Cw and passively Q-switched laser performance of a mixed c-cut Nd:Gd0.33Lu0.33Y0.33VO4 crystal operating at 1.34 μm

Continuous wave and passively Q-switched laser operation with a mixed c-cut Nd:Gd_0.33Lu_0.33Y_0.33VO₄ crystal at 1.34 μm has been realized for the first time as far as we know. The largest output power of the continuous wave was 1.1 W for the output mirror of 5% transmission, with the optical conversion efficiency and the slope efficiency being 15% and 17.2%, respectively. The passive loss and the stimulated emission cross-section of the Nd:Gd_0.33Lu_0.33Y_0.33VO₄ crystal were found to be 0.6% and 0.47 × 10⁻¹⁹ cm². The thermal lens effect that weakened the laser performance has also been measured. For passively Q-switched operation, the shortest pulse duration of 26 ns, the highest peak power of 1.8 kW, along with the pulse energy as large as 47 μJ, were obtained using V:YAG as Q-switch. The experimental results have shown that the passively Q-switched Nd:Gd_0.33Lu_0.33Y_0.33VO₄ laser can generate pulses with larger pulse energy and higher peak power in comparison with the passively Q-switched Nd:GdYVO₄ lasers.     

Present states and future prospect of fast ignition realization experiment (FIREX) with Gekko and LFEX Lasers at ILE

The fast ignition realization experiment (FIREX) project is progressing. The new short pulse laser system, LFEX laser, has been completely assembled and one of the four beamlets is now in operation. A fast-ignition experiment was performed using this single short pulse combined with the Gekko XII implosion laser. The energy of the GXII implosion laser was about 2 kJ and the pulse width was 1.5 ns. The energy of the LFEX laser was increased upto 800 J and two pulse durations 5 and 1.6 ps were compared. Targets were deuterated plastic shells with gold cones. It was found that the neutron yield was increased by a factor of 30 as a result of the fast electron-induced heating in LFEX 1.6 ps shot. The estimated coupling efficiency between the LFEX laser pulse and the compressed fuel was low (less than 5%). This may be due to pre-plasma formed by light arriving at the target before the main laser pulse. Further investigations and attempts to overcome these problems are now in progress.     

Cavitation erosion behavior of laser-clad Ni-Cr-Fe-WC on brass

Laser surface modification of brass with an alloy-ceramic composite powder Ni-Cr-Fe-WC was studied using a two-step process, with powder preplacement followed by laser irradiation. For preplaced coatings of a few 100 μm thick, successfully cladding/alloying was achieved at a laser (CW Nd:YAG) energy density in the range 60-100 J/mm². Under proper processing conditions, the cavitation erosion resistance was increased by a factor of 9.1. This large increase was attributed to the formation of a Ni-rich matrix reinforced by precipitated carbides and tightly bound WC particles.     

Structural and optical properties of tellurite thin film glasses deposited by pulsed laser deposition

Tellurite (TeO₂-TiO₂-Nb₂O₅) thin film glasses have been produced by pulsed laser deposition at room temperature at laser energy densities in the range of 0.8-1.5 J/cm² and oxygen pressures in the range of 3-11 Pa. The oxygen concentration in the films increases with laser energy density to reach values very close to that of the bulk glass at 1.5 J/cm², while films prepared at 1.5 J/cm² and pressures above 5 Pa show oxygen concentration in excess of 10% comparing to the glass. X-ray photoelectron spectroscopy shows the presence of elementary Te in films deposited at O₂ pressures ≤ 5 Pa that is not detected at higher pressures, while analysis of Raman spectra of the samples suggests a progressive substitution of TeO₃ trigonal pyramids by TeO₄ trigonal bipyramids in the films when increasing their oxygen content. Spectroscopic ellipsometry analysis combined with Cauchy and effective medium modeling demonstrates the influence of these compositional and structural modifications on the optical response of the films. Since the oxygen content determines their optical response through the structural modifications induced in the films, those can be effectively controlled by tuning the deposition conditions, and films having large n (2.08) and reduced k (< 10^− 4) at 1.5 μm have been produced using the optimum deposition conditions.     

Spectroscopic properties and energy transfer process in Er doped ZrF4-based fluoride glass for 2.7 μm laser materials

Intense 2.7 μm emission from Er³⁺ doped in a new type of ZrF₄-based fluoride glass is reported. 2.7 μm emission characteristics and energy transfer process upon excitation of a conventional 980 nm laser diode are investigated. Based on the absorption spectra, the Judd-Ofelt parameters and radiative properties were calculated and compared with those of other glass hosts. The prepared glass possesses higher predicted spontaneous transition probability (29.04 s⁻¹) along with larger calculated emission cross section (9.16 × 10⁻²¹ cm²). Besides, the energy transfer coefficient of laser upper level (⁴I_11/2) can reach as high as 6.56 × 10⁻³⁹ cm⁶/s. Hence, these results indicate that this Er³⁺ doped ZrF₄-based fluoride glass has potential applications in 2.7 μm laser.     

All-plastic organic dye laser with distributed feedback resonator structure

We demonstrated an all-plastic waveguide organic dye laser with distributed feedback (DFB) resonator. We fabricated DFB structure on a surface of SU-8 2002 photoresist polymer using the interference of two beams of a frequency-tripled Nd:YAG pulse laser at 355 nm. The typical grating pitch of fabricated DFB structure was 190, 380 and 570 nm, corresponding to the number of mode m, and each corresponding grating amplitude was ca. 1, 4-5, and 25 nm, respectively. Shallow amplitude of 1 or 4-5 nm is ascribed to the wide incidence angle of the interference beams. Threshold of lasing for m = 3 is lower than that for m = 1 or m = 2 depending on the emission wavelength. DFB structure with the smaller amplitude of gratings at m = 1 and 2 is required for the higher threshold of laser emission. By controlling the grating pitch with nanometer scale, we can tune the wavelength of laser emission with 40 to 60 nm tunability. Effective energy transfer via nonradiative Förster transfer mechanism assists in lowering the threshold of laser emission.     

Performance measurements from LYSO scintillators coupled to a CMOS position sensitive SSPM detector

We have designed a 5×5 mm² position sensitive solid-state photomultiplier (PS-SSPM) using a complementary metal-oxide-semiconductor (CMOS) process that provides imaging capability on the micro-pixel level. The PS-SSPM has 11,664 micro-pixels total, with each having an active area and micro-pixel pitch of 30×30 μm² and 44.3 μm, respectively. The PS-SSPM was then examined for its performance characteristics such as its energy and spatial resolution, and LYSO scintillator array imaging capabilities. When coupled to 5×5×3 mm³ LYSO, the energy resolution at 511 keV (²²Na) was measured as a function of bias, and corrected for the PS-SSPM non-linear output. The resolution is 14% (FWHM) at 511 keV with 30 V bias. The LYSO coincidence timing resolution was 9.4 ns (FWHM) at 511 keV. Spatial resolution studies were conducted using a focused (∼30 μm beam spot diameter) pulsed 635 nm diode laser. Scintillator array imaging studies were conducted at 511 keV using a 6×6 LYSO array, having 500 μm pixels (530 μm pitch) and 5 mm tall.     

Diode-pumped efficient laser operation and spectroscopy of Tm,Ho:YVO4

Spectroscopic characterization of co-doped Tm,Ho:YVO₄ crystal grown by the Czochralski method has been performed including absorption spectrum, emitting spectrum and luminescence decay lifetime. The polarization emitting spectrum around 2 μm is accomplished by exciting a singly Ho³⁺ doped YVO₄ crystal to exclude the influence of Tm³⁺³F₄-³H₆ transition and the emission cross section is deduced from both Fuchtbauer-Ladenburg (F-L) equation and reciprocity method (RM). In addition, we report up to 10.4 W continuous wave (CW) output with a conversion efficiency of 40% and 10.3 W Q-Switch output with 12.5 kHz pulse repetition rate of diode-pumped cryogenic Tm,Ho:YVO₄ laser. For Q-Switch operation, the minimum pulse width of 28.2 ns is obtained, all of which demonstrate that the Tm,Ho:YVO₄ is excellent laser material for 2 μm radiation.     

Solution-based aerosol deposition process for synthesis of multilayer structures

In this paper, we report the progress on 3D aerosol deposition of barium titanate (BTO) based multilayer ceramic capacitors (MLCC) using an automated process. Solution-based aerosol deposition is a promising direct-write method to produce three-dimensional (3D) structures at low temperatures. We utilize the combination of solution-based aerosol process and selective laser sintering (SLS) to develop 3D structures. The MLCC structure was synthesized on silicon substrate consisting of alternate layers of 0.5-1.0 μm thick silver electrode layer and 0.5-2 μm thick BTO ceramic. The effect of incident laser power subsequent to deposition (in the range of 105 mW and 930 mW) was systematically investigated revealing the changes in grain morphology as a function of local sintering. The results of this study are promising for development of on-substrate thin/thick film MLCCs.     

Optical propagation modified by Cu nanoparticle grating fabricated by heavy ion implantation

The optical propagation property of a planar waveguide with a periodic nanoparticle grating layer is characterized by using sliding prism method. Here, Cu nanoparticle grating was fabricated on a-SiO₂ substrate by periodic heavy-ion irradiation technique. The pitch of these gratings was 2 μm and 3 μm, respectively. The flux and fluence were at the range of 6-10 μA/cm² and 6 × 10¹⁶-1 × 10¹⁷ ions/cm², respectively. The grating effect, mainly including the mode selection effect, is observed. The effect depends on the pitch of the grating and the morphology of nanoparticles. The propagation loss of the waveguide induced by nanoparticle layer is evaluated.     

Suspended core tellurite glass optical fibers for infrared supercontinuum generation

We report the fabrication and characterization of tellurite TeO₂-ZnO-Na₂O (TZN) microstructured suspended core optical fibers (MOFs). These fibers are designed for infrared supercontinuum generation with zero dispersion wavelength (ZDW) at 1.45 μm. The measured losses at this wavelength are approximately 6 dB/m for a MOF with a 2.2 μm diameter core. The effective area of a particular fiber is 3.5 μm² and the nonlinear coefficient is calculated to be 437 W⁻¹km⁻¹. By pumping a 20 cm long fiber at 1.56 μm with a sub-nJ femtosecond laser source, we generate a supercontinuum (SC) spanning over 800 nm in the 1-2 μm wavelength range.     

Crystallization of amorphous silicon thin-film on glass substrate preheated at 650 °C using Xe arc flash of 400 μs

Experimental and theoretical investigations on flash lamp annealing (FLA) of amorphous silicon (a-Si) film on glass were carried out with a view to practical applications in large-window display industries. A Xe arc flash lamp of 950 mm in length and 22 mm in bore diameter was applied with nominal input voltage of 7 kV and flash duration of 400 μs. Prior to the annealing process, the specimen for FLA was preheated at 650 °C, which was very close to the service temperature of the glass specimen used in this study. By employing a focusing elliptic reflector, maximum light energy density of up to 8.4 J/cm² could be attained with an active exposure width of 2 cm. Crystallization of a-Si could be achieved in solid-phase by applying a flash beam with light density of at least 5 J/cm², and its phase-transition characteristics that varied with energy densities could be explained by theoretically estimated temperature fields. Electron microscopy observations confirmed that solid-phase crystallization preceded melting of a-Si due to relatively long flashing (heating) duration of 400 μs, which was comparable to solid-phase crystal-growth times at elevated temperatures.     

Al-doped ZnO films by pulsed laser deposition at room temperature

Polycrystalline Al-doped ZnO films with good photoluminescence property were successfully deposited on quartz glass substrates by pulsed laser deposition (PLD) at room temperature. The films were obtained by ablating a metallic target (Zn:Al 3 wt%) at various laser energy densities (1.0-2.1 J/cm²) in oxygen atmosphere (9 Pa). The structure of the films was characterized by XRD. Ultraviolet photoluminescence centered at 359-361 nm was observed in the room temperature PL spectra of the Al-doped ZnO films.     

High-quality parallel patterning of carbon nanotube thin films by a pulsed laser beam

Single-walled carbon nanotube thin films solution-deposited on the glass substrate were directly patterned by a spatially-modulated pulsed laser beam (wavelength = 355 nm, pulse width = 5 ns) incident from the backside of the substrate. This method utilizes a ultrashort pulse-induced strong thermo-elastic force exerting on the film which plays a role to detach it from the substrate. The threshold energy density required for patterning was as low as 90 mJ/cm², making it possible to pattern over a few square centimeters by a single pulse with maximum energy of 180 mJ. The irradiated regions of the film were clearly photoetched without leaving any residual nanotubes. High-fidelity patterns could be fabricated with a feature size of 35 μm.     

Femtosecond pulsed laser ablation and deposition of titanium carbide

In this work we have evaporated a titanium carbide target by an Nd:glass laser with 250 fs pulse duration. The plasma produced from the ablation has been characterized by Intensified Coupled Charge Device (ICCD) fast imaging, optical emission spectroscopy and quadrupole mass spectrometry, while X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS) and Scanning and Transmission Electron Microscopy have been used to study the deposited film morphology and composition. The plume shape and front velocity are very similar to those found in other systems and are typical of femtosecond ablation. In particular the front velocity is 1.1 × 10⁷ cm s^− 1 at a laser fluence of 1.9 J cm^− 2, while the value of the cosine exponent is 4.5 in the same conditions. In the TiC system a delayed emission, found by ICCD imaging and emission spectroscopy, is also present. In fact, although the emission involved in the “traditional” plume ends after about 1 μs, the target is still hot and gives origin to another emission, expanding with a velocity that is about two orders of magnitude lower compared to that of the traditional plume (2.2 × 10⁴ cm s^− 1 at a laser fluence of 1.9 J cm^− 2).The results of the analysis of both the gaseous plume and the deposited films seem to indicate that in the case of TiC system the presence of a large number of particles ejected from the target is responsible for the formation of the films. XPS and EDX data indicate that the stoichiometry of the target is preserved in the films, while XRD analysis shows that the films are amorphous in structure.     

Porous nanostructured ZnO films deposited by picosecond laser ablation

Porous nanostructured polycrystalline ZnO films, free of large particulates, were deposited by picosecond laser ablation. Using a Zn target, zinc oxide films were deposited on indium tin oxide (ITO) substrates using a picosecond Nd:YVO₄ laser (8 ps, 50 kHz, 532 nm, 0.17 J/cm²) in an oxygen atmosphere at room temperature (RT). The morpho-structural characteristics of ZnO films deposited at different oxygen pressures (150–900 mTorr) and gas flow rates (0.25 and 10 sccm) were studied. The post-deposition influence of annealing (250–550 °C) in oxygen on the film characteristics was also investigated. At RT, a mixture of Zn and ZnO formed. At substrate temperatures above 350 °C, the films were completely oxidized, containing a ZnO wurtzite phase with crystallite sizes of 12.2–40.1 nm. At pressures of up to 450 mTorr, the porous films consisted of well-distinguished primary nanoparticles with average sizes of 45–58 nm, while at higher pressures, larger clusters (3.1–14.7 μm) were dominant, leading to thicker films; higher flow rates favored clustering.     

Spectral characterization of Yb-doped silica layers for high power laser applications

Thick silica layers, doped with rare-earth elements are required as active media for high power waveguide lasers and amplifiers. In this work, Yb/Al-codoped silica particles were deposited on pure silica wafers, followed by high temperature sintering and post-sinter laser annealing treatment. The optical properties of the layers were monitored at different stages of the process using transmission spectrometry in the near IR to UV range, micro-Raman spectroscopy, fluorescence spectrum, and decay measurements. Evolution of the Yb³⁺ ion fluorescence and stabilization of the Si:O bonds as a result of the sintering process were observed.Measurements of 30 μm thick layers showed high Yb absorption of 500 dB/m at 980 nm. The fluorescence lifetime was close to 1 ms and the propagation loss was less than 20 dB/m at 633 nm, currently limited by the measurement system. The results show that a potential material for high power applications has been achieved.     

Characterization on pulsed laser deposited nanocrystalline ZnO thin films

Nanocrystalline zinc oxide thin films were deposited on glass and silicon substrates by using pulsed laser deposition at different laser energy densities (1.5, 2, and 3 J/cm²). The film thickness, surface roughness, composition, optical and structural properties of the deposited films were studied using an α-step surface profilometer, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), optical transmittance, and X-ray diffraction (XRD), respectively. The film thickness was calculated as 244 nm. AFM analysis shows that the root-mean-square roughness increases with increasing laser energy density. XPS analysis shows that the interaction of zinc with oxygen atoms is greatly increased at high laser energy density. In the optical transmittance spectra, a shift of the absorption edge towards higher wavelength region confirms that the optical band gap increases with an increase in laser energy density. The particle size of the deposited films was measured by XRD, it is found to be in the range from 7.87 to 11.81 nm. It reveals that the particle size increases with an increase in laser energy density.     

Dependence of field-emission threshold in diamond-like carbon films grown by various techniques

In this paper, we report field-emission measurements from ∼0.5-μm-thick hydrogenated amorphous carbon (diamond-like carbon (DLC)) films. These films were grown by a variety of easily implementable plasma-enhanced chemical vapor deposition (PECVD) based techniques and also by a method that uses a saddle-field fast atom beam source. Field-emission behavior in these materials has been discussed in light of residual stress, hardness, optical band gap, and characteristic energy of band tails (Urbach energy). Onset emission-fields as low as ∼6 V/μm, together with low residual stress of 0.25 GPa, hardness of 17.5 GPa, optical band gap of 1.5 eV, and Urbach energy of 165 meV, have been obtained in DLC films grown by pulsed-PECVD at 13.56 MHz. DLC films of comparable quality could also be grown using a saddle-field fast atom beam source, which operates on modest dc power supply and with no heated filaments or magnets.     

Thermal lensing investigation on bulk ceramics and thin-film PLZT using visible and far-infrared laser beams

The real-time thermo-optical properties of thin-film relaxor ferroelectric (9/65/35) lead lanthanum zirconate titanate (PLZT) films (∼2 μm) were investigated and compared to identical composition of thick PLZT ceramics (∼1 mm). Since PLZT absorbs strongly in the far-infrared, a CO₂ laser beam (λ = 10.6 μm) was used to create a Gaussian temperature distribution at the surface of the material, emulating a lens. Subsequently, a low power visible He-Ne laser, travelling along the same path as the CO₂ laser, probed the optical properties of the induced PLZT thermal lens. It was found that identical composition thin-film and thick PLZT ceramics have opposite thermal lensing behaviors. An explanation of this phenomenon is provided from basic physical concepts.