Tunable Detector Research Articles

Narrow-band tunable terahertz detector in antiferromagnets via staggered-field and antidamping torques

We study dynamics of antiferromagnets induced by simultaneous application of dc spin current and ac charge current, motivated by the requirement of all-electrically controlled devices in THz gap (0.1-30 THz). We show that ac electric current, via N\'eel spin orbit torques, can lock the phase of a steady rotating N\'eel vector whose precession is controlled by a dc spin current. In the phase-locking regime the frequency of the incoming ac signal coincides with the frequency of autooscillations which for typical antiferromagnets fall into the THz range. The frequency of autooscillations is proportional to the precession-induced tilting of the magnetic sublattices related to the so-called dynamical magnetization. We show how the incoming ac signal can be detected from the measurement of the dc-current dependencies of the constant dynamical magnetization. We formulate the conditions of phase-locking based on relations between parameters of an antiferromagnet and the characteristics of the incoming signal (frequency, amplitude, bandwidth, duration). We also show that the rotating N\'eel vector can generate ac electrical current via inverse N\'eel spin-orbit torque. Hence, antiferromagnets driven by dc spin current can be used as tunable detectors and emitters of narrow-band signals operating in the THz range.

Broadband tunable absorber for terahertz waves based on isotropic silicon metasurfaces

A broadband tunable absorber is studied based on the silicon metasurface at terahertz frequencies. By tuning the conductivity of silicon, absorptance is more than 90% from 0.497 THz to 1.045 THz with central frequency of 0.771 THz when the conductivity is equal to 2500S/m. Simulated results show that absorptance peak can be modulated from 1% to 100% when the conductivity continuously changes from 0S/m to2500S/m. The central-symmetry structure leads to polarization independence. Our design may be used as optoelectronic modulator, tunable detector, and terahertz switch.

Distributed Target Detection in Partially Homogeneous Environment When Signal Mismatch Occurs

This paper investigates the problem of detecting a distributed target in the presence of signal mismatch in a partially homogeneous environment. To design a selective detector, we modify the original hypothesis test by injecting a fictitious interference under the null hypothesis test. The fictitious interference is assumed to be orthogonal to the nominal signal in the whitened subspace. Then, according to the generalized likelihood ratio criterion, we obtain a detector that has the constant false alarm rate property and is more selective than existing detectors. However, the proposed detector works only in the case of large number of training data. To overcome this limitation, we introduce a tunable detector, which is parameterized by a tunable parameter. It can work under a very loose constraint on a number of training data. More importantly, the directivity property (robustness and selectivity) of the tunable detector can be flexibly adjusted through the tunable parameter. In addition, the tunable detector can achieve roughly the same detection performance as the corresponding generalized likelihood ratio test when no signal mismatch occurs. Numerical examples are given to demonstrate the effectiveness of the proposed detectors.

Plasmonic Absorption Enhancement in Elliptical Graphene Arrays.

In this paper, we come up with a wavelength tunable absorber which is made up of periodically elliptical graphene arrays in the far-infrared and terahertz regions. Through simulation, we find that we can increase the length of long axis of the ellipse, raise the incidence angles of TM- and TE-polarization (TM- and TE-polarization indicate the direction of the incident electric field along the direction of the x and the y axis, respectively.) within certain limits, and increase the chemical potential of graphene, so as to enhance the absorption of light in the elliptical graphene arrays. We also compare the absorption spectra of the original structure and the complementary structure, and find that the absorption of the original structure is higher than that of the complementary structure. In the end, we study the changes in the absorption rate of the double layer structure of the elliptical array with the increase in the thickness of SiO2. The elliptical array structure can be applied to tunable spectral detectors, filters and sensors at far-infrared and terahertz wavelengths.

Tunable Adaptive Detection in Colocated MIMO Radar

We consider the adaptive target detection problem in colocated MIMO radar where disturbance covariance matrix is unknown, and no training data are available. A tunable detector is proposed, which includes the Rao and Wald tests as special cases. The selectivity or robustness of the proposed detector to mismatched signals can be flexibly tuned by adjusting the tuning parameter. An equivalent form of the proposed detector is obtained to provide a deeper insight into how the selectivity or robustness is adjusted by selecting the tuning parameter. We derive analytical expressions for the probability of false alarm and the detection probability to evaluate its detection performance in both matched and mismatched steering vector cases. It is shown that the proposed detector bears a constant false alarm rate property with respect to the disturbance covariance matrix. Numerical results reveal that the selectivity of the proposed detector can be enhanced by increasing the value of the tuning parameter.

Near-infrared absorbers based on the heterostructures of two-dimensional materials.

Although the conductance and dielectric function of graphene can be tuned by applying external voltage, the tunability is less than 3%. Hybridizing graphene with other two-dimensional transition metal dichalcogenides (TMDs) can improve the adjustability and tunability of the optical properties of graphene-based structures at near-infrared frequencies. In this paper, we theoretically compute the dielectric function of graphene-MoTe2-graphene and graphene-MoTe2-graphene heterostructures utilizing the quantum electrostatic heterostructure (QEH) model, which is an ab-initio method. Utilizing the QEH results, we propose a hyper crystal (HC) absorber at near-infrared frequencies. Hence, we use the transfer matrix method to investigate our proposed absorber analytically. Moreover, we simulate the graphene-TMD-graphene (G-TMD-G) absorbers by the numerical finite difference time domain method. The results of the numerical solution are consistent with those of the analytical method. Due to the dependency of the Fermi level of graphene on the direct bandgap of the TMDs, the dielectric function of the G-TMD-G heterostructure can be tuned and enhanced further by changing the number of TMD layers. Finally, we demonstrate that the full absorption of the heterostructures can be achieved at different frequencies for transverse magnetic polarization. Since the thicknesses of the layers in the HC are lower than the wavelength of the light, no diffracted bands are ubiquitous, and the absorption can be observed for a wide range of incidence angles and bandwidths at near-infrared frequencies. Because of utilizing graphene-based HCs, in addition to the feasibility of design compared to the complex metasurfaces, the absorption bandwidth is significant for a wide range of incidence angles. This kind of HC absorber can be used in the design of sensitive optical devices, such as tunable filters, detectors, and photovoltaic applications.

Chiral Molecule-Enhanced Extinction Ratios of Quantum Dots Coupled to Random Plasmonic Structures.

Devices based on self-assembled hybrid colloidal quantum dots (CQDs) coupled with specific organic linker molecules are a promising way to simply realize room-temperature, spectrally tunable light detectors. Nevertheless, this type of devices usually has low quantum efficiency. Plasmonics has been shown as an efficient tool in guiding and confining light at nanoscale dimensions. As plasmonic modes exhibit highly confined fields, they locally increase light-matter interactions and consequently enhance the performance of CQD-based photodetectors. Recent publications presented experimental results of large extinction enhancement from a monolayer of CQDs coupled to random gold nanoislands using a monolayer of organic alkyl linkers. We report here that a twofold larger extinction enhancement in the visible spectrum is observed when a monolayer of helical chiral molecules connects the CQDs to the gold structure instead of a monolayer of achiral linkers. We also show that this effect provides insight into the chirality of the molecules within the monolayer. In future work, we plan to evaluate the potential of these results to be used in the construction of a more efficient and sensitive photon detector based on surface QDs, as well as to supply a simple way to map the chirality of a single chiral monolayer.

INVESTIGATING THE IMPLEMENTATION OF ZnO NANOPARTICLES AS A TUNABLE UV DETECTOR FOR DIFFERENT SKIN TYPES

A facile chemical reduction method was used to synthesize ZnO nanoparticles (NPs) in ethylene glycol solvent at two different calcination temperatures. As a result of variation in the calcination temperature, ZnO NPs with two different sizes were achieved. The NPs were investigated for their structural and optical characteristics using X-ray diffraction and ultraviolet (UV)–Vis spectroscopy. The synthesized ZnO NPs exhibited a hexagonal structure with sizes of 46 and 65[Formula: see text]nm. The synthesized NPs were then used to investigate dye photocatalytic behavior of products as a tunable UV detector for different skin types. The dye degradation and decolorization of methylene blue in the presence of ZnO NP, following UV radiation as a function of time, were studied at different pH levels. The optical absorption spectra were then taken every 15[Formula: see text]min for all samples. The UV–Vis spectroscopy spectra revealed that optical absorption of solution was decreased upon UV exposure as a function of time. Photocatalytic reaction indicated that the dye degradation and decolorization rate were accelerated with the increase of pH level. Therefore, a tunable UV detector for different skin types could be engineered by varying the pH level of solution to avoid human skin burning.

A review on tungsten bronze ferroelectric ceramics as electrically tunable devices

ABSTRACTFerroelectric ceramics are the most promising material for electrically tunable devices and found its application in microwave devices such as phase shifters, varactors and tunable oscillators and detectors. It has been reported that various non-ferroelectric compound such as carbon nanotubes (CNT), ferrites and microwave dielectrics are used for electrically tunable dielectric materials. This review paper gives overall summary of tungsten bronze ferroelectric ceramics on developing electrically tunable ferroelectric material and improving performance of dielectric material for various applications in microwave devices.

Tunable polarization-independent coherent perfect absorber based on a metal-graphene nanostructure

A tunable polarization-independent coherent perfect absorber (CPA) is proposed based on the metal-graphene hollowed-out cross array nanostructure at the mid-infrared region. By adjusting the phase difference between the incident beams, the coherent absorption at 9969nm is all-optically modulated from 99.97% to nearly 0 with the modulation depth over 80dB. The absorption resonance of the CPA is tuned by the gate-controlled Fermi energy with the coherent absorption remaining nearly 100%. The CPA is also found to be angular insensitive to incident beams within the range of −12° to 12°. The central-symmetry nanostructure leads to the polarization independence and the introduced graphene layer results in the electrical tunability of the CPA at the absorption resonance, which have prospects in applications such as for tunable mid-infrared detectors, transducers, modulators, and optical switches.

Quantum Hall Dual-Band Infrared Photodetector

An essential task in infrared (IR) technology is to develop and exploit advanced photodetectors, to enable both fundamental research and applications. The authors utilize quantum Hall states and cyclotron-resonance absorption in a hybrid graphene-GaAs/Al${}_{x}$Ga${}_{1-x}$As system to implement a dual-band, tunable detector covering both mid- and far-IR wavelengths, with a log-periodic antenna to minimize chip size and significantly enhance the photoresponse. These results point toward a panoply of solutions for highly sensitive, tunable, multicolor IR detectors and imaging arrays.

Magnetically Tunable Photocurrent in La0.7Sr0.3MnO3/BaSnO3 Heterojunctions

Artificially constructed oxide heterointerfaces have attracted much attention. Herein, the novel all‐perovskite p‐n heterojunction composed of a colossal magnetoresistive manganite La0.7Sr0.3 MnO3 (LSMO) and an n‐type transparent semiconducting BaSnO3 (BSO) is designed via optimizing the growth condition. This LSMO/BSO p‐n junction exhibits good rectification with a forward‐to‐reverse ratio of 275 at 1 V, high photo detection capability with a photo‐to‐dark current of 581.9 at ‐0.5 V, high ultraviolet light sensitivity with a UV (360 nm)‐to‐visible (532 nm) ratio of ∼2.4 × 103, and a significantly magneto‐tunable photocurrent with a variation ratio of ∼1.25 % under 532 nm illumination and 0.5 T magnetic field. As a result, combining synergistically the functionality of diode and magnetically tunable photo detector, the LSMO/BSO p‐n junction is a promising candidate for advanced magneto‐optoelectronic devices.

Complementary graphene metamaterial with independently tunable dual absorption bands at terahertz frequency

We design an independently tunable dual-band terahertz absorber consisting of graphene ribbons patterned with a combination of two complementary crosses of different sizes within a unit-cell. The Fermi energy of the interdigitated graphene ribbons can be tuned independently via applied gate voltages. Our simulations indicate that the absorption at two resonant bands can be ~75% under normal incidence. In addition, our design allows independent tuning of the resonant frequencies by changing the Fermi energy of graphene via controlling the applied bias voltages. Our results may find application in developing frequency tunable terahertz modulators, sensors, and detectors.

Strong Amplified Spontaneous Emission from High Quality GaAs1–xSbx Single Quantum Well Nanowires

Quantum confinement in semiconductor nanowires is of contemporary interest. Enhancing the quantum efficiency of quantum wells in nanowires and minimizing intrinsic absorption are necessary for reducing the threshold of nanowire lasers and are promising for wavelength tunable emitters and detectors. Here, we report on growth and optimization of GaAs1–xSbx/Al1–yGayAs quantum well heterostructures formed radially around pure zinc blende GaAs core nanowires. The emitted photon energy from GaAs0.89Sb0.11 quantum well (1.371 eV) is smaller than the GaAs core, thus showing advantages over GaAs/Al1–yGayAs quantum well nanowires in photon emission. The high optical quality quantum well (internal quantum efficiency reaches as high as 90%) is carefully positioned so that the quantum well coincides with the maximum of the transverse electric (TE01) mode intensity profile. The obtained superior optical performance combined with the supported Fabry–Perot (F–P) cavity in the nanowire leads to the strong amplified sponta...

Luminescence properties of AgTb(WO4)2 doped with Ce3+: Experimental determination of the Stern-Volmer quenching constant

Scheelite-like AgTb(WO4)2 doped Ce3+ were synthesized by ceramic method. Efficient characteristic bands of Tb3+ were observed in AgTb(WO4)2 compounds. However, the presence of Ce3+ quenched the emission band of Tb3+ which can be caused by metal-to-metal charge transfer between Tb3+/Ce3+. The quenching is more efficient at high temperature which can be attributed to more overlaps of wave functions of Ce3+-W6+-Tb3+ at high temperature. The experimental Stern-Volmer quenching rate constant at room temperature and liquid nitrogen temperature were measured as (376±4)·10−3μs−1 (x content/mol)−1 and (45±4)·10−3μs−1 (x content/mol)−1 respectively. In excitation spectra, evident blue shift of charge transfer band in the case of liquid nitrogen temperature was observed. In addition, the blue shift of CT band also occurred with increasing concentration of Ce3+. The shifting of charge transfer band can be interesting to the design of solar cells and tunable excitation sources and detectors.

SERS Active Nanobiosensor Functionalized by Self-Assembled 3D Nickel Nanonetworks for Glutathione Detection.

We introduce a "non-noble metal" based SERS active nanobiosensor using a self-assembled 3D hybrid nickel nanonetwork. A tunable biomolecule detector fabricated by a bottom-up approach was functionalized using a multiphoton ionization energy mechanism to create a self-assembled 3D hybrid nickel nanonetwork. The nanonetwork was tested for SERS detection of crystal violet (CV) and glutathione (GSH) at two excitation wavelengths, 532 and 785 nm. The results reveal indiscernible peaks with a limit of detection (LOD) of 1 picomolar (pM) concentration. An enhancement factor (EF) of 9.3 × 108 was achieved for the chemical molecule CV and 1.8 × 109 for the biomolecule GSH, which are the highest reported values so far. The two results, one being the CV molecule proved that nickel nanonetwork is indeed SERS active and the second being the GSH biomolecule detection at both 532 and 785 nm, confirm that the nanonetwork is a biosensor which has potential for both in vivo and in vitro sensing. In addition, the selectivity and versatility of this biosensor is examined with biomolecules such as l-Cysteine, l-Methionine, and sensing GSH in cell culture medium which mimics the complex biological environment. The functionalized self-assembled 3D hybrid nickel nanonetwork exhibits electromagnetic and charge transfer based SERS activation mechanisms.

Investigation of tunable terahertz metamaterial perfect absorber with anisotropic dielectric liquid crystal

In this letter, we report the unique design, simulation and experimental verification of an electrically tunable THz metamaterial perfect absorber consisting of complementary split ring resonator (CSRR) arrays integrated with liquid crystal as the subwavelength spacer in between. We observe a shift in resonance frequency of about 5.0 GHz at 0.567 THz with a 5 V bias voltage at 1KHz between the CSRR and the metal backplane, while the absorbance and full width at half maximum bandwidth are maintained at 90% and 0.025 THz, respectively. Simulated absorption spectrum by using a uniaxial model of LC matches perfectly the experiment data and demonstrates that the effective refractive index of LC changes between 1.5 and 1.7 by sweeping a 1 kHz bias voltage from 0 V to 5 V. By matching simulation and experiment for different bias voltages, we also estimate the angle of LC molecules versus the bias voltage. Additionally, we study the created THz fields inside the spacer to gain a better insight of the characteristics of tunable response of this device. This structure and associated study can support the design of liquid crystal based tunable terahertz detectors and sensors for various applications.

Manipulating Smith-Purcell Emission with Babinet Metasurfaces.

Swift electrons moving closely parallel to a periodic grating produce far-field radiation of light, which is known as the Smith-Purcell effect. In this letter, we demonstrate that designer Babinet metasurfaces composed of C-aperture resonators offer a powerful control over the polarization state of the Smith-Purcell emission, which can hardly be achieved via traditional gratings. By coupling the intrinsically nonradiative energy bound at the source current sheet to the out-of-plane electric dipole and in-plane magnetic dipole of the C-aperture resonator, we are able to excite cross-polarized light thanks to the bianisotropic nature of the metasurface. The polarization direction of the emitted light is aligned with the orientation of the C-aperture resonator. Furthermore, the efficiency of the Smith-Purcell emission from Babinet metasurfaces is significantly increased by 84%, in comparison with the case of conventional gratings. These findings not only open up a new way to manipulate the electron-beam-induced emission in the near-field region but also promise compact, tunable, and efficient light sources and particle detectors.

Spectral filtering using active metasurfaces compatible with narrow bandgap III-V infrared detectors

Narrow-bandgap semiconductors such as alloys of InAsAlSb and their heterostructures are considered promising candidates for next generation infrared photodetectors and devices. The prospect of actively tuning the spectral responsivity of these detectors at the pixel level is very appealing. In principle, this could be achieved with a tunable metasurface fabricated monolithically on the detector pixel. Here, we present first steps towards that goal using a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode operating in the long-wave region of the infrared spectrum. We fabricate such a coupled system using the same epitaxial layers used for infrared pixels in a focal plane array and demonstrate the existence of ENZ modes in high mobility layers of InAsSb. We confirm that the coupling strength between the ENZ mode and the metasurface depends on the ENZ layer thickness and demonstrate a transmission modulation on the order of 25%. We further show numerically the expected tunable spectral behavior of such coupled system under reverse and forward bias, which could be used in future electrically tunable detectors.

Active wavelength selection for mixture identification with tunable mid-infrared detectors

This article presents a wavelength selection framework for mixture identification problems. In contrast with multivariate calibration, where the mixture constituents are known and the goal is to estimate their concentration, in mixture identification the goal is to determine which of a large number of chemicals is present. Due to the combinatorial nature of this problem, traditional wavelength selection algorithms are unsuitable because the optimal set of wavelengths is mixture dependent. To address this issue, our framework interleaves wavelength selection with the sensing process, such that each subsequent wavelength is determined on-the-fly based on previous measurements. To avoid early convergence, our approach starts with an exploratory criterion that samples the spectrum broadly, then switches to an exploitative criterion that selects increasingly more relevant wavelengths as the solution approaches the true constituents of the mixture. We compare this “active” wavelength selection algorithm against a state-of-the-art passive algorithm (successive projection algorithm), both experimentally using a tunable spectrometer and in simulation using a large spectral library of chemicals. Our results show that our active method can converge to the true solution more frequently and with fewer measurements than the passive algorithm. The active method also leads to more compact solutions with fewer false positives.