Rolling up MoSe2 Nanomembranes as a Sensitive Tubular Photodetector

Transition metal dichalcogenides, as a kind of 2D material, are suitable for near‐infrared to visible photodetection owing to the bandgaps ranging from 1.0 to 2.0 eV. However, limited light absorption restricts photoresponsivity due to the ultrathin thickness of 2D materials. 3D tubular structures offer a solution to solve the problem because of the light trapping effect which can enhance optical absorption. In this work, thanks to mechanical flexibility of 2D materials, self‐rolled‐up technology is applied to build up a 3D tubular structure and a tubular photodetector is realized based on the rolled‐up molybdenum diselenide microtube. The tubular device is shown to present one order higher photosensitivity compared with planar counterparts. Enhanced optical absorption arising from the multiple reflections inside the tube is the main reason for the increased photocurrent. This tubular device offers a new design for increasing the efficiency of transition metal dichalcogenide–based photodetection and could hold great potential in the field of 3D optoelectronics.     

Spatially variant regularization improves diffuse optical tomography

Diffuse tomography with near-infrared light has biomedical application for imaging hemoglobin, water, lipids, cytochromes, or exogenous contrast agents and is being investigated for breast cancer diagnosis. A Newton-Raphson inversion algorithm is used for image reconstruction of tissue optical absorption and transport scattering coefficients from frequency-domain measurements of modulated phase shift and light intensity. A variant of Tikhonov regularization is examined in which radial variation is allowed in the value of the regularization parameter. This method minimizes high-frequency noise in the reconstructed image near the source-detector locations and can produce constant image resolution and contrast across the image field.     

Study of a dual-frequency laser interferometer with unique optical subdivision techniques

A dual-frequency laser interferometer has been developed based on a low-performance commercial interferometer. An optical resolution of 1.24 nm and a nanometer-scale accuracy have been achieved by using unique techniques to obtain an optical subdivision factor of 1/8. A method for reducing static positioning errors was also shown. The measurement of a free-falling body was performed to test the maximum achievable target velocity of the device. The experimental setup for measuring the static positioning errors was also given. The new interferometer could be widely used in nanometer-scale fabrications and measurements.     

The anomalous optical absorption of discontinuous silver films and the polarizability of small particles

A method of determining the dynamic polarizability of small metal particles is proposed. It is found that the absolute values of the real and imaginary parts of the dynamic polarizability α of small silver particles show large increases with decreasing particle size and that the spectral dependence of the imaginary part of the dynamic polarizability has an additional band in the region of anomalous optical absorption of discontinuous silver films. It is shown that the size dependence of α and of the anomalous optical absorption of discontinous silver films cannot be described in terms of the classical and quantum size effects in small metal particles. It is assumed that, owing to the enormous influence of the particle surface on the properties of small particles, the anomalous absorption of silver films is mainly due to indirect interband transitions.     

Ultrafast Coherent Absorption in Diamond Metamaterials

Diamond is introduced as a material platform for visible/near‐infrared photonic metamaterials, with a nanostructured polycrystalline diamond metasurface only 170 nm thick providing an experimental demonstration of coherent light‐by‐light modulation at few‐optical‐cycle (6 fs) pulse durations. “Coherent control” of absorption in planar (subwavelength‐thickness) materials has emerged recently as a mechanism for high‐contrast all‐optical gating, with a speed of response that is limited only by the spectral width of the absorption line. It is shown here that a free‐standing diamond membrane structured by focused ion beam milling can provide strong, spectrally near‐flat absorption over a visible to near‐infrared wavelength range that is wide enough (wider than is characteristically achievable in plasmonic metal metasurfaces) to facilitate coherent modulation of ultrashort optical pulses comprising only a few oscillations of electromagnetic field.     

Radiation engineering of optical antennas for maximum field enhancement

Optical antennas have generated much interest in recent years due to their ability to focus optical energy beyond the diffraction limit, benefiting a broad range of applications such as sensitive photodetection, magnetic storage, and surface-enhanced Raman spectroscopy. To achieve the maximum field enhancement for an optical antenna, parameters such as the antenna dimensions, loading conditions, and coupling efficiency have been previously studied. Here, we present a framework, based on coupled-mode theory, to achieve maximum field enhancement in optical antennas through optimization of optical antennas' radiation characteristics. We demonstrate that the optimum condition is achieved when the radiation quality factor (Q(rad)) of optical antennas is matched to their absorption quality factor (Q(abs)). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO(2)) coated ground plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve constructive interference, and leads to ～20% higher field enhancement relative to a quarter-wavelength thick dielectric layer.     

Strong Depletion in Hybrid Perovskite p–n Junctions Induced by Local Electronic Doping

A semiconductor p–n junction typically has a doping‐induced carrier depletion region, where the doping level positively correlates with the built‐in potential and negatively correlates with the depletion layer width. In conventional bulk and atomically thin junctions, this correlation challenges the synergy of the internal field and its spatial extent in carrier generation/transport. Organic–inorganic hybrid perovskites, a class of crystalline ionic semiconductors, are promising alternatives because of their direct badgap, long diffusion length, and large dielectric constant. Here, strong depletion in a lateral p–n junction induced by local electronic doping at the surface of individual CH₃NH₃PbI₃ perovskite nanosheets is reported. Unlike conventional surface doping with a weak van der Waals adsorption, covalent bonding and hydrogen bonding between a MoO₃ dopant and the perovskite are theoretically predicted and experimentally verified. The strong hybridization‐induced electronic coupling leads to an enhanced built‐in electric field. The large electric permittivity arising from the ionic polarizability further contributes to the formation of an unusually broad depletion region up to 10 µm in the junction. Under visible optical excitation without electrical bias, the lateral diode demonstrates unprecedented photovoltaic conversion with an external quantum efficiency of 3.93% and a photodetection responsivity of 1.42 A W^?1.     

Static electric and optical properties of two coupled noble metal nanoparticles

The static electric polarizability and absorption spectrum of the coupled noble metal (Ag, Au, Cu) cluster-pair are investigated within density functional theory (DFT). The results show that the HOMO-LUMO gap gradually widens with increasing cluster distance D, and the static polarizability exhibits a maximal value as D is near the average bond length of cluster. In the absorption spectrum, the spectral strength is related to electric dipole moments which sensitively rely on D. With the increase of D the low-energy peaks (LEPs) obviously blue shift while the high-energy peaks (HEPs) keep their positions unmoved. The LEPs (HEPs) mostly comes from the transitions between near (far) the HOMO and LUMO energy levels. In these transitions, the low-energy excitations are dominantly contributed by d-electrons, indicating the fact that intense coupling effects can weaken the screen of d-electrons on sp-electrons. By means of the competition mechanism between charge transfer and electron cloud distortion, the spacing dependence behaviors of both electric and optical properties are explained self-consistently.     

Polymer:Fullerene Bimolecular Crystals for Near‐Infrared Spectroscopic Photodetectors

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and biointegrated electronics, is hampered by the use of bulky dispersive optics. Here, solution‐processed organic donor–acceptor blends are employed in a resonant optical cavity device architecture for wavelength‐tunable photodetection. While conventional photodetectors respond to above‐gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge‐transfer states of the intercalating poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, near‐infrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors, is fabricated by using a blade‐coated PBTTT:PCBM thin film with a thickness gradient. As an application example, a measurement of the transmittance spectrum of water by this device is demonstrated.     

A Scalable Nickel–Cellulose Hybrid Metamaterial with Broadband Light Absorption for Efficient Solar Distillation

Sophisticated metastructures are usually required to broaden the inherently narrowband plasmonic absorption of light for applications such as solar desalination, photodetection, and thermoelectrics. Here, nonresonant nickel nanoparticles (diameters < 20 nm) are embedded into cellulose microfibers via a nanoconfinement effect, producing an intrinsically broadband metamaterial with 97.1% solar-weighted absorption. Interband transitions rather than plasmonic resonance dominate the optical absorption throughout the solar spectrum due to a high density of electronic states near the Fermi level of nickel. Field solar purification of sewage and seawater based on the metamaterial demonstrates high solar-to-water efficiencies of 47.9–65.8%. More importantly, the solution-processed metamaterial is mass-producible (1.8 × 0.3 m²), low-cost, flexible, and durable (even effective after 7 h boiling in water), which are critical to the commercialization of portable solar-desalination and domestic-water-purification devices. This work also broadens material choices beyond plasmonic metals for the light absorption in photothermal and photocatalytic applications.     

Optical properties of HgTe colloidal quantum dots

Room temperature photodetection with HgTe colloidal quantum films is reported between 2 and 5 μm for particles of sizes between ~5 and ~12 nm diameter, and photodetection extends to 7 μm at 80 K. The size-tuning of the absorption of HgTe colloidal quantum dots, their optical cross section and the infrared absorption depth of films are measured. The tuning with radius is empirically given by [see formula in text] where R is in nm. The optical cross section of the colloidal dots at 415 nm is approximately proportional to their volume and given by σ(Hg)(415) = 2.6 ± 0.4 10(-17) cm(2)/mercury atom. The size-dependent optical cross section at the band edge ~1.5 10(-15) cm(2) is consistent with the expected oscillator strength of the quantum dots. The absorption depth of HgTe colloidal dot films is short, about 1-2 μm, which is an advantage for thin film devices. These properties agree rather well with the expectation from the k · p model. HgTe colloidal quantum dot thin films show a strong tuning with temperature with a large positive thermal shift between 0.4 and 0.2 meV K(-1), decreasing with decreasing size within the size range studied and this is attributed primarily to electron-phonon effects.     

Effect of an electrostatic field on the optical properties of a cloud of dielectric particles

The optical properties of a cloud of anisotropic dielectric particles when the orientational distribution is made nonrandom by interaction with an electrostatic field are studied. Since the interaction energy is determined by the polarizability of the particles, a general expression for the polarizability of nonspherical particles is worked out. In particular, we investigated the response to the electrostatic field of two different dispersions whose component particles are built as clusters of four identical spheres. Although in one cloud the clusters were shaped as linear chains, and in the other cloud the clusters were shaped as squares, the optical properties of both dispersions as a function of the static field are rather similar. There are, however, noticeable ranges of size within which the optical response of the two kinds of particles is substantially different.     

Polarizability, volume expansion, and stress contributions to the refractive index change of Cu+-Na+ ion exchanged waveguides in glass

The refractive index of optical waveguides formed by electric field assisted Cu(+)-Na(+) ion exchange in two types of glass is measured. Assuming, as in a previously published work, that the observed refractive index increase is solely due to polarizability changes, the difference in electronic polarizability between Cu(+) and Na(+) ions is determined by applying the Lorentz-Lorenz equation to the data. In our work, the concentration of exchanged ions, which is a necessary input to the Lorentz-Lorenz equation, is determined by combining optical data and electrical data obtained during the exchange. Values for the electronic polarizability difference are in agreement with that in the literature. However, when a correction is made, taking into consideration the measured volume expansion and stress in the glass, the calculated electronic polarizability difference is shown to increase by 19%.     

Photonic Metamaterial Absorbers: Morphology Engineering and Interdisciplinary Applications

Recent advances in nanofabrication technologies have spurred many breakthroughs in the field of photonic metamaterials that provide efficient ways of manipulating light–matter interaction at subwavelength scales. As one of the most important applications, photonic metamaterials can be used to implement novel optical absorbers. First the morphology engineering of various photonic metamaterial absorbers is discussed, which is highly associated with impendence matching conditions and resonance modes of the absorbers, thus directly determines their absorption efficiency, operational bandwidth, incident angle, and polarization dependence. Then, the recent achievements of various interdisciplinary applications based on photonic metamaterial absorbers, including structural color generation, ultrasensitive optical sensing, solar steam generation, and highly responsive photodetection, are reviewed. This report is expected to provide an overview and vision for the future development of photonic metamaterial absorbers and their applications in novel nanophotonic systems.     

Influence of structural and dielectric anisotropy on the dielectrophoresis of single-walled carbon nanotubes

We report on a carbon nanotube network which is composed of aligned metallic and randomly oriented semiconducting single-walled carbon nanotubes. The material is formed by using a novel radio frequency dielectrophoresis setup, which generates very large dielectrophoretic force fields and allows dielectrophoretic assembling of nanotube films up to 100 nm thickness. Polarization dependent absorption measurements provide experimental evidence for the electronic type specific alignment behavior. We explain the experimental data with an advanced model for nanotube dielectrophoresis, which explicitly takes into account both the longitudinal and transversal polarizability. On the basis of this model, we calculate the dielectrophoretic force fields and show that semiconducting nanotubes deposit under very large fields due to their transversal polarizability even for high field frequencies.     

Spectral absorption coefficient measured in situ in the North Sea with a marine radiometric spectrometer system

A submersible marine radiometric spectrometer system, capable of simultaneous measurements of the in situ spectral and angular properties of the underwater oceanic light field, is used to determine spectral inherent optical properties of marine waters. The inversion methods used to convert the sampled light field measurements into estimates of spectral absorption are presented and sample results for three water types obtained during a cruise in the North Sea are given.     

The influence of different alkaline earth oxides on the structural and optical properties of undoped,Ce-doped,Sm-doped,and Sm/Ce co-doped lithium alumino-phosphate glasses

Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO₂ concentration as a result of Ce³⁺/Ce⁴⁺ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg²⁺ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba²⁺ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity R_m and molar polarizability α_m values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm³⁺ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO₂, the absorption of Ce³⁺ hinders the high energy absorption of Sm³⁺ and this effect becomes more obvious with increasing optical basicity.     

Exploiting optical properties of P3HT:PCBM films for organic solar cells with semitransparent anode

In this study, we demonstrate optical properties of multilayer system in an organic solar cell based on poly (3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM) with semitransparent anode through thermal annealing effect. The optical absorption is enhanced via optimizing annealing treatment which further elevates near-field electric field amplitude. The electric field amplitude at the interface (active layer/semitransparent anode) is enhanced after thermal annealing corresponding to effective absorption near to semitransparent anode. Moreover, the thickness of the active layer is optimized via optical thin-film model for enhancing the organic solar cell efficiency.     

Long-path measurement of atmospheric NO2 with an obstruction flashlight and a charge-coupled-device spectrometer

A novel method of differential optical absorption spectroscopy (DOAS) is proposed and demonstrated to monitor the concentration of atmospheric pollutant gas. In contrast to conventional DOAS measurements with continuous light sources, the present method relies on white flashlights such as aviation obstruction lights that are generally installed on tall constructions. A simple detection system is devised by means of a telescope and a compact CCD spectrometer. A path length of 5.5 km allows us to measure atmospheric NO2 concentration with a detection limit of approximately 1 part per billion. We also discuss the possibility of deriving the aerosol optical thickness through the horizontal atmosphere from this pulsed DOAS measurement.     

Sensitivity analysis of differential absorption lidar measurements in the mid-infrared region

The availability of new laser sources that are tunable in the IR spectral region opens new perspectives for differential absorption lidar (DIAL) measurements. A region of particular interest is located in the near IR, where some of the atmospheric pollutants have absorption lines that permit monitoring of emissions from industrial plants and in urban areas. In DIAL measurements, the absorption lines for the species to be measured must be carefully chosen to prevent interference from other molecules, to minimize the dependence of the absorption cross section on temperature, and to optimize the measurements with respect to the optical depth. We analyze the influence of these factors and discuss a set of criteria for selecting the best pairs of wavelengths (lambda(on) and lambda(off)) to be used in DIAL measurements of several molecular species (HCl, CO, CO(2), NO(2), CH(4), H(2)O, and O(2)). Moreover, a sensitivity study has been carried out for selected lines in three different regimes: clean air, urban polluted air, and emission from an incinerator stack.