Visible Laser Research Articles

Experimental and numerical study of Si annealed by laser sources of the visible spectrum (red and blue wavelength)

Laser annealing (LA) is an increasingly important process in the semiconductor manufacturing industry. So far, lasers of nonvisible spectrum have been mostly used for semiconductor applications. The study using a visible wavelength laser is not nearly found though it could be a potential tool to expand applications of the LA. In this study, the silicon (Si) deposited on oxide was annealed using laser sources of visible spectrum. LA of Si was performed using a single laser source of either red or blue wavelengths or a dual-laser source using both wavelengths. The surface morphology and microstructure of the annealed Si were analyzed. A numerical simulation was also conducted to predict the temperature and melt pool characteristics of Si during the annealing process. Surface roughness increased with increasing heat input (or power density) of the laser beam, owing to the considerable melting of Si. The dual laser using a relatively high power density produced Si with high polycrystallinity and large grains. However, the Si lattice constant difference induced by different annealing conditions was negligible. Simulation results showed that temperature, melt pool length, and duration increased with the power density of the dual-laser beam. This contributed to the growth of large Si grains.

Study on Sm3+ ion concentration in physical, structural and optical properties of magnesium-sodium fluoroborate glasses for reddish-orange visible laser applications

The influence of Sm3+ ion concentration on physical, structural and optical properties of magnesium-sodium fluoroborate glasses were studied and analyzed. The XRD, FTIR, SEM with EDS spectra validated the non-crystalline nature, presence of functional groups, morphology and elementary analysis of the prepared glasses. The bonding parameters, oscillator strength, JO intensity parameters and radiative properties were evaluated using absorption and emission analysis. In JO intensity parameters, the lower Ω2 value implied higher symmetry and less covalency at higher Sm3+ concentration. The 4G5/2 → 6H7/2 emission transition possesses higher stimulated emission cross section (15.75 ✗ 10−22) and higher branching ratio 0.533 (βcal), 0.51 (βexp) for BMNNFS1.5 glass. The higher values of radiative parameters witnessed the suitability of prepared glasses for laser applications. Decay measurements were carried out and BMNNFS0.1 glass has higher quantum efficiency as 72%. The color coordinates (x.y) of prepared glasses lie in the reddish-orange region of the CIE diagram this confirmed the reddish-orange visible laser applications.

UV-laser-diode-pumped visible Tb3+:LiLuF4 lasers.

We investigate the dependence of the visible laser performance of Tb3+:LiLuF3 (Tb:LLF) on the ultraviolet (UV) pumping wavelength and present the first, to the best of our knowledge, UV-laser-diode-pumped Tb3+-based laser. We find an onset of thermal effects already at moderate pump power for UV pump wavelengths with strong excited-state absorption (ESA), which vanishes at pump wavelengths with weak ESA. Pumping with a UV laser diode emitting at 378.5 nm enables continuous wave laser operation in a 3-mm short Tb3+(28 at.%):LLF crystal. Slope efficiencies of 36% at 542/544 nm and 17% at 587 nm are obtained with a minimum laser threshold as low as 4 mW.

Thermal effect analysis on cuboid Pr:YLF crystals pumped by blue laser diodes.

Using blue laser diodes (LDs) to pump Pr:YLF crystals can directly realize visible-band laser output. Compared with the traditional frequency doubling and LD direct output method, it has the advantages of simple design, compact structure, and high beam quality. For solid-state lasers, pump-induced thermal effects of gain media are the principal limiting factors for the desired high-power output. In this paper, internal temperature space model distribution of a rectangular cross-section Pr:YLF crystal is established. On this basis, the temperature distribution, thermal stress distribution, and thermal focal length variation of single-end pumped and double-end pumped laser crystals are analyzed. The results are verified by COMSOL simulations and experimental measurements. To our knowledge, this analysis is the first to examine the thermal effect of a rectangular cross-section Pr:YLF crystal, analyzing the limit power that the crystal can withstand, which paves the way for better performances of visible lasers with stable and high-power output.

Comparative studies on CIELab, thermal and optical characteristics of Dysprosium (Dy3+) incorporated phospho-tellurite glasses for white light generation

Concentration dependence of Dy3+ activated strontium lead phospho-tellurite glass series have been synthesized by melt quenching process. To explore the structural, thermal and optical traits of the present glasses we have investigated XRD, FTIR, DTA, UV–Vis–NIR absorption, optical band gap, excitation band, luminescence emission, decay lifetime and CIE chromaticity diagram, and analysed the Judd-Ofelt intensity parameters Ωλ (λ = 2,4,6) and various radiative parameters in detail. The photoluminescence spectra exhibit highly intense blue and yellow emissions at around 483 and 573 nm. The better values of yellow/blue intensity ratio and branching ratio (βexp) suggest that the proposed glass series are efficient candidates for visible laser media. Emission quenching effect has been observed beyond 1 mol% Dy3+ doping concentration. The luminescence lifetime of the 4F9/2 excited level declines with increasing Dy3+ doping level. The experimental lifetime and quantum efficiency of 1 mol% Dy3+ activated glass are acquired to be 0.75 ms and 81%, respectively. The appearance of chromaticity coordinates as well as CIELab coordinates in the white region highlights that the present glass system could be most promising for white light generation.

Development of a compact 406 nm diode laser-based cavity-enhanced spectrometer for high-sensitive detection of NO2 levels in exhaust gas

Nitrogen dioxide (NO2) is an important air pollutant due to its environmental impact and adverse effects on human health. It is released into the atmosphere primarily through anthropogenic activities. Here, we report on the development of a simple, compact, and cost-effective robust optical detection method exploiting cavity-enhanced absorption spectroscopy for high-sensitive and selective measurement of NO2 levels in real-time using a visible diode laser operating at 406 nm. A typical detection limit of ∼330 ppb for NO2 was achieved with an optimum acquisition time of ∼3.9 s, at optimal cavity pressure of 100 Torr. The sensor system demonstrates an effective optical path-length of 180 m in a high-finesse 50 cm long optical cavity in an interference-free spectral region and aerosol-free conditions. The spectrometer was optimized, calibrated, and demonstrated for the detection of NO2 levels in vehicular exhaust gases.

Passive OFE WDM-over-POF Gigabits per Second Performance Comparison of Spatial Diversity and Spatial Multiplexing

We present a novel indoor communication network using 1-mm core diameter step-index plastic optical fibres as a feeder line, and using the light emitted by the POF-end for wireless data transmission, creating a fully passive access point (AP). This has the advantage that it does not require electrical powering, resulting in low maintenance. Due to the large numerical aperture, the light emitted by the POF is extremely divergent, thus a lens must be placed in front of the POF-end to adjust the size of the coverage area to enable users moving within the area or to increase the number of users served by the wireless link. We present a theoretical model for both multiple-input multiple-output (MIMO) transmission modes, spatial diversity (SD) and spatial multiplexing (SM) concepts, using orthogonal frequency division multiplexing (OFDM). We report the system throughput, that is estimated using the singular value decomposition (SVD) of the experimentally measured channel matrix <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf {H}(f)$</tex-math></inline-formula> . We then present the experimental results applying both SD and SM concepts and make a comparison between theoretical and experimental results. We demonstrate that this passive AP concept using OFDM achieves throughputs of around 5.2 Gbps when SM is applied, and 3.8 Gbps when SD is applied. Furthermore, we use eye-safe visible light laser at the beginning of the POF feeder which, due to optical losses by POF and components, limited the wireless link to 1 m only and the coverage area to 45 cm diameter. We believe the proposed system represents a low-cost, high-capacity, highly secure, interference free wireless system, suitable for next generation for industrial and home applications.

Visible Pulsed Laser-Assisted Selective Killing of Cancer Cells with PVP-Capped Plasmonic Gold Nanostars.

A new generation of nanoscale photosensitizer agents has improved photothermal capabilities, which has increased the impact of photothermal treatments (PTTs) in cancer therapy. Gold nanostars (GNS) are promising for more efficient and less invasive PTTs than gold nanoparticles. However, the combination of GNS and visible pulsed lasers remains unexplored. This article reports the use of a 532 nm nanosecond pulse laser and polyvinylpyrrolidone (PVP)-capped GNS to kill cancer cells with location-specific exposure. Biocompatible GNS were synthesized via a simple method and were characterized under FESEM, UV-visible spectroscopy, XRD analysis, and particle size analysis. GNS were incubated over a layer of cancer cells that were grown in a glass Petri dish. A nanosecond pulsed laser was irradiated on the cell layer, and cell death was verified via propidium iodide (PI) staining. We assessed the effectiveness of single-pulse spot irradiation and multiple-pulse laser scanning irradiation in inducing cell death. Since the site of cell killing can be accurately chosen with a nanosecond pulse laser, this technique will help minimize damage to the cells around the target cells.

Infrared laser ablation dynamics using light scattering

The dynamics of infrared laser ablation at atmospheric pressure was studied employing continuous visible laser light scattering in the expanding plume. A pulsed infrared optical parametric oscillator at 2940 nm wavelength and 5 ns pulse width was used to irradiate samples comprising liquid glycerol or 50 µm thick sections of rat liver tissue. The scattered light from the expanding laser ablation plume was measured using a 532 nm continuous laser parallel to the target and several millimeters above the ablated spot. The scattered light was recorded using a photomultiplier detector and the signal used to estimate the time at which the plume front passed through the continuous laser beam. The velocity of the ablation plume was obtained from consecutive time and distance measurements and the fraction of the laser energy transferred to the expanding shock wave was determined using the Taylor shock wave model. Plume modeling calculations indicate that the ablation is driven by phase explosion that is thermally confined and near the stress confinement regime.

Laser-Synthesized 2D-MoS2 Nanostructured Photoconductors.

The direct laser synthesis of periodically nanostructured 2D transition metal dichalcogenide (2D-TMD) films, from single source precursors, is presented here. Laser synthesis of MoS2 and WS2 tracks is achieved by localized thermal dissociation of Mo and W thiosalts, caused by the strong absorption of continuous wave (c.w.) visible laser radiation by the precursor film. Moreover, within a range of irradiation conditions we have observed occurrence of 1D and 2D spontaneous periodic modulation in the thickness of the laser-synthesized TMD films, which in some cases is so extreme that it results in the formation of isolated nanoribbons with a width of ~200 nm and a length of several micrometers. The formation of these nanostructures is attributed to the effect that is known as laser-induced periodic surface structures (LIPSS), which is caused by self-organized modulation of the incident laser intensity distribution due to optical feedback from surface roughness. We have fabricated two terminal photoconductive detectors based on nanostructured and continuous films and we show that the nanostructured TMD films exhibit enhanced photo-response, with photocurrent yield increased by three orders of magnitude as compared to their continuous counterparts.

Growth and optical properties of a Sm3+, Eu3+ co-doped La2CaB10O19 crystal

Due to the excellent spectral properties, Eu3+-doped La2CaB10O19 (Eu3+:LCB) single crystals have shown promising prospect as a medium for laser radiation in the visible region. However, the weak absorption and emission intensities of Eu3+:LCB single crystals limit their laser application. Here, a Sm3+, Eu3+ co-doped La2CaB10O19 (Sm3+, Eu3+:LCB) crystal is grown to improve its optical performance. The intensity parameters, spontaneous transition probability, radiation lifetime and fluorescence branch ratio of the Sm3+, Eu3+:LCB crystal are studied with the Judd–Ofelt theory. Compared with an Eu3+-doped La2CaB10O19 (Eu3+:LCB) crystal, the Sm3+, Eu3+:LCB crystal exhibits higher absorption intensities and optical transition line strengths. Based on emission spectra, an energy transfer process from Sm3+ to Eu3+ ions in the Sm3+, Eu3+:LCB crystal is supposed. Fluorescence lifetimes for the Sm3+ and Sm3+→Eu3+ energy transfer process are calculated to be 1.698 ms and 3.648 ms, and energy transfer efficiency from Sm3+ to Eu3+ ions is calculated to be 13.7%. Compared with the Eu3+:LCB crystal, the emission intensity of the Sm3+, Eu3+:LCB crystal is doubled. Our work shows that the absorption and emission intensities of Eu3+:LCB crystals are significantly enhanced by co-doping with Sm3+ ions, which is conducive to their application for visible lasers.

Photoluminescence of Tb3+/Sm3+ ions co-activated in SrO - Na2O - WO3 - tellurite glasses for multicolor laser applications

A novel series of Tb3+, Sm3+ single doped and Tb3+/Sm3+ co-incorporated tungsten tellurite glasses were synthesized by melt quenching technique and corresponding structural as well as luminescence features of the prepared glasses have been reported here. Spectral overlapping between the luminescence spectra of Tb3+ ions and the excitation spectra of Sm3+ ions manifests that the energy transfer process takes place from Tb3+ ions to Sm3+ ions. By using the dual excitations at 377 and 484 nm, the titled co-doped glasses emit green light of wavelength 542 nm along with reddish – orange colour light at 599 nm. In addition to this, there is no possibility of reverse energy transfer which is validated with the help of excitation at 403 nm (Sm3+ ions) as major evidence. The lifetimes of all co-doped glasses decline with increasing Tb3+ doping level in the ligand matrices, indicating the energy migration process takes place from Tb3+→ Sm3+. The chromaticity coordinates of all synthesized co-doped glasses lie in yellowish-orange region of CIE1931 diagram and it shifts to deep yellow region when Tb3+ ion concentration varies. Our findings propose that the titled glasses can be used as visible laser materials for multicolor laser applications.

Enhanced Ablation of Cornea Doped with a Fluorescein Dye Using a Visible Wavelength Laser.

Myopia (nearsightedness) and presbyopia (aging eye) are the most common refractive errors of the human eye. Technology has advanced toward correcting aberration using various surgical procedures, including laser surgery, as well as prescribing corrective lenses. Hence, the effect of various laser wavelengths on the eye has been extensively studied over the last few decades. Usually, excimer lasers are used for this purpose, which increases the cost of the procedure because they are unique and difficult to manufacture and require regular maintenance. Due to the absorption properties, visible wavelengths do not interact with the corneal layers and hence are currently not used for eye surgery. This study presents the first clinical evidence that a 532 nm laser in combination with an eye-safe fluorescein dye that is in wide clinical use in ophthalmology can be utilized for high-precision ablation purposes due to the photochemical reaction that occurs on an ex vivo porcine eye. Our results show the promise of utilizing inexpensive visible wavelength lasers in the ablation of biological tissues, reducing the high costs of ophthalmological surgical procedures, as well as in other applications.

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.

Laser diodes grown on porous GaN by plasma-assisted molecular beam epitaxy

Porous GaN has been proposed as a novel cladding material for visible light-emitting laser diodes (LDs). Fabrication of nanoporous-GaN bottom-cladding LDs was already realized by selective electrochemical etching (ECE) of the highly n-type doped GaN layer in the LD structure after epitaxy. In this work, we applied a reverse approach: in the first step, locally porous areas in GaN substrate were fabricated, and next, a LD structure was grown on top by plasma-assisted molecular beam epitaxy (PAMBE). We compare the electrical and optical properties of the devices with porous bottom cladding with the devices from the same wafer that was grown on top of a standard GaN layer. Continuous wave (CW) operation is achieved for porous LD at 435.4 nm and slope efficiency of 0.046 W/A. Standard LD was lased in CW mode at 442.6 nm and had a slope efficiency of 0.692 W/A. In porous LD, the internal losses were estimated using the Hakki-Paoli method to be 68 cm−1, while for standard LD, the losses were 25 cm−1. Near-field patterns recorded for the studied devices indicate light scattering on the porous layer to be the possible reason for the increased losses in porous LD.

Fabrication of FeCoNi medium-entropy alloy nanoparticles by high-repetition-rate UV picosecond laser ablation in water

Laser ablation in liquid (LAL) is a rapid heating and cooling process that enables the formation of a variety of nanomaterials. Extensive LAL studies for metals, semiconductors, insulators, and carbons have been reported. Further, alloys are worthwhile target to investigate the applicability of LAL. We report the LAL of equiatomic FeCoNi medium-entropy alloy (MEA) in water using a high-repetition-rate UV picosecond laser with a galvanometric scanner. The extinction spectra of the colloidal solution, morphology, elemental composition, and size distribution of MEA nanoparticles (NPs) were investigated as a function of laser-irradiation time, laser power, and scanning speed. Electron microscopy revealed that chain-like superstructures longer than 10 µm consisting of Ni-rich FeCoNi MEA-core Fe-shell spherical NPs of approximately 50-nm diameter were formed. These superstructures were not originated from the NP agglomeration during sample preparation for microscopy or the successive laser irradiation of colloids, but from the overstriking ablation on the target surface. Generally, the LAL target was irradiated by spatially and temporally separated near-infrared or visible laser pulses to efficiently produce NPs. Overstriking of UV ultrashort laser pulses was shown to be a valuable strategy for modifying the elemental composition of MEA NPs and for forming their superstructures.

Laser performance of high optical quality 4 at.% Pr3+: Sr0.7La0.3Mg0.3Al11.7O19 (Pr:ASL) single crystals.

In this work, we present the visible laser performance of improved optical quality Czochralski-grown 4 at.% Pr3+-doped Sr0.7La0.3Mg0.3Al11.7O19 (Pr:ASL) single crystals in the deep red (726 nm), the red (645 nm) and the orange (620 nm) range using two different pumping sources. Using a high beam quality frequency doubled Ti:sapphire laser with 1 W output power as pump source, deep red laser emission was reached at a wavelength of 726 nm with 40 mW of output power and a laser threshold of 86 mW. The corresponding slope efficiency was 9%. At 645 nm in the red, up to 41 mW of laser output power were obtained with 15% slope efficiency. Moreover, orange laser emission at 620 nm was demonstrated with 5 mW output power and 4.4% slope efficiency. Using a 10 W multi-diode module as pumping source allowed to obtain the highest output power of a red and deep-red diode-pumped Pr:ASL laser to date. The respective output powers at 726 and 645 nm reached 206 mW and 90 mW.

Comparative Performance Analysis of Femtosecond-Laser-Written Diode-Pumped Pr:LiLuF4 Visible Waveguide Lasers

In this work, we present the operation of a femtosecond-laser-written diode-pumped visible waveguide laser based on praseodymium-doped lithium lutetium fluoride. The refractive index modification induced by the femtosecond laser in the crystal exhibits an anisotropic behavior, thus enabling the fabrication of different types of waveguides from single-track structures to stress-induced waveguides and depressed cladding structures. All the waveguides were characterized by realizing transmission measurements and the waveguide design was optimized to obtain extremely low propagation losses, equal to 0.12 dB/cm. Lasing has been achieved at 604 nm and 721 nm from different waveguides. In addition, stable continuous-wave lasing at 698 nm has been obtained in a depressed cladding waveguide. This wavelength corresponds to the one needed for the transition of the atomic clock based on the neutral strontium atom. In the end, we report the observation of laser emission at 645 nm from a depressed cladding waveguide.

Composition dependence visible fluorescence from praseodymium ions doped Al2O3–CaO-M(M=B2O3, GeO2, Ga2O3) glasses

Pr3+ doped Al2O3–CaO based glasses were excellent candidate optical gain mediums for visible solid-state lasers due to its wide transmittance region and excellent mechanical properties. However, poor glass-forming ability limited its further application in visible fiber laser. In this work, B2O3, GeO2, Ga2O3 were employed for modifying the glass-forming ability and lowing the melting temperature of the Al2O3–CaO based glasses. By introducing 15 mol% GeO2 and Ga2O3, the thermal stability ΔT were enlarged to be 199 and 264 °C, respectively. The influence of glass composition on the fluorescence characteristic of Pr3+ were systematically investigated. For lower doping concentration, different multiphonon relaxation rare of 3P0 and 1D2 energy levels accounting for the different fluorescence spectra of Pr3+ doped Al–Ga, Al–Ge and Al–B glasses, while for the higher doping concentration, cross relaxation processes lead to the fluorescence was gradually dominated by the transition of 3P0 with increasing Pr3+ concentration.

Diode-pumped visible lasing in femtosecond-laser-written Pr:LiLuF4 waveguide.

In this Letter we report the realization of a femtosecond-laser-written diode-pumped Pr:LiLuF4 visible waveguide laser. The waveguide studied in this work consisted of a depressed-index cladding, whose design and fabrication were optimized to minimize the propagation loss. Laser emission has been achieved at 604 nm and 721 nm, with output power of 86 mW and 60 mW, respectively, and slope efficiencies of 16% and 14%. In addition, we obtained, for the first time in a praseodymium-based waveguide laser, stable continuous-wave laser operation at 698 nm (3 mW of output power and 0.46% of slope efficiency), corresponding to the wavelength necessary for the clock transition of the strontium-based atomic clock. The waveguide laser emission at this wavelength is mainly in the fundamental mode (i.e., the larger propagation constant mode) showing a nearly Gaussian intensity profile.