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

Photonic cavities that strongly confine light are finding applications in many areas of physics and engineering, including coherent electron-photon interactions, ultra-small filters, low-threshold lasers, photonic chips, nonlinear optics and quantum information processing. Critical for these applications is the realization of a cavity with both high quality factor, Q, and small modal volume, V. The ratio Q/V determines the strength of the various cavity interactions, and an ultra-small cavity enables large-scale integration and single-mode operation for a broad range of wavelengths. However, a high-Q cavity of optical wavelength size is difficult to fabricate, as radiation loss increases in inverse proportion to cavity size. With the exception of a few recent theoretical studies, definitive theories and experiments for creating high-Q nanocavities have not been extensively investigated. Here we use a silicon-based two-dimensional photonic-crystal slab to fabricate a nanocavity with Q = 45,000 and V = 7.0 x 10(-14) cm3; the value of Q/V is 10-100 times larger than in previous studies. Underlying this development is the realization that light should be confined gently in order to be confined strongly. Integration with other photonic elements is straightforward, and a large free spectral range of 100 nm has been demonstrated.

High- Q photonic nanocavity in a two-dimensional photonic crystal

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

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. The unique properties of wide-bandwidth and dispersion-free propagation in photonic-crystal devices have made them a good candidate for slow-light generation. This article gives the background theory of slow light, as well as an overview of recent experimental demonstrations based on photonic-band engineering.

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Slow light in photonic crystals

Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot-cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.

Quantum nature of a strongly coupled single quantum dot–cavity system

We demonstrate that a double heterostructure in a two-dimensional photonic-crystal slab is crucial for the strong confinement of photons in volumes of optical-wavelength size. An extremely high quality factor over 2.7/spl times/10/sup 5/ has been achieved experimentally.

Ultrahigh-Q nanocavity based on photonic-crystal double heterostructure

We experimentally investigate nanoscale local interaction in a composite system consisting of dielectric photonic-crystal nanocavity and metallic meta-atoms. The Q factor of the composite system changes by a maximum of about 10 dB based on the relative position and the type of meta-atoms. The emission by meta-atoms dominates the nanocavity emission when they are in the electric or magnetic antinodes of the cavity field. Circularly polarized emission is achieved by tuning the polarization, the position, and the coupling phase of each meta-atom with respect to the nanocavity. Utilization of the local interaction with meta-atoms is shown to be useful as a new degree of freedom in tailoring the characteristics of nanocavities.

Investigation of electric/magnetic local interaction between Si photonic-crystal nanocavities and Au meta-atoms.

The effects of vertical structural asymmetries in two-dimensional photonic crystal slab waveguides are investigated. It is shown that coupling between TE-like and TM-like modes occurs, which results in large propagation losses.

Theoretical investigation of a vertically asymmetric photonic crystal slab

We demonstrate an improved design to obtain high-Q nanocavity in a two-dimensional photonic crystal. Extremely high-Q factor of 75,000 is experimentally achieved by fine-tuning air-hole positions at cavity edges, which allows gentler confinement of light

Fine-tuned high-Q photonic crystal nanocavity based on a gentle confinement of light

PROBLEM TO BE SOLVED: To provide a two-dimensional photonic crystal to be used for an optical path change switch to change propagation paths. SOLUTION: A first region 121 and a second region 122 having different periods and sizes of holes 131, 132 are formed on a main body 11, and a main waveguide 15 is formed so as to obliquely intersect the boundary 14 of the above regions. A branch waveguide 17 is formed, which branches from the main waveguide 15 into the first region 121, starting from the intersection of the main waveguide 15 and the boundary 14. By heating the second region 122 to change the refractive index of the main body in the region, the frequency band for passing the main waveguide 15 in the second region 122 is varied. Light at a specified frequency propagating from the first region 121 of the main waveguide 15 is extracted from the second region 122 of the waveguide 15 or is not allowed to propagate the waveguide 15 in the second region 122 but extracted from the branch waveguide 17, depending on heating or non-heating. COPYRIGHT: (C)2006,JPO&NCIPI

Bong-Shik Song

Papers

Ultrahigh-Q nanocavity based on photonic-crystal double heterostructure

Investigation of electric/magnetic local interaction between Si photonic-crystal nanocavities and Au meta-atoms.

Theoretical investigation of a vertically asymmetric photonic crystal slab

Fine-tuned high-Q photonic crystal nanocavity based on a gentle confinement of light

Two-dimensional photonic crystal and optical functional element using the same