Direct generation of a terahertz vector beam from a ZnTe crystal excited by a focused circular polarized pulse.

A vector beam is a type of topological beam in which the polarization direction of light rotates around a singularity on the wavefront. This paper proposes a method to generate a vector beam by tightly focusing a pump beam in the crystalline direction such that the second-order nonlinear optical effect is forbidden. The directional dependence of the effective nonlinearity in zincblende crystals, such as ZnTe, was analytically investigated. Two types of nonlinear polarization singularities were found in [111] and [100] directions. Their polarization topological charge ℓ was +1 and -1, respectively. To experimentally demonstrate the proposed method, a (111) cut ZnTe crystal was selected as the nonlinear crystal. The polarization state of the generated terahertz (THz) beams was measured with a custom-built THz spectroscopic polarization imaging system. Radially polarized distributions were observed within the entire generated spectral region. Such a broadband feature of the generated vector beam is likely due to the topological nature of the focused pump beam, where the wavevectors are winding once about the optical axis. This simple method for generating THz vector beams will accelerate its applications.

Terahertz Fresnel-zone-plate thin-film lens based on a high-transmittance double-layer metamaterial phase shifter.

Planar diffractive lenses, with metamaterial artificial structures and subwavelength thickness, provide unique and flexible platforms for optical design in the terahertz (THz) regime. Here, we present a metamaterial-based Rayleigh-Wood Fresnel-zone-plate (FZP) thin-film lens designed to focus a monochromatic THz beam at 1.0 THz with a high transmittance of 80%, short focal length of 24 mm, and subwavelength thickness of 48 µm. Specifically, the FZP lens is composed of 8 alternating concentric zones through a polymer film substrate, where odd zones are patterned with double-layer un-split ring resonators (USRRs) that provide a polarization-independent phase shift of π/2 compared to un-patterned even zones. Both simulation and experiment confirm that our FZP lens creates a focused beam at the designed frequency of 1.0 THz by constructive interference through alternating concentric metamaterial-patterned and un-patterned zones, producing a diffraction-limited resolution of 0.6 mm for imaging applications. In contrast to conventional approaches in which the uniform periodic array of metamaterial unit cells has been treated as an effective material, we newly find that double-layer USRRs can work as an independent meta-atom without degradation of its performances, which benefits the behavior of small arrays of double-layer USRRs located in the outer zones of the FZP lens. Such a planar thin-film lens would enable us to realize compact and lightweight THz systems.

Broadband high-resolution terahertz single-pixel imaging.

We report a simple single-pixel imaging system with a low mean squared error in the entire terahertz frequency region (3-13 THz) that employs a thin metallic ring with a series of directly perforated random masks and a subpixel mask digitization technique. This imaging system produces high pixel resolution reconstructed images, up to 1200 × 1200 pixels, and imaging area of 32 × 32 mm2. It can be extended to develop advanced imaging systems in the near-ultraviolet to terahertz region.

Zero-spindle spectral drill: real-time spectral measurement in a fixed Fabry-Pérot cavity.

An optical configuration for Fabry-Pérot cavity scanning using a geometric phase shifter, known as the "spectral drill," is improved to acquire a spectrum in real-time. Previously, the resonance condition of the spectral drill is swept by the mechanical rotation of a phase plate comprising a geometric phase shifter, and the acquisition time is limited. In this work, using a q-plate and a camera instead of phase plate rotation and a photo detector, we remove all the spinning mechanics and increase the acquisition rate by a factor 720. This technique will be applied to locking laser frequency.

Semiconductor property imaging on as-grown wafer with monochromatic tunable THz-wave source.

In this paper, we propose the use of a tunable monochromatic terahertz (THz) wave source to measure the carrier properties of semiconductors. We also report on the recent improvement of our measurement system and demonstrate its ability to measure an as-grown sample; our system involves reference mirror-free reflective measurement. In our method, carrier properties, such as resistivity and mobility in semiconductors relating to the carrier density, can be determined by reflective measurement. This method was applied to composite semiconductors-GaN, GaAs, and SiC. In the measurement system, we improved the filter to block the infrared beam co-linearly propagating with the desired THz-wave. Consequently, the signal intensity was 13 times higher than that achieved in our previous work. We demonstrated the measured carrier properties in an as-grown n-type GaN wafer. We observed flat features via our THz-reflective measurement, whereas the sample showed prominent surface roughness in a picture taken with visible light.

Nanoscale chiral surface relief of azo-polymers with nearfield OAM light.

An optical vortex with orbital angular momentum (OAM) can be used to induce microscale chiral structures in various materials. Such chiral structures enable the generation of a nearfield vortex, i.e. nearfield OAM light on a sub-wavelength scale, thereby leading to further nanoscale mass-transport. We report on the formation of a nanoscale chiral surface relief in azo-polymers due to nearfield OAM light. The resulting nanoscale chiral relief exhibits a diameter of ca. 400 nm, which corresponds to less than 1/5-1/6th of the original chiral structure (ca. 2.1 µm). Such a nanoscale chiral surface relief is established by the simple irradiation of uniform visible plane-wave light with an intensity of <500 mW/cm2.

Off-resonance and in-resonance metamaterial design for a high-transmission terahertz-wave quarter-wave plate.

This Letter describes a novel metamaterial design by employing off-resonance and in-resonance excitation for a high-transmission terahertz-wave quarter-wave plate (QWP). The device is demonstrated with a thin film metamaterial with double-layer split ring resonators (SRRs). Different from a usual resonant metamaterial device, here we design the work frequency off from the inductor-capacitor (LC) resonance for the TE mode, while in a dipole resonance for the TM mode to obtain the artificial birefringence. Rectangular SRRs in this Letter provide a choice to optimize the off-resonance and in-resonance excitation, to assist the double-layer design for high transmission. Converting a linearly polarized wave to circular polarization with our QWP, the experiment confirms a transmittance of 0.8 and an ellipticity of 0.99 at 0.98 THz. The developed thin film device is flexible and has a thickness of 48 µm (sub-wavelength). This is an advantage for potential integration in systems where overall device compactness is required.

Thin terahertz-wave phase shifter by flexible film metamaterial with high transmission.

Thin terahertz (THz)-wave optical components are fundamentally important for integrated THz-wave spectroscopy and imaging systems, especially for phase manipulation devices. As described herein, a thin THz-wave phase shifter was developed using a flexible film metamaterial with high transmission and polarization independent properties. The metamaterial unit structure employs double-layer un-split ring resonators (USRRs) with a designed distance between the two layers to obtain phase retardance of π/2, thus constituting a THz-wave phase shifter. The metamaterial design keeps the transmission coefficient as high as 0.91. The phase shifter also has polarization independence due to the four-fold symmetry of the USRR structure. Because of the subwavelength feature size of the USRR, this shifter can offer benefits for manipulating the spatial profile for the THz-wave phase through design of a binary optics phase plate by arranging a USRR array. The thickness of 48 µm has benefits for developing integrated THz optics and other applications that demand compactness and flexibility. The developed film size of 5 cm × 5 cm from the device fabrication process is suitable for THz lenses or gratings of large optical components.

Photon Drag Effect due to Berry Curvature.

A theoretical investigation reveals that the photon drag effect (PDE) is induced in a grating slab with deformation by the Berry curvature in phase space. It drifts the momentum of light, and gives asymmetric PDE signals in momentum space. Large PDE signals are observed even near the Γ point. This characteristic agrees well with our theoretical results.

Waveguide-mode interference lithography technique for high contrast subwavelength structures in the visible region.

We explore possibilities of waveguide-mode interference lithography (WMIL) technique for high contrast subwavelength structures in the visible region. Selecting an appropriate waveguide-mode, we demonstrate high contrast resist mask patterns for the first time. TM1 mode in the waveguide is shown to be useful for providing a three-dimensional structure whose cross section is checkerboard pattern. Applying our WMIL technique, we demonstrate 1D, 2D and 3D subwavelength resist patterns that are widely used for the fabrication of metamteterials in the visible region. In addition to the resist patterns, we demonstrate a resonance at 1.9 eV for a split tube structure experimentally.

Tunable Terahertz-wave generation from DAST crystal pumped by a monolithic dual-wavelength fiber laser.

For developing a continuous-wave (CW) tunable Terahertz-wave (THz-wave) source using difference-frequency generation (DFG) in highly nonlinear optical crystals, we proposed and demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on wideband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifier (SOA). Thermo-induced phase shift along the CFBG produces a very sharp transmission spike therefore two lasing wavelengths with single longitudinal mode operation are oscillating simultaneously within the fiber ring cavity. Due to the inhomogeneous gain broadening property of SOA, the wavelength spacing of our dual-wavelength fiber laser can be continuously adjusted from 0.3 to 9.5 nm. By using this single emitter dual-wavelength fiber laser to pump an organic nonlinear DAST crystal, type-0 collinear phase matching of DFG process can be fulfilled and monochromatic THz wave ranging from 0.5 to 2 THz has been successfully generated.

New method to determine the refractive index and the absorption coefficient of organic nonlinear crystals in the ultra-wideband THz region.

A method for simultaneously measuring the refractive index and absorption coefficient of nonlinear optical crystals in the ultra-wideband terahertz (THz) region is described. This method is based on the analysis of a collinear difference frequency generation (DFG) process using a tunable, dual-wavelength, optical parametric oscillator. The refractive index and the absorption coefficient in the organic nonlinear crystal DAST were experimentally determined in the frequency range 2.5-26.2 THz by measuring the THz-wave output using DFG. The resultant refractive index in the x-direction was approximately 2.3, while the absorption spectrum was in good agreement with FT-IR measurements. The output of the DAST-DFG THz-wave source was optimized to the phase-matching condition using the measured refractive index spectrum in THz region, which resulted in an improvement in the output power of up to a factor of nine.

Dual-wavelength single-crystal double-pass KTP optical parametric oscillator and its application in terahertz wave generation.

A tunable dual-wavelength double-pass singly resonant optical parametric oscillator based on a single KTP crystal has been proposed and demonstrated. By setting the rear mirror tuning angle precisely, collinear and noncollinear dual phase matching can be established simultaneously and leads to a two-wavelength oscillation at steady state. With high-speed KTP angle tuning, two wavelengths and their frequency difference can be adjusted continuously and accurately. By using this newly developed dual-wavelength optical parametric oscillator to pump a DAST crystal, a monochromatic terahertz wave ranging from 0.5 to 3 THz has been successfully generated in a difference-frequency generation system.

Optimized terahertz-wave generation using BNA-DFG.

An effective and widely tunable means of producing terahertz (THz) radiation using difference-frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated by optimizing the pump wavelengths. We calculated the (wideband) refractive index and coherence length mapping of BNA to establish the optimum phase-matching condition of the DFG configuration. To satisfy the phase-matching conditions over the range 0.1-20 THz, both pump wavelengths were varied from 780 to 950 nm, and the optical wavelength dependency of the THz-wave output was clearly observed. We expanded the range of THz-wave generation and increased the power output to ten times that obtained using previous methods.

Measurement of polarization dependence of nonlinear susceptibility responsible for Rayleigh-wing and Brillouin scattering.

By using a simple optical geometry based on backward light scattering and employing a Sandercock-type tandem Fabry-Perot interferometer, we measure both the linear and the circular polarization dependences of Rayleigh-wing and Brillouin scattering in a sample of liquid-crystal 4-n-pentyl-4'-cyanobiphenyl. Observed polarization dependences are consistent with the third-order nonlinear susceptibilities, taking into account the traceless symmetric scattering tensor for Rayleigh-wing scattering and the isotropic scattering tensor for Brillouin scattering.