Shape dependence of all-dielectric terahertz metasurface.

All-dielectric metasurfaces have been attracting attention in the terahertz spectral range for low-loss planar optical elements such as lenses, beam splitters, waveplates, vortex plates, and magnetic mirrors. Various shapes of meta-atoms have been used in many studies; however, no systematic comparative study of each shape has been reported. The optical properties of various shapes of metasurfaces are reported in this work using finite difference time domain simulation. The phase of a pillar-type all-dielectric metasurface is mainly determined by the cross-sectional area, rather than its detailed shape. Consequently, in the square lattice geometry, the square shape meta-atom performs best in terms of full phase control at the lowest pillar height with negligible polarization dependence. Furthermore, we compare the transmission, phase, and polarization dependence of the hexagonal and square lattices. Square-shape metasurface successfully realizes subwavelength focusing metalens and vortex plate.

All-solid-state spatial light modulator with independent phase and amplitude control for three-dimensional LiDAR applications.

Spatial light modulators are essential optical elements in applications that require the ability to regulate the amplitude, phase and polarization of light, such as digital holography, optical communications and biomedical imaging. With the push towards miniaturization of optical components, static metasurfaces are used as competent alternatives. These evolved to active metasurfaces in which light-wavefront manipulation can be done in a time-dependent fashion. The active metasurfaces reported so far, however, still show incomplete phase modulation (below 360°). Here we present an all-solid-state, electrically tunable and reflective metasurface array that can generate a specific phase or a continuous sweep between 0 and 360° at an estimated rate of 5.4 MHz while independently adjusting the amplitude. The metasurface features 550 individually addressable nanoresonators in a 250 × 250 µm2 area with no micromechanical elements or liquid crystals. A key feature of our design is the presence of two independent control parameters (top and bottom gate voltages) in each nanoresonator, which are used to adjust the real and imaginary parts of the reflection coefficient independently. To demonstrate this array's use in light detection and ranging, we performed a three-dimensional depth scan of an emulated street scene that consisted of a model car and a human figure up to a distance of 4.7 m.

Anomalous blueshift of aperture resonance enabled by the loss of a thin film.

The substrate effects on aperture resonance have been widely studied because the resonance peak position is key for sensing, communications, and field enhancement applications. So far, the theoretical works have focused on the lossless infinite substrate case, which only explains the resonance peak redshift by the substrate index. The loss effect has not been investigated yet because lossy infinite substrate significantly reduces the aperture transmission. Therefore, this work studied the loss effect on aperture resonance via an analytic model of the transmission though slot antennas on a finite substrate. When the substrate thickness was very thin compared to the wavelength, the transmittance remained high even for a large imaginary part of the refractive index; furthermore, the developed model predicted a strong blueshift when this imaginary part exceeds the real part. Besides, the zero of the imaginary part of the coupling strength was crucial in determining the resonance frequency for both infinite and finite substrates. Thus, this study can contribute to the material characterization, label-free detection, and efficient design of active metamaterials.

Metasurface-driven OLED displays beyond 10,000 pixels per inch.

Optical metasurfaces are starting to find their way into integrated devices, where they can enhance and control the emission, modulation, dynamic shaping, and detection of light waves. In this study, we show that the architecture of organic light-emitting diode (OLED) displays can be completely reenvisioned through the introduction of nanopatterned metasurface mirrors. In the resulting meta-OLED displays, different metasurface patterns define red, green, and blue pixels and ensure optimized extraction of these colors from organic, white light emitters. This new architecture facilitates the creation of devices at the ultrahigh pixel densities (>10,000 pixels per inch) required in emerging display applications (for instance, augmented reality) that use scalable nanoimprint lithography. The fabricated pixels also offer twice the luminescence efficiency and superior color purity relative to standard color-filtered white OLEDs.

Electromagnon with Sensitive Terahertz Magnetochromism in a Room-Temperature Magnetoelectric Hexaferrite.

An electromagnon in the magnetoelectric (ME) hexaferrite Ba_{0.5}Sr_{2.5}Co_{2}Fe_{24}O_{41} (Co_{2}Z-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field (E^{ω}) of the THz light parallel to the c axis (∥ [001]), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes -8.7% and +5.8% under external magnetic field (H) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different H.

Ultrasensitive molecular absorption detection using metal slot antenna arrays.

We theoretically study the transmission reduction of light passing through absorptive molecules embedded in a periodic metal slot array in a near infrared wavelength regime. From the analytically solved transmitted light, we present a simple relation given by the attenuation length of light at the resonance wavelength of the slot antennas with respect to the spectral width of the resonant transmission peak. This relation clearly explains that the control of the transmission reduction even with very low absorptive materials is possible. We investigate also the transmission reduction by absorptive molecules in a real metallic slot antenna array on a dielectric substrate and compare the results with finite difference time domain calculations. In numerical calculations, we demonstrate that the same amount of transmission reduction by a bulk absorptive material can be achieved only with one-hundredth thickness of the same material when it is embedded in an optimized Fano-resonant slot antenna array. Our relation presented in this study can contribute to label-free chemical and biological sensing as an efficient design and performance criterion for periodic slot antenna arrays.

A Vanadium Dioxide Metamaterial Disengaged from Insulator-to-Metal Transition.

We report that vanadium dioxide films patterned with λ/100000 nanogaps exhibit an anomalous transition behavior at millimeter wavelengths. Most of the hybrid structure's switching actions occur well below the insulator to metal transition temperature, starting from 25 °C, so that the hysteresis curves completely separate themselves from their bare film counterparts. It is found that thermally excited intrinsic carriers are responsible for this behavior by introducing enough loss in the context of the radically modified electromagnetic environment in the vicinity of the nanogaps. This phenomenon newly extends the versatility of insulator to metal transition devices to encompass their semiconductor properties.

Epsilon-near-zero meta-lens for high resolution wide-field imaging.

Herein, we will propose a new application possibility of epsilon-near-zero (ENZ) materials: high resolution wide-field imaging. We show that the resolution can be dramatically enhanced by simply inserting a thin epsilon-near-zero (ENZ) material between the sample and substrate. By performing metal half-plane imaging, we experimentally demonstrate that the resolution could be enhanced by about 47% with a 300-nm-thick SiO2 interlayer, an ENZ material at 8-µm-wavelength (1250 cm-1). The physical origin of the resolution enhancement is the strong conversion of diffracted near fields to quasi-zeroth order far fields enabled by the directive emission of ENZ materials.

Selective enhanced resonances of two asymmetric terahertz nano resonators.

We studied the electromagnetic interaction between two asymmetric terahertz nano resonators, rectangular holes which have a few hundred micron lengths but nanoscale widths. We report that the dominant resonant transmission of the structures can be modulated by the horizontal distance between two rectangles due to the different oscillation strength of the asymmetric coupling at two different resonance frequencies. Our results are significant for an optimum design of rectangular holes in terahertz frequency regime for applications such as sensitive nanoparticle detection and terahertz filters.

Electrical control of terahertz nano antennas on VO2 thin film.

We demonstrate an active metamaterial device that allows to electrically control terahertz transmission over more than one order of magnitude. Our device consists of a lithographically defined gold nano antenna array fabricated on a thin film of vanadium dioxide (VO(2)), a material that possesses an insulator to metal transition. The nano antennas let terahertz (THz) radiation funnel through when the VO(2) film is in the insulating state. By applying a dc-bias voltage through our device, the VO(2) becomes metallic. This electrically shorts the antennas and therefore switches off the transmission in two distinct regimes: reversible and irreversible switching.

A reel-wound carbon nanotube polarizer for terahertz frequencies.

Utilizing highly oriented multiwalled carbon nanotube aerogel sheets, we fabricated micrometer-thick freestanding carbon nanotube (CNT) polarizers. Simple winding of nanotube sheets on a U-shaped polyethylene reel enabled rapid and reliable polarizer fabrication, bypassing lithography or chemical etching processes. With the remarkable extinction ratio reaching ∼37 dB in the broad spectral range from 0.1 to 2.0 THz, combined with the extraordinary gravimetric mechanical strength of CNTs, and the dispersionless character of freestanding sheets, the commercialization prospects for our CNT terahertz polarizers appear attractive.

Giant nonlinear response of terahertz nanoresonators on VO2 thin film.

We report on an order of magnitude enhanced nonlinear response of vanadium dioxide thin film patterned with nanoresonators--nano slot antennas fabricated on the gold film. Transmission of terahertz radiation, little affected by an optical pumping for the case of bulk thin film, can now be completely switched-off: DeltaT/T approximately -0.9999 by the same optical pumping power. This unprecedentedly large optical pump-terahertz probe nonlinearity originates from the insulator-to-metal phase transition drastically reducing the antenna cross sections of the nanoresonators. Our scheme enables nanoscale-thin film technology to be used for all-optical switching of long wavelength light.

Active terahertz nanoantennas based on VO2 phase transition.

Unusual performances of metamaterials such as negative index of refraction, memory effect, and cloaking originate from the resonance features of the metallic composite atom(1-6). Indeed, control of metamaterial properties by changing dielectric environments of thin films below the metallic resonators has been demonstrated(7-11). However, the dynamic control ranges are still limited to less than a factor of 10,(7-11) with the applicable bandwidth defined by the sharp resonance features. Here, we present ultra-broad-band metamaterial thin film with colossal dynamic control range, fulfilling present day research demands. Hybridized with thin VO(2) (vanadium dioxide) (12-18) films, nanoresonator supercell arrays designed for one decade of spectral width in terahertz frequency region show an unprecedented extinction ratio of over 10000 when the underlying thin film experiences a phase transition. Our nanoresonator approach realizes the full potential of the thin film technology for long wavelength applications.