Broad Spectral Region Research Articles

Enhanced second-harmonic generation from L-shaped AlGaAs nanoantennas

Nanostructured materials offer many advantages over bulk materials in nonlinear optical phenomena. For example, the subwavelength scale allows the neglect of phase-matching issues. The ability to control tight confinement and large resonant enhancement of electromagnetic fields also results in higher nonlinear frequency conversion efficiencies than bulk materials. Compared with plasmonic nanostructures, high-index dielectric nanostructures render larger mode volumes and show great promise for enhanced nonlinear optical processes. In this work, we present resonantly enhanced second-harmonic generation from L-shaped AlGaAs nanoantennas. By tuning the nanostructure thickness, arm length, and width, the overlap of resonances at fundamental and second-harmonic wavelengths can be achieved using any pump wavelength in a broad near-infrared spectral region. We predict SHG efficiency exceeding 4 × 10 − 3 with a pump intensity of 1 G W / c m 2 at two representative pump wavelengths of 1675 and 1800 nm. Our findings are valuable for applications involving all-dielectric nanoantennas, such as efficient sensing and single biomolecule tracking microscopy.

Localized surface plasmon resonance of Cu-doped ZnO nanostructures and the material’s integration in dye sensitized solar cells (DSSCs) enabling high open-circuit potentials

Herein, we report one step co-precipitation method of synthesis of ZnO and Cu-doped ZnO nanostructures with distinct amount (1, 3 and 5 mol %) of copper. All the synthesized samples are characterized by X-ray powder diffraction, Field emission scanning and Transmission emission microscopy. The optical properties of the samples were evaluated by UV-visible spectroscopy and photoluminescence studies. The presence of Cu atoms in the ZnO matrix enhances a broad spectral absorption upto near-IR region. Suppressed E2high phonon modes are observed in Cu-doped samples through Raman scattering measurements, which furnishes evidence for effective Cu-doping in ZnO crystal lattice. Un-doped and Cu-doped ZnO samples were integrated in Dye sensitized solar cells (DSSCs) as photoanode materials. The device performance and interface charge transfer properties were evaluated through photocurrentdensity-photovoltage measurements and electrochemical impedance spectroscopy respectively. It is noted that the incorporation of Cu2+ into the hexagonal wurtzite structure of ZnO considerably elevates the Fermi levels towards the conduction band edge which is the root cause for high open-circuit potentials of DSSCs integrated with Cu-doped ZnO photoanodes.

Silicon nitride optical phased array based on a grating antenna enabling wavelength-tuned beam steering.

An optical phased array (OPA) in silicon nitride (SiN) is conspicuously highlighted as a vital alternative to its counterpart in silicon. However, a limited number of studies have been conducted on this array in terms of wavelength-tuned beam steering. A SiN OPA has been proposed and implemented with a grating antenna that incorporated an array of shallow-etched waveguides, rendering wavelength-tuned beam steering along the longitudinal direction. To accomplish a superior directionality on a wavelength-tuned beam steering, the spectral beam emission characteristics of the antenna have been explored from the viewpoint of a planar structure that entails a buried oxide (BOX), a SiN waveguide core, and an upper cladding. Two OPA devices having substantially different thicknesses of the resonant cavities, established by combining the BOX and SiN core, were considered theoretically and experimentally to scrutinize the spectral emission characteristics of the antenna on beam steering. Both of the fabricated OPA devices steered light by an angle of 7.4° along the longitudinal direction for a wavelength ranging from 1530 to 1630 nm, while they maintained a divergence angle of 0.2°×0.6° in the longitudinal and lateral directions. Meanwhile, the OPA fabricated on a substantially thick BOX layer featured a limited steering performance to attain a stabilized response over a broad spectral region. We examined the influence of the cavity thickness on the spectral response of the antenna in terms of optical thickness. Based on the two antenna characteristics, it was confirmed that the grating antenna emitted the beam with a higher efficiency when the optical thickness of the cavity corresponded to odd integer multiples of the quarter wavelength. This work is a considerable strategy for designing a stabilized SiN OPA over a desired spectral region.

Performance evaluation of THz Atmospheric Limb Sounder (TALIS) of China

Abstract. THz Atmospheric Limb Sounder (TALIS) is a microwave limb sounder being developed for atmospheric vertically resolved profile observations by the National Space Science Center, Chinese Academy of Sciences (NSSC, CAS). It is designed to measure temperature and chemical species such as O3, HCl, ClO, N2O, NO, NO2, HOCl, H2O, HNO3, HCN, CO, SO2, BrO, HO2, H2CO, CH3Cl, CH3OH, and CH3CN with a high vertical resolution from about 10 to 100 km to improve our comprehension of atmospheric chemistry and dynamics and to monitor the man-made pollution in the atmosphere. Four heterodyne radiometers including several FFT spectrometers of 2 GHz bandwidth with 2 MHz resolution are employed to obtain the atmospheric thermal emission in broad spectral regions centred near 118, 190, 240, and 643 GHz. A theoretical simulation is performed to estimate the retrieval precision of the main targets and to compare them with that of Aura MLS standard spectrometers. Single scan measurement and averaged measurement are considered in the simulation, respectively. The temperature profile can be obtained with a precision of <2 K for a single scan from 10 to 60 km by using the 118 GHz radiometer, and the 240 and 643 GHz radiometers can provide temperature information in the upper troposphere. Chemical species such as H2O, O3, and HCl show a relatively good single scan retrieval precision of <20 % over most of the useful range and ClO, N2O, and HNO3 can be retrieved with a precision of <50 %. The other species should be retrieved by using averaged measurements because of the weak intensity and/or low abundance.

An ultra-high resolution mid-infrared dual-band notch filter based on an asymmetric grating structure

An asymmetric grating structure was investigated for realizing mid-infrared (mid-IR) ultra-high resolution dual-band notch filters. The structure is constructed by an asymmetric gold grating on the stack of an Al2O3 layer and a CaF2 substrate. The grating consists of three gold strips in each unit cell, and the center-to-center distances between two adjacent gold strips are different. The filter provides two high-extinction transmission dips over a broad spectral region. One is at a wavelength of ∼3.8381μm with a full-width at half-maximum (FWHM) of ∼0.7 nm, and the other is at a wavelength of ∼3.8435μm with a FWHM of ∼1.7 nm. The quality-factor (Q-factor) of the filter is >2200. It was found that the two transmission dips result from the excited (+1) and (-1) resonance modes, respectively. Thus, there is an interesting feature that either of the two transmission dips does not split at oblique incidence.

Subtle Molecular Tailoring Induces Significant Morphology Optimization Enabling over 16% Efficiency Organic Solar Cells with Efficient Charge Generation.

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene-electron-acceptor-based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx-2 with quinoxaline-containing fused core and PBDB-TF as donor. The significant increase in photovoltaic performance of AQx-2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx-1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure-morphology-property relationship is established. The stronger π-π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx-2-based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next-generation high-performance OSCs.

Study of optical behaviour of Eu3+ and Tb3+ doped zirconate perovskite phosphors prepared by molten salt technique

This paper gives a keen insight of the study of spectroscopic and optical behavior of Eu3+ and Tb3+ doped calcium zirconate, barium zirconate and strontium zirconate perovskite phosphors synthesized by molten salt synthesis (MSS) technique. Here, the low temperature MSS technique uses molten chloride salt sodium chloride NaCl that increases solid–solid diffusion rate for creating an active reaction medium. These phosphors were subjected to sintering at a temperature of 950 °C with Eu3+ and Tb3+ ions doping concentration varying from 0 to 2 mol%. The Powder X-ray diffractometry technique was used to study of crystallographic behavior of synthesized phosphors. In order to study the morphological and topographical aspects, field-emission scanning electron microscopy and transmission electron microscopy analyses were done. The Fourier transform infrared analysis generates the fingerprint of the synthesized phosphors confirming the presence of all constituent elements. The photoluminescent (PL) characteristics of synthesized phosphors were studied on exciting CaZrO3:Eu3+, Tb3+ with 225 nm, BaZrO3:Eu3+, Tb3+ with 229 nm and SrZrO3:Eu3+, Tb3+ with 235 nm excitation wavelength. The PL emission spectra show variable emissions over a broad spectral region corresponding to 5D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 hypersensitive electronic transitions of Tb3+ ions along with 5D0–7F0, 5D0–7F1 and 5D0–7F2 induced electric dipole and magnetic dipole transitions of Eu3+ ions. These color tunable phosphors with increased PL output confirm their suitability for photonics, optoelectronics and display device applications.

High responsivity and fast UV–vis–short-wavelength IR photodetector based on Cd3As2/MoS2 heterojunction

High responsivity, fast response time, ultra-wide detection spectrum are pursuing goals for state-of-art photodetectors. Cd3As2, as a three-dimensional (3D) Dirac semimetal, has a zero bandgap, high light absorption rate in broad spectral region, and higher mobility than graphene at room temperature. However, photoconductive detectors based Cd3As2 suffer low quantum efficiency due to the absence of high built-in field. Here, a Cd3As2 nanoplate/multilayer MoS2 heterojunction photodetector was fabricated which achieved a quite high responsivity of 2.7 × 103 A W−1 at room temperature. The photodetector exhibits a short response time of in broad spectra region from ultraviolet (365 nm) to short-wavelength-infrared (1550 nm) and reached 65 μs at 650 nm. This work provides a great potential solution for high-performance photodetector and broadband imaging by combining 3D Dirac semi-metal materials with semiconductor materials.

Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe Spectroscopy.

The photochemistry of the cationic diazidocobalt(III) complex, trans-[Co(cyclam)(N3)2]+, following its ligand-to-metal charge transfer (LMCT) excitation is studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond spectroscopy with detection in a very broad spectral region covering the near-ultraviolet (near-UV) all the way to the mid-infrared (MIR), thereby enabling a combined probing of electronic and vibrational degrees of freedom of the dynamically evolving system. The initially prepared singlet LMCT-state decays, via the metal-centered singlet excited state, 1MC(1Eg), into the triplet ground state, 3MC (3Eg/3A1g), on a time scale shorter than 25 ps. During this time period, the vibrational spectrum demonstrates uniquely that the nature of the complex changes from a monoazidocobalt(II) species bearing a neutral azide radical ligand immediately after photon absorption to a metal-centered open-shell diazidocobalt(III) species. At the same time, the 3MC state is characterized by a very strong absorption band centered at 710 nm, which can be assigned to a transition to the triplet LMCT state. The 1LMCT lifetime is about 2 ps, whereas that of the excited state, 1MC, is defined by the primary intersystem crossing time of 6 ps. The ensuing intersystem recrossing from 3MC to the parent's singlet ground state, 1A1g, occurs with a rate of 1/(110 ps). The mid-infrared pump-probe spectrum after 1 ns, gives evidence for a heterolytic Co-N bond fission with a quantum yield of ∼5%, leading to free azide anions and the monoazido species, trans-[Co(cyclam)(N3)(OSMe2)]+, featuring an oxygen-bound DMSO ligand in its coordination sphere.

Transient Absorption Spectroscopy Reveals Performance-Limiting Factors in a Narrow-Bandgap Oxysulfide La5(Ti0.99Mg0.01)2CuS5O6.99 Photocatalyst for H2 Generation

Mg-doped La5(Ti0.99Mg0.01)2CuS5O6.99 (Mg-LTC) is an oxysulfide-based H2 evolution photocatalyst with a bandgap of ≈1.84 eV. In addition, Mg-LTC has also been successfully integrated as a photocathode in a Z-scheme-based overall water-splitting process. Despite 15 years of intensive research, LTC-based photocatalysts are yet to demonstrate high photoconversion efficiency. To this end, transient absorption spectroscopy was employed to unveil the key loss processes reducing the efficiency of H2 evolution in Mg-LTC. Charge carrier dynamics from sub-picosecond to microsecond was probed over a broad spectral region (visible to IR) for both the Mg-LTC powder and Mg-LTC/Au photocathode prepared by the particle-transfer method with Au back contact. A faster decay of hole dynamics (700 nm probe) for the Mg-LTC/Au photocathode than Mg-LTC powder indicated hole transfer from Mg-LTC to Au with a time constant of 0.5 ps and 12.5% efficiency. At least 50% of the electrons probed at 3435 nm decayed by trapping to defects in tens of picosecond. As a result, the key electron-transfer process from Mg-LTC to the Pt cocatalyst, which determines the H2 evolution efficiency, occurred from the microsecond long-lived electrons with a time constant of 0.26 μs and 30.5% efficiency. The results provide insight into the perspective of material-design and photocathode fabrication procedure to further advance the H2-generation efficiency in Mg-LTC.

CdIn2S4 surface-decorated Ta3N5 core-shell heterostructure for improved spatial charge transfer: In-situ growth, synergistic effect and efficient dual-functional photocatalytic performance

Efficient design and construction of core-shell heterojunction is regarded as a promising strategy to promote interface electronic transfer for achieving enhanced photocatalytic hydrogen evolution and pollutant degradation. Thus, in this study, a novel core-shell CdIn2S4/Ta3N5 (CIS/TN) nanocomposite photocatalyst was designed and successfully fabricated via the facile in-situ hydrothermal process, where the CdIn2S4 nano-octahedra densely anchored on the surface of Ta3N5 nanoparticles to form the core-shell heterostructure. The as-prepared CIS/TN heterostructures all showed high dual-functional photocatalytic performances for H2 generation and Methyl Orange (MO) decomposition. Compared with pristine CdIn2S4, the photocatalytic activities of CIS/TN heterostructures were obviously enhanced and the sample CIS/TN-3 with the 3 wt% of Ta3N5 possessed the highest H2 production of 122.6 μmol·h−1·g−1, which was about 2.43 times as high as that of CdIn2S4 nano-octahedra. Also, it presented the optimal MO photodegradation efficiency (92.2%) and its rate constant was 2.79 times higher than that of pure CdIn2S4. The wonderful dual-functional photocatalytic performance could be attributed to the broad spectral adsorption region and efficient transfer of photoinduced charge carriers between the intimate heterogeneous interface, which would lead to the higher quantum yield and efficiency. The detailed charge transfer path and possible reaction mechanism were also proposed based on the results of trapping experiments and ESR analysis. Our research may provide a direct guidance for developing other highly-efficient Ta3N5-based core/shell heterostructure for energy conversion and environmental governance.

Suppressing the unwanted resonance mode in a metal-insulator-metal structure using fine-structured gratings.

We numerically present that suppressing the unwanted resonance mode in a metal-insulator-metal (MIM) structure can be achieved by using a fine-structured gold grating. In each period of the grating, a sub-grating consisting of multiple gold strips is used. Investigations on a high quality-factor (Q-factor) MIM structure with the grating and usual gratings are carried out. Comparisons show that the proposed grating supports different diffraction orders. Moreover, the unwanted mode which exists in the case of usual gratings can be significantly suppressed, and the desired mode can be kept almost unchanged. Thus, the MIM structure with our grating shows a single resonance at ~3.9 μm with a high Q-factor (~260) and an ultra-narrow linewidth (~10 cm-1) over a broad spectral region. This study provides a simple and effective approach to selective manipulating the resonance modes in MIM structures, which is useful for design of mid-infrared narrowband filters, emitters, and absorbers.

Definition of polaritonic fluctuations in natural hyperbolic media

The discovery of photonic hyperbolic dispersion surfaces in certain van der Waals bonded solids, such as hexagonal boron nitride and bismuth selenide (a topological insulator), offers intriguing possibilities for creating strongly modified light-matter interactions. However, open problems exist in quantifying electromagnetic field fluctuations in these media, complicating typical approaches for modeling photonic characteristics. Here, we address this issue by linking the identifying traits of hyperbolic response to a coupling between longitudinal and transverse fields that can not occur in isotropic media. This description allows us to calculate the influence of hyperbolic response on electromagnetic fluctuations without explicitly imposing a characteristic size (model of nonlocality), leading to formally bounded expressions so long as material absorption is included. We then apply this framework to two exemplary areas: the optical sum rule for modified spontaneous emission enhancement in a general uniaxial medium, and thermal electromagnetic field fluctuations in hexagonal boron nitride and bismuth selenide. We find that while the sum rule is satisfied, it does not constrain the enhancement of light-matter interactions in either case. We also show that both hexagonal boron nitride and bismuth selenide possess broad spectral regions where the magnitude of electromagnetic field fluctuations are over 120 times larger, and over 800 times larger along specific angular directions, than they are in vacuum.

Ultrasmall Optical Vortex Knots Generated by Spin‐Selective Metasurface Holograms

AbstractTying isolated vortex knots in complex optical fields has been recently realized using spatial light modulators (SLMs). These traditional devices suffer from low efficiency, narrow bandwidth, and they cannot be integrated to nanophotonic systems. Here, the design and realization of metasurface holograms (MHs) is reported to efficiently generate the optical vortex knots in a broad spectral region. In comparison with conventional optical devices, the thicknesses and pixels of MHs are both in subwavelength scale, and the size of created optical vortex knot can be six orders of magnitude smaller than those produced by SLMs. Such extremely little vortex knots may find widespread applications in templating knotted micro/nano‐structures in photosensitive materials and 3D optical trapping of cold atoms in complex topologies at microscale. In particular, it is found that the spatial position of the generated off‐axis optical vortex knot is able to be symmetrically reversed by just changing the helicity of the incident light. This spin‐selective vortex knot generator can be regarded as a new type of metasurface device for manipulating the topological structure of optical vortex lines, which has potential applications in the field of singular optics.

Preparation of AZO:PDIN hybrid interlayer materials and application in high-gain polymer photodetectors with spectral response from 300 nm to 1700 nm

Abstract Inorganic-organic hybrid interlayer materials were prepared using aluminum doped ZnO nanoparticles (AZO) and amine-containing perylene diimide (PDIN). By employing this hybrid material as cathode interlayer and BCP as anode interlayer, high-gain polymer photodetectors have been realized. The low-bandgap polymer PB-D-T-TPT photodetector exhibited a significant increase in EQE as large as 25982% (−2.0 V, 360 nm), large responsivity of 77.2 A/W (−2.0 V, 830 nm) and high detectivity of >1012 Jones (All the specific detectivity in this paper were calculated from EQE and dark current.) over a broad spectral region (300–1700 nm). Using the same hybrid cathode interlayer, the devices based on P3HT and PTII-TT polymers also showed detectivity of >1015 Jones in 330–640 nm and >1012 Jones in 320–1600 nm, respectively, further demonstrating a unique ability of AZO:PDIN hybrid interlayer to improve the performance of polymer photodetectors.

The carboxyl derivatives of 6,8-di-(tert.-butyl)phenoxazine: Synthesis, oxidation reactions and fluorescence

New carboxyl-containing o-aminophenols and phenoxazines were synthesized by condensation of 3,5-di-(tert.-butyl)-quinone with p-aminobenzoic and anthranilic acids. Oxidative transformations of the o-aminophenols and intermediate o-iminoquinones occur with the formation of the ESR detected phenoxazinyl radicals, which furthermore transform to phenoxazines or the dimeric products emerged through the radical attack at the C1 carbon of a formed phenoxazine. Molecular structure of the dimer obtained by oxidation of methyl ester of 4-[3,5-di-(tert.-butyl)-1-(2′-hydroxyphenyl)amino]benzoic acid was X-ray determined. Reaction of 4-[3,5-di-(tert.-butyl)-1-(2′-hydroxyphenyl)amino]benzoic acid with thionyl chloride gives rise to the formation of a derivative of 2-oxido-3H-benzo[d,j][1,2,3]oxathiazol system, the structure of which was established using X-ray crystallography. Solutions of methyl-6,8-di-(tert.-butyl)-10H-phenoxazine-3-carboxylate solvents display intense fluorescence covering a broad spectral region in the range of 400–600 nm.

Suppressing Effect of 2-Nitrobenzaldehyde on Singlet Oxygen Generation, Fatty Acid Photooxidation, and Dye-Sensitizer Degradation.

2-Nitrobenzaldehyde was found to efficiently block singlet oxygen generation in a series of different test samples upon exposure to UV and visible light under aerobic conditions. The effect of quenching singlet oxygen formation was monitored in the presence of 1, 4-diazabicyclo [2.2.2] octane (DABCO) acting as a well-known singlet oxygen scavenger. A comparison of different nitrobenzaldehyde isomers with other highly effective synthetic antioxidants used in the food industry such as butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tert-butylhydroquinone (TBHQ) revealed that the protection of materials from singlet oxygen decreases in the order of 2-nitrobenzaldehyde > DABCO > TBHQ > 3-nitrobenzaldehyde > BHA > 4-nitrobenzaldehyde > BHT. Upon addition of 2-nitrobenzaldehyde, the oxidation of fatty acids and the degradation of photosensitizers was found to be considerably diminished, which indicates that the presence of 2-nitrobenzaldehyde has a significant protective influence by restricting the singlet oxygen generation and photodegradation of dyes. Moreover, the compound turned out to display its highly suppressing effects on typical singlet oxygen-dependent reactions, such as fatty acid photooxidation and dye photosensitizer degradation, in a rather broad spectral region covering wavelengths from 300 nm (UV-B) to 575 nm (close to the maximum of ambient solar radiation).

Excitonic Luminescence Engineering in Tervalent-Europium-Doped Cesium Lead Halide Perovskite Nanocrystals and Their Temperature-Dependent Energy Transfer Emission Properties

Cesium lead halide (CsPbX3 (X = Cl, Br, I)) perovskite nanocrystals (NCs) have shown excellent prospects in lighting, display, and lasing fields owing to their excellent photoluminescence (PL) properties. To dope rare earth ions into halide perovskite, CsPbX3 (X = Cl, Br, I) NC hosts not only inherit the excellent narrow linewidth excitonic properties but also yield unique photoluminescence emission. Herein, engineering of such excitonic luminescence is achieved for Eu3+-doped CsPbCl3–xBrx (x = 0, 1, 1.5, 2, 3) solid solution NCs for the first time. The singly doped NCs present wide color gamut emission covering the whole visible spectrum. The blue-to-green range (400–520 nm) emission is covered by tunable excitonic photoluminescence of CsPbX3 NCs. Besides, there is a broad red spectral region (590–700 nm) originating from the emission of the tervalent europium ions in NCs. Meanwhile, a noticeable spin-polarized 5D0 → 7F1–6 emission of Eu3+ ions is obtained owing to energy transfer from excitons to dopant...

Fundamental-frequency discrimination based on temporal-envelope cues: Effects of bandwidth and interference.

Both music and speech perception rely on hearing out one pitch in the presence of others. Pitch discrimination of narrowband sounds based only on temporal-envelope cues is rendered nearly impossible by introducing interferers in both normal-hearing listeners and cochlear-implant (CI) users. This study tested whether performance improves in normal-hearing listeners if the target is presented over a broad spectral region. The results indicate that performance is still strongly affected by spectrally remote interferers, despite increases in bandwidth, suggesting that envelope-based pitch is unlikely to allow CI users to perceive pitch when multiple harmonic sounds are presented at once.

Simultaneous Observation of Nonlinear Magneto-Optical Rotation in the Temporal and Spectral Domains with an Electro-Optic Frequency Comb

Highly sensitive magnetic field measurements are desirable for medical diagnoses and imaging, as well as research in geomagnetism and fundamental physics. Atomic magnetometers measure the Larmor frequency of an alkali vapor, and their sensitivity depends on generating a high degree of ground-state coherence in the atomic population via optical pumping. This study uses an electro-optic frequency comb to probe a broad spectral region, yielding measurements of the Larmor frequency that are both temporally and spectrally resolved. The techniques developed here allow real-time analysis of the degree of polarization of the atomic ensemble, and the avenues by which this polarization is probed.