Isotropic Light Research Articles

Customized calibration sources in the JUNO experiment

We customized a laser calibration system and four radioactive γ-ray calibration sources for the Jiangmen Underground Neutrino Observatory (JUNO), a 20-kton liquid scintillator-based neutrino detector. The laser source system was updated to realize the isotropic light emission timing within ±0.25 nsec level and to allow the tuning of the laser intensity covering more than four orders of magnitude. In addition, methods to prepare four different radioactive sources (18F, 40K, 226Ra, and 241Am), covering energies from 0 (10) keV to 0 (1) MeV, for the JUNO detector were established in this study. The radioactivity of each source and the risk of radioactive substance leaking into the detector from the source were confirmed to meet the experimental requirements.

Large Field of View and Isotropic Light Sheet Microscopy with Aberration‐Free Tunable Foci

Large Field of View and Isotropic Light Sheet Microscopy with Aberration‐Free Tunable Foci

Helical hollow channel waveguide in YAG fabricated by femtosecond laser enhanced wet etching.

Three-dimensional optical waveguides with hollow channels have many advantages, such as strong mode confinement and excellent dispersion control ability. Femtosecond laser enhanced wet etching is widely used to fabricate hollow channel waveguides in transparent dielectric materials. We propose a method for fabricating hollow channel waveguides in YAG using femtosecond laser enhanced wet etching with a simpler fabrication process and shorter etching time compared with the previous work. After 90 h of etching, a series of helical hollow channel waveguides with a length of 5 mm and a radius of 32 µm were successfully fabricated. At a pitch of 3 µm, the waveguide exhibited a loss (including coupling loss and transmission loss) as low as 0.68 dB at 1030 nm. The helical hollow channel waveguide also exhibited exceptional isotropic light confinement capability and remarkable supercontinuum-generating properties. Moreover, helical hollow channel waveguides with a radius of 2 µm were successfully fabricated. According to simulations, waveguides of such size can effectively control dispersion. Our work presents, to our knowledge, a novel approach to fabricating hollow channel waveguides with arbitrary lengths using femtosecond laser-enhanced wet etching.

Line shapes of isotropic and depolarized CILS spectra of neon: Interaction potential and pair polarizability

The present work proposes three interaction potentials between two gaseous neon atoms and applies them to the study of models representing the anisotropy and trace of the induced pair polarizability tensor. The proposed empirical interatomic potentials for the gaseous Ne-Ne interaction are developed by fitting the Improved Lennard-Jones (ILJ), Modified Tang-Toennies (MTT) and Barker et al. (BFW) potentials simultaneously to the spectroscopic and transport properties over a wide temperature range. The validity of the proposed interatomic potentials is verified by comparing the calculated and experimental second pressure virial coefficient over a wide temperature range. In addition to this, the results show satisfactory accordance between the experimental and calculated vibrational energy levels using the deduced pair potential parameters compared to those reported for neon gas. These comparisons exhibit acceptable accuracy when the present work potential parameters are inserted to reproduce the transport and thermo-physical properties, besides the spectral moments and the spectral line-shape intensities for neon gas. The isotropic and depolarized collision-induced light scattering (CILS) spectra are produced and analyzed as functions of the potential considered. They exhibit sensitivity to both the attractive part of the potential and the short-range values of the polarizability anisotropy or mean at intermediate and high frequencies. Mean and anisotropy are deduced directly from spectral line shapes using a new inversion method.

Hyperuniform Disordered Solids with Morphology Engineering

AbstractHyperuniform disordered solid (HUDS) structures can provide large, uniform, complete, and isotropic light confinement at the nanoscale after precise design. Based on the HUDS structures, in‐plane light confinement for developing photonic integrated circuits is also explored. To improve the performance of HUDS devices, researchers have mainly focused on cell size or cell distribution optimization in HUDS, which suffers from time‐consuming computation or moderate photonic bandgap (PBG) modification. Here, a morphology engineering method is demonstrated to tailor HUDS PBGs and improve HUDS waveguide devices for transverse electric mode. The results show that the Bezier‐curve‐decorated HUDS devices can achieve a maximum of about 75% PBG width increase, 1.5 dB transmittance improvement in a 10 µm long HUDS waveguide, improved quality factors of HUDS‐cladding microring resonators, and device fabrication compatibility with foundry processing. This study opens new avenues for the development of unprecedented devices for exploring light field regulation, nonlinear optics, and sensing.

A mirror-based 2D isotropic illumination in total internal reflection fluorescence microscopy

In a total internal reflection fluorescence microscope, there is a 3D anisotropy in intensity of the excitation light. Using a mirror in a prism-based total internal reflection fluorescence microscope, we propose a very low-cost method to decrease the anisotropic property of the excitation light. Theoretically, we obtained the electric field of the transmitted light inside the sample as a function of the rotational angle of the mirror, polarization degree of the incident beam, and the refractive indices of the prism, immersion oil, cover glass, and sample. By using the refractive indices that are commercially available, proper practical combinations of the angle of mirror and polarization degree of the incident light are suggested to provide a merely 2D isotropic excitation light at the sample plane of a total internal reflection fluorescence microscope.

Investigation of Aluminum Arsenide Honeycomb Monolayer via Density Functional Theory

First principle calculations are performed to theoretically predict the physical properties of hexagonal aluminium arsenide planar and buckled monolayers. The structural characteristics showed that the buckled parameter is about 0.32 A°. Cohesive energies have favourable values and it indicates the fabrication possibility. Phonon dispersion properties indicated that the planar aluminium arsenic monolayers are dynamically unstable, while the buckled is less dynamically unstable. The elastic constant parameters achieved the required characteristics of stable hexagonal monolayer structures. The study of electronic band structure prefers to indirect semiconductor band gaps, and the density of states showed strong orbital hybridization in the conduction band. Planar structure has isotropic light electron effective mass and anisotropic heavy hole effective mass. The buckled structure has isotropic light electron effective mass and isotropic heavy hole effective mass. The absorption spectra have high absorption coefficient in various visible and ultraviolet wavelength. The absorption coefficient levels off at about direct and indirect band gaps.

Coherent response of the electronic system driven by non-interfering laser pulses

The strength of light–matter interaction in condensed matter is fundamentally linked to the orientation and oscillation strength of the materials’ optical transition dipoles. Structurally anisotropic materials, e.g., elongated molecules, exhibit optical transition dipoles with fixed orientations that govern the angular-dependent light–matter interaction. Contrary, free electron-like metals should exhibit isotropic light–matter interaction with the light fields dictating the orientation of the optical transition dipoles. Here, we demonstrate that an anisotropic direction of the optical transition dipoles even exists in highly free electron-like noble metal surfaces. Our time- and phase-resolved photoemission experiment reveals coherent interference effects on the (110)-oriented silver surface after optical excitation with two non-interfering cross-polarized pulses. We explain this coherent material response within the density matrix formalism by an intrinsic coupling of the non-interfering light fields mediated by optical transition dipoles with fixed orientations in silver.

Isotropic and anisotropic collision-induced light scattering spectra of krypton gas simultaneously fitted by ground state interatomic potential

Isotropic and anisotropic binary collision-induced light scattering spectra of gaseous krypton obtained at room temperature are analyzed in terms of different interatomic potentials, both reference and recent, and trace and anisotropy models of interaction-induced polarizability. The spectral intensities obtained numerically at low frequencies are determined by bound and free transitions. At intermediate and high frequencies, the spectra are sensitive to both the attractive part of the potential and to short-range values of the trace or anisotropy. An empirical interatomic potential for the krypton gas interaction is developed by simultaneously fitting the Barker et al. (BFW) and modified Tang-Toennies (MTT) potentials to thermophysical and transport properties over a wide temperature range. The quality of the present potentials was checked by comparison between the calculated and experimental vibrational energy levels. The results show that these are the most accurate potentials reported to date for this system, both for the reproduction of spectral line shapes and spectral moments.

Deep‐Ultraviolet Micro‐LEDs Exhibiting High Output Power and High Modulation Bandwidth Simultaneously

Deep-ultraviolet (DUV) solar-blind communication (SBC) shows distinct advantages of non-line-of-sight propagation and background noise negligibility over conventional visible-light communication. AlGaN-based DUV micro-light-emitting diodes (µ-LEDs) are an excellent candidate for a DUV-SBC light source due to their small size, low power consumption, and high modulation bandwidth. A long-haul DUV-SBC system requires the light source exhibiting high output power, high modulation bandwidth, and high rate, simultaneously. Such a device is rarely reported. A parallel-arrayed planar (PAP) approach is here proposed to satisfy those requirements. By reducing the dimensions of the active emission mesa to micrometer scale, DUV µ-LEDs with ultrahigh power density are created due to their homogeneous injection current and enhanced planar isotropic light emission. Interconnected PAP µ-LEDs with a diameter of 25µm are produced. This device has an output power of 83.5mW with a density of 405 W cm-2 at 230mA, a wall-plug efficiency (WPE) of 4.7% at 155mA, and a high -3dB modulation bandwidth of 380MHz. The remarkable high output power and efficiency make those devices a reliable platform to develop high-modulation-bandwidth wireless communication and to meet the requirements for bio-elimination.

Wide-angle acousto-optic devices based on isotropic light scattering in biaxial crystals

Using the example of a rhombic crystal of alpha-iodic acid, a detailed study of new variants of acousto-optic isotropic light scattering in oblique sections of biaxial crystals, whose refractive surface contains a concave portion, is carried out. For these cases, the frequency dependences of the Bragg angles and the ranges of acousto-optic interaction are calculated. The possibility of implementing a unique, simultaneously broadband and wide-angle variant of light scattering is established. It is shown that the two-dimensional transfer function of such acousto-optic geometry has a complex X-shaped structure that has no analogues among uniaxial crystals. The possibility of applying this diffraction regime in light beam spatial filtering devices is studied.

Monitoring of optical properties of deep lake water

We present the results of the one year monitoring of absorption and scattering lengths of light with wavelengths in the range of 375 nm to 532 nm within the effective volume of the deep underwater neutrino telescope Baikal-GVD, which were measured by a device named ≪BAIKAL-5D≫. ≪BAIKAL-5D≫ was installed during the 2020 winter expedition at a depth of 1250 m. To carry out the optical measurements, we use a device with a shaded point-like isotropic light source having a spectral resolution of about 3 nm. A wide angle light receiver is moved by a stepper motor so that the distance between the receiver and the light source can vary from 0.9 m to 7.4 m. The absorption and scattering lengths were measured every week in 6 spectral points. The short-time variation of absorption length was estimated.

Fractal correlation of fluctuations of primary differential polarization properties

In this work, we consider the properties of fractal correlation of fluctuations of primary differential polarization parameters. The spatial correlation function for isotropic light scattering is first generalized to light scattering by the anisotropic medium. It is then demonstrated that when there exist finite fractal correlations between fluctuations of the elementary polarization properties the elements of the differential Mueller matrix are functions of the thickness of the layer of the medium. Thus the differential Mueller matrices derived from the logarithm of measured macroscopic Mueller matrices can only be regarded as equivalent matrices. The results obtained in this work are helpful for correctly interpreting measured macroscopic Mueller matrices.

Steady-state Fano coherences in a V-type system driven by polarized incoherent light

We explore the properties of steady-state Fano coherences generated in a three-level V-system continuously pumped by polarized incoherent light in the absence of coherent driving. The ratio of the stationary coherences to excited-state populations $\mathcal{C} = (1+\frac{\Delta^2}{\gamma(r+\gamma)} )^{-1}$ is maximized when the excited-state splitting $\Delta$ is small compared to either the spontaneous decay rate $\gamma$ or the incoherent pumping rate $r$. We demonstrate that an intriguing regime exists where the $\mathcal{C}$ ratio displays a maximum as a function of the dephasing rate $\gamma_d$. We attribute the surprising dephasing-induced enhancement of stationary Fano coherences to the environmental suppression of destructive interference of individual incoherent excitations generated at different times. We identify the imaginary Fano coherence with the non-equilibrium flux across a pair of qubits coupled to two independent thermal baths, unraveling a direct connection between the seemingly unrelated phenomena of incoherent driving of multilevel quantum systems and non-equilibrium quantum transport in qubit networks. The real part of the steady-state Fano coherence is found to be proportional to the deviation of excited-state populations from their values in thermodynamic equilibrium, making it possible to observe signatures of steady-state Fano coherences in excited-state populations. We put forward an experimental proposal for observing steady-state Fano coherences by detecting the total fluorescence signal emitted by Calcium atoms excited by polarized vs. isotropic incoherent light. Our analysis paves the way toward further theoretical and experimental studies of non-equilibrium coherent steady states in thermally driven atomic and molecular systems, and for the exploration of their potential role in biological processes.

Luminescent and Hydrophobic Wood Films as Optical Lighting Materials.

Most materials used for optical lighting applications need to produce a uniform illumination and require high mechanical and hydrophobic properties. However, they are rarely eco-friendly. Herein, a bio-based, polymer matrix-free, luminescent, and hydrophobic film with excellent mechanical properties for optical lighting purposes is demonstrated. A template is prepared by turning a wood veneer into porous scaffold from which most of the lignin and half of the hemicelluloses are removed. The infiltration of quantum dots (CdSe/ZnS) into the porous template prior to densification resulted in almost uniform luminescence (isotropic light scattering) and could be extended to various quantum dot particles, generating different light colors. In a subsequent step, the luminescent wood film is coated with hexadecyltrimethoxysilane (HDTMS) via chemical vapor deposition. The presence of the quantum dots coupled with the HDTMS coating renders the film hydrophobic (water contact angle ≈ 140°). This top-down process strongly eliminates lumen cavities and preserves the orientation of the original cellulose fibrils to create luminescent and polymer matrix-free films with high modulus and strength in the direction of fibers. The proposed optical lighting material could be attractive for interior designs (e.g., lamps and laminated cover panels), photonics, and laser devices.

Harnessing Dynamic Wrinkling Surfaces for Smart Displays.

Reversible and switchable wrinkling surfaces in response to various external stimuli have extensive potential applications. In this Letter, we prepared the reversible wrinkling on poly(dimethylsiloxane) (PDMS) surfaces, responsive to the solvents, by ultraviolet-ozone (UVO) treatment with/without mechanical prestrain. Based on the solvent-responsive wrinkling, three types of optical transparency dynamics were achieved easily in a single and simple film-substrate system, including (I) completely reversible transparency with controlled relaxation time and isotropic light scattering; (II) completely reversible transparency with anisotropic light scattering and tunable diffusion degree; and (III) incompletely reversible transparency. The reversibility and stability of wrinkles can be controlled by tailoring the solvent type, UVO exposure time, and mechanical prestrain. The underlying mechanisms for the three wrinkling dynamics have been clearly elucidated. The extremely simple material system and the facile but efficient technique pave a novel way for realizing versatile optical dynamics for smart displays.

Characterization of the radiative impact of aerosols on CO<sub>2</sub> and energy fluxes in the Amazon deforestation arch using artificial neural networks

Abstract. In vegetation canopies with complex architectures, diffuse solar radiation can enhance carbon assimilation through photosynthesis because isotropic light is able to reach deeper layers of the canopy. Although this effect has been studied in the past decade, the mechanisms and impacts of this enhancement over South America remain poorly understood. Over the Amazon deforestation arch large amounts of aerosols are released into the atmosphere due to biomass burning, which provides an ideal scenario for further investigation of this phenomenon in the presence of canopies with complex architecture. In this paper, the relation of aerosol optical depth and surface fluxes of mass and energy are evaluated over three study sites with artificial neural networks and radiative transfer modeling. Results indicate a significant effect of the aerosol on the flux of carbon dioxide between the vegetation and the atmosphere, as well as on energy exchange, including that surface fluxes are sensitive to second-order radiative impacts of aerosols on temperature, humidity, and friction velocity. CO2 exchanges increased in the presence of aerosol in up to 55 % in sites with complex canopy architecture. A decrease of approximately 12 % was observed for a site with shorter vegetation. Energy fluxes were negatively impacted by aerosols over all study sites.

Nanopatterned sapphire substrates in deep-UV LEDs: is there an optical benefit?

Light emitting diodes (LEDs) in the deep ultra-violet (DUV) offer new perspectives for multiple applications ranging from 3D printing to sterilization. However, insufficient light extraction severely limits their efficiency. Nanostructured sapphire substrates in aluminum nitride based LED devices have recently shown to improve crystal growth properties, while their impact on light extraction has not been fully verified. We present a model for understanding the impact of nanostructures on the light extraction capability of DUV-LEDs. The model assumes an isotropic light source in the semiconductor layer stack and combines rigorously computed scattering matrices with a multilayer solver. We find that the optical benefit of using a nanopatterned as opposed to a planar sapphire substrate to be negligible, if parasitic absorption in the p-side of the LED is dominant. If losses in the p-side are reduced to 20%, then for a wavelength of 265 nm an increase of light extraction efficiency from 7.8% to 25.0% is possible due to nanostructuring. We introduce a concept using a diffuse ('Lambertian') reflector as p-contact, further increasing the light extraction efficiency to 34.2%. The results underline that transparent p-sides and reflective p-contacts in DUV-LEDs are indispensable for enhanced light extraction regardless of the interface texture between semiconductor and sapphire substrate. The optical design guidelines presented in this study will accelerate the development of high-efficiency DUV-LEDs. The model can be extended to other multilayer opto-electronic nanostructured devices such as photovoltaics or photodetectors.

Tunable bandgap and isotropic light absorption from bismuth-containing GaAs core-shell and multi-shell nanowires.

Semiconductor core-shell nanowires based on the GaAs substrate are the building blocks of many photonic, photovoltaic and electronic devices, thanks to their associated direct bandgap and highly tunable optoelectronic properties. The selection of a suitable material system is crucial for custom designed nanowires tailored for optimised device performance. Bismuth-containing GaAs materials are an imminent class of semiconductors which not only enable an exquisite control over the alloy strain and electronic structure but also offer the possibility to suppress internal loss mechanisms in photonic devices. Whilst the experimental efforts to incorporate GaBixAs1-x alloys in the nanowire active region are still at an early stage, the theoretical understanding of the optoelectronic properties of such nanowires is only rudimentary. This work elucidates and quantifies the role of nanowire physical attributes such as its geometry parameters and bismuth incorporation in designing light absorption wavelength and polarisation response. Based on the multi-million atom tight-binding simulations of the GaBixAs1-x/GaAs core-shell and GaAs/GaBixAs1-x/GaAs multi-shell nanowires, our results predict a large tuning of the absorption wavelength, ranging from 0.9 μm to 1.6 μm, which can be controlled by engineering either Bi composition or nanowire diameter. The analysis of their strain profiles indicates a tensile character leading to significant light-hole mixing in the valence band states. This offers a possibility to achieve polarisation-insensitive light interaction, which is desirable for several photonic devices involving amplification and modulation of light. Furthermore, at low Bi compositions, the carrier confinement is quasi type-II, which further broadens the suitability of these nanowires for a myriad of applications requiring large carrier separations. The presented results provide a systematic and comprehensive understanding of the GaBixAs1-x nanowire properties and highlight new possibilities for future technologies in photonics, quantum optics and solar energy harvesting.

3D conical helix metamaterial-based isotropic broadband perfect light absorber.

We present the design and fabrication of an isotropic broadband perfect light absorber in the near-infrared range using 3D metamaterials with a single resonator in the unit cell. The metamaterial resonator is comprised of a gold conical helix supported on a silicon pillar with back reflector realized on a silicon substrate. Simulations and experiments have demonstrated that the proposed absorber achieves a broad absorption band of more than 3 µm in the 1.5-4.5 µm wavelength range with an average absorbance of more than 90%. The numerical and experimental analyses show that the proposed device can provide both incident angle and polarization independent operations, which further widens the application prospects of our device.