Search

We report the extraordinary terahertz (THz) transmission through subwavelength hole array in superconducting NbN film. As the temperature drops below the superconducting transition temperature, the transmission spectra experience distinct changes. The extraordinary transmission is greatly enhanced in superconducting state due to the enhancement of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs). We have also observed temperature-dependent resonance frequency shift, which mainly depends on the coupling between SPPs and LSPs.

Using the method of time-domain spectroscopy, we measure the absorption and dispersion of several non-polar organic solvents from 0.2 to 2.5THz. Based on the result, we believe liquid paraffin might be a proper choice for the core material of THz liquid-core fiber when Teflon is used as cladding material. Transmitting performance of THz liquid-core fiber at 98GHz is tested.

Pulsed terahertz time domain spectroscopy (THz-TDS) is a coherent technique, in which both the amplitude and the phase of a THz pulse are measured. Coherent detection enables direct calculations of both the imaginary and the real parts of the refractive index without using the Kramers-Kronig relations. In this paper, the spectroscopic characteristics of pine wood nematode and diseased pine wood in the THz regime are studied. In order to look for a rapid spectroscopic discrimination method for the disease, we measure the transmission characteristics of a Bursaphelenchus xylophilus (Bx) sample and a Bursaphelenchus mucronatus (Bm) sample by terahertz time domain spectroscopy. The measured time domain spectrums and corresponding frequency domain spectrums are compared to distinguish them from their absorption characteristics. At the same time, we also measure the transmission characteristics of a healthy Pinus massoniana (Pm) wood sample, a Pm with Bm wood sample, and a diseased Pm wood sample by THz-TDS and compare their difference in THz absorption spectrum and diffraction dispersive spectrum to confirm the effect of Bx on Pm by the absorption coefficient and refractive index.

An unusual microwave response of the surface impedance Zs of high-Tc thin films at an applied small dc magnetic field (Bdc) at 77 K, namely a decrease of Zs, is observed with the microstrip resonator technique. The resonant frequency is 1.107 GHz. The direction of Bdc is parallel or perpendicular to the a-b plane. Bdc ranges from 0 to 200 G. It is found that the surface resistance (Rs) at Bdc parallel to the a-b plane first decreases with Bdc and then increases above a crossover field. The Rs behaviour for Bdc perpendicular to the a-b plane is the same but with a different crossover field. The two behaviours can be collapsed to one curve by scaling the crossover fields. The changes of surface reactance Xs correlated linearly with the changes of Rs in the ranges of Bdc. The ratios rH of changes of Rs and Xs (rH = Rs/ Xs) are 0.5 at Bdc less than the crossover field and 0.1 at Bdc greater than the crossover field. The measurements also show that the crossover field is independent of rf input power. A phenomenological model is also proposed to explain this unusual behaviour. By adjusting fitting parameters the computed results agree with the experimental results qualitatively.

Pine wood nematode disease, namely pine wilt disease, is caused by the invasion of Bursaphelenchus xylophilus (Bx) into pines. Once susceptible pines are infected by the nematode, the disease develops rapidly, the infected pines cease to exude oleoresin and die quickly. Hence it is called pine cancer. Given the fact that there are still no good methods in diagnosing the disease, here we propose to study the spectroscopic characteristics of pine wood nematode and diseased pine wood in the THz regime in order to look for a rapid spectroscopic discrimination method for the disease. Firstly, we measure the transmittances of a Bx sample, a B. mucronatus (Bm) sample, a healthy Pinus massoniana wood sample and a diseased P. massoniana wood sample by a superconducting heterodyne mixer at 500 GHz. And their characteristics are compared. Secondly, we measure the transmission characteristics of a Bx sample and a Bm sample by terahertz time domain spectroscopy (THz-TDS). The measured time domain spectrums and corresponding frequency domain spectrums are compared to distinguish them from their absorption characteristics. Thirdly, we measure the transmission characteristics of a healthy P. massoniana wood sample and a diseased P. massoniana wood sample by THz TDS and compare their difference in THz absorption spectrum and diffraction dispersive spectrum to confirm the effect of Bx on P. massoniana by the absorption coefficient and refractive index. Some discussions are given for future development of the diagnostic technique of pine wood nematode disease based on THz spectrum.

Terahertz (THz) time-domain spectroscopy (TDS) is attracting more and more attention recently, and has been growing up as a powerful technique for measuring the material parameters. Indium phosphide (InP), which with short carrier average collision time, high band-gap energy and low effective mass, is growing up as one of the best photoconductive materials for emitting and detecting THz waves. An n-type InP of 0.35 Ω•cm was measured with normal transmitted TDS system for the temperature and frequency ranges of 4.2 - 300 K and 0.2 - 4 THz, respectively. THz beam was placed in a closed box purged with dry nitrogen gas, and the sample was mounted in a MicrostatHe cryostat made by Oxford Instruments. Temperature dependence of THz properties for InP as a function of frequency was characterized.

We present here the temperature dependence of the real part of the microwave conductivity of MgB(2) thin film measured at 7.2 GHz. A pronounced peak occurs at a temperature around 0.6 times the critical temperature, same position as it did at 17.9 GHz [Jin et. al., Phys. Rev. Lett. 91 (2003) 127006]. This frequency independence of the peak position confirms our previous conclusion; i.e., the peak comes from the coherence peak from the n band contribution. Some discussions are given to the measurement and extraction procedures on the microwave conductivity. We conclude that it is crucial to measure the real and imaginary parts of the surface impedance simultaneously at the same frequency to get an accurate temperature dependence of the microwave conductivity through the superconducting transition. (c) 2007 Elsevier B.V. All rights reserved.

The optical properties of three-layered silver–gold–silica (SGS) nanoshells with gain have been investigated theoretically by using Mie theory. Surface plasmon amplification by stimulated emission of radiation (spaser) phenomena can be observed at two plasmon modes of the active SGS nanoshell in the visible region. It is found with the decrease in the radius of the inner Ag core that the critical value of eg″(ωg) for the super-resonance of the low-energy mode increases first and then decreases while that for the high-energy mode decreases. An interesting overlap between the two curves for the critical value of eg″(ωg) can be found at a special core radius. At this point, two super-resonances can be achieved concurrently at the low- and high-energy modes of the active SGS nanoshell with the same gain coefficient. This dual-frequency spaser based on the bimetallic Ag/Au nanoshell may be an efficient candidate for designing the nanolaser.

Versatile devices, especially tunable ones, for terahertz imaging, sensing and high-speed communication, are in high demand. Liquid crystal based components are perfect candidates in the optical range; however, they encounter significant challenges in the terahertz band, particularly the lack of highly transparent electrodes and the drawbacks induced by a thick cell. Here, a strategy to overcome all these challenges is proposed: Few-layer porous graphene is employed as an electrode with a transmittance of more than 98%. A subwavelength metal wire grid is utilized as an integrated high-efficiency electrode and polarizer. The homogeneous alignment of a high-birefringence liquid crystal is implemented on both frail electrodes via a non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Its polarization evolution is directly demonstrated. Furthermore, quarter-wave plates that are electrically controllable over the entire testing range are achieved by stacking two cells. The proposed solution may pave a simple and bright road toward the development of various liquid crystal terahertz apparatuses. Light: Science & Applications (2015) 4, e253; doi:10.1038/lsa.2015.26; published online 27 February 2015

The effect of a grain boundary (GB) in YBa2Cu3O7−δ thin films for guiding vortex motion is studied by investigating transversal (Hall) and longitudinal magnetoresistances, (Rxy) and (Rxx). Here, a GB is inclined to the direction of the applied current with an angle of 30°. We investigate the dependences of both Rxy and Rxx on the temperature, the applied magnetic field and the bias current. It is found that the GB can act as an easy-flow channel for vortex motion. Furthermore, the pinning force in a GB is also discussed.

We report on the simulations of the pinning properties and vortex dynamics in thin superconducting film with three different periodic arrays of magnetic dots: (1) all magnetic dots have the same magnetization orientation (ferromagnetic pinning array); (2) magnetic dots with up and down magnetization orientation are arranged alternatively, to form an antiferromagnetic two-dimensional array; and (3) the same as the previous array, but each dot is replaced by a $2\ifmmode\times\else\texttimes\fi{}2$ magnetic dot subarray. All dots in the subarray have the same magnetization orientation. We calculated the critical depinning force and magnetization as a function of the applied magnetic field for these three arrays. Due to the field polarity-dependent flux pinning effect of the magnetic dots, asymmetric pinning behavior in the first array can be switched to the symmetric one in the latter two arrays while keeping the pronounced matching effects. We also calculated the current-voltage characteristics at both commensurate and incommensurate fields. We found that at the first matching field, the transition from the pinned phase to a vortex motion phase is continuous for the ferromagnetic configuration but discontinuous for the two antiferromagnetic configurations. This can be explained by different vortex dynamical trajectories. Our results indicate that pinning properties and vortex dynamics in thin superconducting films can be manipulated by tuning the configuration of the magnetic dot array.

An electric poling method has been used to prepare microstructured LiTaO3 crystals with periodically inverted-ferroelectric domains. By using these crystals as acoustic superlattices, both an “in-line” scheme and a “cross-field” scheme for acoustic excitation have been realized. The experimental results are in good agreement with the theoretical analysis. It is expected that these results may be applied to a bulk-acoustic device operating at a frequency high above 450 MHz.

We report on terahertz (THz) surface impedance measurement of an epitaxial MgB2 thin film using time domain THz spectroscopy. We show that the surface resistance of the MgB2 film is much lower than that of YBa2Cu3O7-delta and copper in the THz range. A linear dependence of the surface reactance on frequency is observed, yielding a penetration depth of about 100 nm at low temperatures. The measurements agree qualitatively with calculations based on impurity scattering in the Born limit. Our results clearly indicate that MgB2 thin films have a great potential for THz electronic applications.

A 10.6 GHz hybrid superconducting film/GaAs-MESFET microwave oscillator has been designed, fabricated and characterized on a 10 mm x 15 mm LaAlO3 substrate. The oscillator was a reflection mode type using a GaAs MESFET (NE72084) as the active device and a superconducting microstrip resonator as the frequency stabilizing element. By improving the unloaded quality factor of the superconducting microstrip resonator and adjusting coupling coefficient between the resonator and the MESFET, the phase noise of the oscillator was decreased. At 77 K, the phase noise of the oscillator at 10 KHz offset from carrier was -87 dBc/Hz. (C) 1996 Elsevier Science Limited

The microwave surface impedance Z(s) = R(s) + jomegamu(0)lambda was measured with dielectric resonator techniques for two c-axis-oriented MgB(2) thin films. The temperature dependence of the penetration depth, measured with a sapphire resonator at 17.93 GHz can be well fitted from 5 K close to T(c) by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kT(c) to be as low as 1.13 and 1.03 for the two samples. From these fits the penetration depth at zero temperatures was determined to be 102 nm and 107 nm, respectively. The results clearly indicate the s-wave nature of the order parameter. The temperature dependence of surface resistance R(s) measured with a rutile dielectric resonator, shows an exponential behaviour below about T(c)/2 with a reduced energy gap being consistent with the one determined from the, data. The R(s) value at 4.2 K was found to be as low as 19 muOmega at 7.2 GHz, which is comparable with that of a high-quality high-temperature thin film of YBa(2)Cu(3)O(7). A higher-order mode at 17.9 GHz was employed to determine the frequency f dependence of R(s) alpha f(n(T)). Our results revealed a decrease of n with increasing temperature ranging from n = 2 below 8 K to n = 1 from 13 to 34 K.

The microwave surface impedance Z/sub s/=R/sub s/+j/spl omega//spl mu//sub 0//spl lambda/ of MgB/sub 2/ thin films was measured via advanced dielectric resonator (DR) techniques. First, the temperature dependence of the penetration depth /spl lambda/ measured with a sapphire puck at 17.9 GHz can be well fitted from 5 K close to T/sub c/ by the standard BCS integral expression assuming the reduced energy gap /spl Delta/(0)/kT/sub c/ to be as low as 1.0-1.1 assuming /spl lambda/(0)=100--110 nm. These results clearly indicate the s-wave nature of the order parameter. Similar good fits were achieved by an anisotropic one gap and an isotropic two-gap model. Second, the temperature dependence of surface resistance R/sub s/, as measured with a rutile puck, indicates an exponential behavior below about T/sub c//2 with a reduced energy gap being consistent with the one determined from the /spl lambda/ data. The R/sub s/ value at 4.2 K was found to be as low as 19 /spl mu//spl Omega/ at 7.2 GHz, which is comparable with that of a high-quality high temperature thin films of YBa/sub 2/Cu/sub 3/O/sub 7/. A higher-order mode at 17.9 GHz was employed to investigate the frequency f dependence of R/sub s//spl alpha/f/sup n(T)/. Our results revealed an decrease of n with increasing temperature ranging from n=2 below 8 K to n=1 close to T/sub c/. Finally, the microwave power handling of MgB/sub 2/ films was deduced and compared with values for YBa/sub 2/Cu/sub 3/O/sub 7/ films. We found that the power handling of MgB/sub 2/ is comparable or even better than that of YBa/sub 2/Cu/sub 3/O/sub 7/ films for temperature below 30 K.

We have measured the temperature dependence of the microwave surface impedance Z(s)=R(s)+iomegamu(0)lambda of two c-axis oriented MgB(2) films employing dielectric resonator techniques. The temperature dependence of the magnetic-field penetration depth lambda determined by a sapphire dielectric resonator at 17.9 GHz can be well fitted from 5 K close to T(c) by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kT(c) to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 102 and 107 nm were determined from the fit. Our results clearly indicate the s-wave character of the order parameter. A similar fit of the penetration depth data was obtained with an anisotropic s-wave BCS model. Within this model we had to assume a prolate order parameter, having a large gap value in the c-axis direction and a small gap within the ab plane. This is in contrast to recent fits of the anisotropic s-wave model to upper critical-field data, where an oblate order parameter had to be used, and raises interesting questions about the nature of the superconducting state in MgB(2). A rutile dielectric resonator was employed to obtain the temperature dependence of R(s) with high accuracy. Below about T(c)/2, R(s)(T)-R(s)(5 K) exhibits an exponential temperature dependence with a reduced energy gap consistent with that determined from the penetration depth data. The R(s) value at 4.2 K was found to be as low as 19 muOmega at 7.2 GHz, which is comparable with a high-temperature superconducting copper oxide thin film.

Nonlinear terahertz (THz) transmission through subwavelength hole array in superconducting niobium nitride (NbN) film is experimentally investigated using intense THz pulses. The good agreement between the measurement and numerical simulations indicates that the field strength dependent transmission mainly arises from the nonlinear properties of the superconducting film. Under weak THz pulses, the transmission peak can be tuned over a frequency range of 145 GHz which is attributed to the high kinetic inductance of 50 nm-thick NbN film. Utilizing the THz pump-THz probe spectroscopy, we study the dynamic process of transmission spectra and demonstrate that the transition time of such superconducting plasmonic device is within 5 ps.

We report on Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43–0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other “hot” devices, e.g., quantum cascade lasers.

We present the magnetic tunability of a metamaterial made from superconducting niobium nitride film. The inductive-capacitive resonance excited by a normally incident terahertz wave was found to be continuously modulated through an external magnetic field at temperatures below the superconducting transition point. A giant resonance modulation was observed due to a strong magnetic effect, where the variation of the magnetic field alters the intrinsic conductivity of the superconducting film. The high sensitivity of the metamaterial allows us to observe the temperature-dependent magnetic effect, and the magnitude of resonance modulation decreases with increasing temperatures. This work demonstrates that a strong magnetic effect could be implemented as an active control modality in superconducting integrated devices functioning at terahertz frequencies.

We present the experimental demonstration of a quasioptical terahertz (THz) detector. It is based on the series connection of three Nb 5 N 6 microbolometers. This detector is of high responsivity and broadband response to THz signals. The maximum optical responsivity is 428 V/W at 0.245 THz and the minimum is 102 V/W at 0.367 THz. The thermal time constant of the detector has been demonstrated to be 1.3 μs, which is similar to the ones obtained for single-element microbolometers. These results make arrays of antenna-coupled Nb 5 N 6 microbolometers promising for the development of pixels in THz focal-plane arrays.

We studied numerically the temperature dependent extraordinary terahertz transmission through niobium nitride (NbN) film perforated with subwavelength spindle-like apertures. Both the resonant frequency and intensity of extraordinary terahertz transmission peaks can be greatly modified by the transition of NbN film from the normal state to the superconducting state. An enhancement of the (±1,0) NbN/magnesium oxide (MgO) peak intensity as high as 200% is demonstrated due to the combined contribution of both the superconducting transition and the excitation of localized surface plasmons (LSPs) around the apertures. The extraordinary terahertz transmission through spindle-like hole arrays patterned on the NbN film can pave the way for us to explore novel active tuning devices.

Using millimeter-wave spectroscopy, we study the electromagnetic response of asymmetric split-ring resonators made from superconducting niobium. The asymmetry is introduced by using different arm lengths of the U-shape split-ring design. A small asymmetry allows the excitation of an otherwise dark mode with only a weak coupling to electromagnetic field. The radiation losses are strongly reduced in this geometry. The combination of weak electromagnetic coupling and reduced ohmic losses in a superconductor leads to high quality factors of split-ring resonators. Quality factors of up to 150 in an open design are achieved experimentally.

Terahertz plasmon-polariton modes are excited and probed in a grating coupled GaN/AlGaN two-dimensional electron gas embedded in a Fabry-Perot cavity using a terahertz time-domain spectroscopy at 8K. A strong coupling between the plasmon modes and the cavity modes was observed. Electromagnetic simulations confirmed that both the cavity and the grating coupler play important roles in coupling the terahertz field and the plasmons. The finding suggests that the manipulation of terahertz plasmon-polariton could be achieved by engineering the Q-factor and the mode number of the cavity to further enhance the coupling strength. (C) 2013 AIP Publishing LLC.

We investigated the transmission properties of two coplanar waveguide resonators coupled with a flux qubit inductively. It is found that the transmission coefficients spectrum depends on not only the geometric resonator-resonator coupling constant, but also on the qubit-resonator coupling strength. Therefore, we can use the qubit to tune the coupling between the resonators in situ, realizing a quantum switch to generate entanglement of two resonators.

We present the nonlinear response of superconducting niobium nitride (NbN) thin film and NbN metamaterial with different thicknesses under intense terahertz pulses. For NbN thin film, nonlinearity emerges and superconductivity is suppressed with increasing incident terahertz electric field, and the suppression extent weakens as the film thickness increases from 15 to 50?nm. As the variation in intense terahertz fields alters the intrinsic conductivity in NbN, a consequent remarkable amplitude modulation in NbN metamaterial is observed due to the strong nonlinearity. Absorbed photo density in either NbN film or NbN metamaterial is estimated and used to understand the mechanism of nonlinear response. With a thicker NbN film element of 200?nm, the resonance of the metamaterial shows similar nonlinear modulation accompanied by a lower loss and a higher quality factor compared with a thinner NbN film element of 50?nm, which demonstrates the innovative implementation of strongly enhanced nonlinearity with thick superconducting film elements and the potential for novel applications using nonlinear metamaterial.

A novel room-temperature microbolometer array chip consisting of an Nb5N6 thin film microbridge and a dipole planar antenna, which is used as a terahertz (THz) detector, is described in this paper. Due to the high-temperature coefficient of the resistance of the Nb5N6 thin film, which is as high as −0.7% K−1, such an antenna-coupled microbolometer is ideal for detecting signals in a frequency range from 0.22 THz to 0.33 THz. The dc responsivity, calculated from the measured I–V curve of the Nb5N6 microbolometer, is about −760 V/W at a bias current of 0.19 mA. A typical noise voltage as low as 10 nV/Hz1/2 yields a low noise equivalent power (NEP) of 1.3 × 10−11 W/Hz1/2 at a modulation frequency above 4 kHz, and the best RF responsivity, characterized using an infrared device measuring method, is about 580 V/W, with the corresponding NEP being 1.7 × 10−11 W/Hz1/2. In order to further test the performance of the Nb5N6 microbolometer, we construct a quasi-optical type receiver by attaching it to a hyperhemispherical silicon lens, and the result is that the best responsivity of the receiver is up to 320 V/W. This work could offer another way to develop a large scale focal-plane array in silicon using simple techniques and at low cost.

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver.

Complex conductivity of FeSe0.5Te0.5 superconducting films has been investigated using backward-wave-oscillator spectroscopy in the terahertz frequency range. The normal state conductivity corresponds well to the low-frequency limit of a Drude metal. In the superconducting state, the real part of the complex conductivity is not completely suppressed, indicating the existence of a strongly anisotropic gap. Weak features in the conductivity are consistent with the superconducting gap value close to 12 cm−1(1.5 meV). The experimental results are discussed within the framework of a two-gap model based on the s± symmetry of the order parameter.

The coherence peak is observed in the temperature dependence of the real part of the microwave conductivity ( σ 1 ) of c -axis oriented MgB 2 thin film, which is extracted from microwave surface impedance ( Z s ) measurements using a sapphire resonator at 17.9 GHz. The peak appears at a temperature around 0.6 times the critical temperature ( T c ), which is greatly shifted downwards in temperature compared with conventional superconductors for which the peak occurs around 0.9 T c . Calculations based on the two-band model suggest that this peak corresponds to the smaller of the two energy gaps.

We report on fractal-featured square and ring-shaped apertures with a Sierpinski carpet pattern (SCP) on metallic and superconducting NbN films. Multiple extraordinary terahertz (THz) transmission peaks are studied in the transmission spectra using both THz time-domain spectroscopy and numerical simulation. The characteristic transmission peaks are found to be associated with the interaction of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) for ring-shaped apertures. The effect of LSPs is less remarkable in the square apertures. For the superconducting NbN film, when the temperature is slightly lower than the critical transition temperature Tc, the peak magnitude of SPP resonances is most prominent due to the non-monotonic temperature dependence of kinetic inductance. These results provide a new way to design compact and efficient THz devices.

We develop a fluorinated phenyl-tolane based nematic mixture NJU-LDn-4 and evaluate its frequency-dependent birefringence utilizing terahertz time domain spectroscopy (THz-TDS). A large mean birefringence of 0.306 is obtained in a broad range from 0.4 to 1.6 THz, with a maximum of 0.314 at 1.6 THz. Furthermore, relation between molecular structures and birefringence property is discussed. This work reveals new insights for tailing liquid crystal molecules with desirable large birefringence in THz range, which is extremely meaningful for the design and fabrication of fast, compact and tunable terahertz devices.

We designed and fabricated planar terahertz (THz) metamaterials made from superconducting NbN films to mimic electromagnetically induced transparency (EIT) system. They are characterized using THz time domain spectroscopy over a temperature range from 8 to 300 K. High transmittance and large delay-bandwidth product at transparency window are demonstrated, which mainly arise from the enhanced coupling and decreased damping in superconducting state. The EIT-like spectral response could be tuned in a wide frequency range. By applying two dark resonators with different resonance frequencies coupled with a radiative resonator, we experimentally demonstrated the planar metamaterials mimicking four-level EIT system.

Using sub-wavelength metallic gratings as both transparent electrodes and broadband high-efficiency polarizers, a highly-compact self-polarizing phase shifter is demonstrated by electrically tuning the effective birefringence of a nematic liquid crystal cell. The metal grating polarizers ensure a good polarizing efficiency in the range of 0.2 to 2 THz. Phase shift of more than π/3 is achieved in a 256 μm-thick cell with a saturation root mean square voltage of around 130 V in this integrated device.

Superconducting planar terahertz (THz) metamaterials (MMs), with unit cells of different sizes, are fabricated on 200 nm-thick niobium nitride (NbN) films deposited on MgO substrates. They are characterized using THz time domain spectroscopy over a temperature range from 8.1 K to 300 K, crossing the critical temperature of NbN films. As the gap frequency (f(g) = 2Δ0/h, where Δ0 is the energy gap at 0 K and h is the Plank constant) of NbN is 1.18 THz, the experimentally observed THz spectra span a frequency range from below f(g) to above it. We have found that, as the resonance frequency approaches f(g), the relative tuning range of MMs is quite wide (30%). We attribute this observation to the large change of kinetic inductance of superconducting film.

This paper studies the suppression of superconducting transition temperature (T(c)) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data. (C) 2011 American Institute of Physics. [doi:10.1063/1.3518037]

Superconducting terahertz (THz) metamaterial (MM) made from superconducting Nb film has been investigated using a continuous-wave THz spectroscopy with a superconducting split-coil magnet. The obtained quality factors of the resonant modes at 132 GHz and 450 GHz are about three times as large as those calculated for a metal THz MM operating at 1 K, which indicates that superconducting THz MM is a very nice candidate to achieve low loss performance. In addition, the magnetic field-tuning on superconducting THz MM is also demonstrated, which offer an alternative tuning method apart from the existed electric, optical and thermal tuning on THz MM.

Terahertz (THz) time-domain spectroscopy is used as a noncontact method to evaluate a nondoped indium phosphide (InP) wafer for the temperature and frequency ranges of 4.2-300 K and 0.2-4 THz, respectively. The strongly temperature- and frequency-dependent optical constants of the complex refractive index and complex conductivity were observed in the THz region, which were fitted and analyzed with a simple Drude model. The temperature dependence of the carrier density and scattering time are also presented. The shallow donors of impurities are discussed with the obtained results.

We have studied the effects of YBCO films used and stropline edges on the power handling capability of microstrip resonators. We have observed that both of them influence the performance of resonator when input power increases. Therefore, to improve power handling capability we have not only to use high quality superconducting films but also to make better edges of line.

In this paper, we study the correlation between the depositing oxygen pressure and surface resistance R s of YBa 2 Cu 3 O 7-δ (YBCO) films fabricated by pulsed-laser deposition method. It reveals that oxygen pressure has great influence on R s . In terms of minimum R s , there exists optimum value of oxygen pressure which is different from the optimum oxygen pressure for maximum T c . The crystalline orientation and surface morphologies of the films are helpful in understanding the variations of R s with the oxygen pressure.