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

One-dimensional nanostructures, such as nanowires and nanotubes, represent the smallest dimension for efficient transport of electrons and excitons and thus are ideal building blocks for hierarchical assembly of functional nanoscale electronic and photonic structures. We report an approach for the hierarchical assembly of one-dimensional nanostructures into well-defined functional networks. We show that nanowires can be assembled into parallel arrays with control of the average separation and, by combining fluidic alignment with surface-patterning techniques, that it is also possible to control periodicity. In addition, complex crossed nanowire arrays can be prepared with layer-by-layer assembly with different flow directions for sequential steps. Transport studies show that the crossed nanowire arrays form electrically conducting networks, with individually addressable device function at each cross point.

https://science.sciencemag.org/content/sci/291/5504/630.full.pdf

Directed Assembly of One-Dimensional Nanostructures into Functional Networks

The electronic ground state of a periodic system is usually described in terms of extended Bloch orbitals, but an alternative representation in terms of localized "Wannier functions" was introduced by Gregory Wannier in 1937. The connection between the Bloch and Wannier representations is realized by families of transformations in a continuous space of unitary matrices, carrying a large degree of arbitrariness. Since 1997, methods have been developed that allow one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set of maximally localized Wannier functions, accomplishing the solid-state equivalent of constructing localized molecular orbitals, or "Boys orbitals" as previously known from the chemistry literature. These developments are reviewed here, and a survey of the applications of these methods is presented. This latter includes a description of their use in analyzing the nature of chemical bonding, or as a local probe of phenomena related to electric polarization and orbital magnetization. Wannier interpolation schemes are also reviewed, by which quantities computed on a coarse reciprocal-space mesh can be used to interpolate onto much finer meshes at low cost, and applications in which Wannier functions are used as efficient basis functions are discussed. Finally the construction and use of Wannier functions outside the context of electronic-structure theory is presented, for cases that include phonon excitations, photonic crystals, and cold-atom optical lattices.

Maximally-localized Wannier Functions: Theory and Applications

Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n2 ∼ 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between impr...

/pdf/light-trapping-in-silicon-nanowire-solar-cells-eytbh3l72i.pdf

Light Trapping in Silicon Nanowire Solar Cells

By introducing artificial defects and/or light-emitters into photonic bandgap structures1,2, it should be possible to manipulate photons. For example, it has been predicted2 that strong localization (or trapping) of photons should occur in structures with single defects, and that the propagation3,4 of photons should be controllable using arrays of defects. But there has been little experimental progress in this regard, with the exception of a laser5 based on a single-defect photonic crystal. Here we demonstrate photon trapping by a single defect that has been created artificially inside a two-dimensional photonic bandgap structure. Photons propagating through a linear waveguide are trapped by the defect, which then emits them to free space. We envisage that this phenomenon may be used in ultra-small optical devices whose function is to selectively drop (or add) photons with various energies from (or to) optical communication traffic. More generally, our work should facilitate the development of all-optical circuits incorporating photonic bandgap waveguides and resonators.

Trapping and emission of photons by a single defect in a photonic bandgap structure

An artificial crystal structure has been fabricated exhibiting a full three-dimensional photonic bandgap effect at optical communication wavelengths. The photonic crystal was constructed by stacking 0.7-micrometer period semiconductor stripes with the accuracy of 30 nanometers by advanced wafer-fusion technique. A bandgap effect of more than 40 decibels (which corresponds to 99.99% reflection) was successfully achieved. The result encourages us to create an ultra-small optical integrated circuit including a three-dimensional photonic crystal waveguide with a sharp bend.

https://science.sciencemag.org/content/sci/289/5479/604.full.pdf

Full three-dimensional photonic bandgap crystals at near-infrared wavelengths

Lasing action of a surface-emitting laser with a two-dimensional photonic crystal structure is investigated. The photonic crystal has a triangular-lattice structure composed of InP and air holes, which is integrated with an InGaAsP/InP multiple-quantum-well active layer by a wafer fusion technique. Uniform two-dimensional lasing oscillation based on the coupling of light propagating in six equivalent Γ−X directions is successfully observed, where the wavelength of the active layer is designed to match the folded (second-order) Γ point of the Γ−X direction. The very narrow divergence angle of far field pattern and/or the lasing spectrum, which is considered to reflect the two-dimensional stop band, also indicate that the lasing oscillation occurs coherently.

Coherent two-dimensional lasing action in surface-emitting laser with triangular-lattice photonic crystal structure

We demonstrate polarization mode selection in a two-dimensional (2D) photonic crystal laser by controlling the geometry of the unit cell structure. As the band diagram of the square-lattice photonic crystal is influenced by the unit cell structure, calculations reveal that changing the structure from a circular to an elliptical geometry should result in a strong modification of the electromagnetic field distributions at the band edges. Such a structural modification is expected to provide a mechanism for controlling the polarization modes of the emitted light. A square-lattice photonic crystal with the elliptical unit cell structure has been fabricated and integrated with a gain media. The observed coherent 2D lasing action with a single wavelength and controlled polarization is in good agreement with the predicted behavior.

https://science.sciencemag.org/content/sci/293/5532/1123.full.pdf

Polarization Mode Control of Two-Dimensional Photonic Crystal Laser by Unit Cell Structure Design

We present theoretical studies on waveguides and waveguide bends in two-dimensional photonic crystal slabs. The waveguides are created by either filling up or decreasing the sizes of air holes. Our designs focus on using the frequency range where the waveguide mode is nonleaky. It is shown that high transmission through the sharp bend can be obtained for some frequency ranges in the triangular lattice slabs. The waveguides in square lattice slabs are also investigated.

Alongkarn Chutinan

Papers

Trapping and emission of photons by a single defect in a photonic bandgap structure

Full three-dimensional photonic bandgap crystals at near-infrared wavelengths

Coherent two-dimensional lasing action in surface-emitting laser with triangular-lattice photonic crystal structure

Polarization Mode Control of Two-Dimensional Photonic Crystal Laser by Unit Cell Structure Design

Waveguides and waveguide bends in two-dimensional photonic crystal slabs