Natsuki Nemoto

Enantiomeric switching of chiral metamaterial for terahertz polarization modulation employing vertically deformable MEMS spirals

Active modulation of the polarization states of terahertz light is indispensable for polarization-sensitive spectroscopy, having important applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. In the terahertz region, the lack of a polarization modulator similar to a photoelastic modulator in the visible range hampers expansion of such spectroscopy. A terahertz chiral metamaterial has a huge optical activity unavailable in nature; nevertheless, its modulation is still challenging. Here we demonstrate a handedness-switchable chiral metamaterial for polarization modulation employing vertically deformable Micro Electro Mechanical Systems. Vertical deformation of a planar spiral by a pneumatic force creates a three-dimensional spiral. Enantiomeric switching is realized by selecting the deformation direction, where the polarity of the optical activity is altered while maintaining the spectral shape. A polarization rotation as high as 28° is experimentally observed, thus providing a practical and compact polarization modulator for the terahertz range.

Spiral metamaterial for active tuning of optical activity

We propose an electrostatically actuated spiral structure as a metamaterial for circularly polarized light in the terahertz (THz) frequency range. An array of planar spiral structures was fabricated with micro electro mechanical system technology, and the geometry of the structures can be changed by electrostatic actuation. The actuation transforms the planar spirals into three-dimensional helices, resulting in optical activity in which the differential polarization response of the material depends on whether the incident light is left- or right-circularly polarized. THz spectroscopy confirmed that the optical activity can be tuned with the proposed structures.

Highly precise and accurate terahertz polarization measurements based on electro-optic sampling with polarization modulation of probe pulses

We have developed an electro-optic (EO) sampling method with polarization modulation of probe pulses; this method allows us to measure the direction of a terahertz (THz) electric-field vector with a precision of 0.1 mrad in a data acquisition time of 660 ms using a 14.0-kHz repetition rate pulsed light source. Through combination with a THz time-domain spectroscopy technique, a time-dependent two-dimensional THz electric field was obtained. We used a photoelastic modulator for probe-polarization modulation and a (111)-oriented zincblende crystal as the EO crystal. Using the tilted pulse front excitation method with stable regeneratively amplified pulses, we prepared stable and intense THz pulses and performed pulse-by-pulse analog-to-digital conversion of the signals. These techniques significantly reduced statistical errors and enabled sub-mrad THz polarization measurements. We examined the performance of this method by measuring a wire-grid polarizer as a sample. The present method will open a new frontier of high-precision THz polarization sensitive measurements.

High-Sensitivity and Broadband, Real-Time Terahertz Camera Incorporating a Micro-Bolometer Array With Resonant Cavity Structure

Terahertz (THz) cameras comprising an uncooled micro-bolometer array have been developed for simple THz imaging, and the improvement of their sensitivity is one of the important issues. We fabricated a new micro-bolometer array with a resonant cavity structure for a real-time THz camera, alongside a new method for evaluating the sensitivity across a wide range of the THz frequency region. The frequency dependence of the sensitivity of the THz camera is measured in the 0.5-2.0-THz frequency range taking the polarization dependence into account. It was found that the resonant cavity structure effectively increased the sensitivity of the THz camera, and, actually, the improvement by one order of magnitude was achieved in the frequency range below 1 THz. The THz camera with much enhanced sensitivity will expand the frontiers of real-time THz imaging such as molecular imaging and nondestructive inspection.

Real-time broadband terahertz spectroscopic imaging by using a high-sensitivity terahertz camera

Terahertz (THz) imaging has a strong potential for applications because many molecules have fingerprint spectra in this frequency region. Spectroscopic imaging in the THz region is a promising technique to fully exploit this characteristic. However, the performance of conventional techniques is restricted by the requirement of multidimensional scanning, which implies an image data acquisition time of several minutes. In this study, we propose and demonstrate a novel broadband THz spectroscopic imaging method that enables real-time image acquisition using a high-sensitivity THz camera. By exploiting the two-dimensionality of the detector, a broadband multi-channel spectrometer near 1 THz was constructed with a reflection type diffraction grating and a high-power THz source. To demonstrate the advantages of the developed technique, we performed molecule-specific imaging and high-speed acquisition of two-dimensional (2D) images. Two different sugar molecules (lactose and D-fructose) were identified with fingerprint spectra, and their distributions in one-dimensional space were obtained at a fast video rate (15 frames per second). Combined with the one-dimensional (1D) mechanical scanning of the sample, two-dimensional molecule-specific images can be obtained only in a few seconds. Our method can be applied in various important fields such as security and biomedicine.

Ultrafast zero-bias photocurrent and terahertz emission in hybrid perovskites

Methylammonium lead iodide is a benchmark hybrid organic perovskite material used for low-cost printed solar cells with a power conversion efficiency of over 20%. Nevertheless, the nature of light–matter interaction in hybrid perovskites and the exact physical mechanism underlying device operation are currently debated. Here, we report room temperature, ultrafast photocurrent generation, and free-space terahertz emission from unbiased hybrid perovskites induced by femtosecond light pulses. The polarization dependence of the observed photoresponse is consistent with the bulk photovoltaic effect caused by a combination of injection and shift currents. Observation of this type of photocurrents sheds light on the low recombination and long carrier diffusion lengths arising from the indirect bandgap in CH3NH3PbI3. Naturally ballistic shift and injection photocurrents may enable third-generation perovskite solar cells with efficiency exceeding the Shockley–Queisser limit. The demonstrated control over photocurrents with light polarization also opens new venues toward perovskite spintronics and tunable THz devices.The study provides new insight into the light-matter interactions of organic based perovskites. The authors do so by investigating the hybrid perovskite methylammonium lead iodide, which is considered a benchmark material for solar cells with high power conversion efficiency.

Tunable metamaterials by controlling sub-micron gap for the THz range

We propose a tunable metamaterial actuated by pneumatic force. The unit of the proposed metamaterial has a pair of sprit-ring-resonators (SRR), and the gap between them is controllable around sub-micron-order in size. One SRR is formed on a thin cantilever, which can be bent due to differential pressure between the upper and lower surfaces of the cantilever, whereas the other is fixed on a thin membrane. We confirmed that controlling the gap ranging from sub-micron-order to a few-micron-order was suitable for tuning a resonant frequency of a terahertz metamaterial.

Development and evaluation of high-sensitivity terahertz camera

Summary form only given. Terahertz (THz) imaging technology attracts a lot of attentions for security and biochemical applications because many biochemical molecules have characteristic absorption spectra in the THz frequency region. THz camera, which can take real-time THz images is one of the most important detection devices for the development of THz imaging technology. NEC has developed a THz camera called IRV-T0830 consisting of uncooled microbolometer THz focal plane arrays (THz-FPAs) [1]. Although the operation of real-time imaging system with this camera was demonstrated recently [2], improvement of performance is still required. In this study, we have improved structure of the THz-FPAs to increase the sensitivity in the THz frequency region, and evaluated frequency-dependent sensitivity of the improved THz camera systematically.IRV-T0830 detects THz waves by sensing resistance change of the microbolometers. The resistance change is originated from the temperature increase by the absorption of incident THz waves. In addition, reflection layer is formed at the bottom of air gap located under the microbolometer layer in order to increase the THz absorption efficiency. Optical cavity is formed between reflection layer and THz absorption layer and it was found that the thickness of air gap is closely related to sensitivity of the THz camera [1]. Here, we have developed a new THz camera which air-gap thickness is near to wave length of THz frequency region to increase the sensitivity. THz beam used in sensitivity-measurement experiment was generated by tilted-pulse-front excitation of lithium niobate (LN) crystal. Spectrum of generated THz beam was obtained by electro-optic (EO) sampling method. The spectrum extended to 2 THz. We have measured frequency dependence of sensitivity of THz camera by putting THz band-pass filters in front of THz camera, and measured incident polarization direction angle dependence by rotating THz camera. It was found that sensitivities of both the new THz camera and IRV-T0830 depend on polarization of incident THz beam. Therefore, we estimated frequency dependence of sensitivities by averaging count numbers of each THz camera throughout all polarization angles. Frequency dependence of sensitivity for each THz camera is shown in Fig.1. The sensitivity of the new THz camera is three or four times larger than IRV-T0830 in frequency range of 1-2 THz. THz beam-spot images obtained by two cameras are shown in Fig.2. A band-pass filter which has transmission peak at 1.0 THz and full width at half maximum of 0.24 THz was used, and the power of incident THz beam penetrating the band-pass filter is about 100 μW. The image obtained by the new THz camera is much clearer than IRV-T0830. These results indicate that the structural modification of FPA successfully improved the performance of the THz camera, which should become a critical tool for the development of THz imaging technology.

Spiral metamaterial for tunable circular dichroism

We propose an electrostatically actuated spiral structures as a metamaterial for circularly polarized light in Terahertz (THz) frequency. Actuation of the spiral structures changes their geometry, and produces a tunable chirality resulting in circular dichroism, which is the differential absorption between the left and right circularly polarized light. Results of the THz-spectroscopy confirmed that the ON/OFF of circular dichroism was achieved with the proposing structures.