Visible Laser Illumination Research Articles

Optical properties of YMASG:Ce fluorescent ceramics prepared by hot-pressure sintering

Cerium-doped yttrium aluminum garnet (YAG:Ce) based transparent ceramics have been widely used in fluorescent lighting as high-quality inorganic fluorescent conversion materials. This paper further explores the Mg2+-Si4+ ions doped YAG:Ce transparent ceramics by combining the solid-phase reaction method with vacuum hot-pressure sintering and implementing protection measures against hot-pressure mold contamination, and also investigates the effect of different Mg2+-Si4+ doping contents on the structure, transmittance and luminescence properties of the ceramics under hot-pressure sintering. In this work, pure-phase YMASG:Ce transparent fluorescent ceramics with a grain size of about 3-6 μm and clear and clean grain boundaries were obtained with an In-line transmittance of 67% at 800 nm. Under the excitation at 460 nm, the emission peak was red-shifted by 26 nm and the full width at half maxima (FWHM) was broadened with the increase of Mg2+-Si4+ content, which shows that the Mg2+-Si4+ ion pair effectively complements the absence of the red light component in the YAG:Ce emission spectrum. The optimized YMASG:Ce ceramics obtained high-quality warm white light with a low correlated color temperature (CCT) and a high color rendering index (CRI) under the excitation of the blue LED chip. This work proved the feasibility of vacuum hot-pressure sintering to prepare YMASG:Ce transparent fluorescent ceramics, and provided a new approach for studying YMASG:Ce-based ceramics, which was significant for the application of high-power visible laser illumination.

Photonic Encryption Platform via Dual-Band Vectorial Metaholograms in the Ultraviolet and Visible.

Metasurface-driven optical encryption devices have attracted much attention. Here, we propose a dual-band vectorial metahologram in the visible and ultraviolet (UV) regimes for optical encryption. Nine polarization-encoded vectorial holograms are observed under UV laser illumination, while another independent hologram appears under visible laser illumination. The proposed engineered silicon nitride, which is transparent in UV, is employed to demonstrate the UV hologram. Nine holographic images for different polarization states are encoded using a pixelated metasurface. The dual-band metahologram is experimentally implemented by stacking the individual metasurfaces that operate in the UV and visible. The visible hologram can be decrypted to provide the first key, a polarization state, which is used to decode the password hidden in the UV vectorial hologram through the use of an analyzer. Considering the property of UV to be invisible to the naked eye, the multiple polarization channels of the vectorial hologram, and the dual-band decoupling, the demonstrated dual-band vectorial hologram device could be applied in various high-security and anticounterfeiting applications.

Determination of nonlinear refractive index of paprika oil and pepper oil under cw visible laser illumination

We report the observation of multiple diffraction rings patterns in paprika and pepper oils using continuous wave laser beams .The patterns occur at relatively low intensities . The number of rings together with outermost ring diameter of each pattern increases monotonically with increasing input power . The total changes in refractive index , ?n ,and nonlinear refractive index,n2 ,are determined using the number of rings observed in both samples and 473 nm and 532 nm laser beams . We obtained good values of ?n = 0.01 and n2 = 10-6 cm2/w . These large values are attributed to thermal effects .

High Performance Optical Rectenna Arrays Using Multiwall Carbon Nanotube–Insulator–Metal Tunneling Diodes

We report important fundamental advancements in multiwall carbon nanotube (MWCNT) rectenna devices, which have potential applications for infrared and terahertz sensing and energy harvesting. Our rectenna devices incorporate ultrafast metal-insulator-metal tunneling diodes fabricated on the tips of MWCNT antennas arrays to convert high-frequency electromagnetic waves to D.C. electricity. The tunneling barrier geometry and insulator material selection are critical to designing high performance MWCNT diodes. We demonstrate new MWCNT diode structures that incorporate multiple insulating layers to control electron tunneling and achieve excellent current-voltage asymmetry (>20) with air stable metal contacts (Al and Ag). For improving antenna-diode impedance matching, we demonstrate that reducing insulator thickness and etching open the MWCNT tips can lower resistance. Our double-insulator devices show the first evidence of air stable rectification under IR and visible laser illumination (Figure 1). Open-circuit voltage is several orders of magnitude higher than previous reports that utilized single-insulator MWCNT diodes, thereby making our double-insulator MWCNT rectenna devices especially attractive for infrared photodetectors. These results bring to light a new generation of MWCNT rectenna arrays that can use a variety of top electrode materials – including air stable, transparent, and conductive materials – indicating strong potential and efficient utilization in a variety of high-frequency applications. Figure 1

Partially Reversible Photoinduced Chemical Changes in a Mixed-Ion Perovskite Material for Solar Cells.

Metal halide perovskites have emerged as materials of high interest for solar energy-to-electricity conversion, and in particular, the use of mixed-ion structures has led to high power conversion efficiencies and improved stability. For this reason, it is important to develop means to obtain atomic level understanding of the photoinduced behavior of these materials including processes such as photoinduced phase separation and ion migration. In this paper, we implement a new methodology combining visible laser illumination of a mixed-ion perovskite ((FAPbI3)0.85(MAPbBr3)0.15) with the element specificity and chemical sensitivity of core-level photoelectron spectroscopy. By carrying out measurements at a synchrotron beamline optimized for low X-ray fluxes, we are able to avoid sample changes due to X-ray illumination and are therefore able to monitor what sample changes are induced by visible illumination only. We find that laser illumination causes partially reversible chemistry in the surface region, including enrichment of bromide at the surface, which could be related to a phase separation into bromide- and iodide-rich phases. We also observe a partially reversible formation of metallic lead in the perovskite structure. These processes occur on the time scale of minutes during illumination. The presented methodology has a large potential for understanding light-induced chemistry in photoactive materials and could specifically be extended to systematically study the impact of morphology and composition on the photostability of metal halide perovskites.

Ultra-high photocurrent response in a chromia oxide thin film under visible light illumination

In this work, the photoconductivity of chromia (Cr2O3) thin films, which were configured an in-plane electrode, was investigated under visible laser illumination. The fast and slow response components of photocurrent were originated by its exciting defect level. Surprisingly, a high photoresponsivity up to 1.94 × 103 A/W was obtained when the Cr2O3/Pt planar capacitor was applied an incident wavelength of 650 nm at room temperature. Moreover, dielectric characterization implied that the current of channel in Cr2O3 film increased with the increasing of its defects concentrations under laser illumination. Our results indicate that the Cr2O3 thin films have a potential application in the photoelectric field.

Phase retrieval deblurring for imaging of dense object within a low scattering soft biological tissue.

Tissues are characterized by a strong scattering of visible optical radiation, which prevents one from achieving deep-tissue imaging. We propose a computational imaging technique for the inference of specific macroscopic, spatial phase distribution features of the scattering media. The spatial phase distribution is reconstructed from several defocused intensity images. We empirically demonstrate the method by reconstructing the location of two fibula chicken bones, embedded within chicken breast tissue. The suggested technique is safe, using visible laser illumination, and noninvasive. It is also cost-effective since a simple optical system is used and the images are acquired using a conventional camera, and it does not require interferometric detection as well as direct access to the object in absence of the layer.

Light-Driven Reversible Shaping of Individual Azopolymeric Micro-Pillars.

Azopolymers are known to exhibit a strong light responsivity known as athermal photofluidization. Although the underlying physics is still under debate, athermal photofluidization has been demonstrated to trigger mass-migration according to the polarization of a proper illumination light. Here, a polymer blend is proposed wherein a commercial azo-polyelectrolyte is mixed with a passive polymer. The blend is patterned as an array of micro-pillars that are individually exposed to visible laser illumination. Thanks to the interplay between the two blend components, a reversible and controlled deformation of the micro-pillars by periodically tuning the laser polarization in time is demonstrated. A reduced mobility of the azo-compound allows to repeatibly elongate and rotate micro-pillars along specific directions, with no significant material flow outisde the initial volume and no significant degradation of the structure morphology over several cycles. The proposed work suggests new degrees of freedom in controlling the mechanical features of micro-patterned light-responsive materials that can be usefully exploited in many application fields.

Analysis of phase patterns in photochromic polyurethanes by a holographic approach

Photochromic polyurethanes based on diarylethene units show a large reversible modulation of refractive index in the Vis-NIR spectral region. The change of refractive index in the material is easily induced by visible laser illumination, without any optical or chemical post-process. In this paper, patterns at the micron scale range have been written by a suitable direct laser writing machine and characterized at 1550 nm by means of a digital holographic approach. The refractive index profile has been retrieved, its dependence on the film thickness and writing speed was shown. The writing process has also been modelled by means of a kinetic model, showing theoretically the dependence of the pattern width and profile with the writing condition. It is demonstrated how the photochromic films are suitable for developing a reconfigurable platform for complex phase patterns working in the NIR.

Dynamics of Light-Absorption Variations Induced in a Bismuth Silicate Crystal by Visible Laser Illumination

We present the results of experimental studies of the dynamics of the photoinduced optical absorption in a bismuth silicate crystal subject to continuous laser irradiation with wavelengths of 532 and 655 nm. The semiconductor-laser light beam with the wavelength λ = 655 nm causes the crystal bleaching at this wavelength, whereas its exposure to a shorter-wavelength irradiation from the optical-spectrum green region with a wavelength of 532 nm increases the optical absorption at both wavelengths, of 532 and 655 nm. The experimental results are interpreted using the theoretical model which assumes that the crystal has deep defect centers of two types so that an electron at each of these centers can be in one of the states characterized by different photoionization cross sections.

In-situ characterization of trapped charges in amorphous semiconductor films during plasma-enhanced chemical vapor deposition

The subband-gap absorption current in a hydrogenated amorphous silicon film has been measured during plasma-enhanced chemical vapor deposition. The current is probed by a near-infrared laser while photoexcited carriers are generated under visible laser illumination. The trapped charge density is determined from the magnitude of current under the assumption of carrier generation and recombination kinetics. The result indicates that trapped charges are distributed uniformly in the film during growth, and they are reduced after the growth. The trapped charge density is minimized at a growth temperature of ≈ 473 K.

Observation of photoinduced phase transition in phase-separatedPr0.55(Ca1−ySry)0.45MnO3thin films via x-ray photoemission spectroscopy

Perovskite manganite thin films, ${\mathrm{Pr}}_{0.55}{({\mathrm{Ca}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y})}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$, have been studied using x-ray photoemission spectroscopy in order to clarify the consequence of the competition between ferromagnetic metal (FM) and charge-orbital-ordered insulator (COOI). Films with $y=0.40$ undergo a uniform paramagnetic insulator to FM transition. On the other hand, in films with $y=0.25$, the composition near the bicritical point, phase separation of COOI and FM domains is indicated by the spectral change below $125\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Interestingly, between $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the visible laser illumination transfers the COOI-like spectra obtained in cooling process to the FM-like spectra obtained in warming process. This indicates that the photoinduced insulator to metal transition is governed by the increase of the FM volume fraction and is deeply related to the phase separation between the FM and COOI states.

Thermal radiation scanning tunnelling microscopy

In standard near-field scanning optical microscopy (NSOM), a subwavelength probe acts as an optical 'stethoscope' to map the near field produced at the sample surface by external illumination. This technique has been applied using visible, infrared, terahertz and gigahertz radiation to illuminate the sample, providing a resolution well beyond the diffraction limit. NSOM is well suited to study surface waves such as surface plasmons or surface-phonon polaritons. Using an aperture NSOM with visible laser illumination, a near-field interference pattern around a corral structure has been observed, whose features were similar to the scanning tunnelling microscope image of the electronic waves in a quantum corral. Here we describe an infrared NSOM that operates without any external illumination: it is a near-field analogue of a night-vision camera, making use of the thermal infrared evanescent fields emitted by the surface, and behaves as an optical scanning tunnelling microscope. We therefore term this instrument a 'thermal radiation scanning tunnelling microscope' (TRSTM). We show the first TRSTM images of thermally excited surface plasmons, and demonstrate spatial coherence effects in near-field thermal emission.

Photo-oxidation of Ge Nanocrystals: Kinetic Measurements by In Situ Raman Spectroscopy

ABSTRACTGe nanocrystals are formed in silica by ion beam synthesis and are subsequently exposed by selective HF etching of the silica. Under ambient conditions, the exposed nanocrystals are stable after formation of a protective native oxide shell of no more than a few monolayers. However, under visible laser illumination at room temperature and in the presence of O2, the nanocrystals rapidly oxidize. The oxidation rate was monitored by measuring the Raman spectra of the Ge nanocrystals in-situ. The intensity ratio of the anti-Stokes to the Stokes line indicated that no significant laser-induced heating of illuminated nanocrystals occurs. Therefore, the oxidation reaction rate enhancement is due to a photo-chemical process. The oxidation rate varies nearly linearly with the logarithm of the laser intensity, and at constant laser intensity the rate increases with increasing photon energy. These kinetic measurements, along with the power dependencies, are described quantitatively by an electron active oxidation mechanism involving tunneling of optically excited electrons through the forming oxide skin and subsequent transport of oxygen ions to the Ge nanocrystal surface.

Holographic interference filters for infrared communications

We demonstrate that high-quality interference filters for the wavelength range 1300-1600 nm can be holographically fabricated in DuPont HRF-800X001 photopolymer material by use of visible laser illumination. We also summarize a chain-matrix technique, which we call thin-film decomposition, that is useful for modeling multilayer films with an arbitrary index profile n(z). We use the thin-film-decomposition technique to create design curves that allow one to choose the proper exposure angle and film thickness with which to fabricate a holographic interference filter with a desired transmission efficiency and bandwidth at a particular wavelength. These curves are of general utility and are not confined to any particular holographic recording medium. Excellent agreement between theory and experiment is found.

Kinetics and reaction mechanisms of laser-assisted chemical vapor deposition of polycrystalline silicon dots from silane

Polycrystalline silicon dots have been deposited on silicon-coated SiO2 substrates from silane irradiated with a continuous wave (cw) argon ion laser and a cw CO2 laser. Various experimental configurations were adopted for laser-assisted chemical vapor deposition of Si dots. The cw argon ion laser was focused either directly on the surface of absorbent polycrystalline Si films acting as substrates (front side visible laser illumination) or penetrated through the transparent SiO2 plate and focused at the SiO2–Si interface (rear side visible laser illumination). The cw CO2 laser beam penetrated through the transparent polycrystalline Si film and was absorbed into the SiO2 plate (front side IR laser illumination). The deposition rate of polycrystalline Si dots was investigated as a function of the laser power (or surface temperature which was proportional to the laser power) and silane pressure. With the direct visible laser irradiation of Si/SiO2 substrates (front side laser illumination), at a surface temperature in the range 700–1000 °C, the growth kinetics of polycrystalline Si dots was found to be limited by surface reactions. The reaction mechanism proposed for chemical vapor deposition of Si films from pyrolysis of silane in conventional reactors can be invoked to explain the kinetic data, i.e., the rate-limiting step is the release of hydrogen from the Si surface. Visible photon interactions with reactive species were found to promote the release of hydrogen since this kinetic regime was not observed with other experimental configurations investigated. The deposition rate of polycrystalline Si dots produced at a surface temperature in the range 1100–1410 °C was proportional to the laser power or surface temperature and silane pressure. In addition, the deposition rate was independent of the experimental configuration used. In other words, only pyrolytic effects are involved in the laser-induced deposition of Si dots even with the IR laser light which is absorbed by silane molecules. With the gas phase in the fluid regime, the rate-limiting step was probably the diffusion of reactive species through the boundary layer developed in the vicinity of the surface of the deposition zone. Silylene radicals, SiH2, formed in the gas phase from inelastic collisions between excited silane molecules seem to be the major reactive species. A reaction mechanism of laser-assisted chemical vapor deposition of polycrystalline Si dots involving the formation and diffusion of silylene radicals through silane which acts as a buffer gas with respect to silylene radicals is proposed and discussed in this article.

Metastable crystalline state induced in amorphous SiGe layers under cw visible laser illumination

A metastable crystalline state is reported to occur in amorphous SiGe films on oxidized Si under illumination with cw visible laser. This state is reached for laser power below the threshold for irreversible crystallization. The observation of this state is performed by Raman spectroscopy. It is found to be reversible- and temperature and composition-dependent. The main properties of this metastable state are described in terms of the Raman spectral features.