Single Laser Beam Research Articles

On the non-linear spectroscopy including saturated absorption and four-wave mixing in two and multi-level atoms: a computational study

In this paper we report on the study of two and multi-level atoms interacting with multiple laser beams. The semi-classical approach is used to describe the system in which the atoms are treated quantum mechanically via the density matrix operator, while the laser beams are treated classically using Maxwells equations. We present results of a two level atom interacting with single and multiple laser beams and demonstrate Rabi oscillations between the levels. The effects of laser modulation on the dynamics of the atom (atomic populations and coherences) are examined by solving the optical Bloch equations. Plots of the density matrix elements as a function of time are presented for various parameters such as laser intensity, detuning, modulation etc. In addition, phase-space plots and Fourier analysis of the density matrix elements are provided. The atomic polarization, estimated from the coherence terms of the density matrix elements, is used in the numerical solution of Maxwells equations to determine the behaviour of the laser beams as they propagate through the atomic ensemble. The effects of saturation and hole-burning are demonstrated in the case of two counter propagating beams with one being a strong beam and the other being very weak. The above work is extended to include four-wave mixing in four level atoms in a diamond configuration. Two co-propagating beams of different wavelengths drive the atoms from a ground state |1〉 to an excited state |3〉 via an intermediate state |2〉. The atoms then move back to the ground state via another intermediate state |4〉, resulting in the generation of two additional correlated photon beams. The characteristics of these additional photons are studied.

Bi-directional top-hat D-Scan: single beam accurate characterization of nonlinear waveguides

The characterization of a third-order nonlinear integrated waveguide is reported for the first time by means of a top-hat dispersive-scan (D-Scan) technique, a temporal analog of the top-hat Z-Scan. With a single laser beam, and by carrying two counterdirectional nonlinear transmissions to assess the input and output coupling efficiencies, a novel procedure is described leading to accurate measurement of the TPA figure of merit, the effective two-photon absorption (TPA), and optical Kerr (including the sign) coefficients. The technique is validated in a silicon strip waveguide for which the effective nonlinear coefficients are measured with an accuracy of ±10%.

Optical Surface Tension Measurement of Two-Dimensionally Confined Liquid Surfaces.

Surface tension measurement based on spontaneous capillary wave resonance in confined micrometer-sized liquid interfaces was demonstrated. A single-beam quasi-elastic laser scattering method was used to detect the resonance. Characteristic resonant modes were observed on a 44-μm-sized circular water surface. The frequencies of the peaks agreed well with those simulated by assuming planar resonance, and the relationship was further confirmed for triangular, square, and pentagonal water surfaces. Then, the applicability of the method was successfully demonstrated by surface tension measurements of aqueous solutions of sodium dodecyl sulfate. The sensitive detection of capillary resonance opens new possibilities for the chemical and biochemical analysis of liquid interfaces.

Surface quasi periodic and random structures based on nanomotor lithography for light trapping

We compare the characteristics of two types of patterns obtained with two azopolymer materials: a Gaussian random pattern and a quasi-random grating pattern. The surface structurations have been obtained with a simple bottom-up technique, illuminating azopolymer thin films with a single laser beam. We demonstrate the interesting generated properties of these two surfaces. In particular, the surface with quasi-random gratings can address beam splitting for light coupling in different directions in an ultra-thin film. We use these two surfaces as a mold and replicate them on a transparent elastomeric material and demonstrate a very good light entrapment. We also show that the efficiency of light trapping is 20% better with the quasi-random gratings, than with the Gaussian random surface, and is close to 40%.

Two-color monochromatic x-ray imaging with a single short-pulse laser.

Simultaneous monochromatic crystal imaging at 4.5 and 8.0 keV with x-rays produced by a single short-pulse laser is presented. A layered target consisting of thin foils of titanium and copper glued together is irradiated by the 50 TW Leopard short-pulse laser housed at the Nevada Terawatt Facility. Laser-accelerated MeV fast electrons transmitting through the target induce Kα fluorescence from both foils. Two energy-selective curved crystals in the imaging diagnostic form separate monochromatic images on a single imaging detector. The experiment demonstrates simultaneous two-color monochromatic imaging of the foils on a single detector as well as Kα x-ray production at two different photon energies with a single laser beam. Application of the diagnostic technique to x-ray radiography of a high density plasma is also presented.

Comparison of Three Timing Systems: Reliability and Best Practice Recommendations in Timing Short-Duration Sprints.

Bond, CW, Willaert, EM, and Noonan, BC. Comparison of three timing systems: reliability and best practice recommendations in timing short-duration sprints. J Strength Cond Res 31(4): 1062-1071, 2017-With the importance placed on athletic speed, it is important to use a valid and reliable timing system, particularly in sprints of short duration. Unfortunately, many of the commonly used timing systems have not been rigorously evaluated. This study aimed to compare results from a single-beam infrared photocell (PC), single-beam laser with a microprocessor (LA), and a previously validated video camera (VC) sacrum-based timing system; and in doing so, determine these systems' reliabilities, and establish best practices for increasing reliability. It was hypothesized that PC and LA times would be different from VC times and show reduced reliability compared with VC. Fifteen athletes performed 5 repetitions of a 60-foot maximal effort sprint with split times recorded for the first and second half. Photocell and LA full-time and first-half split times were significantly slower than VC (p < 0.001), but almost identical for the second half split (p = 0.08). Repeated sprint analysis showed that VC tended to have smaller SD compared with PC and LA for first-half split (0.05 vs. 0.08 vs. 0.09 seconds, respectively) and total time (0.09 vs. 0.10 vs. 0.11 seconds, respectively). Time differences were more dependent on initial forward lean and varying body segments triggering the beam, than on a systematic instrument error. The increased variability of PC and LA systems dampen the ability to determine whether meaningful change has occurred. The VC system allows for very valid and reliable measurements of an athlete's sprint time, especially in distances <30 feet.

Optical concatenation of a large number of beads with a single-beam optical tweezer.

Optical tweezers consist of the spatial confinement of microscopic dielectric particles by the action of forces produced by the change in momentum of the photons of a highly focused laser beam that are deviated by the particle. In experiments that use a single laser beam, it is common to capture not only one but a few particles in the optical trap. However, to our knowledge, the formation of a long chain of beads optically confined with a single laser beam has never been reported. In this work, up to 73 silica spheres immersed in water are seen concatenated along the propagation direction of a 976-nm wavelength Gaussian laser of 300 mW of power. This long chain of beads is obtained when the laser is focused through an oil-immersion DIN microscope objective with 100× magnification and a numerical aperture of 1.25. When performing the same experiment using an infinity-corrected UplanFLN 100× objective with a numerical aperture of 1.3, the maximum number of concatenated beads is only 14. Our results suggest that the mechanisms responsible for the observed phenomena involve successive refocusing of the laser beam by each trapped sphere, optically induced dipole coupling (commonly referred to as optical binding), and aberrations generated by the DIN microscope objective.

Optimization of a crossed optical dipole trap for loading and confining laser-cooled atoms

We theoretically study the loading of cold atoms into a crossed optical dipole trap (ODT). We compared the loading efficiency dependent on the trap geometry for Nd:YAG and CO2 laser ODTs. By crossing two Nd:YAG laser beams at a small angle, such as 10&#x00B0;, we can shape the geometry of the effective potential in the intersection region and achieve comparable performance to that of a single-beam CO2 laser ODT. We also calculate the optimal experimental parameters for these two laser wavelengths for laser power per beam ranging from 5 W to 100 W. We further experimentally studied the loading of laser-cooled Rb87 atoms into a small-angle crossed Nd:YAG ODT and found that the experimental data agreed with the simulation results.

Grating chips for quantum technologies

We have laser cooled 3 × 106 87Rb atoms to 3 μK in a micro-fabricated grating magneto-optical trap (GMOT), enabling future mass-deployment in highly accurate compact quantum sensors. We magnetically trap the atoms, and use Larmor spin precession for magnetic sensing in the vicinity of the atomic sample. Finally, we demonstrate an array of magneto-optical traps with a single laser beam, which will be utilised for future cold atom gradiometry.

Comparison of single-beam and dual-beam laser welding of Ti–22Al–25Nb/TA15 dissimilar titanium alloys

Laser beam welding (LBW) was used to join Ti–22Al–25Nb/TA15 dissimilar titanium alloys. The microstructure and mechanical properties of the welded joints under single and dual beam welding were analyzed and compared. In the mode of single laser beam, the fusion zone only consisted of B2 phase because of existence of β-phase stabilizer and rapid cooling rate of LBW. However, O phase was formed in the fusion zone while applying dual-beam laser welding due to decrease of the cooling rate. The microhardness distribution of the welded joint in dual-beam welding mode was consistent with that in single mode, but the hardness of the weld under dual laser beam was higher than that of single laser beam. In room-temperature tensile tests, the fractures all occurred in the weld, but the morphology exhibited a quasi-cleavage feature in single mode while the morphology was dimple fracture in the mode of dual laser beam. The tensile strength and elongation were both increased under dual-beam laser welding compared with those under single-beam laser welding.

Analytical calculations of optical trapping forces for drag calibration: Effects of mismatch between beam focus and particle center

We present analytical calculations of optical forces for drag calibrations via the geometrical optics approximation (GOA) for the case in which a single laser beam is strongly focused on a polystyrene microsphere dispersed in an aqueous phase. When the beam focus is mismatched with the particle center owing to the presence of vertical forces, such as gravity, buoyancy, and the radiation force caused by the laser beam, the trapped particle would be displaced in a radial direction when the lateral drag force is applied. Based on the analytical calculations of optical trapping forces and the force balance upon dragging, we found that the critical laser power at which the beam focus matches the particle center exhibits a power-law relationship with the particle radius a according to the empirical expression log(Pcrit (mW)) = 2.899 log(2a (µm)) - 2.316. We also found an empirical expression for the mechanical equilibrium position of the vertical displacement Δzeq0(Δy=0) as functions of the laser power P and the particle radius, \(\Delta z_{eq}^0 = \frac{{\left[ {{F_{gb}}\left( {pN} \right) - 0.068P\left( {mW} \right)} \right]\left[ {2.7a\left( {\mu m} \right) - 0.252} \right]}}{{P\left( {mW} \right)}}\), where Fgb is the sum of the gravitational and buoyant forces.

Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler

Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media.

Single femtosecond laser beam pumped transient diffraction and transient lens effects for ultrafast measurement in background-free geometry

We proposed an ultrafast optical gating configuration for ultrafast measuring the characters of a supercontinuum (SC) probe beam. Single femtosecond laser beam pumped transient diffraction (TrD) and transient lens (TrL) effects were realized at the same time, which ensured the measurement in background-free geometry. Experimental results showed that the proposed ultrafast optical gating was robust for very high signal-to-noise ratio (SNR) detecting.

Performance of micro-dent array fabricated by laser shock peening on the surface of A304 stainless steel

A technology called net mask laser shock peening (NMLSP) is presented in this paper. A metallic mesh grid was used as a mask. When a high energy laser beam travels through the mask, it is divided into several beams of a micrometer-scale spot size. Shock waves induced by these micrometer-scale laser beams, were used to fabricate micro-dent arrays on the surface of a A304 stainless steel. Due to the interference of shock wave released by adjacent surfaces, surface topography, microhardness and residual stress distribution of the as-fabricated surface were different from the distribution of the surfaces fabricated by a single laser beam. Moreover, the friction wear experiments indicated that the wear performance of as-fabricated surfaces was remarkably improved when the load was low. As load and sliding speed increased, the untreated samples wear was altered from abrasive to adhesive, whereas the NMLSP treated samples still exhibited abrasive wear. Therefore, NMLSP can substantially improve friction and wear performance of the A304 stainless steel.

Effects of beam configurations on wire melting and transfer behaviors in dual beam laser welding with filler wire

Butt joints of 2mm thick stainless steel with 0.5mm gap were fabricated by dual beam laser welding with filler wire technique. The wire melting and transfer behaviors with different beam configurations were investigated detailedly in a stable liquid bridge mode and an unstable droplet mode. A high speed video system assisted by a high pulse diode laser as an illumination source was utilized to record the process in real time. The difference of welding stability between single and dual beam laser welding with filler wire was also compartively studied. In liquid bridge transfer mode, the results indicated that the transfer process and welding stability were disturbed in the form of “broken-reformed” liquid bridge in tandem configuration, while improved by stabilizing the molten pool dynamics with a proper fluid pattern in side-by-side configuration, compared to sigle beam laser welding with filler wire. The droplet transfer period and critical radius were studied in droplet transfer mode. The transfer stability of side-by-side configuration with the minium transfer period and critical droplet size was better than the other two configurations. This was attributed to that the action direction and good stability of the resultant force which were beneficial to transfer process in this case. The side-by-side configuration showed obvious superiority on improving welding stability in both transfer modes. An acceptable weld bead was successfully generated even in undesirable droplet transfer mode under the present conditions.

Photothermal heating of nanoribbons

Nanoscale optical materials are of great interest for building future optoelectronic devices for information processing and sensing applications. Although heat transfer ultimately limits the maximum power at which nanoscale devices may operate, gaining a quantitative experimental measurement of photothermal heating within single nanostructures remains a challenge. Here, we measure the nonlinear optical absorption coefficient of optically trapped cadmium-sulfide nanoribbons at the level of single nanostructures through observations of their Brownian dynamics during single-beam laser trapping experiments. A general solution to the heat transfer partial differential equation is derived for nanostructures having rectilinear morphology including nanocubes and nanoribbons. Numerical electromagnetic calculations using the discrete-dipole approximation enable the simulation of the photothermal heating source function and the extraction of nonlinear optical absorption coefficients from experimental observations of single nanoribbon dynamics.

Phase sensitive chirped laser dispersion spectroscopy under high absorbance conditions

A whole-fiber methane sensor under high absorbance based on phase sensitive chirped laser dispersion spectroscopy is presented in this paper. The laser source of the sensor is a tunable distributed feedback diode laser with a frequency of 1653.7 nm. A telecom-based electro-optical intensity Mach-Zehnder modulator working in carrier suppression mode is adapted to modulate the single frequency laser beam for generating a dual-sideband spectrum beside the carrier wave. Unlike previous proposed phase sensitive chirped laser dispersion spectroscopy scheme, the beatnote signal generated by the two sidebands is detected experimentally. The refractive index fluctuation around the 23 transition of methane is measured by detecting the phase variation of the dual-sideband beatnote signal through using the heterodyne interferometric method. A lock-in amplifier is employed in the phase demodulation process. By connecting the refractive index (the real part of the complex refraction index) and the absorption coefficient (the imaginary part of the complex refraction index) via Kramers-Kroning relation, the gas concentration information is retrieved from the optical dispersion measurement. Absorption-based wavelength modulation spectroscopy measures the gas concentration encoded in the optical intensity based on Beer-Lambert's law. However, the signal sensitivity of wavelength modulation spectroscopy decreases, and the signal even decreases while the gas concentration is raised in high absorbance condition, which leads to an uncertainty in concentration measurement. Experimental results demonstrate that wavelength modulation spectroscopy has better performance in low absorbance condition. The detection limit is about 38.1 ppmm. However, because the sensitivity decreases in high absorbance conditions, the upper detection limit of wavelength modulation spectroscopy is only 1500 ppmm. The dynamic range is defined through dividing the upper detection limit by the detection limit. Therefore, the wavelength modulation spectroscopy obtains a linear measurement dynamic range of 16 dB. Nevertheless, under the same experimental condition, the phase sensitive chirped laser dispersion spectroscopy has a much larger linear measurement range from 47.3 ppmm to 174825 ppmm with a dynamic range higher than 35 dB. Absorption-based gas measurement technique such as wavelength modulation spectroscopy can achieve a low detection limit by using long optical path at the expense of lower upper limit concentration. Phase sensitive chirped laser dispersion spectroscopy appears to be effective in high absorbance condition, which may be caused by high concentration or long optical path. Furthermore, by combining phase sensitive chirped laser dispersion spectroscopy and long optical path technique such as multi pass cell in sensor design, large linear measurement dynamic range and low detection limit can be obtained at the same time.

基于单激光束旋转的深孔直线度测量方法

基于单激光束旋转的深孔直线度测量方法

A spherical hohlraum design with tetrahedral 4 laser entrance holes and high radiation performance

As usual cylindrical hohlraum with double laser ring cones may lead to serious laser-plasma interaction, such as the simulated Raman scatter and cross-beam energy transfer effect, spherical hohlraum with octahedral 6 Laser Entrance Holes (LEHs) and single cone laser beams, was investigated and reported to have a consistent high radiation symmetry during the whole implosion process. However, it has several potential challenges such as the smaller space left for diagnosis and the assembly of centrally located capsule. In this paper, based on the view-factor model, we investigate the radiation symmetry and the drive temperature on the capsule located in the spherical hohlraum with tetrahedral 4 LEHs and single cone laser beams, since there is more available space for laser disposition and diagnosis. Then, such target is optimized on the laser beam pointing direction to achieve a high radiation performance, i.e., the radiation symmetry and drive temperature on the capsule. Finally, an optimal spherical hohlraum with optimal laser beam pointing has been demonstrated and compared with the spherical hohlraum with octahedral 6 LEHs. The resulting radiation symmetry and the drive temperature shows that it has almost a similar radiation symmetry (the radiation asymmetry variation is no more than 0.2%), and higher drive temperature (the temperature has been increased by 1.73%, and an additional 133 kJ energy of 2 MJ energy for fusion can be saved).

Dynamics of polystyrene beads linking to DNA molecules under single optical tweezers: A numerical study using full normalized Langevin equation

The dynamics of a polystyrene bead linking to a [Formula: see text]-phage deoxyribonucleic acid (DNA) molecule under single optical tweezers using CW laser beam is analyzed by the full normalized Langevin equation with elastic and Brownian forces. The obtained results show that the stable time and stable position of polystyrene bead depend on its beginning position, laser beam waist, and laser beam power. The oscillation of polystyrene beads at stable position is observed with different parameters of optical tweezers. Finally, the conditions to stabilize the bead and to stretch the [Formula: see text]-phage DNA by single laser beam optical tweezers are discussed.