Terahertz split-ring metamaterials as transducers for chemical sensors based on conducting polymers: a feasibility study with sensing of acidic and basic gases using polyaniline chemosensitive layer

We report on the first application of terahertz metamaterials acting as transducers for chemical sensors based on conducting polymers. In our feasibility study aimed at sensing of gaseous hydrochloric and ammonia, a two-dimensional sensor metamaterial consisting of an array of split-ring resonators on the surface of undoped silicon wafer was prepared. The surface of the resonator was coated with a 150-μm layer of polyaniline. Binding of hydrogen chloride to polyaniline leads to distinct changes in the resonance frequency of the metamaterial. Measurements can be performed both in the reflection and transmission mode. A numerical simulation of the response revealed an increase of both the real and the imaginary components of the dielectric function of the polyaniline film. These changes are attributed to the transition from emaraldine base to emeraldine salt. The results demonstrate a new approach for formation of highly sensitive transducers for chemical sensors.

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Response of tunnel schottky-barrier junctions to radiation pressure of fir laser radiation

The response of tunnel Schottky junctions due to plasma reflection of laser radiation has been investigated in the far infrared. The signal was found to be independent of the laser radiation frequency below the plasma edge. At high power levels a nonlinearity of the response was observed. It is assumed that this nonlinearity is caused by near-zone field effects which lead also to a substantial enhancement of the responsivity.On leave from A. F. Ioffe Physico-Technical Institute of the Russian Academy of Sciences St. Petersburg 194021, Russia     

Photoconductivity of the narrow-gap Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te(In) semiconductors in the terahertz spectral range

The spectral dependence of photoconductivity in the doped narrow-gap semiconductor Pb_0.75Sn_0.25Te(In) under the action of terahertz laser radiation pulses has been studied at temperatures 4.2–30 K. It is shown that the photoconductivity spectrum of the semiconductor spreads at least up to the wavelength of 500 μm. This value is more than twice higher than the red cutoff wavelength of 220 μm in uniaxially stressed Ge(Ga) which is known as the most long-wavelength photodetector among sensitive photon detectors of radiation. Mechanisms responsible for photosensitivity of PbSnTe(In) in the terahertz spectral range are discussed.     

Dynamic Hall effect driven by circularly polarized light in a graphene layer

We report the observation of the circular ac Hall effect where the current is solely driven by the crossed ac electric and magnetic fields of circularly polarized radiation. Illuminating an unbiased monolayer sheet of graphene with circularly polarized terahertz radiation at room temperature generates--under oblique incidence--an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. Alike the classical dc Hall effect, the voltage is caused by crossed E and B fields which are, however rotating with the light's frequency.     

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.     

Visible Light Emission due to Resonant CO2 Laser Excitation of Dental Hard Tissue

Visible light emission of dental hard substances excited by high-power infrared pulses of a tunable TEA CO₂ laser has been investigated. A clear correlation between observed visible light emission, plasma formation as well as ablation of dental hard tissue has been demonstrated. Both, the highly nonlinear infrared to visible upconversion process and the ablation efficiency show a sharp spectral resonance close to a vibrational mode of PO₄ at 1090 cm^-1 in dental enamel and dentin. The influence of strong infrared light impulses on dental hard tissue is examined by performing upconversion studies of visible light emission of human dental enamel and dentin. Our experimental setup allows one to determine the plasma formation threshold being important in dental surgery.     

Terahertz Cyclotron Photoconductivity in a Highly Unbalanced Two-Dimensional Electron–Hole System

The magnetic properties of the α-Fe₂O₃ hematite at a high hydrostatic pressure have been studied by synchrotron Mössbauer spectroscopy (nuclear forward scattering (NFS)) on iron nuclei. Time-domain NFS spectra of hematite have been measured in a diamond anvil cell in the pressure range of 0–72 GPa and the temperature range of 36–300 K in order to study the magnetic properties at a phase transition near a critical pressure of ~50 GPa. In addition, Raman spectra at room temperature have been studied in the pressure range of 0–77 GPa. Neon has been used as a pressure-transmitting medium. The appearance of an intermediate electronic state has been revealed at a pressure of ~48 GPa. This state is probably related to the spin crossover in Fe³⁺ ions at their transition from the high-spin state (HS, S = 5/2) to a low-spin one (LS, S = 1/2). It has been found that the transient pressure range of the HS–LS crossover is extended from 48 to 55 GPa and is almost independent of the temperature. This surprising result differs fundamentally from other cases of the spin crossover in Fe³⁺ ions observed in other crystals based on iron oxides. The transition region of spin crossover appears because of thermal fluctuations between HS and LS states in the critical pressure range and is significantly narrowed at cooling because of the suppression of thermal excitations. The magnetic P–T phase diagram of α-Fe₂O₃ at high pressures and low temperatures in the spin crossover region has been constructed according to the results of measurements.     

Circular photogalvanic effect at inter-band excitation in semiconductor quantum wells

We observed a circular photogalvanic effect (CPGE) in GaAs quantum wells at inter-band excitation. The spectral dependence of the CPGE is measured together with that of the polarization degree of the time-resolved photoluminescence. A theoretical model takes into account spin splitting of conduction and valence bands.     

Generation of far-infrared pulses by use of a passively mode-locked high-pressure CO2 laser

Summary Our present study demonstrates for the first time generation of frequency-tunable intense infrared radiation pulses in the nanosecond time regime by mode-locking a high pressure CO₂ laser using p-doped germanium as a saturable absorber. These pulses were transferred into the FIR region via stimulated Raman scattering of the CO₂ laser radiation in CH₃F resulting in subnanosecond FIR laser pulses.