Efficiency of Mode Excitation in an Open Resonator Connected to a Waveguide

Based on an analysis of the equivalent circuit of a reflection open resonator (OR), we derive an expression for the efficiency of mode excitation in such a resonance system and compare the efficiency of excitation of the lowest OR mode by aperture and slot couplers. It is shown that the excitation efficiency of the same mode by each of the considered couplers is high for small diffraction losses and amounts to no less than 95%. To excite an OR with a large distance between the mirrors, it is expedient to use an aperture coupler ensuring a higher efficiency of excitation of the studied mode.     

High quality resonances for terahertz radiation diffraction at periodically corrugated semiconductor interfaces

It is shown that the resonance features analogous to the well known optic Wood-type anomalies can be observed in the THz region for diffraction at periodically profiled semiconductor surfaces. The analytical theory of such resonance processes caused by excitation of surface plasmon polaritons (SPPs) is developed. It is shown that strong resonance effects such as total suppression of the specular reflection (TSSR) can be achieved for rather small inclinations of harmonic gratings. The analytical theory predictions are confirmed by strict numerical simulations. The analytical approach presented allows one to find parameters of the gratings so that the resonance diffraction results in specific redistributions of the reflected energy between different diffraction channels. As an example we demonstrate parameters of the InSb biharmonic grating responsible for the TSSR accompanied by 50% reflection in the minus first diffraction order when the SPP is excited in the plus first diffraction order.     

Profiled hologram diffraction gratings based on As2Se3 inorganic photoresist

We devised a method of producing blaze hologram diffraction gratings by transforming symmetric grooves of the original grating into asymmetric ones using an additional oblique irradiation by monochromatic or polychromatic light and a repeated chemical etching of the additionally irradiated grating. Photosensitive As₂Se₃ film layers are used as a photoconductive material for recording gratings. A numerical computer modeling of the formation of asymmetric grooves is conducted. The calculated shape of the groove profile is in good agreement with experimental data. The manifestation of asymmetry of the profile shape of the produced gratings in angular and spectral relations for their diffraction efficiency is considered. It is shown that by varying the parameters of producing the original gratings and the conditions of their additional treatment it is possible to obtain hologram gratings with the required profile shape and blaze angle. To whom correspondence should be addressed. Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 45, Nauka Ave., Kiev, 252028, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 587–590, July–August, 1999.     

Control of light transmission in laser-written phase-shifted Bragg grating couplers

We demonstrate the fabrication by direct laser writing and the operation of a directional coupler containing Bragg gratings in each waveguide. We achieve high-precision control over the longitudinal shift between the gratings, which feature first-order Bragg resonance at telecommunication wavelengths. We observe fundamental differences between light transmission characteristics in couplers with unshifted and shifted gratings in agreement with theoretical predictions.     

Negative role of surface plasmons in the transmission of metallic gratings with very narrow slits

It is generally admitted that the extraordinary transmission of metallic grating with very narrow slits is mainly due to the excitation of surface plasmons on the upper and lower interfaces of the grating. We show that the surface plasmon contribution is not the prime effect and that waveguide mode resonance and diffraction are responsible for the extraordinary transmission. Additionally and surprisingly, we reveal that the transmittance of subwavelength metallic gratings is always nearly zero for frequencies corresponding to surface plasmon excitation. This finding implies that surface plasmons play a negative role in the transmission.     

Polarization effects of electron current settling when the cerenkov radiation is excited by a nonrelativistic electron beam in an open dielectric resonator

We study the effect of electron current settling of a strip nonrelativistic electron beam (EB) on the Cerenkov radiation (CR) in a metal-dielectric channel formed by the metal surface and the surface of a rectangular open dielectric resonator having isotropic properties and operated at millimeter wavelengths (3.6–5.8 mm). The choice of this channel was substantiated by theoretical studies of the CR energy density. We experimentally examined several effects, including the neutralization of the dielectric surface dipole charge by electrons, the accumulation of electron charge at the dielectric surface, and the influence of the electron charge on the EB and CR. Conclusions are drawn on the prospects of using short pulses of EB and special metal-dielectric diffraction gratings for the excitation of CR. Institute of Radioengineering and Electronics, Ukrainian National Academy of Sciences, Kharkov, Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 4, pp. 491–505, April, 1997.     

Diffraction from phase modulated apertures

We apply the Rayleigh-Sommerfeld diffraction integral to solve the diffracted fields of periodically phase-modulated apertures. We show that the diffraction of small phase modulated apertures (with few refractive index modulation periods) behaves as for phase gratings, but the sharp diffraction peaks may already appear in the Fresnel region behind the aperture. Thus, these components may serve as lensless miniature interconnection gratings in micro-optics.     

Laser induced gratings in CdS

The diffraction from laser-induced gratings is observed in CdS at low temperature in the spectral region of the absorption edge. The excitation intensity is varied from about 100 W/cm² to 1 MW/cm². The reasons for the formation of gratings are a broadening of the excitonic absorption band, a new resonance due to two polariton transitions to the biexciton and the formation of an electron-hole plasma with increasing excitation intensities.     

Characteristics of filament induced Dammann gratings fabricated using femtosecond laser

To establish optimal processing conditions for direct write fabrication of diffractive optical elements such as gratings, waveguides, lenses, we have investigated the effect of process parameters such as scan speed, numerical aperture (NA) of objective lens, pulse energy on the characteristics of the filament induced inside fused silica with a femtosecond Ti:sapphire laser. The optimum process parameters were used to fabricate a number of Dammann gratings, 6×6 array, having different thicknesses and number of layers. The performance of these optical elements was evaluated by measuring their diffraction efficiencies. All gratings fabricated were strongly birefringent, the zero order spot with high intensity was not separated from the spot array, and the intensity distribution of 6×6 spot array exhibited some degree of nonuniformity. The single layer Dammann grating fabricated with a thickness of 80 μm attained a maximum diffraction efficiency of 38.8%.     

AN OPEN RESONATOR FOR PHYSICAL STUDIES

The excitation efficency of the TEM_01q oscillation of an two-mirror hemispherical open resonator (OR) is studied. The resonator is excited by the TE₀₁ wave of a circular waveguide joined in the middle of the OR plane mirror. Given the waveguide optimum size, the TEM_01q mode excitation efficiency reaches 78%. Analysis of the resonant system spectrum in the 4-mm wave region shows that this waveguide-OR system offers a single mode resonance curve across almost a 10-GHz tuning range. The TEM₀₁₁₀ mode field distribution with and without the circular waveguide in the middle of the OR plane mirror is available due to the small scatterer method. It is shown that the considered open system is suitable for measuring electromagnetic characteristics of high-loss substances and metamaterials in the short-wave end of the millimeter (mm) region as well as in the submillimeter (submm) wave region.     

Effects of thermal treatment on femtosecond laser fabricated diffraction gratings in polystyrene

We report the fabrication of efficient, buried diffraction gratings and micro-craters in bulk polystyrene using femtosecond laser direct writing technique. We recorded a maximum diffraction efficiency of 10% for a buried grating fabricated at 1 μJ energy, 1 mm/s speed, and a period of 30 μm. Buried micro-craters, with typical dimensions of ∼2 μm, were achieved at low energies and high scanning speeds. From the field emission scanning electron microscope studies, the observed emission is attributed as due to the inner surface modifications and the debris settled around the voids. The fabricated gratings subjected to heat treatment were tested for the diffraction efficiency and emission at different excitation wavelengths and the observed results are presented. Raman spectra collected from the femtosecond laser modified regions revealed the disappearance of few Raman modes at high peak intensities associated with incident Gaussian laser pulse. Potential applications of these luminescent micro-craters are highlighted.     

New approach to spectroscopy of surface plasmons

We report a novel approach to spectroscopy of surface plasmons based on the simultaneous excitation of surface plasmons by a polychromatic light and the dispersion of light on a diffraction grating coupler of a special design. This approach shows promise for the development of new, miniaturized, spectroscopic surface plasmon resonance (SPR) sensors. Potential of this approach for SPR sensing is demonstrated in a model refractometric experiment.     

Tunable optical add-drop multiplexer based on long-period fiber gratings for coarse wavelength division multiplexing systems

We propose and experimentally demonstrate a voltage-controllable add-drop multiplexer with a tunable coupler based on long-period fiber gratings (LPFGs). The tunable coupler is based on cladding mode coupling between two parallel LPFGs with divided coil heaters, which controls the resonant wavelength positions of the two LPFGs. The wavelength tunability of the tunable coupler is measured to be -0.45 nm/degrees C in the temperature range from 20 to 120 degrees C. The large tuning range of optical signals (approximately 50.54 nm) from 1502.32 to 1552.86 nm, which covers both the short (S) and the conventional (C) bands, is achieved. The channel isolation is as high as approximately 40 dB. It has several advantages, including a broad tuning range of wavelength in both the S- and C-bands, high channel isolation, no backreflection, and so on.     

Mode excitation in a generator of diffraction radiation with a multistage interaction space

We present the results of studying theoretically and experimentally the peculiarities of mode excitation in a generator of diffraction radiation with the two- and three-cascade interaction space in one open resonator. To describe the multistage interaction mechanism, the theoretical model used considers the excitation of the TEM_0nq modes, which have n in-phase field “spots,” i.e., cascades, along the direction of the electron beam motion. It is shown that even the use of the two-cascade interaction space with the TEM_01q modes allows one to reduce the starting current of the generator by 1.91 times. The results of studying experimentally the two-cascade generator in the 5 and 1.5-mm wavelength ranges are presented. The in-phase property of the electron beam with the resonance field was achieved by disrupting the periodicity of the lattice slots at the center of the interaction space. The output power of the two-stage generator of 5-mm diffraction radiation in the TEM013 mode ranged from 3 to 5 W. We also observed a 4–5-fold increase in the starting current for the regime of surface waves.     

Integration of optical pulses by resonant diffraction gratings

The possibility of integrating optical pulses by resonant diffraction gratings has been considered. It has been shown that a diffraction grating provides integration of the pulse envelope in the vicinity of quasiguided-mode resonances. The integration is performed with an exponential weight function, whose decay rate is deter-mined by the quality factor of the resonance. Metallic diffraction gratings for integration of picosecond pulses have been computed. The calculation of the grating eigenmodes with the use of the scattering-matrix method has shown that the integration is performed in the vicinity of the resonances corresponding to the excitation of surface plasmon polaritons at the grating boundary. According to the results of numerical simulation, the integration quality is quite high.     

High-purity transmission of a slow light odd mode in a photonic crystal waveguide

We demonstrate a novel scheme to control the excitation symmetry for an odd mode in a photonic crystal waveguide and investigate the spectral signature of this slow light mode. An odd-mode Mach-Zehnder coupler is introduced to transform mode symmetry and excite a high-purity odd mode with 20?dB signal contrast over the background. Assisted by a mixed-mode Mach-Zehnder coupler, slow light mode beating can be observed and is utilized to determine the group index of this odd mode. With slow light enhancement, this odd mode can help enable novel miniaturized devices such as one-way waveguides.     

Resolution of 90 nm (lambda/5) in an optical transmission microscope with an annular condenser

Resolution of 90 nm was achieved with a research microscope simply by replacing the standard bright-field condenser with a homebuilt illumination system with a cardioid annular condenser. Diffraction gratings with 100 nm width lines as well as less than 100 nm size features of different-shaped objects were clearly visible on a calibrated microscope test slide. The resolution increase results from a known narrower diffraction pattern in coherent illumination for the annular aperture compared with the circular aperture. This explanation is supported by an excellent accord of calculated and measured diffraction patterns for a 50 nm radius disk.     

孔缝对金属腔体强电磁脉冲耦合特性影响研究

核电磁脉冲和高功率微波等强电磁脉冲易造成电子设备功能失效甚至损毁,在实际工程实施中用金属腔体对电子设备进行屏蔽是常用的强电磁脉冲抑制手段。基于电磁仿真计算,对含矩形孔缝金属腔体的强电磁脉冲耦合特性进行了系统研究,阐述了孔缝宽长比、腔体尺寸等因素对多种不同类型强电磁脉冲（核电磁脉冲、宽带高功率微波、窄带高功率微波）作用下腔体内耦合场的影响;并以此为基础,重点分析了强电磁脉冲与含孔缝金属腔体之间的作用机制。研究结果表明:不同类型强电磁脉冲耦合信号差异明显,金属腔体对强电磁脉冲的响应是腔体谐振模式、孔缝谐振频率与强电磁脉冲共同作用的结果;当腔体谐振模式、孔缝谐振频率在强电磁脉冲的带内时,腔体内部的耦合场会出现增强效应;特别地,腔体与孔缝间的相互作用还可造成腔体与缝隙的谐振频率发生偏移。因此,在为电子设备设计金属屏蔽外壳时,应基于不同强电磁脉冲的频带范围,对腔体与孔缝的尺寸进行综合设计,抑制腔体、孔缝谐振及谐振频率偏移,提升其强电磁脉冲防护性能。     

基于非线性定向耦合器的光二进制逻辑器件

本文提出了一种新的非双稳态光学逻辑器件,它由一个双端输入的非线性光波导耦合器产生相应的输入输出波形,再经过多级单端输入的非线性耦合器进行波形限幅和整形,以获得阶跃开关特性.通过理论计算说明适当选择输入条件,这种器件可以实现所有六种基本的二进制逻辑运算功能:与、或、与非、与或、异或和同或.     

一维金属光栅的光学反射吸收

利用RCWA(rigid coupled-wave analysis)方法研究了一维金属光栅的反射特性,考察了 瑞利反常、表面等离激元驻波共振和几何共振三种共振吸收机理,分析了这三种机理的相互作用,如表面等离激元驻波共振和几何共振可以形成混合模式. 在反射式复合金属光栅中,确认了第四种共振形式,即相位共振. 数值计算表明相位共振对光学吸收的影响有两种形式: 当光栅周期大于一个波长时,相位共振导致尖锐的吸收峰,峰位在几何共振吸收峰一侧;当光栅周期小于一个波长时,相位共振导致混合模式的共振吸收峰发生劈裂. 对一维金属光栅反射特性的研究增加了对金属光栅共振吸收模式及其相互作用的认识. 关键词： 一维金属光栅 瑞利反常 表面等离激元 相位共振