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

Improvements in the channel drop efficiency of an in-plane drop filter in a two-dimensional photonic crystal slab are presented, using a device consisting of two photonic crystal slabs with different lattice constants. It is theoretically shown that drop efficiencies much higher than the maximum of 25% for a conventional configuration are achievable when utilizing reflections at the photonic crystal heterostructure interface. Additionally, the higher drop efficiency is found to be less sensitive to structural fluctuations. Drop operations with efficiencies of more than 80% are experimentally demonstrated by the fabricated devices.

Highly efficient in-plane channel drop filter in a two-dimensional heterophotonic crystal

The optical properties of line-defect waveguides in two-dimensional photonic crystal slabs are investigated using picosecond light pulses. Time-domain waveforms of the light pulse propagating through the waveguide are successfully observed using an autocorrelation method. The group velocity of the waveguide is directly determined from the group delay time for light pulses reflected back and forth along the waveguide. A small group velocity of one-twentieth the speed of light in vacuum is observed at a frequency near the edge of the waveguide mode. The frequency dependence of the group velocity is also measured, and the group-velocity dispersion is found to be larger than that of normal single-mode optical fibers by a factor of 104–105.

/pdf/time-domain-measurement-of-picosecond-light-pulse-26cjhe55ty.pdf

Time-domain measurement of picosecond light-pulse propagation in a two-dimensional photonic crystal-slab waveguide

To realize nanophotonic devices that operate in both the infrared and visible wavelength ranges on a single wafer, we investigated the optical characteristics of silicon carbide (SiC)-based photonic crystal nanocavities. By fabricating nanocavities with lattice constants ranging from 150 to 600 nm, we experimentally demonstrated resonant wavelengths of individual cavities ranging from 550 to 1450 nm on a single SiC wafer. Furthermore, this ultra-broadband operation reveals the material dispersion of the thin SiC wafer, which is estimated as nSiC = 2.34 + 9.18 × 104/λ2, over the wide range of aforementioned wavelengths.

Silicon carbide-based photonic crystal nanocavities for ultra-broadband operation from infrared to visible wavelengths

We theoretically and experimentally investigate the characteristics of a multichannel add/drop filter which utilizes in-plane hetero photonic crystals (IP-HPCs). The structure consists of an in-plane array of photonic crystals with different lattice-constants. Finite-difference time-domain calculations reveal that optimal performance in terms of wavelength resolution and efficiency can be kept almost constant at different wavelengths even though slab thickness is not changed. The multichannel add/drop device fabricated consists of seven photonic crystals, with a lattice parameter difference of 1.25 nm between neighboring regions. This device demonstrated wavelength spacing /spl sim/5nm with almost constant Q factor and efficiency. It is also shown that the boundary between different PCs (the heterointerface), plays an important role not only in improving add/drop efficiency but also in performing directional filtering.

Multichannel add/drop filter based on in-plane hetero photonic Crystals

We report transmission and reflection characteristics of in-plane hetero-photonic crystals (IP-HPCs) consisting of two serially connected photonic crystal waveguides with differing lattice constants. We show experimentally and theoretically that the transmission spectrum of the structure corresponds to the transmission frequency range common to both waveguides. Also, there exists a frequency gap where the structure has a guiding mode for one waveguide but not for the neighboring one. Our calculated results reveal that light within the common frequency range is transmitted with approximately 100% efficiency through the waveguides while light within the gap is almost perfectly reflected.

Bong-Shik Song

Papers

Highly efficient in-plane channel drop filter in a two-dimensional heterophotonic crystal

Time-domain measurement of picosecond light-pulse propagation in a two-dimensional photonic crystal-slab waveguide

Silicon carbide-based photonic crystal nanocavities for ultra-broadband operation from infrared to visible wavelengths

Multichannel add/drop filter based on in-plane hetero photonic Crystals

Transmission and reflection characteristics of in-plane hetero-photonic crystals