Degenerate spontaneous parametric down-conversion in nonlinear metasurfaces.

We propose a simple scheme of degenerate spontaneous parametric down-conversion (SPDC) in nonlinear metasurfaces or photonic crystal slabs with quasi-guided modes. It employs a band crossing between even- and odd-parity quasi-guided mode bands inside the light cone (above the light line) and a selection rule in the conversion efficiency of the SPDC. The efficiency can be evaluated fully classically via the inverse process of noncollinear second-harmonic generation (SHG).

Patchwork metasurface quantum well photodetectors with broadened photoresponse.

Complex lightwave manipulation such as broadband absorption has been realized with metasurfaces based on laterally arranged metal-dielectric-metal cavities with different geometries. However, application of these metasurfaces for optoelectronic devices by incorporating functional dielectrics remains challenging. Here, we integrate a quantum well infrared photodetector (QWIP) with a metasurface made of a patchwork of square cavities with different dimensions arranged in a subwavelength unit cell.

Synchronously wired infrared antennas for resonant single-quantum-well photodetection up to room temperature.

Optical patch antennas sandwiching dielectrics between metal layers have been used as deep subwavelength building blocks of metasurfaces for perfect absorbers and thermal emitters. However, for applications of these metasurfaces for optoelectronic devices, wiring to each electrically isolated antenna is indispensable for biasing and current flow. Here we show that geometrically engineered metallic wires interconnecting the antennas can function to synchronize the optical phases for promoting coherent resonance, not only as electrical conductors.

Synthetic gauge field and pseudospin-orbit interaction in a stacked two-dimensional ring-network lattice.

We study the effects of a synthetic gauge field and pseudospin-orbit interaction in a stacked two-dimensional ring-network model. The model was introduced to simulate light propagation in the corresponding ring-resonator lattice, and is thus completely bosonic. Without these two items, the model exhibits Floquet-Weyl and Floquet-topological-insulator phases with topologically gapless and gapped band structures, respectively.

Floquet-Weyl and Floquet-topological-insulator phases in a stacked two-dimensional ring-network lattice.

We show the presence of Floquet-Weyl and Floquet-topological-insulator phases in a stacked two-dimensional ring-network lattice. The Weyl points in the three-dimensional Brillouin zone and Fermi-arc surface states are clearly demonstrated in the quasienergy spectrum of the system in the Floquet-Weyl phase. In addition, chiral surface states coexist in this phase.

Non-reciprocity and topology in optics: one-way road for light via surface magnon polariton.

We show how non-reciprocity and topology are used to construct an optical one-way waveguide in the Voigt geometry. First, we present a traditional approach of the one-way waveguide of light using surface polaritons under a static magnetic field. Second, we explain a recent discovery of a topological approach using photonic crystals with the magneto-optical coupling.

Topological properties of bulk and edge states in honeycomb lattice photonic crystals: the case of TE polarization.

Topological properties of bulk and edge states in honeycomb lattice photonic crystals are investigated theoretically for transverse-electric (TE) polarization. Breaking of space-inversion and time-reversal symmetries is considered at optical frequencies. The bulk band structure exhibits a topological phase transition by changing the degree of the broken symmetries.

Imitating the Cherenkov radiation in backward directions using one-dimensional photonic wires.

A novel radiation emission from traveling charged particles in vacuum is theoretically demonstrated. This radiation is conical as in the Cherenkov radiation, but emerges in backward directions of the particle trajectories. The basic mechanism of the radiation is the Smith-Purcell effect via the interaction between the charged particles and a circular-symmetric photonic wire with a one-dimensionally periodic dielectric function.

Designing spinning Bloch states in 2D photonic crystals for stirring nanoparticles.

Based on an optical analogy of spintronics, the generation of spinning Bloch states is theoretically investigated in two-dimensional photonic crystals without space-inversion symmetry. We address its close relation to the Berry curvature in crystal momentum space, which represents the nontrivial geometric property of a Bloch state. It is shown that the Berry curvature is easily controlled by tuning two types of dielectric rods in a honeycomb photonic crystal.

Evaluation of effective electric permittivity and magnetic permeability in metamaterial slabs by terahertz time-domain spectroscopy.

We established a novel method to evaluate effective optical constants by terahertz (THz) time domain spectroscopy and suggested a strict definition of optical constants and an expression for electromagnetic energy loss following the second law of thermodynamics. We deduced the effective optical constants of phosphor bronze wire grids in the THz region experimentally and theoretically. The results depend strongly on the polarization of the THz waves.

Theory of unconventional Smith-Purcell radiation in finite-size photonic crystals.

Unusual emission of light, called the unconventional Smith- Purcell radiation (uSPR) in this paper, was demonstrated from an electron traveling near a finite photonic crystal (PhC) at an ultra-relativistic velocity. This phenomenon is not related to the accepted mechanism of the conventional SPR and arises because the evanescent light from the electron has such a small decay constant in the ultra-relativistic regime that it works practically as a plane-wave probe entering the PhC from one end. We analyze the dependence of the SPR spectrum on the velocity of electron and on the parity of excited photonic bands and show, for PhCs made up of a finite number of cylinders, that uSPR probes the photonic band structure very faithfully.

Scaling law of enhanced second harmonic generation in finite Bragg stacks.

The second-harmonic generation (SHG) in finite Bragg stacks of alternating linear and nonlinear optical films is studied with the exact Green function under the assumption of perturbation theory. Various mechanisms of enhanced SHG are investigated analytically, and the scaling law with respect to the number N of stacking layers is derived for each mechanism. In particular, it is shown that there is a simple mechanism of the enhanced SHG having N;4 scaling, in which both the enhancement of the Green function and the phase matching condition peculiar to finite Bragg stacks are fulfilled simultaneously.

Electron energy loss and Smith-Purcell radiation in two- and three-dimensional photonic crystals.

A theoretical description of the electron energy loss and the Smith-Purcell radiation is presented for an electron moving near a two-dimensional photonic crystal slab and a three-dimensional woodpile photonic crystal. The electron energy loss and the Smith-Purcell radiation spectra are well correlated with the photonic band structures of these crystals and thus can be used as a probe of them. In particular, there is a selection rule concerning the symmetries of the photonic band modes to be excited when the electron moves in a mirror plane of the crystals.

Self-assembly of concentric quantum double rings.

We demonstrate the self-assembled formation of concentric quantum double rings with high uniformity and excellent rotational symmetry using the droplet epitaxy technique. Varying the growth process conditions can control each ring's size. Photoluminescence spectra emitted from an individual quantum ring complex show peculiar quantized levels that are specified by the carriers' orbital trajectories.