Frequency-selective Detection Research Articles

Reconfigurable terahertz light harvesting with MoTe2 hybrid metasurface.

Near-perfect light harvesting of a metasurface-based absorber paves the way for achieving numerous potential applications in sensing, cloaking, and photovoltaics. Here, we present a reconfigurable perfect absorber based on a molybdenum ditelluride (MoTe2) hybrid metasurface at terahertz (THz) frequency. By investigating the optical response of metasurface-based absorbers, a reconfigurable switching of dual-frequency perfect absorption to a new single-frequency absorption takes place when light illuminates MoTe2. Moreover, the absorption mechanism of the hybrid metasurface is well demonstrated with the analytical coupled-dipole model and impedance analysis. The proposed reconfigurable THz meta-absorber provides a new, to the best of our knowledge, route for active radar stealth, frequency-selective detection, and next-generation wireless communication.

P-Site Structural Diversity and Evolution in a Zeosil Catalyst.

Phosphorus-modified siliceous zeolites, or P-zeosils, catalyze the selective dehydration of biomass derivatives to platform chemicals such as p-xylene and 1,3-butadiene. Water generated during these reactions is a critical factor in catalytic activity, but the effects of hydrolysis on the structure, acidity, and distribution of the active sites are largely unknown. In this study, the P-sites in an all-silica self-pillared pentasil (P-SPP) with a low P-loading (Si/P = 27) were identified by solid-state 31P NMR using frequency-selective detection. This technique resolves overlapping signals for P-sites that are covalently bound to the solid phase, as well as oligomers confined in the zeolite but not attached to the zeolite. Dynamic Nuclear Polarization provides the sensitivity necessary to conduct 29Si-filtered 31P detection and 31P-31P correlation experiments. The aforementioned techniques allow us to distinguish sites with P-O-Si linkages from those with P-O-P linkages. The spectra reveal a previously unappreciated diversity of P-sites, including evidence for surface-bound oligomers. In the dry P-zeosil, essentially all P-sites are anchored to the solid phase, including mononuclear sites and dinuclear sites containing the [Si-O-P-O-P-O-Si] motif. The fully-condensed sites evolve rapidly when exposed to humidity, even at room temperature. Partially hydrolyzed species have a wide range of acidities, inferred from their calculated LUMO energies. Initial cleavage of some P-O-Si linkages results in an evolving mixture of surface-bound mono- and oligonuclear P-sites with increased acidity. Subsequent P-O-P cleavage leads to a decrease in acidity as the P-sites are eventually converted to H3PO4. The ability to identify acidic sites in P-zeosils and to describe their structure and stability will play an important role in controlling the activity of microporous catalysts by regulating their water content.

Polarization insensitive, metamaterial absorber-enhanced long-wave infrared detector.

Detecting low energy photons, such as photons in the long-wave infrared range, is a technically challenging proposition using naturally occurring materials. In order to address this challenge, we herein demonstrate a micro-bolometer featuring an integrated metamaterial absorber (MA), which takes advantage of the resonant absorption and frequency selective properties of the MA. Importantly, our micro-bolometer exhibits polarization insensitivity and high absorption due to a novel metal-insulator-metal (MIM) absorber design, operating at 8-12 µm wavelength. The metamaterial structures we report herein feature an interconnected design, optimized towards their application to micro-bolometer-based, long-wave infrared detection. The micro-bolometers were fabricated using a combination of conventional photolithography and electron beam lithography (EBL), the latter owing to the small feature sizes within the design. The absorption response was designed using the coupled mode theory (CMT) and the finite integration technique, with the fabricated devices characterized using Fourier-transform infrared spectroscopy (FTIR). The metamaterial-based micro-bolometer exhibits a responsivity of approximately 198 V/W over the 8-12 µm wavelength regime, detectivity of ∼ 0.6 × 109 Jones, thermal response time of ∼ 3.3 ms, and a noise equivalent temperature difference (NETD) of ∼33 mK under 1mA biasing current at room-temperature and atmosphere pressure. The ultimate detectivity and NETD are limited by Johnson noise and heat loss with thermal convection through air; however, further optimization could be achieved by reducing the thermal conductivity via vacuum packaging. Under vacuum conditions, the detectivity may be increased in excess of two-fold, to ∼ 1.5 × 109 Jones. Finally, an infrared image of a soldering iron was generated using a single-pixel imaging process, serving as proof-of-concept of this detection platform. The results presented in this work pave the road towards high-efficiency and frequency-selective detection in the long-wave infrared range through the integration of infrared MAs with micro-bolometers.

Multi-spectral frequency selective mid-infrared microbolometers.

Frequency selective detection of low energy photons is a scientific challenge using natural materials. A hypothetical surface which functions like a light funnel with very low thermal mass in order to enhance photon collection and suppress background thermal noise is the ideal solution to address both low temperature and frequency selective detection limitations of present detection systems. Here, we present a cavity-coupled quasi-three dimensional plasmonic crystal which induces impedance matching to the free space giving rise to extraordinary transmission through the sub-wavelength aperture array like a "light funnel" in coupling low energy incident photons resulting in frequency selective perfect (~100%) absorption of the incident radiation and zero back reflection. The peak wavelength of absorption of the incident light is almost independent of the angle of incidence and remains within 20% of its maximum (100%) up to θi≤45˚. This perfect absorption results from the incident light-driven localized edge "micro-plasma" currents on the lossy metallic surfaces. The wide-angle light funneling is validated with experimental measurements. Further, a super-lattice based electronic biasing circuit converts the absorbed narrow linewidth (Δλ/λ0< 0.075) photon energy inside the sub-wavelength thick film (< λ/100) to voltage output with high signal to noise ratio close to the theoretical limit. Such artificial plasmonic surfaces enable flexible scaling of light funneling response to any wavelength range by simple dimensional changes paving the path towards room temperature frequency selective low energy photon detection.

Mutual 3:1 subharmonic synchronization in a micromachined silicon disk resonator

We demonstrate synchronization between two intrinsically coupled oscillators that are created from two distinct vibration modes of a single micromachined disk resonator. The modes have a 3:1 subharmonic frequency relationship and cubic, non-dissipative electromechanical coupling between the modes enables their two frequencies to synchronize. Our experimental implementation allows the frequency of the lower frequency oscillator to be independently controlled from that of the higher frequency oscillator, enabling study of the synchronization dynamics. We find close quantitative agreement between the experimental behavior and an analytical coupled-oscillator model as a function of the energy in the two oscillators. We demonstrate that the synchronization range increases when the lower frequency oscillator is strongly driven and when the higher frequency oscillator is weakly driven. This result suggests that synchronization can be applied to the frequency-selective detection of weak signals and other mechanical signal processing functions.

Mechanically tunable metamaterials terahertz dual-band bandstop filter**Project supported by the National Natural Science Foundation of China (Grant Nos. 61265005 and 11574059), the Natural Science Foundation of Guangxi, China (Grant Nos. 2015GXNSFDA19039 and 2014GXNSFAA118376), the Foundation from Guangxi Key Laboratory of Automatic Detection Technology and Instrument, China (Grant Nos. YQ14114 and YQ15106) and the Innovation Project of Guangxi Graduate Education, China

We experimentally demonstrate a mechanically tunable metamaterials terahertz (THz) dual-band bandstop filter. The unit cell of the filter contains an inner aluminum circle and an outside aluminum Ohm-ring on high resistance silicon substrate. The performance of the filter is simulated by finite-integration-time-domain (FITD) method. The sample is fabricated using a surface micromachining process and experimentally demonstrated using a THz time-domain-spectroscopy (TDS) system. The results show that, when the incident THz wave is polarized in y-axis, the filter has two intensive absorption peaks locating at 0.71 THz and 1.13 THz, respectively. The position of the high-frequency absorption peak and the amplitude of the low-frequency absorption peak can be simultaneously tuned by rotating the sample along its normal axis. The tunability of the high-frequency absorption peak is due to the shift of resonance frequency of two electrical dipoles, and that of the low-frequency absorption peak results from the effect of rotationally induced transparent. This tunable filter is very useful for switch, manipulation, and frequency selective detection of THz beam.

Design of tunable terahertz bandstop filter based on electrostatically actuated reconfigurable metamaterials

A tunable terahertz (THz) bandstop filter based on electrostatically actuated microelectromechanical systems (MEMS) reconfigurable metamaterials is presented. The central part of the filter is a periodic array structure, and each unit cell consists of a movable bar and a fixed inverted trapezoidal ring. When direct current (DC) voltage is applied to actuate the electrostatic comb actuators symmetrically positioned on both sides, the movable frame and all movable bars can move in-plane. This will reconfigure the unit cell of the filter, and thus change its resonance frequency. Finite integration time domain (FITD) method is used to study the working mechanism and the influence of structure parameters. The results show that the performance of the filter strongly depends on the distance between the trapezoidal ring and the cross bar, the width of inverted trapezoidal ring and the basic angle of trapezoidal ring. A finite element analysis (FEA) method is introduced to study the electromechanical performance of the actuator, and a displacement of 3µm is achieved at DC 30V. The corresponding modulation of central frequency is about 24%. The potential applications of this tunable THz bandstop filter are THz communication, THz beam control and frequency selective detection.

Homogeneous Biosensing Based on Magnetic Particle Labels.

The growing availability of biomarker panels for molecular diagnostics is leading to an increasing need for fast and sensitive biosensing technologies that are applicable to point-of-care testing. In that regard, homogeneous measurement principles are especially relevant as they usually do not require extensive sample preparation procedures, thus reducing the total analysis time and maximizing ease-of-use. In this review, we focus on homogeneous biosensors for the in vitro detection of biomarkers. Within this broad range of biosensors, we concentrate on methods that apply magnetic particle labels. The advantage of such methods lies in the added possibility to manipulate the particle labels by applied magnetic fields, which can be exploited, for example, to decrease incubation times or to enhance the signal-to-noise-ratio of the measurement signal by applying frequency-selective detection. In our review, we discriminate the corresponding methods based on the nature of the acquired measurement signal, which can either be based on magnetic or optical detection. The underlying measurement principles of the different techniques are discussed, and biosensing examples for all techniques are reported, thereby demonstrating the broad applicability of homogeneous in vitro biosensing based on magnetic particle label actuation.

YBa2Cu3O7-x bicrystal Josephson junctions with high IcRn-products and wide-ranging resistances for THz applications

Josephson junctions with high characteristic voltages $I_{c}R_{n}$ and resistances $R_{n}$ ranging from subohm values to several hundred ohms are required for various terahertz (THz) applications. Due to optimization of technology, [100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions with an average $I_{c}R_{n}$ (5 K) value of 7 mV at an $R_{n}$ -range from 0.4 to 400 $\Omega$ have been fabricated on (320) bicrystal NdGaO3 substrates. Classical and frequency-selective detection of THz radiation have been demonstrated with the fabricated Josephson junctions. The main characteristics of Josephson oscillators, spectrum analyzers, admittance spectrometers, and classical detectors have been evaluated within this range of junction parameters, and promising results have been obtained.

Nuts and bolts of chemical exchange saturation transfer MRI.

Chemical exchange saturation transfer (CEST) has emerged as a novel MRI contrast mechanism that is well suited for molecular imaging studies. This new mechanism can be used to detect small amounts of contrast agent through the saturation of rapidly exchanging protons on these agents, allowing a wide range of applications. CEST technology has a number of indispensable features, such as the possibility of simultaneous detection of multiple 'colors' of agents and of changes in their environment (e.g. pH, metabolites, etc.) through MR contrast. Currently, a large number of new imaging schemes and techniques are being developed to improve the temporal resolution and specificity and to correct for the influence of B0 and B1 inhomogeneities. In this review, the techniques developed over the last decade are summarized with the different imaging strategies and post-processing methods discussed from a practical point of view, including the description of their relative merits for the detection of CEST agents. The goal of the present work is to provide the reader with a fundamental understanding of the techniques developed, and to provide guidance to help refine future applications of this technology. This review is organized into three main sections ('Basics of CEST contrast', 'Implementation' and 'Post-processing'), and also includes a brief Introduction and Summary. The 'Basics of CEST contrast' section contains a description of the relevant background theory for saturation transfer and frequency-labeled transfer, and a brief discussion of methods to determine exchange rates. The 'Implementation' section contains a description of the practical considerations in conducting CEST MRI studies, including the choice of magnetic field, pulse sequence, saturation pulse, imaging scheme, and strategies to separate magnetization transfer and CEST. The 'Post-processing' section contains a description of the typical image processing employed for B0 /B1 correction, Z-spectral interpolation, frequency-selective detection and improvement of CEST contrast maps.

Thermal infrared detection using dipole antenna-coupled metal-oxide-metal diodes

This work focuses on dipole antenna-coupled metal-oxide-metal diodes, which can be used for the detection of long wave infrared radiation. These detectors are defined using electron beam lithography and fabricated with shadow evaporation metal deposition. Along with offering complementary metal oxide semiconductor compatible fabrication, these detectors promise high speed and frequency selective detection without biasing, a small pixel footprint, and full functionality at room temperature without cooling. Direct current current-voltage characteristics are presented along with detector response to 10.6μm radiation. The detection characteristics can be tailored to provide for multispectral imaging in specific applications by modifying device geometries.

Classical and Josephson detection of terahertz radiation using YBa2Cu3O7-x [100]-tilt bicrystal junctions

Detection of terahertz radiation using YBa2Cu3O7-x [100]-tilt bicrystal junctions with high IcRn- product up to 6 mV has been studied. Two types of the responses ΔV (V) have been observed. At low frequencies up to some frequency fl, the responses ΔV (V) were proportional to the second derivative of the V(I)-curve. At high frequencies f > fl, the responses ΔV (V) demonstrate odd-symmetric resonances at V = hf/2e. For low-resistance junctions, Josephson frequency-selective detection was found up to the frequency of 5.3 THz, which is above the frequency of the strongest optical mode (f0 = 4.6 THz) in YBa2Cu3O7-x. The frequency fl was found to increase with the resistance Rn and the IcRn-product of the junction. For high-resistance junctions with Rn = 23 Ω, classical detection was observed up to 0.4 THz range. The difference in detection mechanisms is attributed to a continuous type of spectra of Josephson oscillations at low voltages and to a discrete type of Josephson spectra at high voltages. Computer simulation of a Josephson detector has been carried out, considering intensive thermal fluctuations and a frequency-dependent impedance of the Josephson junction. It is shown that a classical detector, based on the [100]-tilt Josephson junctions with Rn = 100 Ohm at T = 40 K, might reach the values of noise equivalent power NEP < 2· 10−14 W/Hz½ at the frequencies up to 1.8 THz.

Frequency-selective detection of nuclear quadrupole resonance signals

Nuclear quadrupole resonance (NQR) offers an unequivocal method of detecting and identifying both hidden explosives, such as land mines, and a variety of narcotics. Unfortunately, the practical use of NQR is restricted by a low signal-to-noise ratio (SNR), and means to improve the SNR are vital to enable a rapid, reliable, and convenient system. In this paper, we introduce a frequency-selective approximate maximum-likelihood (FSAML) detector, operating on a subset of the available frequencies, making it robust to the typically present narrow-band interference. The method exploits the inherent temperature dependency of the NQR frequencies as a way to enhance the SNR. Numerical evaluations, using both simulated and real NQR data, indicate a significant gain in probability of accurate detection as compared to a current state-of-the-art approach.

Bandwidth and Noise of Submillimeter Wave Cuprate Bicrystal Josephson Junction Detectors

Detectors made from superconducting cuprate YBa/sub 2/Cu/sub 3/O/sub x/ bicrystal Josephson junctions (BJJs) on sapphire and NdGaO/sub 3/ substrates have been fabricated and characterized in the frequency band 200-900 GHz. Junctions on sapphire substrates had a normal state resistance R/sub N//spl ap/15-60 /spl Omega/, and I/sub C/R/sub N/ product up to 2.5 mV at T=4.2 K. Junctions on NdGaO/sub 3/ substrates had lower R/sub N/=1-5 /spl Omega/ and I/sub C/R/sub N/=0.4-0.9 mV at T=77 K. Three types of detecting devices have been investigated in both the broadband and the frequency-selective detection modes. One type was patterned with log-periodic antenna, and two others with Pt-metal double-slot antenna designed for a central frequency f=300 GHz and f=400 GHz, respectively. Measurements at f=320 GHz of the reception bandwidth /spl Delta/f for a device with double-slot antenna gave a quality factor Q=f//spl Delta/f/spl ap/10. A low-noise cooled 1-2 GHz bandwidth amplifier enables a better sensitivity in the self-pumping frequency mixing mode, avoiding the 1/f noise. The dependence of the spectral density of noise on voltage was compared to the data for the Josephson emission linewidth obtained by the selective detector response method. Also discussed are measurements at 500 GHz of the NEP values carried out at different experimental conditions.

Hilbert spectroscopy from gigahertz to terahertz frequencies by high- Tc Josephson junctions

Hilbert spectroscopy is based on a frequency-selective detection of electromagnetic radiation by a Josephson junction described by the resistively shunted junction (RSJ) model. The YBa 2Cu 3O 7− x grain-boundary junctions fabricated on NdGaO 3 bicrystals were found to be close to the RSJ model. General-purpose Hilbert spectrometers were developed and characterized using voltage-biased low-resistance junctions. The spectral bandwidth of Hilbert spectroscopy was shown to be of several frequency decades for any junction temperature between 48 and 85 K. A total bandwidth from 6 GHz to 2.5 THz has been covered using one junction at different temperatures. The spectral resolving power f/δ f of Hilbert spectroscopy was found to be of three orders in the terahertz range. The dynamic range of intensities of electromagnetic radiation in Hilbert spectroscopy was shown to be of five orders.

Dynamics of monochromatically generated nonequilibrium phonons in LaF3:Pr3+

The temporal evolution of nonequilibrium phonon populations in ${\mathrm{LaF}}_{3}$:${\mathrm{Pr}}^{3+}$ is investigated at low temperatures (1.8 K) utilizing pulsed, tunable, monochromatic generation and time-resolved, tunable, narrow-band detection. High occupation number, narrow-band phonon populations are generated via far-infrared pumping of defect-induced one-phonon absorption. Time-resolved, frequency-selective detection is provided by optical sideband absorption. Nonequilibrium phonon decay times are measured and attributed to anharmonic decay.

Atomic magneto-optical rotation spectroscopy (AMORS) with magnetic field modulation and an offset angle

Magnetic field modulation is used in conjunction with an offset angle on the polarizers to improve the analytical capabilities of atomic magneto-optical rotation spectroscopy (AMORS). The offset angle method linearizes the quadratic response to density of atoms and lowers the detection limit. However, d.c. photocurrents induced by leakage of source light due to the offset angle can produce varying baselines because of background absorption or light source fluctuations. The application of a modulated magnetic field to the atomic vapor and frequency selective detection enables the elimination of the d.c. offset signal.

A Modulated Sample-Introduction System for ICP-Atomic Emission Spectrometry

A modulated sample-introduction device for use in inductively coupled plasma emission spectrometry has been constructed and evaluated. With this device, the flow of aerosol to the excitation source is modulated by applying at a specific frequency a pressure pulse to the nebulizer chamber; the use of frequency-selective detection then permits efficient signal recovery. Unfortunately, background emission from the ICP was also found to be modulated at the sample-introduction frequency; the magnitude of background modulation depends on the depth of sample modulation and the types of spectral features present at the analytical line of interest. The sample-modulation device discriminated best between analyte emission and continuum background radiation, but performed most poorly when OH band emission was the dominant background interference. The construction and operation of the device are described and its performance for several elements is evaluated in terms of detection limits, S/B enhancement, and working-curve linearity.

A Modulated Sample Introduction System for Atomic Emission Spectrometry

A modulated sample-introduction device for use in flame emission spectrometry has been constructed and evaluated. With this device, the flow of aerosol to the excitation source is modulated by the application, at a specific frequency, of a pressure pulse to the nebulizer chamber; the use of frequency-selective detection then permits efficient signal recovery. If source background remains at a constant level, it is not detected and the signal-to-background ratio (S/B) is consequently improved. Because the technique provides a decrease in the instrument 1/f and flame-background flicker noise, detection limits are improved. These features are especially prominent when the analyte line of interest is in a spectral region of high source background. Modulation frequencies of 8 to 20 Hz have been explored and have resulted in good depth of modulation (&gt;99%) and signal-pulse separation. The construction and operation of the device will be described and its performance for several elements evaluated in terms of detection limits, S/B enhancement, and working curve linearity. Also, the extension of the technique to selective spectralline modulation schemes and to inductively coupled plasma emission spectrometry will be considered.

Children’s frequency-selective detection of signals in noise1

Five children, median age 7 years, and six adults, median age 20 years, served as Os in a task requiring the detection of auditory signals presented against a continuous background of noise. Two-alternative, temporal-forced-choice trials of 5-sec duration were presented periodically in blocks of 50 during two or three 1-h sessions. During a block, some 70% of the signals were of 1,000 Hz and approximately 30% were probe signals of a frequency other than 1,000 Hz. After only rudimentary description of the task, the children and adults performed adequately. The results from the children and adults were similar, showing differential detection as a function of signal frequency. The demonstrated frequency selection is consistent with a sensory-filter model of the O’s auditory behavior.