Masanobu Iwanaga

Photonic metamaterials: a new class of materials for manipulating light waves

Abstract A decade of research on metamaterials (MMs) has yielded great progress in artificial electromagnetic materials in a wide frequency range from microwave to optical frequencies. This review outlines the achievements in photonic MMs that can efficiently manipulate light waves from near-ultraviolet to near-infrared in subwavelength dimensions. One of the key concepts of MMs is effective refractive index, realizing values that have not been obtained in ordinary solid materials. In addition to the high and low refractive indices, negative refractive indices have been reported in some photonic MMs. In anisotropic photonic MMs of high-contrast refractive indices, the polarization and phase of plane light waves were efficiently transformed in a well-designed manner, enabling remarkable miniaturization of linear optical devices such as polarizers, wave plates and circular dichroic devices. Another feature of photonic MMs is the possibility of unusual light propagation, paving the way for a new subfield of transfer optics. MM lenses having super-resolution and cloaking effects were introduced by exploiting novel light-propagating modes. Here, we present a new approach to describing photonic MMs definitely by resolving the electromagnetic eigenmodes. Two representative photonic MMs are addressed: the so-called fishnet MM slabs, which are known to have effective negative refractive index, and a three-dimensional MM based on a multilayer of a metal and an insulator. In these photonic MMs, we elucidate the underlying eigenmodes that induce unusual light propagations. Based on the progress of photonic MMs, the future potential and direction are discussed.

Dual-band infrared metasurface thermal emitter for CO2 sensing

Polarization- and angle-independent, dual-band metasurface thermal emitter was developed. The metasurface emits radiation at 4.26 μm and 3.95 μm, conventionally used for CO2 sensing. The metasurface is based on a planar Au/Al2O3/Au structure, in which orthogonal rectangular Au patches are arrayed alternately, and generates nearly perfect blackbody radiation with an emittance as high as 0.97. The metasurface is integrated on a resistive heater mounted on a SiN membrane, so that the infrared waves are produced by applying a voltage. The metasurface emitter was incorporated into an actual CO2 sensing system and was demonstrated to reduce the electric power needed by about 30% compared with a conventional blackbody emitter by suppressing unnecessary radiation.

Photoluminescence-enhanced plasmonic substrates fabricated by nanoimprint lithography

Abstract. We fabricated large-area stacked complementary plasmonic crystals (SC PlCs) by employing ultraviolet nanoimprint lithography. The SC PlCs were made on silicon-on-insulator substrates consisting of three layers: the top layer contacting air was a perforated Au film, the bottom layer contacting the buried oxide layer included an Au disk array corresponding to the holes in the top layer, and the middle layer was a Si photonic crystal slab. The SC PlCs have prominent resonances in optical wavelengths. It is shown that the fabricated PlCs were precise in structure and uniform in their optical properties. We examined the photoluminescence (PL) enhancement of monolayer dye molecules on the SC PlC substrates in the visible range and found large PL enhancements of up to a 100-fold in comparison with dye molecules on nonprocessed Si wafers.

Optical rectification effect in 1D metallic photonic crystal slabs with asymmetric unit cell

Photo-induced voltage due to photon drag effect is investigated for metallic photonic crystal slabs (PCS) with symmetric and asymmetric unit cells. In the symmetric structure, the signal is antisymmetric as a function of the incident angle, while in the asymmetric structure it is asymmetric. When the laser beam is normally incident to the sample, the photovoltage is observed only for PCS with asymmetric unit cells and its laser wavelength dependence is readily described in terms of uneven diffraction. The phenomenon can be referred to as optical rectification due to photonic scale asymmetry.

Effective optical constants in stratified metal-dielectric metameterial

Effective optical constants of stratified metal-dielectric metameterial are presented. The effective constants are determined by the two-complex reflectivity method (TCRM). The TCRM reveals the full components of the effective permittivity and permeability tensors and indicates the remarkable anisotropy of metallic and dielectric components below the effective plasma frequency. On the other hand, above the plasma frequency, one of the effective refractive indices takes a positive value less than unity and is associated with small loss. The photonic states are confirmed by the distribution of electromagnetic fields.

Subwavelength electromagnetic dynamics in stacked complementary plasmonic crystal slabs.

Resonant electromagnetic fields in stacked complementary plasmonic crystal slabs (sc-PlCSs) are numerically explored in subwavelength dimensions. It is found that the local plasmon resonances in the sc-PlCSs are composite states of locally enhanced electric and magnetic fields. Two sc-PlCSs are analyzed in this paper and it is shown that each sc-PlCS realizes a resonant electromagnetic state suggested by one of Maxwell equations. It is moreover clarified that the local plasmons open efficient paths of Poynting flux, those result in high-contrast polarized transmission.

Self-trapped states and related luminescence in PbCl2 crystals

We have comprehensively investigated localized states of photoinduced electron-hole pairs with the electron-spin-resonance technique and photoluminescence (PL) in a wide temperature range of 5-200 K. At low temperatures below 70 K, holes localize on Pb 2 + ions and form self-trapping hole centers of Pb 3 + . The holes transfer to other trapping centers above 70 K. On the other hand, electrons localize on two Pb 2 + ions at higher than 50 K and form self-trapping electron centers of Pb 3 + 2 . From the thermal stability of the localized states and PL, we clarify that the blue-green PL band at 2.50 eV is closely related to the self-trapped holes.

Heteroplasmon Hybridization in Stacked Complementary Plasmo-Photonic Crystals

We constructed plasmo-photonic crystals in which efficient light-trapping, plasmonic resonances couple with photonic guided resonances of large density of states and high-quality factor. We have numerically and experimentally shown that heteroplasmon hybrid modes emerge in stacked complementary (SC) plasmo-photonic crystals. The resonant electromagnetic-field distributions evidence that the two hybrid modes originate from two different heteroplasmons, exhibiting a large energy splitting of 300 meV. We further revealed a series of plasmo-photonic modes in the SC crystals.

Ultracompact waveplates : Approach from metamaterials

Ultracompact waveplates working in subwavelength scale are substantiated in realistic simulation. The optical elements have been designed employing transparent photonic metamaterials of remarkably strong anisotropy. The waveplates are far more efficient at visible wavelengths than usual bulk waveplates and are shown to be compact down to subwavelength dimensions. Similar designs for the telecommunication wavelengths are also presented.

Self-trapped electrons and holes in PbBr2 crystals

We have directly observed self-trapped electrons and holes in PbBr 2 crystals with the electron-spin-resonance (ESR) technique. The self-trapped states are induced below 8 K by two-photon interband excitation with pulsed 120-fs-width laser light at 3.10 eV. Spin-Hamiltonian analyses of the ESR signals have revealed that the self-trapping electron centers are the dimer molecules of Ph 3 + 2 along the crystallographic a axis and the self-trapping hole centers are those of Br 2 - with two possible configurations in the unit cell of the crystal. Thermal stability of the self-trapped electrons and holes suggests that both of them are related to the blue-green luminescence band at 2.55 eV coming from recombination of spatially separated electron-hole pairs.