of the Purcell effect.When two indistinguishable single photons enter separately, but simultaneously, into the two input ports of a beam splitter, they both emerge together, along the same output port of the beam splitter, as if they had "coalescedЉ into a two-photon Fock state. 1 This second-order interference phenomenon has been used to highlight many fundamental aspects of quantum optics, such as the nonlocality of quantum mechanics, 2 or the measurement of the photon transit time in superluminal photon tunneling. 3 Moreover, the interference of two single photons on a beam splitter plays a central role in recent proposals for the realization of two-qubit gates as key elements of photon-based quantum computing schemes. 4 The two-photon interference phenomenon was first observed using pairs of twin photons produced simultaneously by parametric down conversion. 1 More recently, it was demonstrated using two single photons originating from two distinct emitters: two independent but synchronized optical parametric oscillators, 5 two sequential emission events of a single atom, 6 and a single semiconductor quantum dot 7 excited by a pair of laser pulses.The situation in which two-photon interference occurs between truly independent photons presents an important difference from that of twin parametric photons. Each individual emitter may be subject to fluctuations independently of the other, thus "marking" each of the two photons differently and destroying their indistinguishability. For the case of a semiconductor quantum dot, for example, such fluctuations are due to the exciton-phonon interaction which causes the dephasing of the emitting exciton state, with a characteristic dephasing time T 2 * . In order to reduce the impact of dephasing on the emission process, and thus restore the indistinguishability of the emitted photons, the radiative lifetime of the emitter ͑denoted by T 1 ͒ must be shortened, so that it dominates over T 2 * in determining the overall coherence time of the photon wave train, T 2 , defined byThis can be achieved by embedding the quantum dot in a microcavity, 8 thus taking advantage of cavity quantum electrodynamics effects ͑Purcell effect 9 ͒. The spontaneous emis-sion rate of a dipole is given by Fermi's Golden Rule aswhere E vac ͑r͒ is the vacuum electric field a...

By using scanning near-field optical microscopy, we directly map the subwavelength confinement of light around a point defect in a two-dimensional photonic crystal microresonator. Comparison of our results with the outcome of three-dimensional finite-difference time domain calculations allows us to identify small imperfections in the structure that result in the spatial modification of the intensity distribution.

Photonic crystals based on macroporous silicon with fundamental band gaps in the middle infrared region 3.4-5.8 m were fabricated. Scanning probe optical microscopy and laser spectroscopy were combined to examine a deep two-dimensional photonic crystal microresonator based on a single point defect. Two sharp resonances were recorded in the band gap, in excellent agreement with the results of numerical simulations. Such a microresonator with high-quality factors and a subwavelength mode extension could be used for a range of applications including integrated optical gas sensors.

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