High Spectral Selectivity Research Articles

Metallic Photonic Crystal Absorber‐Emitter for Efficient Spectral Control in High‐Temperature Solar Thermophotovoltaics

A high‐temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance 0.86 and thermal emittance = 0.26 at 1000 K, using high‐temperature material properties, is achieved resulting in a thermal transfer efficiency more than 50% higher than that of a blackbody absorber. Furthermore, an integrated double‐sided 2D PhC absorber/emitter pair is demonstrated for a high‐performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double‐side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a flat Ta absorber and a nearly blackbody absorber composed of an array of multiwalled carbon nanotubes (MWNTs). At an irradiance of 130 kW m−2 the PhC absorber enables more than a two‐fold improvement in measured STPV system efficiency (3.74%) relative to the nearly blackbody absorber (1.60%) and higher efficiencies are expected with increasing operating temperature. These experimental results show unprecedented high efficiency, demonstrating the importance of the high selectivity of the 2D PhC absorber and emitter for high‐temperature energy conversion.

A Monochromatic Electrochemiluminescence Sensing Strategy for Dopamine with Dual-Stabilizers-Capped CdSe Quantum Dots as Emitters

A promising electrochemiluminescence (ECL) sensing strategy was proposed with dual-stabilizers-capped CdSe quantum dots (QDs) as ECL emitters. The dual-stabilizers-capped CdSe QDs were covalently immobilized onto p-aminobenzoic acid modified glass carbon electrode with ethylenediamine as a link molecule. This strategy can preserve the completely passivated surface states of dual-stabilizers-capped CdSe QDs, so that the sensor demonstrated eye-visible greenish, band gap engineering and monochromatic ECL emission at 546 nm with a fwhm of 35 nm. Moreover, the proposed sensor could accurately quantify dopamine from 10.0 nM to 3.0 μM with a detection limit of 3.0 nM in practical drug, human urine, and cerebrospinal fluid samples without any signal amplification techniques. This strategy is promising for developing ECL sensors with high sensitivity and spectral selectivity.

High Detectivity Solar‐Blind High‐Temperature Deep‐Ultraviolet Photodetector Based on Multi‐Layered (l00) Facet‐Oriented β‐Ga2O3 Nanobelts

Fabrication of a high-temperature deep-ultraviolet photodetector working in the solar-blind spectrum range (190-280 nm) is a challenge due to the degradation in the dark current and photoresponse properties. Herein, β-Ga2O3 multi-layered nanobelts with (l00) facet-oriented were synthesized, and were demonstrated for the first time to possess excellent mechanical, electrical properties and stability at a high temperature inside a TEM studies. As-fabricated DUV solar-blind photodetectors using (l00) facet-oriented β-Ga2O3 multi-layered nanobelts demonstrated enhanced photodetective performances, that is, high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, importantly, at a temperature as high as 433 K, which are comparable to other reported semiconducting nanomaterial photodetectors. In particular, the characteristics of the photoresponsivity of the β-Ga2O3 nanobelt devices include a high photoexcited current (>21 nA), an ultralow dark current (below the detection limit of 10(-14) A), a fast time response (<0.3 s), a high R(λ) (≈851 A/W), and a high EQE (~4.2 × 10(3)). The present fabricated facet-oriented β-Ga2O3 multi-layered nanobelt based devices will find practical applications in photodetectors or optical switches for high-temperature environment.

Photoelectric properties of n-TiO2/p-Co0.7Ni0.3O heterostructures

The main photoelectric parameters of n-TiO2/p-Co0.7Ni0.3O film heterostructures obtained by sequential sputtering of corresponding metal targets in oxidizing atmosphere have been determined. The heterostructures are photosensitive in a wavelength range of 225–385 nm and exhibit no photoresponse in the visible spectral range. The proposed structures can be used for creating photodetectors with high spectral selectivity.

Surface-Enhanced Infrared Spectroscopy Using Metal Oxide Plasmonic Antenna Arrays

We successfully demonstrate surface-enhanced infrared spectroscopy using arrays of indium tin oxide (ITO) plasmonic nanoantennas. The ITO antennas show a strongly reduced plasmon wavelength, which holds promise for ultracompact antenna arrays and extremely subwavelength metamaterials. The strong plasmon confinement and reduced antenna cross section allows ITO antennas to be integrated at extremely high densities with no loss in performance due to long-range transverse interactions. By further reducing the spacing of antennas in the arrays, we access the regime of plasmonic near field coupling where the response is enhanced for both Au and ITO devices. Ultracompact ITO antennas with high spatial and spectral selectivity in spectroscopic applications offer a viable new platform for infrared plasmonics, which may be combined with other functionalities of these versatile materials in devices.

Enhanced photodetection in graphene-integrated photonic crystal cavity

We demonstrate the controlled enhancement of photoresponsivity in a graphene photodetector by coupling to slow light modes in a long photonic crystal linear defect cavity. Near the Brillouin zone (BZ) boundary, spectral coupling of multiple cavity modes results in broad-band photocurrent enhancement from 1530 nm to 1540 nm. Away from the BZ boundary, individual cavity resonances enhance the photocurrent eight-fold in narrow resonant peaks. Optimization of the photocurrent via critical coupling of the incident field with the graphene-cavity system is discussed. The enhanced photocurrent demonstrates the feasibility of a wavelength-scale graphene photodetector for efficient photodetection with high spectral selectivity and broadband response.

Green-Sensitive Organic Photodetectors with High Sensitivity and Spectral Selectivity Using Subphthalocyanine Derivatives

Green-sensitive organic photodetectors (OPDs) with high sensitivity and spectral selectivity using boron subphthalocyanine chloride (SubPc) derivatives are reported. The OPDs composed of SubPc and dicyanovinyl terthiophene derivative (DCV3T) demonstrated the highest green-sensitivity with maximum external quantum efficiency (EQE) of 62.6 % at an applied voltage of -5 V, but wide full-width-at-half-maximum (FWHM) of 211 nm. The optimized performance considering spectral selectivity was achieved from the composition of N,N-dimethyl quinacridone (DMQA) and SubPc showing the high specific detectivity (D*) of 2.34 × 10(12) cm Hz(1/2)/W, the EQE value of 60.1% at -5 V, and narrow FWHM of 131 nm. In spite of the sharp absorption property of SubPc with the maximum wavelength (λmax) at 586 nm, the EQE spectrum showed favorable green-sensitivity characterized by smooth waveform with λmax at 560 nm, which is induced from the high reflectance of SubPc centered at 605 nm. The photoresponsivity of the OPD devices was found to be consistent with their absorptance. Optimized DMQA/SubPc device showed the lowest value of blue crosstalk (0.42) and moderate red crosstalk (0.37), suggesting its promising application as a green-sensitive OPD.

P-type ZnS:N nanowires: Low-temperature solvothermal doping and optoelectronic properties

Nitrogen doped p-type ZnS nanowires (NWs) were realized using thermal decomposition of triethylamine at a mild temperature. Field-effect transistors made from individual ZnS:N NWs revealed typical p-type conductivity behavior, with a hole mobility of 3.41 cm2V−1s−1 and a hole concentration of 1.67 × 1017 cm−3, respectively. Further analysis found that the ZnS:N NW is sensitive to UV light irradiation with high responsivity, photoconductive gain, and good spectral selectivity. The totality of this study suggests that the solvothermal doping method is highly feasible to dope one dimensional semiconductor nanostructures for optoelectronic devices application.

High-detectivity nanowire photodetectors governed by bulk photocurrent dynamics with thermally stable carbide contacts

Photodetectors fabricated from one-dimensional semiconductors are always dominated by the surface states due to their large surface-to-volume ratio. Therefore, the basic 5S requirements (high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability) for practical photodetectors are difficult to satisfy. We report on high-temperature and high-detectivity solar-blind deep-ultraviolet (DUV) photodetectors based on β-Ga2O3 nanowires, in which the photoresponse behavior is dominated by the bulk instead of the surface states. Ohmic contact to the β-Ga2O3 nanowires was achieved by using a thermally stable tungsten carbide electrode. As a result, the DUV responsivity at 250 nm shows the highest values—4492 A W−1 at room temperature (RT) and 3000 A W−1 at 553 K (280 ° C)—among the DUV photodetectors. The detectivity is as high as 1.26×1016 cm Hz1/2 W−1 at RT, and still remains 4.1×1014 cm Hz1/2 W−1 at as high a temperature as 553 K. The photocurrent dynamics from the β-Ga2O3 nanowire is discussed in terms of the bulk dominated photoresponse behavior. Other wide bandgap DUV detectors based on nanostructures could also be developed for high-temperature applications based on this work.

Real Time FPGA-Based Crosstalk Elimination for Multichannel Interferometry Systems in Fusion Diagnostics

Infrared (IR) interferometry is a well-known method for measuring the Line-Integrated electron Density (LID) of fusion plasmas. In the TJ-II stellarator, an FPGA-based IR-interferometer has recently been installed to accurately measure the LID of the plasma in real time. To guarantee the correct functionality of the interferometer and achieve high precision rates, it is essential to maximize the Signal-to-Noise Ratio (SNR) of the output density signal. In the measurement process, one of the most important distortion sources is the crosstalk or interchannel interference. Thus, in order to increase the SNR of the system, a crosstalk reduction stage has been designed and implemented in an FPGA. This paper presents a novel crosstalk elimination algorithm that provides very high spectral selectivity while preserves the phase linearity. Experimental results have shown that the proposed algorithm successfully separates the desired channels, improving the SNR with 30 dB, and keeping track of the phase of the original signal. In addition, the proposed solution has been optimized for its high-performance hardware implementation in the TJ-II. Resource consumption in the FPGA is now 22% smaller and 71% faster than with the traditional implementation based on linear-phase digital filtering.

Photothermal diffuse reflectance: a new tool for spectroscopic investigation of scattering samples

A new method named photothermal diffuse reflectance (PTDR) is presented. This method combines the diffuse reflectance spectroscopy with the photothermal technique and is particularly suited for the investigation of strongly scattering samples. This method takes advantage of the high spectral selectivity and absorption of the mid-infrared region with the larger scattering cross section and high detector sensitivity available in the near-infrared. A model describing the PTDR method is proposed and supported with experimental results. The potential of the PTDR technique is illustrated by experimental signals obtained from various scattering media like polymers, liquids and powders.

Comparative performance evaluation of data-driven causality measures applied to brain networks

In this article, several well-known data-driven causality methods are revisited and comparatively evaluated. These are the Granger–Geweke Causality (GGC), the Partial Directed Coherence (PDC), the Directed Transfer Function (DTF) and the Direct Directed Transfer Function (dDTF). The robustness of the four causality measures against two degradation factors is quantitatively evaluated. These are: the presence of realistic biological/electronic noise at various SNR levels, as recorded on a MagnetoEncephalography (MEG) machine, and the presence of a weak node in the brain network where the causality analysis is applied. The causality measures are evaluated in terms of the relative estimation error and the compromise between true and fictitious causal density in the brain network. Both parametric and non-parametric causality analysis is performed. It is illustrated that the non-parametric method is a promising alternative to the more commonly applied MVAR-model based causality analysis. It is also demonstrated that, in the presence of both tested degradation factors, the DTF method is the most robust in terms of low estimation error, while the PDC in terms of low fictitious causal density. The dDTF provides lower fictitious causal density and higher spectral selectivity as compared to DTF, at high enough SNR. The GGC exhibits the worst compromise of performance. An application of the causality measures to a set of MEG resting-state experimental data is accordingly presented. It is demonstrated that significant contrast between the Eyes-Closed and Eyes-Open rest condition in the alpha frequency band allows to detect significant causality between the occipital cortex and the thalamus.

Wavelength-stabilised external-cavity laser diode using cavity resonator integrated guided mode filter

The first wavelength-stabilised external-cavity laser diode using a new generation of resonant grating filters called the cavity resonator integrated guided mode filter (CRIGF) is reported. The CRIGF combines high reflectivity and spectral selectivity for tightly focused beams allowing the simplified `cat's eye' external cavity to be used. Output powers of more than 10 mW together with a side mode suppression ratio larger than 35 dB for a drive current of 50 mA are achieved. CRIGF-controlled operation at a wavelength spanning a 20 nm range is also demonstrated, with a linear relationship between the CRIGF period and output wavelength.

Selective enhancement of infrared absorption with metal hole arrays

We use a surface-enhanced infrared absorption (SEIRA) spectroscopy, a useful sensing and surface analysis method complimentary to the Raman scattering spectroscopy, for the individual enhancement of specific molecular vibration bands and fingerprinting of molecular vibrations. SEIRA spectroscopic measurement using the metal hole array (MHA) is demonstrated with high spectral selectivity. The molecular IR absorption peaks are enhanced up to 10 times at the transmission peak of MHA structure when electromagnetic field enhancement is localized on the walls inside the holes. Experimental and numerical simulations results are in a good qualitative agreement. Selective IR band enhancement can be used for identification of specific molecules within complex mixtures and it can be extended to the longer wavelengths at THz molecular bands.

Comprehensive Investigation of Single Crystal Diamond Deep-Ultraviolet Detectors

The wide bandgap of diamond, along with its extreme semiconductor properties, offers the promising route for deep-ultraviolet (DUV) detection, especially under solar-blind condition and harsh environments. The ideal photodetector should generally satisfy the 5S requirements such as high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability. In this paper, we comprehensively investigate the DUV detectors fabricated from various kinds of single crystal diamonds such as boron-doped diamond homoepitaxial layer, intrinsic diamond homoepitaxial layers with different thicknesses, and single crystal diamond substrates. The post process such as hydrogen plasma treatment on the performance of the DUV detectors is also examined. The strategies to develop high-performance diamond DUV detectors are provided.

Extremely high transmission of light through a nanohole inside a photonic crystal

The transmission of light through single nanoholes with diameters considerably smaller than the wavelength of light (smaller than λ/10) is experimentally studied. The nanoholes were made in a gold film, which is a part of a photonic crystal forming a microcavity with the quality factor Q ≈ 100. A 28-fold increase in the transmission of light through a nanohole inside the microcavity compared to transmission through a nanohole in a gold film is demonstrated. The high spectral selectivity of light transmission through a nanohole is discovered, which is characterized by two features: (i) the transmission maximum is located at the resonance wavelength of the microcavity and (ii) the peak full width at half-maximum is about λ/90.

Optimized Use of Reversible Binding for Fast and Selective NMR Localization of Caged Xenon

The NMR signal of hyperpolarized 129Xe trapped in cryptophane cages in different solvents experiences different chemical shifts. An encoding method is presented that involves the optimal use of reversible Xe binding and efficiently uses hyperpolarization. This method is utilized in nanomolar imaging, subsecond imaging, and time-resolved studies while maintaining high spectral selectivity.

Single-layer resonant-waveguide grating for polarization and wavelength selection in Yb:YAG thin-disk lasers

A single-layer resonant-waveguide grating consisting of a sub-wavelength grating coupler etched into a waveguide is proposed in order to achieve high polarization and high spectral selectivity inside an Yb:YAG thin-disk laser resonator. The designed structure was fabricated with the help of a Lloyd's-mirror interference lithography setup followed by reactive ion beam etching down to the desired grating groove depth. The wavelength and polarization dependent reflectivity is measured and compared to the design results. The behaviour of the device at higher temperatures is also investigated in the present work. The device is introduced as the end mirror of an Yb:YAG thin-disk laser cavity. Output powers of up to 123 W with a spectral bandwidth of about 0.5 nm (FWHM) is demonstrated in a multimode configuration (M2~6). In fundamental-mode operation (TEM00 with M2~1.1) 70 W of power with a spectral bandwidth of about 20 pm have been obtained. Moreover, the degree of linear polarization was measured to be higher than 99% for both multimode and fundamental mode operation.

Low-temperature combustion synthesis of CuCr2O4 spinel powder for spectrally selective paints

CuCr2O4 spinel powder with high quality black hue, investigated as solar-absorbing pigment for spectrally selective paint, was synthesized by an environmental friendly sol–gel combustion process using citric acid as the fuel and metal nitrates as oxidizers. Single-phase CuCr2O4 spinel crystals were obtained after heat treatment of the as-burnt powder at a low temperature (600 °C) and the average crystallite size of the CuCr2O4 powders increased with the calcining temperature. Morphological analysis of powders calcined at various temperatures was done by field emission scanning electron microscopy. CuCr2O4 powder calcined at 700 °C was chosen as pigment to fabricate thickness sensitive spectrally selective paint coatings by simple spray-coating technique. For the sake of comparison, the as-burnt powder composed of mixed metal oxides (i.e., CuO and Cr2O3) was also used as pigment. The results reveal that the spinel CuCr2O4 based paint coatings exhibit much higher spectral selectivity (α s = 0.88–0.91, e 100 = 0.27–0.35) which is depending on the coating thicknesses than that of coatings using as-burnt powder as pigment (α s = 0.83–0.88, e 100 = 0.60–0.66). The CuCr2O4-based paint coatings showed no visible degradation after 600 h of condensation test and the performance criterion value is 0.04, indicating that the coatings have excellent long term stability.

Near‐Room‐Temperature Mid‐Infrared Quantum Well Photodetector

We demonstrate InGaAs mid-infrared quantum well infrared photodetectors (MIR PV-QWIPs) that enable cost-effective mature GaAs-based detection and imaging technologies, with exceptional material uniformity, reproducibility, and yield, over a large area, with high spectral selectivity, innate polarization sensitivity, radiation hardness, high detectivity, and high speed operation at TEC temperatures without bias.