THz Bandpass Filters Research Articles

Fabrication of freestanding photonic devices combining polymer films with microfabrication techniques and 3D printing.

We report a technological concept for freestanding photonic elements based on metamaterials fabricated on polymer films by clean-room processes and framed using 3D printing. A spin-coated cyclic olefin copolymer (TOPAS) of variable thickness down to one micrometer was used as the substrate onto which metamaterials were fabricated using optical lithography. We demonstrate the possibility of applying a second TOPAS layer to protect the device or to allow for stacking another metamaterial layer. To obtain freestanding elements, frames were 3D printed directly on top of the metamaterial before lift-off from the carrier wafer. This ensured maintaining the flatness of the elements. Both the cleanroom process and the 3D printing enabled the design and manufacturing of elements in different sizes and shapes, e.g., to adapt to specific experimental set-ups and holder geometries or to be compatible with standard optical mounts. While TOPAS is transparent for wavelengths from UV to the far infrared, except for a few infrared absorption lines, we illustrate the concept with the simulation and manufacturing of THz band-pass filters. The performance of the fabricated filters was assessed using THz time-domain spectroscopy. The process is scalable to other wavelength ranges and has the potential for upscaling in manufacturing.

Efficient optimal design of mosaic-like PPDW devices for THz application using the adjoint variable method.

In the development of THz-wave circuits, parallel plate dielectric waveguide (PPDW) is a promising platform and recently some fundamental devices have been reported. In order to realize high performance PPDW devices, optimal design methods are crucial and as out-of-plane radiation does not occur in PPDW, mosaic-like optimal design appears to be appropriate for PPDW platform. In this paper, we present a novel and efficient mosaic-like design approach based on gradient method with adjoint variable method (AVM) to realize high performance PPDW devices for THz circuit applications. The design variables in the design of PPDW devices are efficiently optimized by utilizing the gradient method. The mosaic structure in the design region is expressed by using density method with an appropriate initial solution. In the optimization process, AVM is employed for an efficient sensitivity analysis. The usefulness of our mosaic-like design approach is confirmed by designing several PPDW devices, T-branch, three branch, mode splitting device, and THz bandpass filter. The proposed mosaic-like PPDW devices except bandpass filter achieved high transmission efficiencies at single frequency operation as well as at broadband operation. Furthermore, the designed THz bandpass filter achieved the desired flat top transmission property at the targeted frequency band.

THz Filters Made by Laser Ablation of Stainless Steel and Kapton Film.

THz band-pass filters were fabricated by femtosecond-laser ablation of 25-m-thick micro-foils of stainless steel and Kapton film, which were subsequently metal coated with a ∼70 nm film, closely matching the skin depth at the used THz spectral window. Their spectral performance was tested in transmission and reflection modes at the Australian Synchrotron’s THz beamline. A 25-m-thick Kapton film performed as a Fabry–Pérot etalon with a free spectral range () of 119 cm, high finesse , and was tuneable over ∼m (at ∼5 THz band) with tilt. The structure of the THz beam focal region as extracted by the first mirror (slit) showed a complex dependence of polarisation, wavelength and position across the beam. This is important for polarisation-sensitive measurements (in both transmission and reflection) and requires normalisation at each orientation of linear polarisation.

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Tunable THz band-pass filter by a ternary photonic crystal

A 1D ternary photonic crystal (PC) with a Silicon (Si)/Magnetized Cold Plasma (MCP)/Superconductor (SC) ternary is considered as a terahertz tunable bandpass filter. The results show that the Photonic Bandgap (PBG) frequency location can be tuned by variations of the magnetic field, the electron density and the plasma layer thickness as well. However, variation in SC temperature and thickness leads to the PBG width change. Therefore, the proposed PC structure could be useful for the independent control of PBG location and PBG width by the plasma and SC parameters. The proposed structures could provide an alternative for the design of narrow band-pass filters at THz.

Tailoring the band-pass properties of FSS-based THz filters

High-performance THz band-pass filters are highly demanded for THz spectroscopy and imaging applications. In this work, we have analyzed the spectral features of a THz band-pass filter and divided the spectrum into three spectral regions, including a pure waveguide region, a subwavelength region and a cut-off region. Based on the understanding of the spectrum, we have proposed and fabricated two kinds of THz band-pass filters, which were characterized by time-domain THz spectroscopy and show narrow bandwidths and high stopband suppression.

Design and simulation of a new narrow terahertz bandpass filter

In this paper, a new THz bandpass filter, referred to as ‘a spider filter,’ is designed, simulated and analyzed. According to the study of different samples of bandpass filters in the range of 0.1–3 terahertz, the proposed filter has proved to have a very good transmission coefficient and much lower unwanted frequencies than the other bandpass filters ever made.

Resonant Coupling Effects in a Double-Layer THz Bandpass Filter

Cascading similar frequency selective surfaces (FSSs) improves the roll-off rate and frequency selectivity of THz bandpass filters. However, resonant coupling between FSS layers causes anomalous transmission peaks. In this work, we have employed an equivalent circuit model to analyze a double-layer FSS structure with periodic cross apertures at different interlayer spacing. We found that the anomalous peaks can be attributed to the resonant coupling between the upper and lower layer FSSs, and their periodic appearance with changing interlayer spacing is related to the half-wavelength repeatability of the circuit. We have fabricated a double-layer FSS sample using femtosecond laser micromachining and measured it using time-domain THz spectroscopy. The results using the equivalent circuit model agree with the Finite-Difference Time-Domain (FDTD) and experimental results.

Analysis of a Novel Singlet and Its Application in THz Bandpass Filter Design

In this paper, a novel singlet based on TE301 mode resonant cavity is first introduced and analyzed. By using the mode cancelling between the dominant resonance and the spurious resonance, a transmission zero (TZ) is then produced. Furthermore, the position of TZ can be controlled properly by changing the dimensions of the resonant cavity. Based on the novel singlet, a fifth-order filter centered at 140 GHz with two TZs at 131 and 151 GHz has been presented and fabricated by conventional computer numerical control-metal-milling technology. The measured results are in excellent agreement with the simulated ones with a low insertion loss of 0.52 dB and a 3-dB bandwidth of 13 GHz. A high selectivity with a 40 dB outband rejection at 132 and 149 GHz is achieved.

Free-standing double-layer terahertz band-pass filters fabricated by femtosecond laser micro-machining.

We report on the fabrication and transmission properties of free-standing single-layer and double-layer THz bandpass filters. These filters are fabricated on aluminum foils using femtosecond laser micro-machining. The aluminum foils are periodically patterned with cross apertures with a total area of 1.75×1.75 cm2, also known as frequency-selective surfaces. Their terahertz transmission properties were simulated using the FDTD method and measured using a time-domain terahertz spectroscopy system. The simulation results agree with the measurements results very well. The performance of single-layer bandpass filters is as good as the commercial equivalents on the market. The double-layer filters show extraordinary transmission peaks with changing spacing between the two layers. We show the contour map of the electric field distribution across the apertures, and ascribe the new transmission peaks to the interference and coupling of surface plasmon polaritons between the two layers.

Generation and Characterization of Strong Terahertz Fields From kHz Laser Filamentation

We report the generation of intense broadband THz fields (>16 MV/cm) at a 1-kHz repetition rate via two-color femtosecond laser filamentation in air. We find that the output THz energy can be enhanced by optimizing the plasma filament length via weak focusing. More importantly, the radiated THz beam can be refocused to near its diffraction limit even in long-length filamentation. We also examine the divergence angle of THz radiation emitted from short- and long-length filamentation. A real-time lock-in imaging technique is also applied to characterize frequency-dependent THz radiation profiles by using a microbolometer along with THz bandpass filters.

Voltage-tunable terahertz metamaterial based on liquid crystal material for bandpass filters and phase shifters

ABSTRACTIn this paper, we proposed a voltage-tunable terahertz metamatrial, which is composed of the circular-ring deep-slot array fabricated by UV-LIGA process to form a controlled cavity embedded with the liquid crystal (LC). By changing the biased voltage, the transmission peak of the terahertz metamaterial can be continuously tuned within a frequency range of 38 GHz. Moreover, this metamaterial can work over a wide range of incident and polarization angle for both TE and TM waves due structure symmetry. In addition, a differential phase of 74° is obtained at 0.45 THz by tuning the biased voltage. Therefore, our results open new opportunities for the tunable THz bandpass filters and phase shifters.

DAST crystal based terahertz generation and recording of time resolved photoacoustic spectra of N2O gas at 0.5 and 1.5 THz bands

We report the THz generation using optical rectification process in an indigenously grown organic salt 4-N, N-dimethylamino-4′-N′-methyl-stilbazoliumtoyslate (DAST). A 60 fs, 1 kHz optical parametric amplifier, tunable between 0.8 and 1.6 μm range was employed to generate THz radiation. It is observed that the THz signal is accompanied with weak second harmonic signal of pump wavelength. The conversion efficiency (η) of generated THz is 0.0161%. The radiation generated from DAST crystal is only THz signal, which is confirmed theoretically by choosing the crystal thickness is equivalent to be its optimum coherence length. In addition, we have selected two rotational lines i.e. J = 19, 58 of pure N2O gas by using 0.5 and 1.5 THz band pass filters for recording the time resolved photoacoustic (PA) spectra at 1 atm pressure for the first time.

THz photometers for solar flare observations from space

Fil: Kaufmann, Pierre. Universidade Estadual de Campinas; Brasil. Universidade Presbiteriana Mackenzie; Brasil

太赫兹波导滤波器的分析与设计

Tolerance analysis of a filter with different cavity widths is presented, showing that different widths should be used for filters in different frequency bands in order to degrade manufacturing tolerance. Moreover, the effect of various resonant modes is also analyzed. It is found that in terahertz, the cavity length of TE101 mode is smaller than the width of the cavity, and in this case higher resonant modes should be used to increase the Q factor and degrade the tolerance of manufacturing. A 4th order 0.34 THz band pass filter is illustrated. Its measurement shows that the minimum loss is -0.73 dB and all loss is within -2 dB in the frequency range of 0.335 THz to 0.349 THz.

Cross-Shaped Terahertz Metal Mesh Filters: Historical Review and Results

Terahertz frequencies experiments has motivated the development of new sources, detectors and optical components. Here we will present a review of THz bandpass filters ranging from 0.4 to 10 THz. We also demonstrate our fabrication process, simulations and experimental results.

Ultra-broad and sharp-transition bandpass terahertz filters by hybridizing multiple resonances mode in monolithic metamaterials

We present three monolithic metamaterial-based THz bandpass filters, the skewed circular slot rings, meandered slots and Jerusalem cross slots, to fit in the THz gap. These THz bandpass filters are comprised of a metal-dielectric-metal (MDM) structure that supports multiple resonances of electric dipole, magnetic dipole, and standing-wave-like modes. By exciting and further hybridizing these individual resonance modes, we demonstrate excellent performance of broad bandwidth and sharp band-edge transition beyond conventional bandpass filters. By further employing our ad hoc Genetic Algorithm (GA) and Periodic Method of Moments (PMM) to optimize our designs, we achieve an ultra-broad 3dB fractional bandwidth and sharp band-edge transition up to 82.2% and 58.3 dB/octave, respectively, benefiting the practical applications such as material recognition in security systems, imaging, and absorbers.