Probe Laser Beam Research Articles

Reducing Number Fluctuations in an Ultracold Atomic Sample Using Faraday Rotation and Iterative Feedback

We demonstrate a method to reduce number fluctuations in an ultracold atomic sample using real-time feedback. By measuring the Faraday rotation of an off-resonant probe laser beam with a pair of avalanche photodetectors in a polarimetric setup, we produce a proxy for the number of atoms in the sample. We iteratively remove a fraction of the excess atoms from the sample to converge on a target proxy value in a way that is insensitive to environmental perturbations and robust to errors in light polarization. Using absorption imaging for out-of-loop verification, we demonstrate a reduction in the number fluctuations from $3\mathrm{%}$ to $0.45\mathrm{%}$ for samples at a temperature of $16.4\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{K}$ over the time scale of several hours, which is limited by temperature fluctuations, beam-pointing noise, and photon shot noise.

Dual-wavelength flash Raman mapping method for measuring the thermal diffusivity of a single supported nanowire

The heat dissipation of electronic devices has become an enormous challenge with the dramatic increasing of integrated transistors on a chip. Nanowires are viewed as promising materials for solving this problem. The accurate measurement of thermal diffusivity is a precondition for further applications. This paper presents a non-destructive method to measure the thermal diffusivity of a single supported nanowire, which is called dual-wavelength flash Raman (DFR) mapping method. This method has a 100 ps temporal resolution based on the minimum time delay between the heating and probing laser pulses and 50 nm spatial resolution based on the minimum step of the probing pulse. Compared with previously proposed concentric DFR method, the probing pulse can move along the nanwire length and the movable probing laser beam allowed temperature measurements at the most sensitive position,which improves the measurement accuracy by 50%. Moreover, instead of the normalizaiton temperature method in concentric DFR method, a temperature phase method was proposed for analyzing the temperature variation curves. The measurements on a single supported silicon nanowire verified the reliability of the DFR mapping method. One thousand virtual experiments showed that the uncertainty of this method can achieve ±5%.

Photoinduced conductivity during sub-bandgap illumination in periodically poled MgO:LiNbO3 with charged domain walls

We report observations of a spectral dependence of the conductivity in 5 mol.% MgO doped LiNbO3 single crystals with periodical domain structure (PDS) of stripe domains with charged domain walls during sub-bandgap illumination by probing laser beams. The photoinduced conductivity is manifested through a comparatively slow relaxation of the first-order Bragg electrooptical diffraction efficiency after application of an external electric field to the crystal. The relaxation rate connected with screening of the external field due to redistribution of the space charge over the conductive charged domain walls linearly increased in the initial stage with the probe beam intensity. The parameters of the effective photoinduced conductivity averaged over the PDS period that describe its dependence on the intensity decreased from 10.9⋅10−14 to 1.79⋅10−16 Ω−1W−1m as the wavelength of the probe beam increased from 457 to 980 nm.

Electric-field-profile measurement along a probing laser path based on electric-field-induced second-harmonic generation

Electric field measurement based on electric-field-induced second-harmonic generation (E-FISHG) of gas molecules is a promising method that enables noninvasive field measurement in gases. Although the probing laser beam is focused on the target area by a lens, the signal detected by the E-FISHG method is affected by the electric field along the probing laser path. The influencing path spans many tens of times the Rayleigh length of the front and rear of the focused spot. The E-FISHG signal has been frequently calibrated under uniform electric fields. However, this can yield a significantly incorrect calibrated value if the difference between the electric-field profiles in the measurement and calibration is not considered. In this paper, we propose a method to calibrate and furthermore restore the electric-field profile along a probing optical path by measuring a series of E-FISHG signals by changing the focusing spot position. The electric-field profile along the probing laser path is successfully restored from the measured E-FISHG signal sequence, which is verified by an experiment.

An estimate of size of copper nanoparticles levitating over the melt surface using the measurements of spectral reflectance

A strong decrease in normal reflectance of a probe laser beam of 660 nm wavelength reflected from the surface of copper sample just after the beginning of the sample melting in a rarefied argon atmosphere has been observed recently by the authors. A similar time dependence of the reflectance is obtained in the laboratory experiments of the present paper at the wavelengths of 532 nm. The additional spectral measurements enable the authors to estimate the size of condensed nanoparticles levitating over the copper melt.

Compact Probe for Non-Contact Ultrasonic Inspection with the Gas-Coupled Laser Acoustic Detection (GCLAD) Technique

BackgroundGas-Coupled Laser Acoustic Detection (GCLAD) is a non-contact ultrasonic detection technique whose functioning relies on the deviation that a probe laser beam sustains when intersected by an acoustic wavefront propagating in a fluid. The maximum sensitivity of the technique is typically obtained when the ultrasound insists on an ample portion of the probe laser beam extension, but such a condition can be unfeasible in several non-destructive testing applications (as in case of limited accessibility to the component).ObjectiveIn the present work, a solution is provided enabling transformation of the GCLAD device in a point detector. This is based on the use of two mirrors for confining the laser beam in an area with limited width and depth, where reflections however maximize the portion of the probe laser beam subjected to ultrasonic oscillation.MethodsThe characteristics of the obtained GCLAD probe are thoroughly analysed by applying the device to the detection of surface acoustic waves, propagating on a metal bar and refracting into the air. Two different inspection configurations are considered, whose difference lies in the mutual orientation between laser beam and solid surface. The effect on the received signal amplitude of the number of beam reflections, the dimensions of the resulting device, and the bar axisimmetry is investigated in both configurations. ResultsThe optimization of all the analysed standpoints enables obtaining a compact GCLAD probe that features the same signal amplitude of the non-compact alternative. To obtain maximum responsivity of the system, the number of reflections must be maximized, while the distance between the mirrors must be carefully set based on the employed inspection configuration and the eventual axisimmetry of the specimen. The devised GCLAD compact probe is capable of expanding the application range of the technique also to those cases in which the use of point detectors is desirable, without compromising the signal-to-noise ratio of the resulting acquisitions compared to the non-compact alternative.

IR photothermal and spectroscopic analysis of proton-irradiated 4H-SiC

The effect of the proton irradiation dose on thermal transport anisotropy, free carrier density, and defect formation in 4H-SiC is studied by thermal wave scattering and infrared spectroscopy. Thermal waves are generated by infrared laser pulses, and the in-plane and cross-plane thermal diffusion length are measured using the deflection of a laser probe beam due to the mirage effect. The effect of proton irradiation dose on in-plane and cross-plane thermal diffusivity is measured as a function of depth. Proton irradiation is shown to cause significant damage primarily in the direction perpendicular to the sample surface. Irradiation-induced free carriers contributing to heat transport in the sample plane are revealed by Kramers-Kronig analysis of the infrared reflectivity spectra of the studied samples. Measurements of thermal diffusion lengths in the irradiated samples are converted to depth profiles of defect density. It is shown that the highly damaged zone in irradiated 4H-SiC thickens and approaches the proton reaching depth with increasing the irradiation dose. Thermal wave scattering complemented by infrared spectroscopy is proposed as an effective approach for the directional analysis of irradiation-induced changes in physical and structural properties of materials.

Third harmonic generation studies of 1.06 μm Nd:YAG laser beam in Li2SO4–MgO–P2O5 glass system-influence of CuO

Glasses of the composition Li2SO4–MgO–P2O5:CuO were synthesized. The spectroscopic investigations on these samples revealed that there is a gradual reduction of Cu2+ ions in to Cu + ions (that occupy tetrahedral sites) in the glass matrix as the content of CuO is increased. Infrared spectroscopy studies have clearly demonstrated an increased degree of connectivity between various structural groups of the glass network with increase of CuO concentration. Third harmonic generation (THG) beam intensity of Nd:YAG laser (λ = 1064 nm) probing beam (of the power 0–100 J/m2) induced due to the interaction of inherent dipoles with the incident laser beam, is observed to decrease as the concentration of CuO is increased. The analysis of the results of these studies indicated that Li2SO4–MgO–P2O5 glass samples doped with small content of CuO ~ 0.2 mol% are more favorable for achieving high power THG signal with low absorption due to phonons. Hence, such glasses may be useful for optoelectronic devices. However, the spectroscopic studies indicated that in the samples containing higher content of CuO there is higher fraction of Cu+ ions that occupy network forming (Td) positions; such increased degree of polymerization in the glass matrix appears to be a hindrance for accomplishing higher intensity of THG signal due to large phonon losses. These results were found to be in concurrence with the inferences from other studies that include electrical properties, positron annihilation lifetime (PAL) spectroscopy and mechanical properties.

Calibration of spin-light coupling by coherently induced Faraday rotation.

Calibrating the strength of the light-matter interaction is an important experimental task in quantum information and quantum state engineering protocols. The strength of the off-resonant light-matter interaction in multi-atom spin oscillators can be characterized by the readout rate ΓS. Here we introduce the method named Coherently Induced FAraday Rotation (CIFAR) for determining the readout rate. The method is suited for both continuous and pulsed readout of the spin oscillator, relying only on applying a known polarization modulation to the probe laser beam and detecting a known optical polarization component. Importantly, the method does not require changes to the optical and magnetic fields performing the state preparation and probing. The CIFAR signal is also independent of the probe beam photo-detection quantum efficiency, and allows direct extraction of other parameters of the interaction, such as the tensor coupling ζS, and the damping rate γS. We verify this method in the continuous wave regime, probing a strongly coupled spin oscillator prepared in a warm cesium atomic vapour.

Application of the Gas-Coupled Laser Acoustic Detection technique to non-destructive monitoring of mechanical components

Non-contact ultrasonic techniques are fundamental to devise online monitoring systems for moving or difficult to access structures. Gas-Coupled Laser Acoustic Detection (GCLAD) is an unestablished, non-contact detection technology which relies on measuring the deviation affecting a laser beam when travelling across an ultrasonic wavefront propagating in a fluid. The aim of the work is to provide in-depth highlights on the principles on which the technique leverages, with a view towards how several laser beam and ultrasonic wave features reflects on the signal acquired by the GCLAD device. By numerical and experimental approaches, parameters needing to be specifically addressed and suitably set during the investigation phase are highlighted, which enable amplitude maximization of the acquired signal. Specifically, effect of the probe laser beam spot size is thoroughly analyzed, as well as the mutual orientation between the beam and the ultrasonic propagation directions. Three test configurations are lastly proposed, providing different results in terms of GCLAD sensitivity to the acoustic waves; such differences are highlighted by applying the technique to a railway axle on which an artificial crack has been machined, providing a first assessment of the GCLAD capabilities in the non-destructive testing field.

Optical coherence microangiography of the mouse kidney for diagnosis of circulatory disorders

Optical coherence tomography (OCT) has become widespread in clinical applications in which precise three-dimensional functional imaging of living organs is required. Nevertheless, the kidney is inaccessible for the high resolution OCT imaging due to a high light attenuation coefficient of skin and soft tissues that significantly limits the penetration depth of the probing laser beam. Here, we introduce a surgical protocol and fixation scheme that enables functional visualization of kidney's peritubular capillaries via OCT microangiography. The model of reversible/irreversible glomerulus embolization using drug microcarriers confirms the ability of OCT to detect circulatory disorders. This approach can be used for choosing optimal carriers, their dosages and diagnosis of other blood flow pathologies.

Pure modulation and accurate measurement of optical beam's tilt and displacement.

We developed a tilt modulation technique of a laser beam with a wedged crystal. Combined with a phase-compensating crystal, a pure tilt modulation with a wide bandwidth (actually determined by the bandwidth of electro-optic crystals) is realized. By Fourier transformation with a lens, the tilt signal is transformed into displacement. With homodyne detection using a local oscillator of the first-order Hermite-Gauss mode (HG10) and a 4F phase-monitoring system, we measure the displacement and tilt of a laser probe beam. This technique can be used in metrology, such as Newtonian gravitational constant determination and gravitational wave detection, or the calibration of a spatial sensor, such as tilt/displacement sensors.

Comparison of the expansion behavior of atoms and droplets ablated from a β-tricalcium phosphate target under low-pressure ambient H2O gas for hydroxyapatite coating

Visualizing the expansion of particles of different size in a laser-ablated plume is useful for understanding the mechanism of the pulsed-laser deposition process. In our previous work that the crystalline hydroxyapatite content in a coating layer decreased with increasing H2O gas pressure. We concluded that ablated atoms and ions, which were immediately changed into crystalline hydroxyapatite, were stopped before reaching the substrate in the H2O pressure range. In the present study, the expansion behavior of ablated droplets and atoms in H2O gas was measured independently. Droplets were also measured via Mie scattering images using a second probe laser beam and were found to not be affected by the ambient gas pressure. However, the excited Ca atoms and ions in this H2O pressure range were effectively prevented from reaching the substrate. These confirm that the decrease in the crystalline hydroxyapatite content was caused by the increase in the ambient gas pressure.

Insight into replication effectiveness of laser-textured micro and nanoscale morphology by injection molding

Abstract Detailed analysis and characterization of injection-molded polypropylene (PP) specimens produced with laser-textured molds has been performed. Ultrashort pulsed laser surface structuring was exploited to produce sub-micrometric surface features on injection molds over areas of 2400 mm2, including two-dimensional hole and cone arrays, and ridges parallel and perpendicular to the injection direction. Replication effectiveness was evaluated in terms of surface roughness parameters, Filling Volume Fraction (FVF) and Power Spectral Density (PSD), after which detailed comparison of the topography in the same region on plastic samples and molds was performed. A scatterometry setup was employed based on optical diffraction of a probe laser beam to assess replication fidelity over the entire surface of each PP sample with a measurement technique suitable for optimization and quality control in an industrial production environment. Of the tested surface morphologies, greatest replication effectiveness was achieved with LIPSS oriented parallel to the injection direction, where the arithmetic mean height of plastic samples (Sa = 37 ± 4 nm) was 97 % of the corresponding value of the mold (Sa = 38 ± 6 nm), while the average FVF was 86 ± 9 % and the PSD indicated ripple-like features with a spacing of λ ≃ 0.9 μm. Direct comparison of the topography in selected regions highlighted local variability in the transfer effectiveness of individual surface features. The investigation demonstrated that micro and nanoscale morphology can effectively be transferred via injection molding over large areas relevant to industrial applications, while careful attention must be paid to the size and nature of defects in relation to the specific functional surface under consideration.

Characterization of signals for a Divertor Tokamak Test facility interferometer/polarimeter system.

In magnetically confined fusion experiments, laser interferometer/polarimeter systems allow one to determine plasma density, give valuable information on the internal magnetic fields, and contribute to the evaluation of the plasma magnetic equilibrium and to the real-time estimation of the q profile to allow feedback configuration control. This work presents an analysis of the interferometric and polarimetric signals of a multi-chord far-infrared interferometer/polarimeter for the divertor tokamak test facility, the new tokamak device currently under construction in Italy. The polarimetric signals are calculated both with approximate formulas and by solving the equation describing the evolution of the laser beam polarization inside the plasma using the Mueller formalism. The latter method correctly accounts for crosstalk between Faraday rotation and the Cotton-Mouton effect. The impact of the plasma birefringence on the interferometric phase shift is also studied, and it is found that a perturbation of the interferometric phase shift is present also in the case of an initial fixed linear polarization of the probe laser beam.

Optical control of high-density alkali atom vapor in antirelaxation coated cells

In this work, we report on our investigations on LIAD (Light Induced Atomic Desorption) aimed at achieving high density of alkali atoms vapor in a coated cell at room temperature. The experimental results show the possibility to reach a density up to the limit when the medium becomes optically thick by applying highly efficient homogeneous illumination. The photon reabsorption mechanism prevents the precise evaluation of the density by measuring the absorption of a probe laser beam, but there is clear evidence that densities can be achieved higher by two orders of magnitude than the thermodynamic equilibrium value.

Cross-Beam Energy Transfer Saturation by Ion Heating.

We measure cross-beam energy transfer (CBET) saturation by ion heating in a gas-jet plasma characterized using Thomson scattering. A wavelength-tunable ultraviolet (UV) probe laser beam interacts with four intense UV pump beams to drive large-amplitude ion-acoustic waves. For the highest-intensity interactions, the power transfer to the probe laser drops, demonstrating ion-acoustic wave saturation. Over this time, the ion temperature is measured to increase by a factor of 7 during the 500-ps interaction. Particle-in-cell simulations show ion trapping and a subsequent ion heating consistent with measurements. Linear kinetic CBET models are found to agree well with the observed energy transfer when the measured plasma conditions are used.

First measurements of line-integrated electron density in an ITER-like configuration using the JET far infrared polarimeter diagnostic

Polarimetry exploits the optical activity and birefringent properties of thermonuclear plasmas to calculate some important quantities for their control like the line-integrated electron density and magnetic field distribution. The Joint European Torus (JET) far infrared polarimeter shares the same probing laser beams of the interferometer, with eight channels, four vertical and four lateral. While the vertical channels were already optimised to provide accurate measurements of the Faraday rotation angle, Cotton–Mouton (CM) phase shift and ellipticity, the lateral channels had been only optimised for Faraday rotation angle alone. By setting the initial polarisation angle of the lateral channel at zero degrees the CM effect is minimised, the ellipticity is almost zero, and the CM phase shift angle is impossible to measure. During the recent JET experimental campaign (C38 in 2019–2020), the input polarisation for the lateral channels was altered for a class of pulses to analyse the possibility to measure the CM phase shift angle and the ellipticity, and, more important, to assess if it is possible to provide information of line-integrated electron density using the lateral channels of JET polarimetry in an ITER-like configuration. As a note, ITER will have only tangential channels with reflectors buried deep inside the first wall, so this setup was truly an ITER-like configuration. The results clearly show huge improvements, which can be achieved by just changing the input polarisation. Moreover, the analysis of the measurements shows that the polarimetric measurements have a systematic error, which is probably due to the effect of refraction and to the in-vessel mirrors, which was only partially taken into account during the calibration phase before the plasma. Thus, a new calibration method was developed and the results presented on a statistical basis. It has been demonstrated that, varying the input polarisation of the polarimeter and using the new calibration method, it is possible to measure the line-integrated electron density, using the CM phase shift (or the ellipticity) of the lateral channels, with good accuracy with respect to the electron density measured by the interferometer that was considered the reference.

Multiphoton nonlinear frequency mixing effects on the coherent electromagnetically induced absorption spectra of Rb85 atoms under a longitudinal magnetic field: Theory and experiment

Multiphoton nonlinear frequency mixing effects on coherent electromagnetically induced absorption spectra of $^{85}\mathrm{Rb}$ atoms using two orthogonal linear polarizations of strong-coupling and weak probe beams are investigated theoretically and experimentally with respect to an applied longitudinal magnetic field and coupling powers. Herein, we confirm that at least five-photon interactions in solving density matrix equations for the ${F}_{g}=3\ensuremath{\rightarrow}{F}_{e}=4$ transition of $^{85}\mathrm{Rb}$ atoms are required to explain experimentally observed coherent electromagnetically induced absorption spectra when a quantum axis is selected as the propagation direction of co-propagating coupling and probe laser beams. Distinct calculated spectral differences owing to variations in the magnetic field and coupling power between three- and five-photon interactions are confirmed. The obtained asymmetrical spectral shapes match very well with those calculated from five-photon interactions considering the off-resonant ${F}_{g}=3\ensuremath{\rightarrow}{F}_{e}=2,3$ transitions. Genuine coherent spectral shapes are observed with a single laser combined with two acousto-optic modulators, wherein the spectral resolution is limited because of the decoherence rate between Zeeman sublevels in the ground state from transit-time relaxation.

Ethane detection with mid-infrared hollow-core fiber photothermal spectroscopy.

We report a compact mid-infrared (MIR) photothermal spectroscopic ethane (C2H6) sensor with a hollow-core negative-curvature-fiber (HC-NCF) gas cell. The HC-NCF supports low-loss transmission of an MIR pump (3.348 µm) and a near-infrared (NIR) probe (1.55 µm). The pump and probe laser beams are launched into the gas cell from the opposite ends of the HC-NCF, allowing independent MIR pump delivery and NIR fiber-optic probe circuitry. The use of Fabry-Perot as the probe interferometer simplifies the sensor design and suppresses the common-mode noise in the lead in/out single-mode fiber. With a 14-cm-long HC-NCF, an ethane sensor system with the limit of detection (LOD) of 13 parts-per-billion (ppb) is achieved with 1 s lock-in time constant. The LOD goes down to 2.6 ppb with 410 s average time, which corresponds to noise equivalent absorption (NEA) of 2.0×10-6 and is a record for the hollow-core fiber MIR gas sensors. The system instability is 2.2% over a period of 8 hours.