Solid-state Ultraviolet Laser Research Articles

(Invited) III-Nitride Ultraviolet LEDs and Lasers for Applications in Biology and Medicine

Ultraviolet (UV) light emitters with large-energy photons are being developed for emerging biology and medicine applications such as sterilization, water/air purification, and medical treatments, as a replacement for mercury vapor lamps. Solid-state UV light-emitting diodes (LEDs) and lasers based on wide bandgap III-Nitrides offer compact sizes, wavelength tuning, long lifetimes, and sustainability over mercury vapor lamps. While visible LEDs and lasers with longer emission wavelengths find widespread commercial uses for solid-state lighting and display, UV emitters with AlGaN heterostructures struggle with poor external quantum efficiencies of typically less than 10%, which drop further when wavelengths get shorter. Therefore, it is extremely challenging to realize high-efficiency UV emitters despite the demand for compact UV light source, especially for disinfection and sterilization from the pandemic as those high-energy photons can break DNA/RNA bonds and deactivate virus/bacteria. Here, I will discuss our recent developments toward realization of high-efficiency UV LEDs and lasers.To enhance the radiative recombination rate (R sp) from the AlGaN quantum well (QW) active region, we had pursued novel QW design to overcome issues from the existence of valence subbands crossover. Specifically, the arrangement of the three energy-degenerate valence subbands - heavy hole (HH), light hole (LH), and the crystal-field split-off hole (CH) bands for high Al-content and low Al-content AlGaN QWs are completely different. The ordering of the valence bands strongly affects the optical matrix element via the oscillator strength, as well as the direction of the photon emission. Our work has proposed promising solution on the use of delta-QW design to address the valence subbands crossover issue, in order to significantly boost up the R sp. In addition, to enhance the extraction efficiency of UV emitters, we demonstrated the realization and characterization of top-down fabricated, electrically driven UV micropillar array LEDs emitting at 286 nm with a narrow, 9 nm linewidth and output power densities up to 35 mW/cm2. Top-down fabrication allows for formation of perfectly uniform arrays of micropillars with easily tunable diameter and pitch, which cannot be achieved with epitaxially grown nanowires without the use of selective area epitaxy. Dry etch damage induced surface losses were avoided through the use of a two-step etch process combining a Cl dry etch and a hydroxyl-based wet etch to form the micropillar arrays. We also revealed that the unique inverse-taper profile of our micropillars could produce significant enhancements in the extraction efficiency with the increased taper angle. Lastly, our developed III-Nitride wet etch by the use of hydroxyl-based chemicals such as potassium hydroxide (KOH) investigated the effects of temperature, concentration, and the damage recovery aspects of hydroxyl etching of III-Nitride nano and micro structures, leading to important smooth surface profiles for micropillar/nanowire LEDs and optimized mirror facets for ridge-emitting UV lasers.

Research on All Solid-State Ultraviolet Laser at 289.9 nm

Research on All Solid-State Ultraviolet Laser at 289.9 nm

Growth of SrB4O7 Crystal Fibers along the c-Axis by Micro-Pulling-Down Method

SrB4O7 (SBO) receives much attention as solid-state ultraviolet lasers for micro-machining, photochemical synthesis, and laser spectroscopy. For the application of SBO, the SBO crystals require the control of twinning to amplify the conversion light. We also expected that the inhibitation of the SrB2O4 appearance was essential. Here, we show the growth of SBO crystals along the c-axis through the micro-pulling-down method while alternating the application of electric fields (E). Without the application, single crystals were grown. At E ≧ 400 V/cm no needle domains of SrB2O4 inside SBO crystals existed; however, composition planes were formed and twin boundaries did not appear. In contrast, the inversion of surface morphology emerged, and the convex size was especially large at 1000 V/cm. These results demonstrate that convection is generated perpendicular to the growth front by alternating the application of electric fields. This surface morphological change contradicts the conventional concept of growth through the micro-pulling-down method. The distance from seed crystals vs. grain density plot also showed that the density did not decrease with a sufficient slope. Consequently, we concluded that the selection of the c-axis as growth faces is not fruitful to fabricate twins, and the selection of the growth condition, under which geometrical selection strongly affects, is the key.

Effects of the Application of Electric Fields on the Growth of SrB4O7 Crystals by the Micro-Pulling-Down Method

SrB4O7 (SBO) has received much attention as a solid-state ultraviolet laser crystal for a wide variety of applications such as micromachining and spectroscopy. Here we demonstrate the effects of electric fields E on the growth of SBO crystals along the b axis by the μ-pulling-down method to fabricate SBO with periodic twins, which increases the conversion light intensity. During the growth, the direction of electric fields was alternated for 30 min periods. The crystal fibers were evaluated by polarized optical microscopy, X-ray diffraction, and transmission electron microscopy. Without an external voltage, single SBO crystals grew. In contrast, at E ≥ 400 V/cm needles of SrB2O4 appeared inside SBO fibers. At E ≥ 1000 V/cm, new domains of SBO grew from the seed crystals, and interestingly, growth twins were formed perpendicular to growth fronts. Thus, we found that the growth of SBO by the μ-PD method via alternating the application of opposing electric fields exhibits two mechanisms. In contrast, when the electrodes in melt and growth fronts switched to + and −, respectively, growth fronts changed from concave into convex shapes, irrespective of E. We could explain the relation between the two mechanisms and the inversion of surface shapes, using the convection.

A Fundamental Study of a Surface Modification on Silicon Wafer Using Direct Laser Interference Patterning with 355-nm UV Laser

The growing need for precision machining, which is difficult to achieve using conventional mechanical machining techniques, has fueled interest in laser patterning. Ultraviolet (UV) pulsed-lasers have been used in various applications, including the micro machining of polymers and metals. In this study, we investigated direct laser interference patterning of a silicon waver using a third-harmonic diode-pumped solid-state UV laser with a wavelength of 355 nm. Direct laser lithography is much more simple process compare to other submicro processing method. We have studied interference patterning for silicon wafers as a basic research for direct laser interference patterning on wafer surfaces without mask. And Finite element analysis (FEA) was performed for a 150° biprism using modeling software (COMSOL Multiphysics 5.4) to determine changes in the periodic patterns according to the focusing distance in the direct interference lithography experiment. In further study, we expect this technique to be applied to direct laser interference lithography on metals.

First-principles calculations of electronic and optical properties of LiCaAlF6 and LiSrAlF6 crystals as VUV to UV solid-state laser materials

We report the density functional calculations of the electronic and optical properties of perfect LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF) crystals. The calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional employing 35% exact exchange. Using optimized unit crystal volumes and equilibrium lattice constants, both LiCAF and LiSAF are found to have indirect band gaps of 12.23 and 11.79 eV, respectively. The band gap energies of these fluoride crystals are also observed to increase upon application of pressure by uniform volume compression. Moreover, their bulk moduli are determined to be 108.01 (LiCAF) and 83.75 GPa (LiSAF), while their static dielectric constants are 1.27 (LiCAF) and 1.26 (LiSAF). Considering the dielectric functions, refractive indices, and absorption coefficients, both perfect LiCAF and LiSAF crystals are viable vacuum ultraviolet (VUV) to ultraviolet (UV) laser host media. With knowledge of the different optical transitions and pressure dependence, our results yield helpful insights on the use of these fluoride compounds as effective solid-state laser materials in the VUV region.

Optimization of diode pumped solid state ultraviolet laser dicing of silicon carbide chips using design of experiment methodology

This paper investigates the laser micromachining of SiC-4H wafers with an ultraviolet (355 nm) nanosecond source. The design of experiment methodology was used to study the effect of pulse energy, number of passes, defocus, and scanning speed on scribing depth and was modeled with Design Expert® software. Pulse energy and scanning speed were found to have a significant effect on scribe depth (by a factor of 10 compared to defocus), while the effect of the number of passes varied with pulse energy and scanning speed. The model predicted a narrow set of processing parameters to scribe half of the wafer thickness (180/360 μm). A more fundamental study was also conducted in order to validate literature observations on the ablation mechanism. A brief comparison between conventional blade dicing and the laser scribe-break method was performed on a 3 in. diameter wafer. It was discovered that a possible gain of gross die (number of usable chips per wafer) of 7% could be achieved which could provide a useful economic benefit.

Near-ultraviolet laser diodes for brilliant ultraviolet fluorophore excitation.

Although multiple lasers are now standard equipment on most modern flow cytometers, ultraviolet (UV) lasers (325-365 nm) remain an uncommon excitation source for cytometry. Nd:YVO4 frequency-tripled diode pumped solid-state lasers emitting at 355 nm are now the primary means of providing UV excitation on multilaser flow cytometers. Although a number of UV excited fluorochromes are available for flow cytometry, the cost of solid-state UV lasers remains prohibitively high, limiting their use to all but the most sophisticated multilaser instruments. The recent introduction of the brilliant ultraviolet (BUV) series of fluorochromes for cell surface marker detection and their importance in increasing the number of simultaneous parameters for high-dimensional analysis has increased the urgency of including UV sources in cytometer designs; however, these lasers remain expensive. Near-UV laser diodes (NUVLDs), a direct diode laser source emitting in the 370-380 nm range, have been previously validated for flow cytometric analysis of most UV-excited probes, including quantum nanocrystals, the Hoechst dyes, and 4',6-diamidino-2-phenylindole. However, they remain a little-used laser source for cytometry, despite their significantly lower cost. In this study, the ability of NUVLDs to excite the BUV dyes was assessed, along with their compatibility with simultaneous brilliant violet (BV) labeling. A NUVLD emitting at 375 nm was found to excite most of the available BUV dyes at least as well as a UV 355 nm source. This slightly longer wavelength did produce some unwanted excitation of BV dyes, but at sufficiently low levels to require minimal additional compensation. NUVLDs are compact, relatively inexpensive lasers that have higher power levels than the newest generation of small 355 nm lasers. They can, therefore, make a useful, cost-effective substitute for traditional UV lasers in multicolor analysis involving the BUV and BV dyes.

Insights into ultraviolet-induced electrical degradation of thermally grown SiO2/4H-SiC(0001) interface

The harmful impact of ultraviolet (UV) light irradiation on thermally grown SiO2/4H-SiC structures was investigated by means of electrical measurements of metal-oxide-semiconductor (MOS) capacitors. Unlike Si-based MOS devices, significant electrical degradation, such as positive flatband voltage (VFB) shift and hysteresis in the capacitance-voltage (C-V) curves of SiC-MOS capacitors was induced by UV irradiation with a low-pressure mercury lamp. The interfacial fixed charge density increased with UV-irradiation (22.6 mW/cm2 for 16 h) to 1.7 × 1012 cm−2, which was an order of magnitude larger than that of the as-grown SiO2/SiC interface. A detailed study based on single wavelength solid-state UV lasers revealed that there was a threshold photon energy at around 5 eV and a moderate dependence of UV-induced degradation on temperature. These experimental findings imply that pre-existing inactive defects accumulated at the thermally grown SiO2/SiC interface were transformed to active carrier traps with high-energy UV irradiation through transparent SiO2 layers.

Corneal stromal ablation with femtosecond ultraviolet pulses in rabbits

To determine the effectiveness of femtosecond ultraviolet (UV) pulses in ablating corneal stroma in a rabbit model and to compare the healing response between eyes treated with femtosecond UV pulses and eyes treated with standard excimer photorefractive keratectomy. Laser Research Center, Vilnius University, Vilnius, Lithuania. Experimental study. Myopic photoablation using a femtosecond UV solid-state laser system was applied to corneas of pigmented rabbits. Experiments in 16 eyes were performed for optimization of the laser system parameters (fluence, spot size, pulse repetition rate) and calibration of ablation rate. In 7 rabbits, deep femtosecond UV ablation (∼130 μm) in 1 eye and shallow ablation (∼30 μm) in the contralateral eye were performed. Nine rabbits received an approximately 30 μm ablation with femtosecond UV pulses in 1 eye and with a conventional excimer system in the contralateral eye. Two eyes were used as controls. The ablation process and surface-temperature dynamics were monitored and recorded. Surface quality and haze development were evaluated. Rabbits were humanely killed 0 to 6 months after surgery, and eyes were enucleated for histological examination. Rabbit corneas ablated with femtosecond UV pulses or excimer laser radiation were similar in terms of the corneal wound-healing process, surface quality, and histology. The experiments indicate the feasibility of clinical application of femtosecond UV lasers for stromal ablation. The ability to switch between laser harmonics allows fast changeover from infrared to the UV mode, implying that a wide range of ophthalmic procedures can be performed using a single solid-state laser device. Ms. Gabryte and Mr. Danielius are paid employees of Light Conversion Ltd. Mr. Danielius is a shareholder of Light Conversion Ltd. No other author has a financial or proprietary interest in any material or method mentioned.

Low-threshold ultraviolet solid-state laser based on a Ce^3+:LiCaAlF_6 crystal resonator

A low-threshold solid-state UV laser using a whispering gallery mode (WGM) resonator constructed from UV transparent crystalline material is demonstrated. Using a Ce3+:LiCaAlF6 resonator, we observe broad bandwidth lasing (280-330 nm) with a low threshold intensity of 7.5×10(9) W/m(2) and an effective slope efficiency of ~25%. The lasing time delay dynamics in the pulsed operation mode are also observed and analyzed. Additionally, a LiCaAlF(6) WGM resonator with Q=2×10(7) at 370 nm is realized. The combination of this high Q and the small WGM mode volume significantly lowers the pump power threshold compared to traditional cavity designs, opening the door for both tunable continuous-wave and mode-locked operation.

Effect of wavelength on the laser patterning of a cholesteric liquid crystal display electrode

A 1064nm fiber laser and a 355nm ultraviolet solid-state laser with galvanometer scanners and F-theta lenses are used to rapidly etch the silver electrode pattern of a cholesteric liquid crystal display. These results of the two laser processes are that the 1064nm laser effectively reduces the damage to the liquid crystal layer and underlying indium tin oxide layer, and reduces the heat-affected zone of the ablated silver electrode.

Ultraviolet upconversion luminescence of Gd 3+ and Eu 3+ in nano-structured glass ceramics

Ultraviolet multiphoton upconversion emissions of Eu 3+ ( 5 H 3–7, 5 G 2–6, 5 L 6 → 7 F 0) and Gd 3+ ( 6 I J , 6 P J → 8 S 7/2) are studied in the Eu 3+ (or Gd 3+) doped SiO 2–Al 2O 3–NaF–YF 3 precursor glasses and glass ceramics containing β-YF 3 nanocrystals, under continuous-wavelength 976 nm laser pumping. It is experimentally demonstrated that energy transfer from Yb 3+ to Tm 3+, then further to Eu 3+ or Gd 3+ is responsible for the upconversion process. Compared to those in the precursor glasses, the upconversion emission intensities in the glass ceramics are greatly enhanced, owing to the participation of rare earth ions into the low-phonon-energy environment of β-YF 3 nanocrystals. Hopefully, the studied glass ceramics may find potential applications in the field of ultraviolet solid-state lasers.

Investigation of Profile Cutting on Glass Plates Using a Pulsed UV Laser System

A solid-state ultraviolet laser with a circular processing path was used for profile cutting on soda-lime glass plates. Simulation results show that decreasing scanning speed and path spacing and increasing circular processing path frequency yields a smooth cutting edge for the same processing length. Profile glass-plate cutting shows the machined surface with more obvious roughness and numerous microcracks near areas of laser focus than in areas of laser defocusing. This phenomenon is attributed to high laser-energy density and strongly affecting surface roughness.

Fabrication of electrodes on the aluminum doped zinc oxide thin films using an ultraviolet laser direct-patterning technology

Because transparent conductive oxide (TCO) thin films have more than 80% transmittance in visible spectrum, and high electrical conductivity, the TCO films are widely applied to flat panel displays and solar cells as transparent electrode materials. This study aims to develop a direct patterning technology on ZnO:Al (AZO) thin films by a diode-pumped solid state ultraviolet laser. The electrode patterns with array structures on AZO thin films were generated by a high-speed galvanometric scanning system. The optoelectronic properties of a patterned electrode have strong relation with the laser pulse frequency, the scan speed, and the patterning time. The surface morphology and roughness of patterned electrode were measured by three dimension confocal microscope and field emission scanning electron microscope, respectively. The resistivity of AZO thin films before and after laser patterning was measured by a four point probe instrument. The optical transmittance was recorded by a UV/VIS/NIR spectrophotometer. The experimental results indicated that the edge line width and depth decreased with increasing the scan speed. After the array patterns structure were formed by laser dry etching, the roughness Ra values of patterned area increased from 0.06μm to 0.16μm. These transmittances of patterned structure from 400nm to 800nm wavelengths averagely reached to 82%. The measured results of electrical conductively revealed that the resistivity gradually increased with increasing the pulse repetition frequency. In addition, surface morphologic examination on the straight lines, corners, and etched regions of patterned films revealed no micro-cracks observed which meant the patterned surface had a better surface quality.

Corneal shaping and ablation of transparent media by femtosecond pulses in deep ultraviolet range

To assess the performance of a newly developed solid-state femtosecond ultraviolet (UV) laser system in corneal ablation. Vilnius University, Laser Research Centre, Vilnius, Lithuania. Femtosecond pulses in the deep UV range (205 nm) were obtained by the generation of the fifth-harmonic of an amplified Yb:KGW laser system (fundamental output at 1027 nm). Coupled with galvanometric beam-scanning mirrors, this system allowed ablation shaping of transparent media, including poly(methyl methacrylate) (PMMA), collagen, and ex vivo porcine corneas. The surfaces of ablated structures were characterized using a noncontact confocal optical profiler. Spherical structures were successfully formed in all 3 materials tested. A 10.0 diopter refraction change in the cornea was produced in 180 seconds. The resulting surface quality was significantly higher (roughness length >100 microm versus approximately 6 microm) in gelatin and ex vivo corneas than in PMMA. The solid-state femtosecond UV laser system seems an attractive alternative to the currently used ophthalmic argon-fluoride excimer laser system because of its small footprint, silent operation, and ability to generate femtosecond light pulses at both 1027 nm (suitable for flap creation) and 205 nm (corneal ablation).

${\hbox {Nd}}^{3+}{\hbox {:LaF}}_{3}$ as a Step-Wise Excited Scintillator for Femtosecond Ultraviolet Pulses

Vacuum ultraviolet fluorescence from Czochralski method-grown Nd3+:LaF3 excited by 290 nm femtosecond pulses from a Ti:sapphire laser is reported. Peak emission at 175 nm with 7 ns lifetime is observed. This decay time would be one of the fastest among solid-state, inorganic scintillators. Laser emission in the VUV region may also be achievable by pumping with existing ultraviolet solid-state lasers.

Photorefractive Keratectomy Using Solid State Laser 213 nm and Excimer Laser 193 nm: A Randomized, Contralateral, Comparative, Experimental Study

To compare histopathologic changes after photorefractive keratectomy (PRK) for myopia using either a solid state laser (SSL) at 213 nm or an excimer laser (EL) at 193 nm in an experimental model. Forty pigmented rabbits (80 eyes) underwent myopic PRK for the correction of 6 D. Photoablation was randomly assigned, with a solid state laser used in one eye and an EL in the fellow eye. Rabbits were killed immediately after ablation (n = 10) or at 7 days (n = 10), 1 month (n = 8), 3 months (n = 6), or 12 months (n = 6) after surgery. Corneal tissue was preserved for light microscopy and transmission electron microscopy at all postoperative intervals. All eyes reepithelialized in 1 week with no early or late postoperative complication. Immediately after ablation, light, scanning, and transmission electron microscopy revealed relatively smooth ablation surfaces in both groups. Seven days after surgery, epithelium in the SSL specimens appeared thinner than in the EL specimens. Activated keratocytes were observed adjacent to the epithelium in both groups, whereas endothelial cells demonstrated normal morphology. At 1, 3, and 12 months after surgery, tissue appearance of all corneal layers was similar in both groups. PRK in rabbits using ultraviolet SSL and EL revealed similar histopathologic findings up to 1 year after surgery.

Early diagnosis of bladder cancer by special fluorescence spectrum: an in vitro experiment

To investigate the value of special fluorescence spectrum in the early diagnosis of bladder cancer. Hematoporphyrin monomethyl ether (HMME) of the concentrations of 10, 100, and 200 microg/ml, as photosensitizer, was added into the sample vessels of bladder cancer tissue to be incubated for 1 - 3 h. A detecting system consisting of solid state ultraviolet laser with the wavelength of 355 nm was used to measure the fluorescence spectrum. The laser-induced auto-fluorescence (LIAF) of the bladder cancer tissue had characteristic double peaks at 445 - 490 nm. There was a remarkable characteristic peak, drug peak, located at 628.25 nm in the laser-induced drug-fluorescence (LIDF) of the bladder cancer tissue soaked by HMME. Photosentisizer raise the sensitivity and specificity of fluorescence spectrum in diagnosis of bladder cancer. Stable, and with high strength and visible drug peak, the LIDF is may help improve the early diagnosis of bladder cancer.

Diode-end-pumped solid-state ultraviolet laser based on intracavity third-harmonic generation of [formula omitted] in YCa 4O(BO 3) 3 crystal

Intracavity type-I sum-frequency mixing of 1.06 μm and 532 nm with a ( θ, ϕ)=(106°,77.2°)-cut YCOB crystal was performed in a compact laser-diode-pumped solid-state laser. Three type-II phase-matching KTP crystals with different length were used to generate 532 nm light by frequency-doubling of 1.06 μm . The 355 nm output power was measured with the three KTP crystals for Q-switched and continuous-wave (CW) operation, respectively. The maximum ultraviolet output power of 1305 μW was obtained with a 15 mm KTP crystal for CW operation, while the maximum ultraviolet average output power of 124 mW was obtained with a 10 mm KTP crystal for Q-switched operation.