Tunable Laser Beam Research Articles

Phase-tuned acousto-optic spatial filter.

An acousto-optic (AO) tunable filter with a phase-controlled dual-section piezoelectric transducer is designed and created for laser beam shaping (LBS). Owing to the acoustic beam steering effect, we experimentally observe splitting of the two-dimensional transfer function. As a result, we demonstrate generation of tunable bottle laser beams and dual-ring intensity distributions for the diffracted beam.

Controlling the Glassy State toward Structural and Mechanical Enhancement: Additive Manufacturing of Bulk Metallic Glass Using Advanced Laser Beam Shaping Technology

AbstractBulk metallic glasses (BMGs) offer exceptional physical/mechanical properties enabling them to be highly desirable for a variety of applications. Laser powder bed fusion (LPBF) has great promise for producing large and intricate BMG structures. However, using non‐optimal energy distribution in current additive manufacturing machines leads to extensive reheating of previously solidified layers. As a result, the mechanical characteristics can be significantly impacted by structural relaxation and partial crystallization. Here, a tunable advanced laser beam shaping technology is employed to overcome the difficulties originating from non‐optimal energy distribution in current additive manufacturing machines. This study fabricates fully amorphous/dense BMG samples using the shaped laser beam and established optimized atomic‐scale short‐and medium‐range ordering along with improved yield/fracture compressive strength. Formation of a shallow and wide melting pool geometry using the beam shaping allows to increase hatching distances to better control the thermal history introducing improved amorphicity and rejuvenation. This higher rejuvenation and disordering allow for increased atomic mobility, which facilitates the creation and spread of shear bands, thus enhancing the mechanical strength and ductility of the material. The current work demonstrates that BMG parts can be fabricated using flexible beam‐shaping technology allowing to go beyond the capabilities of state‐of‐the‐art additive manufacturing techniques.

Broadband ultrafast ultraviolet laser output by using β-BaB2O4 crystal

In this paper, a broadband ultrafast ultraviolet (UV) laser was generated through sum-frequency method with β-BaB2O4 (BBO) crystal by using a femtosecond (fs) Ti:sapphire laser as the fundamental laser source. We studied the influence of the power ratio between the fundamental frequency and second harmonic laser beams on output power and conversion efficiency of UV laser. The time delay method was used to change the optical path difference in the sum-frequency generation (SFG). At the same time, the cutting angle of the crystal was optimized and the effect of different focal lengths of lenses on the experimental results was studied. Finally, we obtained a tunable UV laser beam with a tuning range from 226.7 nm to 360 nm generated by the BBO crystal. The laser had high conversion efficiency, and the maximum output power was 540 mW at 270 nm.

Air resonance enhanced multiphoton ionization tagging velocimetry.

Air resonance enhanced multiphoton ionization (REMPI) tagging velocimetry (ART) was demonstrated in quiescent and supersonic flows. The ART velocimetry method utilizes a wavelength tunable laser beam to resonantly ionize molecular oxygen in air and generate additional avalanche-type ionization of molecular nitrogen. The fluorescence emissions from the first negative and first positive bands of molecular nitrogen are, thus, produced and used for flow tagging. Detailed characterization of ART was conducted, including the effects of oxygen resonance to fluoresce nitrogen, nitrogen fluorescence spectrum, laser energy deposition into quiescent flow showing minimal perturbations in flow, fluorescence lifetime study at various pressures, and line tagging without breakdown. Pointwise velocity measurements within a supersonic flow from a nominal Mach 1.5 nozzle have been conducted and characterized.

Excitation of the Modes of an Optical Resonator by a Tunable Laser Beam

Excitation of the Modes of an Optical Resonator by a Tunable Laser Beam

Optically active selenium vacancies in BaGa4Se7 crystals

Barium gallium selenide (BaGa4Se7) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is the production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa4Se7 crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize these defects. Five distinct EPR spectra, each representing an electron trapped at a selenium vacancy, are observed at low temperature (there are seven crystallographically inequivalent selenium sites in the crystal). One spectrum is stable at room temperature and is present before illumination. The other four are produced at lower temperatures with 532 nm laser light and are thermally unstable at room temperature. Each S = 1/2 singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site. A significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion and gives principal values of the hyperfine matrices in the 3350–6400 MHz range. Broad photoinduced optical absorption bands in the visible and near-infrared are assigned to the selenium vacancies.

Multilayer dielectric coated Fabry Perot spectral beam combiner for high power, narrowband, widely tunable laser applications

Spectral beam combination of tunable laser beams demonstrated, using a multilayer dielectric coated single cavity Fabry Perot (FP) filter. A stack of 19 layers using TiO2 and SiO2 was optimally designed and fabricated using an electron beam evaporation technique. This filter was characterized for transmission spectrum, surface morphology and laser induced damage threshold. The sharp transmission spectrum facilitated the spectral combination of closer wavelengths (<10 nm) around 632.8 nm. This filter was utilized as an efficient spectral beam combiner as well as an amplified spontaneous emission (ASE) filter for the transmitting wavelength. This study finds direct application for tunable, multi-wavelength laser-based differential absorption lidar (DIAL) and atmospheric remote sensing applications. The application of this device was successfully demonstrated to spectrally combine high power, tunable copper vapor laser pumped dye laser beams. A mechanism to tune the FP transmission profile along with laser beam tunability was established.

Photodissociation branching ratios of 12C16O from 108000 cm−1 to 113200 cm-1 measured by two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method: Observation of channels for producing O(1D)

The photoabsorption and photodissociation of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region is one of the most important photochemical processes in the interstellar medium, thus it has attracted numerous experimental and theoretical studies. Here, we employed the two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method to measure the relative branching ratios [C(3P0)+O(1D)]/ {[C(3P0)+O(3P)]+ [C(3P0)+O(1D)]} and [C(3P2)+O(1D)]/ {[C(3P2)+O(3P)]+[C(3P2)+O(1D)]} in the VUV photoexcitation energy range of 108000−113200 cm−1. Here, one tunable VUV laser beam is used to excite CO to specific rovibronic states, and a second independently tunable VUV laser beam is used to state-selectively ionize C(3P0) and C(3P2) for detection. State-selective photoionization through the 1VUV+1UV/visible resonance-enhanced multiphoton ionization scheme has greatly enhanced the detection sensitivity, which makes many new weak absorption bands observable in the current study. The branching ratio measurement shows that the spin-forbidden channels C(3P0)+O(1D) and C(3P2)+O(1D) only open at several discrete narrow energy windows. This might be caused by certain accidental resonance-enhanced spin-orbit interactions between the directly excited Rydberg states and valence states of triplet type which finally dissociate into the spin-forbidden channels.

Translation of two-photon microscopy to the clinic: multimodal multiphoton CARS tomography of in vivo human skin.

.Two-photon microscopes have been successfully translated into clinical imaging tools to obtain high-resolution optical biopsies for in vivo histology. We report on clinical multiphoton coherent anti-Stokes Raman spectroscopy (CARS) tomography based on two tunable ultrashort near-infrared laser beams for label-free in vivo multimodal skin imaging. The multiphoton biopsies were obtained with the compact tomograph “MPTflex-CARS” using a photonic crystal fiber, an optomechanical articulated arm, and a four-detector-360 deg measurement head. The multiphoton tomograph has been employed to patients in a hospital with diseased skin. The clinical study involved 16 subjects, 8 patients with atopic dermatitis, 4 patients with psoriasis vulgaris, and 4 volunteers served as control. Two-photon cellular autofluorescence lifetime, second harmonic generation (SHG) of collagen, and CARS of intratissue lipids/proteins have been detected with single-photon sensitivity, submicron spatial resolution, and picosecond temporal resolution. The most important signal was the autofluorescence from nicotinamide adenine dinucleotide [NAD(P)H]. The SHG signal from collagen was mainly used to detect the epidermal–dermal junction and to calculate the ratio elastin/collagen. The CARS/Raman signal provided add-on information. Based on this view on the disease-affected skin on a subcellular level, skin areas affected by dermatitis and by psoriasis could be clearly identified. Multimodal multiphoton tomographs may become important label-free clinical high-resolution imaging tools for in vivo skin histology to realize rapid early diagnosis as well as treatment control.

Simulation of Intensities of High Resolution Lines of Nitrogen in the Region of 2045–2305 cm−1

Quasi-random model of molecular band absorption is used to simulate the intensities of high resolution lines of nitrogen in the frequency region 2045–2305 cm−1. A tunable laser beam of frequency between 3 and 5 μm is assumed to be passed through the atmosphere. Values of transmittance, averaged over intervals of 20 cm−1, are obtained for absorber nitrogen thicknesses 500 and 5000 atm-cm. Line-by-line model for calculating the absorption coefficient is next used to find the values of transmittance at the wavelengths of interest. Absorbance values obtained by both the models are compared vis-a-vis the experimental data.

Atomically resolved phase transition of fullerene cations solvated in helium droplets.

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called ‘Atkins snowballs' have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.

Standoff Mechanical Resonance Spectroscopy Based on Infrared-Sensitive Hydrogel Microcantilevers.

This paper reports a highly sensitive and selective remote chemical sensing platform for surface-adsorbed trace chemicals by using infrared (IR)-sensitive hydrogel microcantilevers. Poly(ethylene glycol) diacrylate (PEG-DA) hydrogel microcantilevers are fabricated by ultraviolet (UV) curing of PEG-DA prepolymer introduced into a poly(dimethylsiloxane) mold. The resonance frequency of a PEG-DA microcantilever exhibits high thermal sensitivity due to IR irradiation/absorption. When a tunable IR laser beam is reflected off a surface coated with target chemical onto a PEG-DA microcantilever, the resonance frequency of the cantilever shifts in proportion to the chemical nature of the target molecules. Dynamic responses of the PEG-DA microcantilever can be obtained in a range of IR wavelengths using a tunable quantum cascade laser that can form the basis for the standoff mechanical resonance spectroscopy (SMRS). Using this SMRS technique, we have selectively detected three compounds, dimethyl methyl phosphonate (DMMP), cyclotrimethylene trinitramine (RDX), and pentaerythritol tetranitrate (PETN), located 4 m away from the PEG-DA microcantilever detector. The experimentally measured limit of detection for PETN trace using the PEG-DA microcantilever was 40 ng/cm2. Overall, the PEG-DA microcantilever is a promising candidate for further exploration and optimization of standoff detection methods.

Adaptable beam profiles from a dual-cavity Nd:YAG laser.

We report a technique to tailor a laser beam profile from a donut to quasi-top-hat intensity distribution, directly from the laser, simply achieved by simultaneous excitation and control of the relative contributions of the fundamental (TEM00) and first-order Laguerre-Gaussian (LG01) transverse modes. Exploiting a dual-cavity configuration with a single Nd:YAG gain element, adaptable continuous-wave laser beam profiles from the primary cavity could be obtained by varying the diffraction loss of an acousto-optic modulator in the secondary cavity. We investigate the resultant beam profiles as a function of pump power and the AOM diffraction loss, and discuss the prospects for tunable laser beams profiles.

Interaction of a tunable laser beam with high-resolution lines of ozone in 4220 – 4226 cm −1

Interaction of a tunable narrow linewidth-laser beam with the infrared absorption lines of ozone in the region of 4220-4226 cm -1 is studied  using the quasi-random model of molecular band absorption. Values of transmittance, computed at intervals of 0.1 cm −1 , are obtained for three different absorber thicknesses. The generated absorptance values, which simulate the intensities of the high-resolution absorption lines of ozone, are compared with those generated by the line-by-line (LBL) method. Keywords Line by line method, ozone, quasi-random model, transmittance, tunable laser beam.

O2 rotational temperature measurements in an atmospheric air microdischarge by radar resonance-enhanced multiphoton ionization

Nonintrusive spatially resolved rotational temperature measurements in an atmospheric air microdischarge are presented. The measurements were based on coherent microwave Rayleigh scattering (Radar) from resonance-enhanced multiphoton ionization of molecular oxygen. The open air DC microdischarge source operated in a stable “normal-glow” mode and pin-to-pin electrodes spaced 1.3 mm apart. The second harmonic of a tunable dye laser beam was focused between the two electrodes and scanned between 286 and 288 nm. Coherent microwave Rayleigh scattering was used to collect the two-photon rotational spectra of O2 at C3Π(v = 2)←X3Σ(v′ = 0) transitions. The Boltzmann plots from analyses of the O2 rotational lines determined local rotational temperatures at various axial locations between the electrodes. The molecular oxygen rotational temperature varied from ∼1150 K to ∼1350 K within the discharge area. The measurements had an accuracy of ∼±50 K.

Experimental generation of tunable star-shape laser beam from Gaussian beam

Optical beam plays an important role in many optical systems, and beam construction has attracted much attention. In this paper, we proposed a new kind of tunable star-shape laser beam whose phase wavefront distribution is the sine function of azimuthal angle. And a simple method was also carried out to generate this kind of beam based on computer-generated holograms. Experimental results show that the transverse intensity pattern of the tunable star-shape laser beam changes very considerably on increasing beam parameter that contained in the sine function term of phase wavefront distribution. When beam parameter is smaller than 3, and the number of intensity peak equals beam parameter. When beam parameter is bigger than 4, transverse intensity pattern turns star shape, and its branch number equals beam parameter.

Wavelength tunable laser beam shaping

Laser beam shaping by phase-only transformations, often referred to as field mapping, has for a long time been considered wavelength dependent. In this Letter we outline a simple mathematical argument that shows how the problem may be formulated in a wavelength tunable manner, requiring only a minor adjustment in the observation plane. We verify the theoretical prediction by experiment using the example of a Gaussian-to-flattop-beam transformation, and we show that the shaping is valid across a wide range of wavelengths for a single diffractive optical element.

Br2 molecular elimination in photolysis of (COBr)2 at 248 nm by using cavity ring-down absorption spectroscopy: A photodissociation channel being ignored

A primary dissociation channel of Br(2) elimination is detected following a single-photon absorption of (COBr)(2) at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br(2) fragment in the B(3)Π(ou)(+)-X(1)Σ(g)(+) transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br(2) contribution from a secondary reaction. By means of spectral simulation, the ratio of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 ± 0.09):(0.34 ± 0.07), corresponding to a Boltzmann vibrational temperature of 893 ± 31 K. The quantum yield of the ground state Br(2) elimination reaction is determined to be 0.11 ± 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr)(2) via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br(2) + 2CO. The resulting Br(2) is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism.

Laser-excited PEEM using a fully tunable fs-laser system

The ferroelectric domain structure on a (001) surface of a BaTiO3 W. Widdra single crystal prepared under ultrahigh vacuum conditions is imaged by laser-excited photoemission electron microscopy (PEEM). The PEEM images allow for discrimination of three domain types by their different photoemission yields. To characterize the contrast between the different ferroelectric domains of BaTiO3(001) in the region of 4.0-4.6 eV close to the photoemission threshold, the broad wavelength tunability of the femtosecond laser system is used. The femtosecond time resolution within two-photon PEEM experiments, using two fully independently tunable laser beams, is demonstrated for the first image potential state of Ag(001). In a pump-probe setup with a cross-correlation full-width at half-maximum of 70 fs, an angle-integrated apparent lifetime of 40 fs is derived from the PEEM intensities. In contrast with energy-resolved two-photon photoemission measurements, PEEM data reveal a 17-fs shorter lifetime. This difference is discussed by considering the co-excitation of bulk states, which leads to an additional time-dependent photoelectron contribution in PEEM.

I2 molecular elimination in single-photon dissociation of CH2I2 at 248 nm by using cavity ring-down absorption spectroscopy

Following single-photon dissociation of CH(2)I(2) at 248 nm, I(2) molecular elimination is detected by using cavity ring-down absorption spectroscopy. The technique comprises two laser beams propagating in a perpendicular configuration, in which a tunable laser beam along the axis of the ring-down cell probes the I(2) fragment in the B (3)Π(ou)(+) - X (1)Σ(g)(+) transition. The nascent vibrational populations for v = 0, 1, and 2 levels are obtained with a population ratio of 1:(0.65 ± 0.10):(0.30 ± 0.05), corresponding to a Boltzmann-like vibrational temperature of 544 ± 73 K. The quantum yield of the ground state I(2) elimination reaction is determined to be 0.0040 ± 0.0025. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state CH(2)I(2) via internal conversion, followed by asynchronous three-center dissociation. A positive temperature effect supports the proposed mechanism.