太赫兹波段下金属狭缝的透射增强特性研究

用太赫兹时域光谱技术研究了具有不同宽度的金属狭缝透射光谱特征. 实验结果显示在实验测量有效光谱范围(02 THz—26 THz)内,当狭缝宽度小到一定程度,在频谱中出现了共振增强,具有明显的带通带阻现象,并在理论上分析了这种共振透射现象的原因. 关键词： 表面等离子体 太赫兹脉冲 时域光谱技术 金属狭缝     

Resonant terahertz transmission in subwavelength metallic hole arrays of sub-skin-depth thickness

We study surface-plasmon-enhanced terahertz transmission through subwavelength metallic hole arrays of sub-skin-depth thickness. Dynamic evolution of surface-plasmon resonance in terms of array thickness is characterized by use of terahertz time-domain spectroscopy in the frequency range 0.1-4.5 THz. A critical thickness of lead array film is observed, above which surface-plasmon coupling of terahertz pulses begins and is enhanced rapidly as the array thickness is increased toward the skin depth. The experimental results indicate that high-efficiency extraordinary terahertz transmission can be achieved at an array thickness of only one third of skin depth.     

Millimeter wave twin slot antennas on layered substrates

We have constructed a planar, monolithic twin slot antenna integrated with a bismuth microbolometer detector for operation near 94 GHz. In this paper, we briefly discuss the theory of operation for planar antennas on a dielectric stack. We then cover the fabrication details, including a short discussion of a photoresist bridge technique used to fabricate the microbolometer detectors. Measured receiver gain beam patterns are then compared with theoretical patterns for a number of cases.This work was supported by the National Science Foundation under grant number ECS-8552868 and by the Joint Services Electronic Program under grant number AFOSR 49620-89-0044.     

Coherent plasmonic enhanced terahertz transmission through random metallic media

We experimentally show coherent, enhanced terahertz transmission through dense 3D random metallic media having subwavelength heterogeneity. Preservation of the incident polarization state and strong dispersion of the transmitted radiation indicate that the enhanced transmission is due to delocalized plasmonic propagation over distances 5 orders of magnitude greater than the skin depth. The experimental observations are supported by numerical finite difference time-domain simulations.     

Resonant transmission of microwaves through a narrow metallic slit

Strong resonant transmission of microwave radiation through a very narrow (much less than the radiation wavelength) metallic slit is recorded. The results show that a set of resonant self-coupled surface plasmons are excited within the small gap, giving a Fabry-Pérot-like behavior in accord with analytical results published earlier [Y. Takakura, Phys. Rev. Lett. 86, 5601 (2001)]. The metallic slit, formed by two thick metal plates spaced apart by tens of microns, is inserted in a wavelength-sized aperture. On resonance the transmissivity through the metal slit is more than 2 orders of magnitude greater than the radiation impinging on the slit area.     

Anomalous transmission of terahertz wave through one-dimensional lamellar metallic grating

The anomalous transmission through one-dimensional lamellar metallic gratings was investigated in terahertz (THz) regime. The extraordinary optical transmission (EOT) is identified to originate from two possible ways: coupling of incident light with waveguide resonances and coupling of surface plasmon polaritons (SPPs) at the upper and lower interfaces of metal grating. The dual effects of SPPs have been clarified in this study: (i) the excitation of SPP modes at each individual interface results in the weakness of the THz wave transmission; and (ii) the coupling of SPP modes at two interfaces of metal grating is attributed to enhancement of THz wave transmission. The enhanced transmission is dominated by the coupling of incident light with transverse waveguide resonances. Numerical simulation based on finite-difference time-domain (FDTD) agrees well with experimental results.     

Modified slot antennas on high-permittivity substrates for the millimeter-wave range

We consider the possibility of manufacturing a planar antenna on the dielectric layer with high permittivity, which is intended for a monolithic planar detector of the millimeter-wave range. The previous design of the slot antenna on a foil-cladded dielectric with low permittivity (ε = 2.2) is analyzed. The simulation results show that a transition from the substrate with low dielectric permittivity to a semi-insulating GaAs substrate (ε ≈ 13) with the directivity kept at a level of about ∼10 at the resonant frequency is possible for antennas with external sizes comparable with the wavelength. The parameters of the planar detectors on semi-insulating GaAs substrates in the 3-mm wavelength range (94–97 GHz) were measured. Directional patterns of the planar antennas at the resonant frequencies reasonably coincide with the calculated data. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 864–871, October 2008.     

Self and mutual admittance of slot antennas on a dielectric half-space

In this paper, an efficient implementation of the spectral domain moment technique is presented for computing the self and mutual coupling between slot antennas on a dielectric half-space. It is demonstrated that by the proper selection of the weighting functions in the method of moments, the analytic evaluation or simplification of the transverse moment integrals is enabled. This results into a significant reduction of the required computational labor. The method is then utilized in order to provide design data for the self and mutual admittances between two slot antennas on a dielectric substrate lens in the case of fused quartz ( _r =3.80), crystal quartz ( _r =4.53), silicon ( _r =11.9) and GaAs ( _r =12.8). The presented technique and associated results are useful when designing twin slot quasi-optical receivers, imaging arrays, phased arrays or power-combining arrays of slot elements at millimeter-wave frequencies.     

Broadband transmission enhancement of acoustic waves through tapered metallic gratings

The phenomenon of the broadband transmission enhancement of acoustic waves is important for its prospective applications in various devices. To achieve broadband transmission enhancement of acoustic waves, we investigate the acoustic transmission in a metallic grating device with linearly tapered slits through eigen-mode matching theory (EMMT) and finite element (FE) simulation. The tapering provides a gradual filling fraction variation from the entrance to the exit of the slits, destroying the Fabry–Perot resonance and thus leading to broadband and wide-angle enhanced transmission in the audible regime. In addition, acoustic waves are strongly localized and enhanced at the slit exits, in contrast with straight slits.     

Resonant transmission of acoustic waves through an elastic plate quasiperiodically corrugated on surfaces

In this work, we study the resonant transmission of acoustic waves through a plate with quasiperiodic surface corrugations. The transmission spectrum shows peculiar transmission peaks, which cannot be simply attributed to the coherent diffraction as recognized previously in structured hard plates (without sustaining internal modes), whereas come from the resonant excitation of the coupled Stoneley surface modes in the elastic plate. The excitation frequencies can be determined by the geometrical structure factor of the quasiperiodic lattice, combining with the dispersion relation of the surface modes.     

Resonant transmission of light through finite chains of subwavelength holes in a metallic film

In this Letter we show that the extraordinary optical transmission phenomenon found before in 2D hole arrays is already present in a linear chain of subwavelength holes, which can be considered as the basic geometrical unit showing this property. In order to study this problem, we have developed a new theoretical framework, able to analyze the optical properties of finite collections of subwavelength apertures and/or dimples (of any shape and placed in arbitrary positions) drilled in a metallic film.     

Radiation from slot antennas through a nonlinear plasma layer

Based on a numerical solution of the nonlinear Helmholtz equation, we consider the increasedpower radiation through a layer of plasma from a slot antennas excited by an H-wave. The cases of positive (/|E|>0) and negative (/|E|<0) nonlinearities are analyzed. The effect of restriction of the transmitted power is investigated for negative nonlinearity and the formation of the self-supported waveguide channel is considered for positive nonlinearity. The existence of bistability with variation in the feeding power is shown. The plasma structure near the antenna is analyzed for the high level of the radiated power.Energy Institute, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 5, pp. 436–443, May, 1995.     

Resonant detection of terahertz radiation utilizing plasma waves in high-electron-mobility transistors

This paper theoretically studies the high-electron-mobility transistor （HEMT） driven by the terahertz radiation. It calculates the gate-to-source/drain admittance and the detection responsivity of the HEMT as a function of the signal frequency. It finds that the peaks of the admittances and the responsivity show redshift, and the heights of the peaks decrease with increasing the lengths of the source-to-gate and gate-to-drain spacing. Such phenomena may be useful in designing different HEMTs by utilizing the effects associated with the plasma oscillations excitation.     

Phase resolved near-field imaging of propagating waves in infrared tapered slot antennas

Tapered slot antennas (TSAs) consist of a planar non-resonant structure which couples incident radiation to a propagating waveguide mode. They are commonly used at microwave and radio frequencies because they are fundamentally broadband and have small profiles. Because of their planar layout and broadband response they have recently been scaled to infrared frequencies where they have advantages for sensing and energy harvesting. We use scattering-type scanning near-field optical microscopy (s-SNOM) to study the mode transformation of two types of TSA operating in the thermal infrared (λ₀ = 10.6 μm) with respect to electric field amplitude and phase. The results agree well with simulation showing both the phase reversal across the tapered slot and the traveling of wave fronts along the tapered slot, yet they also reveal high sensitivity of device performance to inhomogeneities in the geometry or illumination. This study will aid future design and analysis of practical non-resonant antennas operating at optical and infrared frequencies.     

Plasmon spectroscopy of metallic nanoparticles above flat dielectric substrates

We numerically analyze the spectral properties of localized plasmon resonances in metal nanoparticles when these are above a dielectric substrate. This analysis is performed as a function of the various parameters involved in the problem (relative optical properties, particle-substrate separation, angle of incidence, etc.). It can be shown that from the spectral behavior of the resonance in the far field, information about particle near-field interactions can be obtained.     

Resonant infrared transmission through SiC films

Resonant transmission through metallic films is observed when a periodic array of holes is drilled. This phenomenon has been attributed to surface plasmon polaritons. We study a similar system made of an array of slits in a SiC film supporting surface phonon polaritons. We find a resonant transmission in the infrared. The role of surface waves is analyzed. We find that surface waves are excited at resonance but are not a necessary condition to obtain a resonant transmission.     

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.     

Resonant amplification of frustrated evanescent waves by single dielectric coating

Resonant amplification and enhancement of evanescent wave’s frustration across an optical barrier by using single dielectric coating is numerically demonstrated in this article. With further tuning of the thickness values of the second and the third medium (optical barrier) of the proposed stratified four-media configuration, it is shown that it may be possible to achieve unity transmittance at discrete, but closely spaced incident angles within the full range defined by the theoretical cut-off limit. The designed configuration may have other potential applications than lithography and waveguide design is one such example.     

Extraordinary resonance transmission of two-dimensional terahertz metallic photonic crystals without defect

In this paper, we have demonstrated a kind of extraordinary transmission in the photonic first pass band of two-dimensional terahertz metallic photonic crystals: in the regime of low-middle metal filling radio, resonances among the metallic cylinders lead to sharp and high resonance peaks of Lorentz type, layer dependency of the peak numbers. The effect of positional disorders of metallic cylinders has been investigated as well. Based on a simple model proposed in this paper, we can intuitively understand the extraordinary transmission and roughly estimate resonance frequencies. Our results agree qualitatively with the experimental data reported in Appl. Phys. Lett. 65, 645.     

Use of broadside twin element antennas to increase efficiency on electrically thick dielectric substrates

This paper describes a technique for using twin element antennas (dipoles and slots) to increase the efficiency of antennas fabricated on electrically thick dielectric substrates. We present calculations which show that the efficiency of both the slot and dipole antennas can be increased by the proper spacing of elements placed broadside to each other. We consider the use of substrates that are odd integral multiples of a quarter of a dielectric wavelength thick and give results for an =4 substrate with thicknesses of one, three, and five quarter wavelengths. These thicknesses can be used when working at millimeter wave frequencies and yield substrate dimensions which can be handled and processed easily, while still yielding radiation-to-air efficiencies of about 70%. We also show calculated beam patterns for the elements which appear to be suitable for imaging array applications.This work was supported by the Joint Services Electronics Program, program no. AFOSR-F49620-86-C0045, and the National Science Foundation under grant number ECS-8552868.