High Spectral Selectivity Research Articles

ZnO Hollow‐Sphere Nanofilm‐Based High‐Performance and Low‐Cost Photodetector

The first high-performance ZnO hollow-sphere nanofilm-based photodetector is constructed by 'water-oil interfacial self-assembly' of polystyrene (PS)/ZnO core/shell nanospheres and subsequent thermal treatment. The versatile growth substrate, general fabrication strategy, and high performance including excellent sensitivity, high spectral selectivity, and fast response times show the current work to be a new paradigm in the preparation of hollow-sphere nanofilm-based photodetectors.

Choosing optical materials for diagnostics of the solar atmosphere in the wavelength range of 6–60 nm

We present our calculations for the reflection characteristics of multilayer mirrors for a series of wavelengths in the interval of 6–60 nm. Interest in these wavelengths is due to preparations for prolonged studies of the Sun within Russia’s ARCA project. For each structure, we suggested a relationship of material layer thicknesses in the period that ensures the optimum reflection coefficient for high spectral selectivity.

High-speed, efficient metal - semiconductor - metal photodetectors

Design principles and the fabrication technique of highly efficient, high-speed photodetectors based on MSM nanostructures are developed. To efficiently confine light in the region of the strong field as well as to decrease light losses due to reflection from the diode contacts, use is made of a nanoscale interdigital diffraction grating and a multilayer Bragg grating. Measurements of the reflection coefficients and the quantum efficiency for a multilayer structure are in good agreement with theoretical estimates. A record-high quantum efficiency (QE = 46 %) is obtained for high speed MSM photodetectors. The detector has a high spectral selectivity (Δλ1/2 = 17 nm) at a wavelength of 800 nm. Taking into account the diode capacitance and the drift time of photogenerated carriers, the performance of the detectors under study is ~ 500 GHz. The low level of the dark current density in the structures under study (j="1" pA μm-2) makes it possible to realise on their basis highly sensitive, high-speed selective detectors of optical radiation.

Photoacoustic investigation of QCL modulation techniques

High detection sensitivity and spectral selectivity is important for gas analysers to identify the measured compound and to detect low concentrations. We investigated three different modulation methods – pulse gate modulation, pulse frequency modulation and chopper modulation – for a new pulsed quantum cascade laser based photoacoustic sensor. The spectral selectivity and the detection limit for the three modulation methods are compared by measuring nitric oxide absorption lines and different concentrations. The highest detection sensitivity of 70 ppb was achieved with pulse gate modulation but at the lowest spectral resolution. The highest spectral resolution was achieved with chopper modulation but at the lowest detection sensitivity. It is demonstrated that for the three modulation methods a compromise has to be made between selectivity and sensitivity for each measuring task.

Narrow linewidth high output-coupling dual VBG-locked Yb-doped fiber laser

Two equal highly reflective volume Bragg gratings (VBGs) were used to lock an Yb-doped fiber laser. By heating one of the VBGs, its center wavelength was shifted and the laser was locked on the overlap between the main peak of one grating and the side-lobe of the other creating a large outcoupling with high spectral selectivity. With this simple arrangement, unidirectional output is achieved with a narrow linewidth (<2.5 GHz), high efficiency (>70%) and with an output power above 7 W.

High-Performance Blue/Ultraviolet-Light-Sensitive ZnSe-Nanobelt Photodetectors.

Single-crystalline zinc selenide (ZnSe) nanobelts were fabricated via the ethylenediamine (en)-assisted ternary solution technique and subsequent thermal treatment. Individual ZnSe nanobelts were assembled into nanoscale devices, showing a high spectral selectivity and photocurrent/immediate-decay ratio and a fast time response, justifying effective utilization of the ZnSe nanobelts as blue/UV-light-sensitive photodetectors.

Single‐Crystalline ZnS Nanobelts as Ultraviolet‐Light Sensors

Single-crystalline ZnS nanobelts with sharp ultraviolet-light emission (∼337 nm) at room temperature have been assembled as UV Sensors. The high spectral selectivity, combined with high photosensitivity and fast response time, justifies the effective utilization of the present ZnS nanobelts as “visible-light-blind” UV photodetectors in different areas.

Design and performance of two-channel EUV multilayer mirrors with enhanced spectral selectivity

In this paper, we present a study on two-channel multilayer mirrors which can operate at two wavelengths in Extreme Ultraviolet (EUV) spectral range. We propose a new method to design two-channel EUV multilayer mirrors with enhanced spectral selectivity. The mirror structure is a stack of two periodic multilayers separated by a buffer layer. We have defined the main parameters which allow adjustment of the distance between different order Bragg’s peak and of wavelength positions of reflectivity minima. Two mirrors have been designed and deposited for solar EUV telescope applications by using this method. The first mirror reflects Fe IX–X line (17.1 nm) and Fe XVI (33.5 nm) lines with attenuation of the He II line (30.4 nm). The second mirror reflects Fe IX–X and He II lines with attenuation of Fe XV (28.4 nm) and Fe XVI lines. Measurements with synchrotron radiation source confirm that, in both cases, for these mirrors, we are able to adjust reflectivity maxima (Bragg peak position) and minima. Such multilayers offer new possibilities for compact design of multi-wavelength EUV telescopes and/or for high spectral selectivity.

Targeted Optical Control of an Active Medium Embedded In Photonic Crystals

We demonstrate optical control of an active medium, such as a quantum dot or an impurity atom, embedded in three-dimensional photonic crystals. The optical control provides a very high spectral selectivity, which allows us to control confined light near a targeted active medium with a specific resonant frequency. Using such an optical control, even if the other active mediums are embedded nearby, we may selectively control only the targeted active medium, leading to a basic operation of an optical memory on the nanoscale below the light wavelength.

Characteristics of ultra-compact polymer modulators based on silicon photonic crystal ring resonators

Ultra-compact polymer modulators were proposed and simulated, based on the hybrid integration of functional polymer materials with Si based photonic-crystal ring resonators (PCRRs). The simulations were carried out on device characteristics with an effective ring radius of 2.3 µm and tunable polymer index from 1.785 to 1.805. Investigating the loss, the cavity quality factor Q, and the free spectral range of such PCRRs, we found a 0.02 dB intrinsic loss that is independent of diameter, unlike the loss that varies inversely with diameter in microstrip resonators. Close to 100% drop efficiency at the drop channel of 1557.5 nm was obtained by design with a high spectral selectivity of Q greater than 1319 in the single-ring PCRR-based add-drop filters with ring radius of 1.2 µm.

Wide-field depth-sectioning fluorescence microscopy using projector-generated patterned illumination

We present a simple and cost-effective wide-field, depth-sectioning, fluorescence microscope utilizing a commercial multimedia projector to generate excitation patterns on the sample. Highly resolved optical sections of fluorescent pollen grains at 1.9 microm axial resolution are constructed using the structured illumination technique. This requires grid excitation patterns to be scanned across the sample, which is straightforwardly implemented by creating slideshows of gratings at different phases, projecting them onto the sample, and synchronizing camera acquisition with slide transition. In addition to rapid dynamic pattern generation, the projector provides high illumination power and spectral excitation selectivity. We exploit these properties by imaging mouse neural cells in cultures multistained with Alexa 488 and Cy3. The spectral and structural neural information is effectively resolved in three dimensions. The flexibility and commercial availability of this light source is envisioned to open multidimensional imaging to a broader user base.

Dynamic contrast enhancement in widefield microscopy using projector-generated illumination patterns

We present a simple and cost-effective optical protocol to realize contrast-enhancement imaging (such as dark-field, optical-staining and oblique illumination microscopy) of transparent samples on a conventional widefield microscope using commercial multimedia projectors. The projector functions as both light source and mask generator implemented by creating slideshows of the filters projected along the illumination planes of the microscope. The projected optical masks spatially modulate the distribution of the incident light to selectively enhance structures within the sample according to spatial frequency thereby increasing the image contrast of translucent biological specimens. Any amplitude filter can be customized and dynamically controlled so that switching from one imaging modality to another involves a simple slide transition and can be executed at a keystroke with no physical filters and no moving optical parts. The method yields an image contrast of 89–96% comparable with standard enhancement techniques. The polarization properties of the projector are then utilized to discriminate birefringent and non-birefringent sites on the sample using single-shot, simultaneous polarization and optical-staining microscopy. In addition to dynamic pattern generation and polarization, the projector also provides high illumination power and spectral excitation selectivity through its red-green-blue (RGB) channels. We exploit this last property to explore the feasibility of using video projectors to selectively excite stained samples and perform fluorescence imaging in tandem with reflectance and polarization reflectance microscopy.

Spectrally selective infrared absorption in defect-mode photonic-crystal-slab cavity

Significantly enhanced absorption at the defect mode can be obtained at surface-normal direction in a dielectric single-defect photonic-crystal-slab, with an absorption enhancement factor greater than 4,000. Complete absorption suppression within the photonic bandgap region can also be observed in defect-free photonic crystal cavities. High spectral selectivity and tunability is feasible with defect mode engineering, making photonic crystal defect cavities a promising nanophotonic platform for the spectrally selective infrared sensing and hyper-spectral imaging, with the incorporation of quantum well or quantum dot infrared photodetector heterostructures.

Precise Control of Molecular Dynamics with a Femtosecond Frequency Comb

We present a general and highly efficient scheme for performing narrow-band Raman transitions between molecular vibrational levels using a coherent train of weak pump-dump pairs of shaped ultrashort pulses. The use of weak pulses permits an analytic description within the framework of coherent control in the perturbative regime, while coherent accumulation of many pulse pairs enables near unity transfer efficiency with a high spectral selectivity, thus forming a powerful combination of pump-dump control schemes and the precision of the frequency comb. Simulations verify the feasibility and robustness of this concept, with the aim to form deeply bound, ultracold molecules.

Coherent anti-Stokes Raman scattering microscopy (CARS): Instrumentation and applications

Recent advances in laser physics have permitted the development of a new kind of microscopy based on stimulated Raman scattering. This new technique known as Coherent anti-Stokes Raman scattering (CARS) microscopy allows vibrational imaging with high sensitivity, high spectral resolution and three-dimensional sectioning capabilities. We review recent advances in CARS microscopy, with applications to chemical and biological systems. We also present an application of CARS microscopy with high optical resolution and spectral selectivity, in resolving structures in surface ex vivo stratum corneum by looking at the CH 2 stretching vibrational band. A strong CARS signal is backscattered from an intense forward generated CARS signal in thick samples. This makes noninvasive imaging of deep structures possible, without labeling or chemical treatments.

Single-frequency Sb-based distributed-feedback lasers emitting at 23 μm above room temperature for application in tunable diode laser absorption spectroscopy

GaInAsSb/GaAlAsSb/GaSb distributed-feedback (DFB) laser diodes based on a type I active region were fabricated by molecular beam epitaxy at the Centre d'Electronique et de Micro-Optoélectronique de Montpellier (CEM2). The DFB processing was done by Nanoplus Nanosystems and Technologies GmbH. The devices work in the continuous-wave regime above room temperature around an emission wavelength of 2.3 microm with a side-mode suppression ratio greater than 25 dB and as great as 10 mW of output power. The laser devices are fully characterized in terms of optical and electrical properties. Their tuning properties made them adaptable to tunable diode laser absorption spectroscopy because they exhibit more than 220 GHz of continuous tuning by temperature or current. The direct absorption of CH4 is demonstrated to be possible with high spectral selectivity.

Crossed-gratings volume hologram: backward reflection with high angular and spectral selectivity

A new type of reflective hologram with both high angular and spectral selectivity is suggested. With novel design of crossed gratings inside the volume hologram, unusual angular selectivity is achieved via coupling of four waves. Analytic solution with spectral and angular detuning from Bragg regime is derived. The compensation between these two detuning forms an interesting figure of saddle shape.

Solar blind AlGaN photodetectors with a very high spectral selectivity

Solar blind detectors based on AlGaN heterostructures grown on sapphire by Molecular Beam Epitaxy and with a dielectric interference filter deposited on the back side are demonstrated to provide record spectral selectivity. Rejection ratios of 2 x 10(4), and better than 5 x 10(4), measured between 280 and 320 nm, are achieved in Metal Semiconductor Metal detectors and Schottky diodes respectively. The whole detector process is fully compatible with low cost array fabrication.

Tunable high-finesse InP/air MOEMS filter

We have fabricated Fabry-Pe/spl acute/rot tunable filters based on a multi-air-gap/InP suspended structure. High spectral selectivity is achieved by displacing the resonant cavity into the high index material rather than in air. This configuration reduces the lateral losses of the cavity. The filters feature a bidirectional electrostatic actuation. A spectral linewidth better than 0.15 nm over a tuning range of 40 nm is experimentally demonstrated.

Highly sensitive UV photodetectors fabricated using high-quality single-crystalline CVD diamond films

We have characterized the diamond-based photodetectors (DBPs) fabricated using high-quality single crystalline diamond film. The diamond film was homoepitaxially grown at high growth rate of ∼2 μm/h by high-power microwave plasma chemical vapor deposition (CVD). The grown film had a substantially flat surface without non-epitaxial crystallites and gave strong free-exciton recombination emission from almost whole homoepitaxial region. Planar-type DBPs with interdigitated electrodes were fabricated using the diamond film with 20-μm thickness by standard photolithograph techniques. The DBP with 43-μm electrode spacing revealed small noise equivalent power (NEP), <1 pW, for 220-nm ultraviolet (UV) light. The NEP was four orders smaller than that for UV-sensitive Si photodetector (SP), that means high performance to faint UV light detection. Photo responsivity of the DBP for 400-nm visible light was at least four orders smaller than that for 220-nm UV light. The low NEP for UV light and high spectral selectivity of the present DBPs reflects high crystalline quality of the diamond film.