STUDY ON EXCESS CONDUCTION LOSS IN NORMAL METALS AT/BELOW SUB-MILLIMETER WAVELENGTHS

We have used the spatial dispersion theory to develop a rigorous model to investigate excess conduction loss in normal metals. We have used the model to account excess conduction loss and dissect the discrepancies between excess conduction loss measurements and classical theoretical predictions in normal metals at/below sub-millimeter wavelengths. Moreover, we have compared the results of this model with the results of the classical skin-effect model and classical relaxation-effect model. Our analysis shows that the conductivity is not only frequency but also wave vector dependent. The results of the calculations indicate a good quantitative agreement with the published experimental data for the room temperature excess conduction loss of normal metals at/below sub-millimeter wavelengths.     

THE SPATIAL DISPERSION THEORY OF EXCESS CONDUCTION LOSS IN NORMAL METALS AT SUB-MILLIMETER WAVELENGTHS

On using the spatial dispersion theory, we have developed a rigorous model for analyzing excess conduction loss in normal metals. We have used the model to account excess conduction loss and dissect the discrepancies between excess conduction loss measurements and classical theoretical predictions in normal metals for sub-millimeter wavelengths. Our analysis shows that the conductivity is not only frequency but also wave vector dependent. We demonstrate good quantitative agreement with the published experimental data for the room temperature excess conduction loss of normal metals for sub-millimeter wavelengths.     

A New Theoretical Modeling of Surface Resistance in Normal Metals at Terahertz Frequencies

A number of researchers have reported discrepancies between surface resistance measurements and classical theoretical predictions for simple mm-wave and sub-mm-wave structures. We have developed a rigorous phenomenological model for analyzing surface resistance of normal metals. The model is based on quantum mechanical analysis for spatial dispersion within the metal. We have used the model to predict the surface resistance in normal metals at terahertz frequencies. Our analysis shows that the conductivity is not only frequency but also wave vector dependent. Moreover, we have compared the results of this model with the results of the classical skin-effect model and classical relaxation- effect model. We demonstrate that our model has good quantitative agreement with the published experimental data for the room temperature surface resistance of normal metals at terahertz frequencies range from the literature.     

Effect of dielectric properties of metals on terahertz transmission subwavelength hole arrays

We study the influence of dielectric function of metals on the transmission properties of terahertz pulses through periodically patterned subwavelength holes. Because of a drastic increase in the value of dielectric constants, most metals become highly conductive at terahertz frequencies. Extraordinary terahertz transmission is observed in subwavelength hole arrays made from both good and poor electrical conductors. The measured transmittance of terahertz pulses is found to be enhanced with increasing ratio of the real to the imaginary dielectric constant of the constituent metals, for which the dielectric function follows the Drude model.     

Direct creation of black silicon using femtosecond laser pulses

Using a direct femtosecond laser surface structuring technique, an array of equally spaced parallel nanostructure-textured microgrooves on silicon was produced that causes a dramatic reduction of the treated silicon reflectance. The processed area appears velvet black at all viewing angles. Throughout the visible region, the reflectance of the blackened surface is less than 5%. The antireflection effect of the processed surface also extends to the mid-infrared wavelength range. Furthermore, this technique has a potential in reducing silicon reflectance at terahertz frequencies and even in millimeter wavelength range.     

欧姆损耗对太赫兹频段同轴表面波振荡器的影响

为了研究欧姆损耗对太赫兹波段真空电子器件工作特性的影响, 本文推导了2.5维全电磁粒子模拟软件UNIPIC的表面阻抗边界条件, 并采用软件对不同金属材料慢波结构的同轴结构表面波振荡器进行了数值模拟研究, 分析了不同金属材料慢波结构器件的输出功率与电导率的关系, 模拟结果表明: 金属电导率对器件的输出功率有非常大的影响, 对于0.14 THz 同轴表面波振荡器, 铜材料和不锈钢材料慢波结构器件的输出功率分别下降13.4%和63.9%, 起振时间分别延迟0.4 ns 和15 ns.     

Photoluminescence of gold, copper and niobium as a function of temperature

A specially constructed instrument for measuring the low intensity photoluminescence emission spectra of metals is described. It uses low luminescence optical components and dedicated sample mounting techniques. Room temperature measurements agree closely with literature spectra for high-purity gold and are found to be sensitive to 100 ppm impurities. Detailed spectra are presented, which are weakly temperature dependent, for gold, copper and unpolished niobium between room temperature and 100 K. We conclude that this work provides accurate luminescence data for Au from 300 K down to 100 K. Although the (variable temperature) luminescence data for Cu are consistent both with the room temperature experimental data in the literature and theory, we conclude the role of surface adsorbates and/or oxides cannot be ruled out. Theory suggests that Nb has a factor ∼50 lower luminescence intensity than Au and Cu because the real part of the refractive index is a factor ∼5 higher and the density of states ∼2 eV below the Fermi energy is a factor of ∼4 lower than Au and Cu. Measurements are presented for unpolished Nb, but given the lack of signal detection for polished Nb and that theory predicts very weak signals, we conclude that the luminescence signals from pure Nb still remain below the sensitivity of our instrument.     

Giant optical activity and circular dichroism in the terahertz region based on bi-layer Y-shaped chiral metamaterial

We present a bi-layer Y-shaped chiral metamaterial (CMM) that can realize a giant optical activity and circular dichroism (CD) effect to the incident linear polarization wave in the terahertz (THz) region. Numerical simulation results exhibit that the pronounced CD effect with a great difference between the transmission coefficients for the circularly polarized waves can be obtained at 5.06 THz, meanwhile the 90°-polarization rotation can be observed at 5.2 THz when a y-polarized wave is incident to this CMM propagating along the −z-axis. The mechanism of the optical activity and giant CD effect is illustrated by simulated surface current distributions. Further, the influences of the structural parameters of the proposed CMM to the optical activity and CD effect have been investigated numerically.     

Nonlinear optical properties of commercial semiconductor-doped glasses

We report new observations made on semiconductor-doped glasses commercially available. We have observed that the modulus of the Kerr susceptibility is proportional to the absorption coefficient below, across and above the gap. We have measured the phase of this susceptibility for various values of the excess energy. We have also observed a blue shift of the absorption edge following pulsed excitation and have measured the diameter of the crystallites to be 70–80 Å. The band-filling model is the most appropriate to account for the nonlinear properties of these glasses.     

Thermal switching of the enhanced transmission of terahertz radiation through subwavelength apertures

We demonstrate that the extraordinary transmission of terahertz radiation through semiconductor gratings of subwavelength apertures can be switched completely by varying the temperature. The enhanced transmission, which is due to the resonant tunneling of surface plasmon polaritons that can be excited in semiconductors at terahertz frequencies, is controlled by thermally modifying the density of free carriers. The transmission through metal gratings cannot be switched in the same way since the carrier density is temperature independent. Thus semiconductors offer an interesting alternative to metals in enhancing the transmission of electromagnetic radiation.     

Accurate calculation of terahertz attenuation of a dielectric-coated metal wire with an intervening air gap using the engineering approach

A dielectric-coated metal wire with an intervening air gap between the conductor and inner surface of the dielectric is presented and demonstrated by theoretical calculation at terahertz frequencies. The characteristic equation of such a modified Goubau surface-wave transmission line is derived for the general case of a lossy dielectric and imperfect conductor. The terahertz attenuation of the modified Goubau line is investigated by using the accurate classical relaxation-effect frequency dispersion model. The influences of the different dimensions, different metal and dielectric materials on terahertz attenuation are also analyzed. In addition, the errors introduced by adopting the traditional and much simpler classical skin-effect model are also quantified. By using various conductivity models, the variation of the conductor loss is changed from 2.8% to 5.5%, and the variation of total loss is changed from 2.4% to 4.7%. It is shown that for certain combinations of the electrical dimensions of the structure the improvement in the attenuation constant over the Goubau line can be higher than 5 dB and realize stronger field confinement at terahertz frequency. The numerical results are very useful for the development of the surface plasmon polaritons (SPPs) devices in the fields of terahertz spectroscopy, sensors and detectors.     

Far-infrared signature of animal tissues characterized by terahertz time-domain spectroscopy

We present terahertz time-domain spectroscopy characterization of various animal tissues obtained from pork and rats. As the sensitivity of terahertz radiation to polar molecules of water is very high, biological tissues with high level of hydration show strong absorption at terahertz frequencies. The experimental data indicate that skin, fat and lean pork tissues have different frequency-dependent response to terahertz radiation due to the variation in water content. The same type tissue from different animals, however, is observed to show very similar water absorption.     

A Method for Quantitative Analysis of Chemical Mixtures with THz Time Domain Spectroscopy

A method for analysing chemical mixtures quantitatively with terahertz time domain spectroscopy is proposed. The experimental results demonstrate the feasibility of this technique. Transmission coefficient of THz wave at the sample surface is taken into account to improve the analytic precision, lsomer mixtures are chosen as the experimental samples. Compared to similar techniques, the analytic precision could be improved evidently in this method.     

Dielectric Properties of GaN in THz Frequencies

The complex refractive indices and the dielectric function of GaN for frequencies ranging from 0.25 to 1.22THz are obtained using THz time-domain spectroscopy. The real part of the dielectric function first decreases from 0.25 to 0.42THz and then oscillates from 0.42 to 1.22THz, whereas the imaginary part of the dielectric function is oscillating within the whole range of frequency. The simple Drude model is extended to take into account the effect of defects on the dielectric function. The extended model is in agreement with the experimental data.     

A synthesis technique of single square loop frequency selective surface at terahertz frequency

In this paper, we have explored and extended the use of frequency selective surface towards the terahertz regime of the electromagnetic spectrum where interesting applications such as imaging, sensing and communication exist. We have discussed a synthesis technique to design the single square loop frequency selective surface (SSLFSS) at 150 and 300 GHz which have found suitable application in the fast analysis and fabrication of the frequency selective surface. Moreover, the analytical results have been supported by the CST Microwave Studio and Ansoft HFSS commercial simulators. We have discussed the angular insensitivity of the SSLFSS at 150 GHz as well as 300 GHz. However, the specific problems arise at terahertz frequencies as compared to the radio and microwave frequencies are the ohmic losses. The proposed analysis has been extended from 100 GHz to 350 GHz to discuss the ohmic and dielectric losses. We have also discussed the other important issues which are very much significant in the terahertz regime of the spectrum such as skin depth and surface roughness.     

Theory of surface polaritons associated with the extreme anomalous skin effect in normal and superconducting metals

Surface polaritons associated with the extreme anomalous skin effect in metals have been theoretically investigated for the normal and superconducting states. The Pippard expression for the current density was used to obtain the nonlocal conductivity tensor for the extreme anomalous limit. From the resultant components of the dielectric tensor, the specular reflection-mirror image technique of Kliewer and Fuchs was used to obtain the surface polariton dispersion relation; calculations were performed for aluminium. For frequencies above the superconductor gap frequency but much less than the plasma frequency, the surface polariton behaviour is nearly the same for the normal and superconductor (T = 0 K) cases. For frequencies below the gap frequency, the surface polariton damping is zero for the superconductor case, and is nonzero and decreases rapidly with frequency for the normal conductor case.     

基于Kramers-Kronig关系建立金属太赫兹色散模型

提出了一种基于测量反射率谱、使用Kramers-Kronig(KK)关系建立金属太赫兹色散模型的方法.结合合金铝和合金铜4—40 THz的测量反射率谱,通过反射系数振幅和相位的KK关系,采用高频端指数外推,低频端常数外推的方法,反演金属复折射率.以KK反演的复折射率作为实验值,以拟合复折射率和实验值误差最小为准则,使用遗传优化算法,拟合了合金铝和合金铜的Drude色散参数(等离子频率和碰撞频率).基于优化的Drude模型计算了0.1—40 THz材料的复折射率,与椭偏仪的实测结果符合,验证了模型的准确性.该方法理论与实验相互验证,以测量的复折射率作为实验定标,将远红外频段的色散信息拓展到太赫兹频域,确定了太赫兹频段金属的微观物理参数,提供了太赫兹频段色散和散射机理的研究依据.     

The influence of surface roughness on conductor at terahertz frequencies

In this paper, a theoretical study of the influence of surface roughness on conductor is proposed at terahertz frequencies. By using the analytic small perturbation method, the effects of a random rough surface on the absorption by a metallic surface at terahertz frequencies are analyzed. And the effect of rough surface on reflectivity and power spectral density are also demonstrated. The numerical results are very useful for the development of terahertz devices and terahertz material.     

Development of high-throughput combinatorial terahertz time-domain spectrometer and its application to ternary composition-spread film

We have developed a high-throughput combinatorial terahertz (THz) time-domain spectrometer (CTTDS) and applied to a ternary composition-spread film. This technique has possibilities to reveal a variety of physical properties such as complex refractive index, complex dielectric constant, and complex electrical conductivity. Further, this method is a non-contact and non-destructive way to map those physical properties. The demonstration of THz transmittance mapping of ternary composition-spread film, with a spatial resolution of 1 mm, reveals metallic behavior in specific range of film compositions. This prospective technique may serve as a convenient tool for the high-throughput, non-contact, non-destructive, and spatially resolved characterization suited for combinatorial composition-spread films.     

Multi-band terahertz two-handed metamaterial based on the combined ring and cross pairs

We present the design of a Multiband two-handed metamaterial (MM) with the composite structure combined ring and cross pairs (RP + CP) in the terahertz regime. The dual-band left-handed and single-band right-handed transmission properties of this composite structure were explored by the FDTD method. The MM exhibits left-handed and right-handed transmission pass bands around the frequencies of 0.43 THz, 1.32 THz and 0.85 THz for the terahertz electromagnetic (EM) wave normal incidence, respectively. The surface currents distributions are demonstrated to discuss the physical mechanism of the left-handed properties of the proposed MM. The retrieved equivalent EM parameters and the refraction phenomenon based on a wedge-shaped model verify the left-handed properties. Furthermore, the dual-band left-handed transmission properties of RP + CP structure can be tunable individually by changing the structural parameters. The presented design of terahertz two-handed MM offers a flexibility for investigation of their novel EM properties, and important terahertz device applications.