Quasi-optical Filters Research Articles

Quasi-Optical Four-Port Acoustic Filters Based on NEMS Coupled Beam Arrays.

Theoretical models are presented for quasi-optical four-port acoustic devices based on NEMS-coupled beam arrays. Analogies with coupled mode devices in microwaves, ultrasonics, optics, and electron wave optics are first reviewed, together with coupled beam filters. Power transfer between two mechanically coupled, electrostatically driven, coupled beam arrays is then demonstrated using a lumped element model, and the conditions for full power transfer are established. Four-port devices, including directional couplers and coupler filters with complementary transmission ports, are then demonstrated. Predictions are verified for realistic device layouts using the stiffness matrix method.

Design of a Narrowband Filter for Implementing the Undersamping Method in Terahertz Time-Domain Spectrometers

In this paper, we propose a design of a narrow-band quasi-optical filter with a central frequency ν = 806 GHz (λ = 372 μm) suitable for the implementation of the undersampling technique in terahertz time-domain spectroscopy. The frequency was chosen to monitor the absorption line of the molecular gas CO in the local transparency window of the atmosphere. The filter is designed as the Fabry-Perot etalon based on a polypropylene film with frequencyselective surfaces (FFS) on both sides consisting of square slots in a sprayed aluminum layer. Through numerical simulation of the transmission of the proposed structure, we defined that the optimum ratio of the width of the metal bridge a to the pitch g of the FSS lies in the vicinity of a/g = 0.5. For g less than half of the operating wavelength λ, the FWHM of the filter is less than 4%, which is sufficient for the implementation of the undersampling method in terahertz time-domain spectroscopy.

Arrays of Sub-Terahertz Cryogenic Metamaterial

Integrated quasi-optical cryogenic terahertz receivers contain arrays of detectors, quasi-optical filters, interferometers, and other metamaterials. Matrices of quasi-optical band-pass, low-pass, and high-pass filters, Fabry–Perot grid interferometers, and arrays of half-wave and electrically small antennas with superconductor-insulator-normal metal-insulator-superconductor (SINIS) sub-terahertz wavelength range detectors were fabricated and experimentally studied on the same computational, technological, and experimental platform. For the design of the filters, we used the periodic frequency-selective surfaces (FSS) approach, contrary to detector arrays that can be presented in a model of distributed absorbers. The structures were fabricated using direct electron beam lithography, thermal shadow evaporation, lift-off, alternatively magnetron sputtering, and chemical and plasma etching. The numerical simulation methods of such structures are sufficiently different: for the reactive matrices with low losses, the approximation of an infinite structure with periodic boundary conditions is applicable, and for the arrays of detectors with dissipative elements of absorbers, a complete analysis of the finite structure with hundreds of interacting ports is applicable. The difference is determined by the presence of dissipation in the detector arrays, the phase of the reflected or re-emitted signal turned out to be undefined and the Floquet periodic boundary conditions are correct only for a phased array antenna. The spectral characteristics of the created filters, interferometers, and antenna arrays were measured in the frequency range 50–600 GHz.

Spectral filtering with diffractive reflection gratings for fast superconducting detectors in the THz frequency range

In this work we describe the design process of a grating spectrometer with fast superconducting THz detectors based on Niobiumnitride. The main application is the use as a diagnostic tool for accelerator-based sources of THz radiation like synchrotrons, where interactions of the electron bunches in storage ring lead to fluctuations in spectrum and intensity of the emitted radiation.Guidelines for the design of the two main parts, grating and detector design, are given. Here, a blazed reflection grating for a frequency range of 1 THz to 5 THz has been developed and successfully fabricated in a high-precision milling process out of brass. Also three different planar antenna designs (double-slot, annular slot and log-per spiral) in combination with quasioptical bandpass filters for the THz range have been simulated and evaluated for best spectral resolution and power coupling.Finally, we have verified grating performance for pulsed THz radiation of 1 THz in an experiment conducted at the synchrotron light source of the KIT. Excellent agreement of the grating performance to the simulations has been found.

Demonstration of wideband metal mesh filters for submillimeter astrophysics using flexible printed circuits.

We developed a wideband quasi-optical band-pass filter covering 170-520 GHz by exploiting the recent advancements in commercially available flexible printed circuit (FPC) fabrication technologies. We designed and fabricated a three-layered stack of loaded hexagonal grid metal meshes using a copper pattern with a narrowest linewidth of 50µm on a polyimide substrate. The measured frequency passband shape was successfully reproduced through a numerical simulation using a set of parameters consistent with the dimensions of the fabricated metal meshes. FPC-based metal mesh filters will provide a new pathway toward the on-demand development of millimeter/submillimeter-wave quasi-optical filters at low cost and with a short turnaround time.

Implementation of Terahertz High-Pass Filters Based on All-Metal Microstructures using Deep X-ray Lithography

A method for fabricating high-pass terahertz quasi-optical filters in the form of thick (up to 1 mm in thickness) self-bearing copper microstructures of subwavelength topology is described. This method is based on forming a high-aspect-ratio mask of SU-8 resist on a silicon wafer via deep X-ray lithography through a tungsten X-ray mask followed by electroplating a copper layer through the resistive mask. An example of a 212-µm thick structure with a cutoff frequency of 0.42 THz having the geometry of hexagon-shaped through-holes arranged on a triangular lattice is considered. The results of broadband THz characterization and electromagnetic analysis of the structure fabricated are presented.

Asymmetrical Impedance Inverter for Quasi-Optical Bandpass Filters With Transmission Lines of Fixed Length

Quasi-optical (QO) filter design is commonly based on the direct coupled bandpass filter topology with distributed resonator elements. Conventionally, prototypes with symmetrical impedance inverters are employed during synthesis, and as a result, this leads to resonators with custom lengths across the filter. This can be impractical from a QO filter implementation perspective, since spacers of arbitrary thickness may not be readily available. In this paper, we propose a novel impedance inverter equivalent network, which allows the development of prototypes where transmission lines across the filter structure to have predefined fixed lengths. Based on this technique, we developed a fourth-order QO filter prototype that composed of 2-D periodic arrays with subwavelength features. Its performance is verified with free-space measurement, and the measured results show good agreement with both full-wave and circuit simulation. Thus, it is shown that the proposed impedance inverter circuit can simplify the implementation of QO filters of higher order.

Implementation of high-pass subterahertz filters using high-aspect-ratio polimeric structures

This paper describes a technological approach to the implementation of quasioptical high-pass subterahertz filters with the use of high-aspect pseudometallic structures. The approach is based on micropatterning a solid polymer layer of polymethylmethacrylate by means of synchrotron X-ray lithography with subsequent metallization of the entire surface of the structure. A fabricated specimen and operational characteristics for the filter with a cutoff frequency of 0.275 THz, which has the thickness of 1 mm and is formed by hexagon-shaped through-holes arranged on a triangular lattice with 70-μm wide crosspieces, are demonstrated.

Investigation of Spectral Characteristics for Microstructured Quasi-Optical Bandpass Subteraherz Filters

The electrodynamic features of quasi-optical bandpass filters based on multilayer microstructured frequency selective surfaces intended for effective spectral selection of subteraherz electromagnetic radiation are considered. As an optimal solution, the multiplex filters are highlighted wherein tripole-slot self-bearing copper microstructures free from dielectric substrates are employed. The results of the experimental development for such filters are presented by the example of structures with resonant transmission at the frequencies of 0.6 and 1 THz, and the details of testing their spectral performance in the range of 0.1–1.5 THz using a BWO spectroscopy technique are described. Good agreement between the experimental data and the results of theoretical predictions is demonstrated.

Terahertz frequency selective surface for future wireless communication systems

An overview of recent progress in research and development of various design issues as well as the technical advancements experienced by the frequency selective surfaces (FSSs) in terms of the mechanical integrity, choice of the numerical techniques, reflection/transmission bandwidth, angular/polarization stability, complexity and the cost of the fabrication in the millimetre and terahertz (THz) wave regime of the electromagnetic spectrum for future wireless communication system is discussed. In addition to this, the limitations of various computational techniques, which have been used to analyse the electromagnetic problems such as scattering, reflection/transmission and losses of the FSSs in THz regime, have also been discussed. However, the performance of the FSSs has widely been affected by the ohmic losses and various other factors such as surface roughness, dispersion through the conductor and the skin depth in the THz regime of the electromagnetic spectrum and it has also been exploited. Along with the other various applications of FSS structures as the Fabry–Perot resonators and as quasi-optical filters, we have also discussed a novel FSS design of spatial bandpass filter which can be used for THz communication systems.

Infrared measurements and simulations of metal meshes in a focused beam

Infrared transmittance measurements of quasioptical filters are often restricted to a focused beam due to the optical design of the spectrometer. In contrast, numerical simulations assume an incident plane wave, which makes it difficult to compare theory with experimental data. We compare transmittance measurements with numerical simulations of square arrays of circular holes in 3-μm thick Cu sheets at angles of incidence from 0° to 20° for both s and p polarizations. These simple structures allow detailed tests of our electromagnetic simulation methods and show excellent agreement between theory and measurement. Measurements in a focused beam are accurately simulated by combining plane wave calculations over a range of angles that correspond to the focal ratio of the incident beam. Similar screens have been used as components of narrow bandpass filters for far-infrared astronomy, but these results show that the transmittance variations with angle of incidence and polarization limit their use to collimated beams at near normal incidence. The simulations are accurate enough to eliminate a costly trial-and-error approach to the design of more complex and useful quasioptical infrared filters and to predict their in-band performance and out-of-band blocking in focused beams.

Development of a Transportable Telescope for Galactic Survey at 500 GHz in Antarctica

We have developed a transportable 30-cm submillimeter-wave telescope to operate at the Dome Fuji station in the Antarctic plateau. Transportability is an important requirement in the design; the telescope can be divided into several subsystems by hands. The maximum weight of the subsystems is restricted to be below 60 kg, so that the telescope can be assembled without a lifting machine. A small 4 K mechanical cryocooler is used for cooling down a SIS mixer. Total power consumption was designed to be less than 2.5 kW. The optical system was designed to satisfy the frequency independent matching condition at the subreflector and the feed horn of the SIS mixer, so we could accommodate a higher frequency receiver without changing mirrors. A quasi-optical filter was employed for the single sideband operation in observations of the CO (J = 4-3) line at 461.04 GHz and the [CI] (3P1-3P0) line at 492.16 GHz. It was equipped with a 1 GHz width spectrometer that covers a velocity width of 600 km/s with a velocity resolution of 0.04 km/s at 461 GHz. We carried out test observations at a 4400-m altitude site in northern Chile during winters of 2010 and 2011. The typical system noise temperature, including atmospheric loss, was 3000 K (SSB) at 461 GHz, that is mainly limited by atmospheric opacity. The beam size of the 30-cm offset Cassegrain antenna at 0.65 mm of wavelength was measured to be 9'.4±0'.4 by cross scanning of the sun. This angular resolution of the 30-cm telescope is same as those of the Columbia-CfA-U. Chile CO (J = 1-0) surveys. We estimated the main beam efficiency to be 87±5% by observing the new moon. We succeeded in mapping Orion Molecular Cloud A and M17 SW in CO (J = 4-3) followed by test observations toward Orion KL in both CO (J = 4-3) and [CI] (3P1-3P0) .

Elaboration of Microstructured Subterahertz Filters Via Liga Technology and Investigation of Their Selective Properties

We discuss the peculiar features of the LIGA-technology methods elaborated in the Siberian Synchrotron and Terahertz Radiation Centre (SSTRC, BINP SB RAS) and applied for producing quasi-optical subterahertz filters based on self-bearing microstructures of the metallic and pseudo-metallic types. The results of manufacturing the low- and highaspect inductive microstructures with topology of dipole slots utilized as the selective elements in the band-pass filters for the frequency range 0,25–0,45 THz are presented. The details of spectral characterization of the developed filters using a BWO-spectroscopy technique are considered. A good agreement between the results of measurements and numerical simulations is demonstrated

THz Frequency Selective Surface Filters for Earth Observation Remote Sensing Instruments

The purpose of this paper is to review recent developments in the design and fabrication of Frequency Selective Surfaces (FSS) which operate above 300 GHz. These structures act as free space electromagnetic filters and as such provide passive remote sensing instruments with multispectral capability by separating the scene radiation into separate frequency channels. Significant advances in computational electromagnetics, precision micromachining technology and metrology have been employed to create state of the art FSS which enable high sensitivity receivers to detect weak molecular emissions at THz wavelengths. This new class of quasi-optical filter exhibits an insertion loss <;0.3 dB at 700 GHz and can be designed to operate independently of the polarization of the incident signals at oblique incidence. The paper concludes with a brief overview of two major technical advances which will greatly extend the potential applications of THz FSS.

Multichannel Radiometric System for Registering Submillimeter-Wave Radiation at Beam-Plasma Interaction

We describe a 4-channel radiometric system developed for registering pulses of submillimeter-wave radiation in the frequency band 210-530 GHz. The system employs a quasi-optical scheme with polarization demultiplexed channels, wherein subsequent frequency filtering and spectral signal detection are provided by quasi-optical bandpass filters and high-speed Schottky detectors respectively. The system has frequency resolution of the channels ~30-50 GHz and response time better than 2 ns and destined for registering emission of electromagnetic radiation in the vicinity of a double plasma frequency, which is produced at turbulent heating of dense plasma in the multimirror magnetic trap GOL-3 by a powerful relativistic electron beam of 10 microsecond duration.

Development and Characterization of Quasi-Optical Mesh Filters and Metastructures for Subterahertz and Terahertz Applications

We review our recent results on development of passive quasi-optical selective devices based on metallized subwavelength microstructure arrays designed for controlling radiation beams at frequencies from a few tens GHz up to ten THz: filters, polarizers, metasurfaces, ET-metamaterial lenses. The methods of electromagnetic simulation, technological implementation and (sub)THz-characterization of microstructure devices, as well as their applications, are discussed.

Toward a 1550 nm InGaAs photoconductive switch for terahertz generation

We report a terahertz (THz) photoconductive switch made from a composite of metal ErAs nanoparticles embedded in In(0.53)Ga(0.47)As and coupled to a square spiral antenna. The THz output power was measured in a 77 K cryostat by using a standard hyperhemisphere-lens package, a Golay cell outside the cryostat, and a quasi-optical filter bank for spot frequency spectral measurements. Results indicate an average output power of approximately 12 microW at 22 V bias using 140 mW of optical pump power from a subpicosecond fiber mode-locked laser. In addition, the THz spectra displayed invariance to bias voltage despite operating near impact ionization.

Advancements in electron cyclotron emission imaging demonstrated by the TEXTOR ECEI diagnostic upgrade

A new TEXTOR electron cyclotron emission imaging system has been developed and employed, providing a diagnostic with new features and enhanced capabilities when compared to the legacy system it replaces. Optical coupling to the plasma has been completely redesigned, making use of new minilens arrays for reduced optical aberration and providing the new feature of vertical zoom, whereby the vertical coverage is now remotely adjustable on a shot-by-shot basis from 20-35 cm. Other innovations, such as the implementation of stacked quasioptical planar notch filters, allow for the diagnostic to be operated without interruption or degradation in performance during electron cyclotron resonance heating. Successful commissioning of the new diagnostic and a demonstration of the improved capabilities are presented in this paper, along with a discussion of the new technologies employed.

Nonreciprocal microwave transmission in metastructures with transversely magnetized ferrite plate and a grating of resonant elements

Nonreciprocal transmission of a linearly polarized electromagnetic wave at a ferromagnetic resonance (FMR) frequency has been observed in a metastructure comprising a transversely magnetized ferrite plate and a grating of resonant elements. The nonreciprocity of wave transmission is observed for the metastructure arranged along the axis of a rectangular waveguide and even in the free space between transmitting and receiving waveguides, where the effect does not take place when there is no grating. The observed phenomenon is explained by the formation of a surface wave with elliptic or circular polarization on the grating. The nonreciprocity reaches maximum (>35 dB) under the conditions of mutual influence between the FMR and the resonance of grating elements for certain values of the certain frequency and magnetic field, which depend on the distance between the ferrite plate and the grating. The nonreciprocal effects have been observed for grating elements in the form of double split rings, polyhedral loops, and dipoles. The results may be of interest for the development of new nonreciprocal devices and multifunctional metastructures such as decoupling elements for quasi-optical systems and two-frequency decoupling filters for counterpropagating waves in the gigahertz and terahertz range.

Quasi-optical diplexer and filter based on a polarization ring interferometer

The operation of a quasi-optical polarization ring interferometer (PRI) performing the function of a diplexer, bandpass filter, and band-stop filter is demonstrated for the first time. It is shown that the beam focusing losses in this interferometer can be reduced using spherical low-permittivity lenses. The results of measurements of the transmission characteristics of a PRI prototype in a 3-mm wavelength range are presented.