Transmission Of Terahertz Radiation Research Articles

THz Laser Beam Profiling by Homogeneous Photodoping of High Resistivity Silicon in a Compact Single-Pixel Detection Setup

A compact single-pixel detection scheme for terahertz (THz) beam profiling is demonstrated. It incorporates an optical amplitude modulator that uses photodoping of a high resistivity Silicon (HR-Si) wafer with near-infrared (NIR) binary intensity masks, to modulate transmission of THz radiation through the semiconductor. A 270 nm thick indium-tin-oxide thin film deposited on glass by radio-frequency magnetron sputtering is used as dichroic mirror, such that the NIR radiation is brought at the Si wafer and the THz radiation is directed toward the detector, in a compact and efficient design. Given that homogeneous and well-defined binary masks in the form of structured intensity profiles of the NIR laser beam were needed, a beam homogenizer based on the Kohler integrator is also implemented. The profile of a THz laser beam with the wavelength of 118.8 μm was recorded using a resolution of 32 × 32 pixels and a square imaging area of 169 and 400 mm2, respectively. Also, due to the tunable nature of the laser system and symmetry of the laser cavity, more than one transverse modes of the laser cavity were imaged, namely, TEM00 and TEM01. A beam diameter of 3.25 mm in the TEM00 mode was recorded and a modulation depth of 52% at 2.5 THz was achieved with the present setup. With this paper, a THz spatial-domain imaging setup based on single-pixel method and having a compact design is proposed. Its functionality and possibility of further development are shown.

Suppression of transparency of thin modulated slabs of layered superconductors in the terahertz frequency range

A reduction in the transmission of terahertz radiation through a periodically modulated slab of layered superconductor owing to diffraction of the incident wave and resonance excitation of eigenmodes is predicted and studied theoretically. The slab thickness is assumed to be much less than the skin depth and when there is no modulation the slab transmission is close to unity.

On the transmission of terahertz radiation through silicon-based structures

We report on the transmission of a terahertz (THz) radiation through prototype structures based on a p-type silicon substrate. In particular, the bare substrate and progressively more complicated multilayer structures were investigated, allowing to address the effect on the transmission of different factors, such as the orientation of interdigitated contacts with respect to the polarized beam, the temperature, and the current flowing through a conductive SnO2 nanorods layer. A suitable experimental set-up was developed for the direct spectral measurement of transmission in the range of 0.75–1.1 THz at room and low temperatures. A simple Drude-Lorentz model was formulated, finding a quantitative agreement with the experimental transmission spectrum of the bare substrate at room temperature. For the multilayer structures, the spectra variations observed with temperature are well accounted by the corresponding change of the mobility of holes in the silicon p-type substrate. The influence of the contact orientation is consistent with that of a polarizing metallic grating. Finally, Joule heating effects are observed in the spectra performed as a function of the current flowing through the SnO2 nanorods layer. The experimental results shown here, together with their theoretical interpretation, provide insights for the development of devices fabricated on conductive substrates aimed to absorb/modulate radiation in the THz range.

Transmission of terahertz radiation by anisotropic MWCNT/polystyrene composite films

Anisotropic composite materials have been prepared by repeated forge rolling of polystyrene and carbon nanotubes (CNTs) with length of ∼65 µm. Transmission spectra of the composites were recorded for two different polarizations of the electric field. Obtained data indicated that the forge rolling resulted in a predominant orientation of CNTs in polymer matrix. Anisotropic response of the composites was measured at 130 µm wavelength on the Novosibirsk terahertz free electron laser and angular dependence of the transmitted light was determined. Absorption spectrum showed no strong resonance features and it was interpreted by CNTs breaking and agglomeration of CNT fragments during the composite fabrication procedure. Based on classical theory of scattering, considered the scatters as electromagnetic antennas, the size distribution of CNTs in composites was found. Anisotropy of terahertz radiation transmitted from MWCNT/polystyrene composite film on the Novosibirsk free electron laser at 130 µm wavelength.

Giant nonlinear response of terahertz nanoresonators on VO_2 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.

Optical control over transmission of terahertz radiation through arrays of subwavelength holes of varying size

We modulate the transmission of terahertz (THz) radiation through periodic arrays of subwavelength holes in a metallic film by using pulses of visible-wavelength light to photoexcite the semiconducting substrate of the hole arrays. By varying the photodoping level of the semiconductor we are able to switch off the resonant transmission of THz radiation through the array. By varying the size of the holes, we demonstrate the crucial role that surface modes play in the resonant transmission and ultimately in the photomodulation behavior of these structures. We demonstrate that the surface-wave transmission mechanism can allow for very efficient optical modulation of radiation transmission.