Recently, strongly modulated photonic crystals, fabricated by the state-of-the-art semiconductor nanofabrication process, have realized various novel optical properties. This paper describes the way in which they differ from other optical media, and clarifies what they can do. In particular, three important issues are considered: light confinement, frequency dispersion and spatial dispersion. First, I describe the latest status and impact of ultra-strong light confinement in a wavelength-cubic volume achieved in photonic crystals. Second, the extreme reduction in the speed of light is reported, which was achieved as a result of frequency dispersion management. Third, strange negative refraction in photonic crystals is introduced, which results from their unique spatial dispersion, and it is clarified how this leads to perfect imaging. The last two sections are devoted to applications of these novel properties. First, I report the fact that strong light confinement and huge light–matter interaction enhancement make strongly modulated photonic crystals promising for on-chip all-optical processing, and present several examples including all-optical switches/memories and optical logics. As a second application, it is shown that the strong light confinement and slow light in strongly modulated photonic crystals enable the adiabatic tuning of light, which leads to various novel ways of controlling light, such as adiabatic frequency conversion, efficient optomechanics systems, photon memories and photons pinning.

https://iopscience.iop.org/article/10.1088/0034-4885/73/9/096501/pdf

Manipulating light with strongly modulated photonic crystals

Here, we demonstrate a compact photonic crystal wavelength demultiplexing device based on a diffraction compensation scheme with two orders of magnitude performance improvement over the conventional superprism structures reported to date. We show that the main problems of the conventional superprism-based wavelength demultiplexing devices can be overcome by combining the superprism effect with two other main properties of photonic crystals, i.e., negative diffraction and negative refraction. Here, a 4-channel optical demultiplexer with a channel spacing of 8 nm and cross-talk level of better than -6.5 dB is experimentally demonstrated using a 4500 microm(2) photonic crystal region.

Compact wavelength demultiplexing using focusing negative index photonic crystal superprisms.

An ultra compact photonic crystal wavelength division demultiplexer using resonance cavities in a modified Y-branch structure

We present a concept of graded photonic crystals used to enhance the control of light propagation. Gradual modifications of the lattice periodicity make it possible to bend the light at the micrometer scale. This effect is tailored by parametric studies of the isofrequency curves. As a demonstration, we propose a two-dimensional graded photonic crystal that could provide frequency-selective tunable bending.

Graded photonic crystals

The propagation of surface electromagnetic waves along photonic crystal (PC) boundaries is examined. It is shown that in a number of cases, these are backward waves. The nature of surface electromagnetic states localized at the PC boundary is discussed; these states transfer no energy along the boundary (their tangential wave number is zero). An analogy with the well-known Tamm and Shockley surface states in solid state physics is drawn. It is shown that in the case of a PC, both types of states can be regarded as Tamm states. Experimental results on the observation of surface states are presented. A system using an external magnetic field to control a surface state is considered.

https://iopscience.iop.org/article/10.3367/UFNe.0180.201003b.0249/pdf

Surface states in photonic crystals

We theoretically investigated the performance of the photonic crystal superprism, that is, the propagating beam quality, the wavelength sensitivity, and the resolution as a narrow band filter at 1.5-μm-wavelength range. First, we defined the equi-incident-angle curve in the Brillouin zone. Next, we mapped three parameters that represented the abovementioned performance over the Brillouin zone. As a result, we found a narrow design window that allows a high resolution of 0.4 nm along an equi-incident-angle curve but requires an incident beam width of over 100 μm and a device length of centimeter order. It can be an essential high efficiency filter if the input end of the crystal is optimized and the propagation loss is suppressed.

Resolution of photonic crystal superprism

We propose a novel compact wavelength demultiplexer, for which two functions arising from the anomalous dispersion characteristics of photonic crystals are combined. One is the superprism that exhibits large angular dispersion and expansion of light beam. The other is the superlens used for the focusing of the expanded light beam. Theoretically, a high resolution of 0.4 nm will be realized in the 1.55 μm wavelength range with device areas of 0.2 and 2.0 mm2, respectively, for available bandwidths of 3 and 35 nm. Also, a low insertion loss of less than 1 dB is expected by the optimization of input and output ends of the photonic crystals. The demultiplexing function is clearly demonstrated in finite-difference time-domain simulation.

Wavelength demultiplexer consisting of Photonic crystal superprism and superlens

We experimentally demonstrate the light focusing by negative refraction in a photonic crystal slab superlens at wavelengths lambda of 1.26-1.42 microm. The photonic crystal slab was fabricated on silicon-on-insulator substrate with an interface structure optimized for low reflection and diffraction losses. The light focusing in the photonic crystal slab was clearly observed through the intentional out-of-plane radiation or scattering of guided light in the slab. The minimum focused spot width was limited to 1.8 microm(1.4 lambda) owing to aberrations. The focusing characteristics were in good agreement with those obtained from photonic band and finite-difference time-domain analyses.

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Focusing of light by negative refraction in a photonic crystal slab superlens on silicon-on-insulator substrate.

Negative refraction of light was observed at near-infrared wavelengths in a silicon-on-insulator photonic-crystal-slab superprism having low-loss interface structures. It was used for wavelength demultiplexing as a diffraction grating, in combination with a photonic-crystal superlens used for focusing light beams. Coarse wavelength demultiplexing action with a channel spacing of 7–27nm was demonstrated in a device whose size was only 80×100μm2 (excluding input and output waveguides). These results agree well with those obtained by a finite-difference time-domain simulation.

Experimental demonstration of a wavelength demultiplexer based on negative-refractive photonic-crystal components

Input and output boundaries of a photonic crystal (PC) are optimized so that the superprism exhibits low insertion loss. It is shown that projected-airhole and half-circular airhole interfaces achieve transmission loss of 0.3 dB and 1.0 dB, respectively, for small and large incident angles of light against normal to boundaries. The finite-difference time-domain simulation shows that a low loss is essentially realized by a periodic phase modulation of the incident beam by the interfaces. It also demonstrates the clear steering of collimated light beam with varying wavelength. The enhancement of angular dispersion is also demonstrated by a PC composed of a dispersive medium.

Takashi Matsumoto

Papers

Resolution of photonic crystal superprism

Wavelength demultiplexer consisting of Photonic crystal superprism and superlens

Focusing of light by negative refraction in a photonic crystal slab superlens on silicon-on-insulator substrate.

Experimental demonstration of a wavelength demultiplexer based on negative-refractive photonic-crystal components

Finite difference time domain study of high efficiency photonic crystal superprisms