Narrow Spectral Bandwidth Research Articles

Two-photon fluorescence imaging using a tunable spectral window based on fiber supercontinuum.

Two-photon excitation fluorescence (TPEF) microscopy has evolved into a versatile tool in biological research. However, the multiplexing capability of TPEF microscopy is limited by the narrow spectral bandwidth of the light source. In this study, we apply a photonic crystal fiber in TPEF microscopy to broaden the excitation source bandwidth. We tuned the spectral window using a spatial light modulator as a programmable diffraction grating that was placed behind a prism pair. In addition, we combined a grating pair to compensate for dispersion to improve the two-photon excitation efficiency. The combination of a broad spectrum and a programmable grating enabled fast spectral window tuning rate on a time scale of tens of milliseconds. We demonstrate the performance of our method by imaging live B16 cells labeled with four emission spectrum overlapped fluorescent proteins.

Simultaneous Dual-Band Hyperspectral Stimulated Raman Scattering Microscopy with Femtosecond Optical Parametric Oscillators.

Stimulated Raman scattering (SRS) microscopy is a label-free quantitative optical technique for imaging molecular distributions in cells and tissues by probing their intrinsic vibrational frequencies. Despite its usefulness, existing SRS imaging techniques have limited spectral coverage due to either a wavelength tuning constraint or narrow spectral bandwidth. High-wavenumber SRS imaging is commonly used to map lipid and protein distribution in biological cells and visualize cell morphology. However, to detect small molecules or Raman tags, imaging in the fingerprint region or "silent" region, respectively, is often required. For many applications, it is desirable to collect SRS images in two Raman spectral regions simultaneously for visualizing the distribution of specific molecules in cellular compartments or providing accurate ratiometric analysis. In this work, we present an SRS microscopy system using three beams generated by a femtosecond oscillator to acquire hyperspectral SRS image stacks in two arbitrary vibrational frequency bands, between 650-3280 cm-1, simultaneously. We demonstrate potential biomedical applications of the system in investigating fatty acid metabolism, cellular drug uptake and accumulation, and lipid unsaturation level in tissues. We also show that the dual-band hyperspectral SRS imaging system can be adapted for the broadband fingerprint region hyperspectral imaging (1100-1800 cm-1) by simply adding a modulator.

A broadband high-brightness quantum-dot double solid immersion lens single photon source

High-brightness single photon sources (SPSs) are key components for practical quantum information processing systems. Although the performances of recently reported high-brightness SPSs are excellent, it remains challenging to match the emission wavelength of a quantum dot (QD) to the cavity since the high-Q cavity structures have narrow spectral bandwidths. Here, we propose a highly bright and broadband QD SPS that can be deterministically fabricated with a simple yet precise method. The optimized GaAs-polymer double solid immersion lens structure is capable of a brightness of 88% at 0.5 NA and has an operation band of 65 nm with a brightness of over 80% from numerical simulations. Experimentally, we achieved a brightness of 51.6% ± 2% and pure single photon emission [g(2)(0) = 0.029 ± 0.005] at saturation. We believe that our result can pave the way to a practical high-brightness QD SPS, considering its simple QD geometry together with its low cost and precise deterministic fabrication without using expensive and complicated e-beam lithography and dry etching processes.

Daylight entanglement-based quantum key distribution with a quantum dot source

Entanglement-based quantum key distribution can enable secure communication in trusted node-free networks and over long distances. Although implementations exist both in fiber and in free space, the latter approach is often considered challenging due to environmental factors. Here, we implement a quantum communication protocol during daytime for the first time using a quantum dot source. This technology presents advantages in terms of narrower spectral bandwidth—beneficial for filtering out sunlight—and negligible multiphoton emission at peak brightness. We demonstrate continuous operation over the course of three days, across an urban 270 m-long free-space optical link, under different light and weather conditions.

Wide-spectrum optical synthetic aperture imaging via spatial intensity interferometry

High resolution imaging is achieved using increasingly larger apertures and successively shorter wavelengths. Optical aperture synthesis is an important high-resolution imaging technology used in astronomy. Conventional long baseline amplitude interferometry is susceptible to uncontrollable phase fluctuations, and the technical difficulty increases rapidly as the wavelength decreases. The intensity interferometry inspired by HBT experiment is essentially insensitive to phase fluctuations, but suffers from a narrow spectral bandwidth which results in a lack of effective photons. In this study, we propose optical synthetic aperture imaging based on spatial intensity interferometry. This not only realizes diffraction-limited optical aperture synthesis in a single shot, but also enables imaging with a wide spectral bandwidth, which greatly improves the optical energy efficiency of intensity interferometry. And this method is insensitive to the optical path difference between the sub-apertures. Simulations and experiments present optical aperture synthesis diffraction-limited imaging through spatial intensity interferometry in a 100 nm spectral width of visible light, whose maximum optical path difference between the sub-apertures reaches 69<italic>λ</italic>. This technique is expected to provide a solution for optical aperture synthesis over kilometer-long baselines at optical wavelengths.

Wavelength‐stabilized ns‐pulsed 2.2 kW diode laser bar with multiple active regions and tunnel junctions

The improvement of the performance of a distributed Bragg reflector laser bar emitting near 905 nm through the use of multiple epitaxially stacked active regions and tunnel junctions is reported. The bar consisting of 48 emitters (each having an aperture of 50 µm) emits an optical power of 2.2 kW in 8 ns long pulses at an injection current of 1.1 kA. This corresponds to an almost threefold increase of the pulse power compared to a bar with lasers having only a single active region. Due to the integrated surface Bragg grating, the bar exhibits a narrow spectral bandwidth of about 0.3 nm and a thermal tuning of only 68 pm/K.

High-sensitivity waveguide-integrated bolometer based on free-carrier absorption for Si photonic sensors.

Conventional photon detectors necessarily face critical challenges regarding strong wavelength-selective response and narrow spectral bandwidth, which are undesirable for spectroscopic applications requiring a wide spectral range. With this perspective, herein, we overcome these challenges through a free-carrier absorption-based waveguide-integrated bolometer for infrared spectroscopic sensors on a silicon-on-insulator (SOI) platform featuring a spectrally flat response at near-infrared (NIR) range (1520-1620 nm). An in-depth thermal analysis was conducted with a systematic investigation of geometry dependence on the detectors. We achieved great performances: temperature coefficient of resistance (TCR) of -3.786%/K and sensitivity of -26.75%/mW with a low wavelength dependency, which are record-high values among reported waveguide bolometers so far, to our knowledge. In addition, a clear on-off response with the rise/fall time of 24.2/29.2 µs and a 3-dB roll-off frequency of ∼22 kHz were obtained, sufficient for a wide range of sensing applications. Together with the possibility of expanding an operation range to the mid-infrared (MIR) band, as well as simplicity in the detector architecture, our work here presents a novel strategy for integrated photodetectors covering NIR to MIR at room temperature for the development of the future silicon photonic sensors with ultrawide spectral bandwidth.

16.4: Bandwidth effect correction used for wide color gamut display measurement

With the development of display technology, several new displays have attracted much attention such as OLED display, QLED display, Laser display and Mini/MicroLED display. Displays with high brightness, wide color gamut, high efficiency and long lifetime are our ultimate goal. Defect inspections and color measurements are important for us to evaluate display quality. To obtain wide color gamut, we usually use narrow spectral bandwidth light source. However, it's not easy for traditional color camera or filter camera to measure narrow bandwidth light source because the accuracy is limited. In this paper, we simulate color measurement accuracy by color camera and filter camera. We analyze the influence of bandwidth effect on the color measurement and carry out bandwidth effect correction method. With the correction, the measurement accuracy improves obviously. In addition, different displays with different spectral bandwidth have been measured to validate the corrections. The results show significant improvement in color measurement accuracy. Our bandwidth effect correction can be used for wide color gamut display measurement.

Spatial spectral contextual image analysis approach to map water quality of inland waterbody using AVIRIS-NG datasets

Remote sensing has already proven its capability in water quality studies. The technology enables the mapping of water quality parameters that affect its optical properties. Generally, multi-spectral remote sensing data has been utilized for such analysis. Mostly, the band rationing or simple regression approaches have been adapted to map the water quality, however, they result in qualitative estimates rather than quantitative. On the other hand, sensor technology has made steep advancements from multi-spectral to hyperspectral. The hyperspectral data provides contiguous spectral information over several hundred narrow spectral bands. This emerging technique provides a more in-depth means of investigating spatial, spectral, and temporal variations, leading to more quantifiable/accurate estimates of the process under consideration. At present, very few satellite-based hyperspectral data are available. Recently, the Indian Space Research Organisation has initiated an airborne hyperspectral remote sensing campaign to cover the important earth features in India using Airborne Visible/Infrared Imaging Spectrometer – Next Generation (AVIRIS-NG) sensor in collaboration with the National Aeronautics and Space Administration (NASA). As the AVIRIS-NG hyperspectral data measures the spectral response in the wavelength range from 380 to 2510 nm at very narrow spectral bandwidth of 5 nm. These advanagtes of AVIRIS-NG datasets enable researchers to analyse the spectra of each and every pixel of the image spatially to match its spectral characteristics with the spectra collected in the field. In the present study, this spectral similarity approach has been expoilted within the context of turbidity, the optical property of water for Chilika Lake, Odisha, India; using the AVIRIS-NG data and field reference spectra. The lake was surveyed synchronous to AVIRIS-NG flights over it during the month of December 2015. A field-based spectral library with respect to various concentrations of turbidity in the lake has been generated using sophisticated instruments like field spectro-radiometer, handheld Global Navigation Satellite System (GNSS) receiver, and turbidimeter. The spectral angle mapper (SAM) spatial-spectral contextual image analysis has been carried out to map the turbidity of the lake. It was observed that almost at each and every pixel the image spectra were matching with the field-collected spectra. Further, the match between two spectra (field and image) measured in terms of SAM score was as high as 0.9 for most of the instances. Moreover, the estimated turbidity was in the agreement (R2 = 0.96) with the turbidity measured on the field. It was realized that the spectral similarity approach using hyperspectral remote sensing data provides more quantitative estimates as compared to multi-spectral data analysis.

Direct patterning of colloidal quantum dots with adaptable dual-ligand surface.

Colloidal quantum dots (QDs) stand at the forefront of a variety of photonic applications given their narrow spectral bandwidth and near-unity luminescence efficiency. However, integrating luminescent QD films into photonic devices without compromising their optical or transport characteristics remains challenging. Here we devise a dual-ligand passivation system comprising photocrosslinkable ligands and dispersing ligands to enable QDs to be universally compatible with solution-based patterning techniques. The successful control over the structure of both ligands allows the direct patterning of dual-ligand QDs on various substrates using commercialized photolithography (i-line) or inkjet printing systems at a resolution up to 15,000 pixels per inch without compromising the optical properties of the QDs or the optoelectronic performance of the device. We demonstrate the capabilities of our approach for QD-LED applications. Our approach offers a versatile way of creating various structures of luminescent QDs in a cost-effective and non-destructive manner, and could be implemented in nearly all commercial photonics applications where QDs are used.

Microdisk array based Weyl semimetal nanofilm terahertz detector

Abstract High-performance terahertz wave detectors at room temperature are still urgently required for a wide range of applications. The available technologies, however, are plagued by low sensitivity, narrow spectral bandwidth, complicated structure, and high noise equivalent power (NEP). Here, we have demonstrated a Weyl semimetal surface plasmon-enhanced high-performance terahertz wave detectors which are based on microdisk array deposited WTe2 nanofilm epitaxially grown on GaN substrate for room temperature operation. With the microdisk array combined the WTe2 layer, strong terahertz wave surface plasmon polaritons can be generated at the WTe2–air interfaces, which results in significant improvement in detecting performance. For the 40 μm diameter microdisk array, a detectivity (D *) of 5.52 × 1012 cm Hz1/2 pW−1 at 0.1 THz is achieved at room temperature. In addition, the responsivity (R A) of 8.78 A W−1 is also obtained. Such high-performance millimeter and terahertz wave photodetectors are useful for wide applications such as high capacity communications, walk-through security, biological diagnosis, spectroscopy, and remote sensing.

Wavelength switching in 2 µm narrow spectral bandwidth Tm:YAP laser

A birefringent-etalon was used to achieve 2 µm narrow spectral bandwidth laser wavelength switching. The 660 mW narrow spectral bandwidth laser was achieved at the wavelength of 1987.6 nm by adjusting the inclination of the birefringent-etalon in relative to the optical axis of the resonator. The beam quality was M2 = 1.89 and M2 = 1.96 respectively in the x direction and y direction. The wavelength switching was accomplished by rotating the birefringent-etalon along its plane. Four thicknesses of birefringent-etalons were used to achieved switching wavelengths at 1988.7 nm, 1938 nm, 1964.7 nm and 1983 nm with the maximum power of 603 mW, 537 mW, 575 mW and 447 mW, respectively.

Spectral synthesis of multimode lasers to the Fourier limit in integrated Fabry–Perot diamond resonators

Fourier-limited nanosecond pulses featuring narrow spectral bandwidths are required for applications in spectroscopy, sensing, and quantum optics. Here, we demonstrate a direct and simple route for the generation of single-frequency light relying on phonon-resonant Raman interactions within a monolithic diamond resonator. The technique enables the production of nearly Fourier-limited nanosecond optical pulses (15 ns), with an overall spectral bandwidth of down to 180 MHz, which is nearly two orders of magnitude narrower than the pump laser linewidth used (12 GHz). The power conversion efficiency was 47%, yielding a power spectral brightness enhancement of &gt; --> 50 × compared to the pump. Our results pave the way to the integration of pulsed widely tunable, power scalable, narrow linewidth light sources into integrated photonic platforms. Furthermore, the device does not need elaborate mechanical feedback loops for cavity length or frequency stabilization, or any additional optical components.

Methodological approach to assessing the influence of ionizing radiation on the efficiency of functioning of free­space optical communication

The methodological approach to the assessment of the influence of ionizing radiation on the efficiency of functioning of free-space optical communication, taking into account technogenically loaded territories of Ukraine, is considered. It is indicated that the efficiency of functioning of free-space optical communication (FSO) should be methodologically examined through the influence of external factors using an integrated approach taking into account the concept of uranium heritage in Ukraine and Europe. It is proved that the protection of FSO transceivers from gamma radiation will be determined by the attenuating ability of their design elements and the direction of the radiant flux vector. The conclusions that it is possible to reduce the level of ionizing radiation impact on the FSO receiver by selecting a receiver with the required value of integral sensitivity, as well as by reducing the bandwidth, are confirmed. The dependence of the average energy of gamma radiation on the angle of incidence at the observation point is established. It is pointed out that the influence of ionizing radiation leads to a decrease in the signal-to-noise ratio and effective sensitivity of the receiver. In some cases, ionizing radiation can cause communication interruptions due to receiver saturation. In addition to the useful signal, the receiver lens also collects some unwanted background radiation, which may consist of direct sunlight, reflected sunlight or scattered sunlight from hydrometeors or other objects. Their effect can be reduced by using a narrow spectral bandwidth and spatial filtering before photodetection. However, minor background noise can enter the spatial and frequency ranges of the detector, which can limit system performance by causing a variable offset in the converted electrical signal.

High pulse energy, narrow-linewidth all-fiber 1064 nm picosecond master oscillator power amplifier system

We demonstrated the generation of narrow spectral bandwidth, high energy picosecond pulses from a low repetition rate SESAM mode-locked laser seeded Yb-doped fiber MOPA system. A short large mode field diameter gain fiber (DCF-30/250) was adopted to efficiently reduce the influence of nonlinear effects. The maximum pulse energy of 12.36 μJ, ∼60 ps pulses with a narrow bandwidth was obtained at a repetition rate of 5 MHz, corresponding to the peak power of 0.2 MW and average output power of 64 W. The optical-to-optical conversion efficiency was ∼64% in such a low repetition rate all-fiber laser design. The output pulse spectra and temporal trace of the MOPA system for different output powers was investigated by simulation. The simulation results were in agreement with experiment but further showed that the spectral splitting would occur as output power increased.

An All-Fiber Mode-Locked Pulse Laser by Fiber Bragg Grating-Based Acousto-Optic Frequency Shifter

An all-fiber frequency shifted feedback laser is developed to export mode-locked pulses with a narrow spectral bandwidth (238 MHz). A simple cavity structure is established by employing a fiber Bragg grating-based acousto-optic frequency shifter. An exotic reflection peak in a large spectral separation from the Bragg reflection peak is generated by applying flexural acoustic waves, and is utilized for narrow-band filtering and frequency shifting in the laser cavity. Stable mode-locked pulse output can be obtained with the cavity length as short as 16.5 m, whereas increasing the cavity length can reduce the pump power to 37 mW for mode-locked pulse output. A maximum power conversion efficiency of 17% with an average output power of 103 mW were obtained experimentally at a pump power of 600 mW, with corresponding pulse width, pulse energy and peak power of 2 ns, 88 nJ and 4.5 W, respectively. A time-bandwidth product of mode locked pulses of 0.476 denotes nearly Fourier-transform-limited performance. This work provides a new technique to implement efficient, compact and low-cost all-fiber FSFLs for narrow spectral bandwidth mode-locked pulse output.

Near-ideal energy modulator for tunable THz pulse generation using sectioned hollow channel plasma system

To break the bottleneck in generating high-power tunable coherent THz radiation with a narrow spectral bandwidth, we propose a novel scheme of using a hollow channel plasma to generate energy modulation in relativistic electron beams. Using magnetic optics, the energy modulation is then effectively converted into beam density modulation, forming microbunches with high-quality and very high bunching factors. The arrangement of segmented multistage hollow channels can further improve the bunching factor, thus significantly increasing the THz energy. We show both theoretical analyses and simulations of this scheme, showing that scheme can be realized in accelerator facilities with repetition rates on the order of kHz, providing great potential in revolutionizing science and related technologies.

Ultra-wide sensing range plasmonic refractive index sensor based on a two-dimensional circular-hole grating engraved on a gold film

We propose a simple structure with a two-dimensional (2D) circular-hole grating engraved on a gold film for ultra-wide range plasmonic refractive index (RI) sensing. The electromagnetic field distributions and reflection spectra of the structure are obtained using the finite-difference time-domain approach for resonance mode analysis and sensing capability characterization. The simulation results show that a high-quality surface plasmon resonance with a narrow spectral bandwidth and strong dip strength can be excited using a 2D grating after compensating for a suitable optical momentum. The structure exhibits a high sensitivity of 1131 nm/RIU and stable sensing performance over an ultra-wide RI range of 1.0–2.0. An improved figure of merit is used to more comprehensively characterize the structural sensing performance. This sensor has the potential to be widely used in the field of biochemistry because of its simple preparation, high sensitivity, and ultra-wide sensing range.

Measuring color capability of wide color gamut near‐eye displays

AbstractNear‐eye displays often use narrow spectral bandwidth light sources to create virtual images. These sources provide wider color gamuts. However, the measurement instrument's spectral bandwidth and spectral stray light need to be considered. We evaluate these aspects in characterizing the display's color capability.

51‐2: Distinguished Paper: Measuring Color Capability of Wide Color Gamut Near‐Eye Displays

Near‐eye displays often use narrow spectral bandwidth light sources to create virtual images. These sources provide wider color gamuts. However, the measurement instrument's spectral bandwidth and spectral stray light need to be considered. We evaluate these aspects in characterizing the display's color capability.