Probe Radiation Research Articles

Ultrafast spallation dynamics of a thin gold film characterized by imaging pump-probe interferometry

Interferometry allows to measure microscopic displacements in the nanometer scale. This work introduces an imaging pump-probe interferometer that enables to detect the spatially resolved phase shift of the probe radiation being reflected on a sample surface after ultrashort pulsed laser excitation with a temporal resolution of 40 fs and a maximum temporal delay of 3 ns. The capability of the pump-probe interferometer is demonstrated on the spallation of a thin gold film upon femtosecond laser irradiation. The pump-probe interferometer enables to measure a minimum phase change of Δφ<π/10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta \\varphi <\\pi /10$$\\end{document} which corresponds to a displacement of Δh<\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta h<$$\\end{document} 12.5 nm for the applied probe wavelength of 500 nm. Upon irradiation, two distinct phase changes are observable: First, an abrupt minor phase shift has been measured in the femtosecond range, which is attributed to the changed optical properties of the sample surface after excitation. Second, a more pronounced continuing phase shift increase is detectable after a few tens of picoseconds resulting from the onset of spallation. Based on the measured spatially resolved phase shift, the transient surface topography during the spallation is reconstructed. The determined velocity of the ablated material reaches a maximum of a few 1000 m/s at ten picosecond after irradiation and decreases to 250 m/s afterwards. Consequently, the introduced imaging pump-probe interferometer provides important insights into the physical processes during laser excitation as well as the subsequent laser-induced ablation, and will enable to validate theoretical models quantitatively in following studies.

Transitions between triplet Rydberg states of the alkaline-earth-metal atoms for precision measurements of microwave fields

We calculated the frequencies and amplitudes of the microwave (µw) dipole transitions between triplet Rydberg states of n3S1, n3P1, n3D2, and n3F3 series in the alkaline-earth atoms. Numerical values of the amplitudes determine the µw electric-field-dependent Rabi frequency of the Rydberg level shift in the field of a resonance radiation. This shift, observed as splitting of resonance in electromagnetically induced transparency for intensely absorbed probe radiation, enables measuring the µw electric field strength. For numerical calculations, we used single-electron methods and the most reliable data on quantum defect values from the literature. We derived the asymptotic approximation formulas of the µw frequencies and amplitudes.

Electromagnetically induced transparency with magnetically induced ΔF=0,mF=0→mF=0 probe transition

Interest in magnetically induced (MI) transitions of alkali metal atoms is caused by the fact that their intensities can exceed the intensities of ‘regular’ atomic transitions in a wide magnetic field range (200–4000 G). The goal of this work was to form and study, for the first time, electromagnetically induced transparency (EIT) resonance in a strong magnetic field using probe radiation tuned to |Fg=F,mF=0⟩→|Fe=F,mF=0⟩ MI transition, which is forbidden in zero magnetic fields. Two narrow-band linearly polarized cw diode lasers were used to form EIT resonance on a Λ-type system of a Cs atomic D2 line in a strong transverse magnetic field (up to 1000 G). The resonance was formed in a Cs atomic vapor nanocell with an atomic vapor column thickness of 850 nm.

Influence of inhomogeneous temperature and field distribution on sound generation and its effect on reflectivity of a thin film heated by a femtosecond pulse

One of the main methods for obtaining information about the generation of sound pulses in metals is to measure the reflection coefficient of a probe wave. Various theoretical models are used to interpret the results of measuring the contribution to reflection coefficient ΔR(t) due to sound-generated displacements of lattice atoms. The purpose of this paper is to establish the degree of accuracy of models used in the case of sound generation in thin films exposed to a femtosecond pulse. It is shown below that the assumption of uniform heating used for thin films is justified if the film thickness is less than the film heating depth and for thicker films at times greater than the film heating time over the entire thickness. For optically thick films, a relatively simple expression for the field can be used. If the film thickness is less than the skin layer depth of the pump field, then it is necessary to consider the field reflection from a substrate. In this case, depending on the optical properties of the metal and the substrate, taking into account reflection can lead to either an increase or a decrease in ΔR(t). It has been established that if the skin layer at the frequency of probe radiation is less than the film heating depth, then taking into account temperature gradients in the equation for the displacement of lattice atoms leads to small changes in ΔR(t). This makes it possible to significantly simplify calculations of the displacement of lattice atoms.

Mid-Infrared Generated Laser-Induced Grating Signals in Methane-Containing Gas Mixtures as Indicators of Composition, Pressure, and Temperature.

The present work is aimed at studying how spatially periodic modulations of the refractive index of the medium, i.e., laser-induced gratings (LIGs), generated in a gas mixture containing methane (CH4) by nanosecond pulses of resonant mid-infrared laser radiation, can be used to measure various gas parameters. It is investigated to what extent the temporal profiles of the LIG signals, recorded as the power of the diffracted by LIGs continuous wave probe radiation, are specific to the composition, pressure, and temperature of a selected buffer gas. This specificity is illustrated by the LIG signal profiles recorded in the experiments in different gas mixtures under various conditions. Experimental data show that large LIG signals can be obtained even in mixtures with CH4 concentrations as low as ∼100 parts per million due to the strong absorption of the excitation light and subsequent rapid, highly exothermic, and partner-dependent collisional energy exchange of the laser-excited molecules with the environment. These two factors ensure high LIG generation efficiency by a small number of CH4 molecules and high sensitivity of signal strength and profile to variations of gas parameters.

Transient ablation topography of a thin chromium film after ultrashort pulsed laser irradiation in the spallation and phase explosion regime

Time-resolved imaging pump–probe reflectometry is an established technique to characterize the dynamic processes occurring during the material ablation upon ultrashort pulsed laser irradiation. In the phase explosion regime, however, the presence of an expanding liquid–vapor mixture on top of the ablated material makes an unambiguous determination of the transient ablation topography almost impossible, especially when only front-side pump–probe reflectometry is used. To circumvent this limitation, the front-side reflectometry was complemented by rear-side reflectometry, where the probe radiation does not interact with the liquid–vapor mixture, and the underneath ablation kinetics of the material becomes observable. To interpret the measured front- and rear-side reflectances, an optical multilayer model of the ablation process was developed. Its applicability is demonstrated on a thin chromium film that was ablated by a single pump pulse. The optical model replicates the Newton rings measured during spallation and phase explosion very well, which allows the reconstruction of the transient ablation topography. The combination of the front- and rear-side reflectometry with modeling offers a valuable tool for studying the ablation processes in thin metal films, because it overcomes the challenges of the phase explosion regime, and thus enhances the understanding of material dynamics upon laser irradiation.

Selective reflection study of excitation-dependent dipole-dipole interaction in dense rubidium vapour

We conducted a comprehensive study of excitation dependent dipole-dipole interaction in dense rubidium gas using the resonant selective reflection of weak probe radiation from the dielectric-rubidium vapor interface. The excitation was created by the pump laser beam. The study involved exploring a diverse range of pumping radiation intensity and detuning for the number density of the rubidium vapour 2.8 × 1016 cm-3. We have measured experimental selective reflection spectra in both linear and nonlinear pumping modes.

Problem of Measuring Absorption Using Time-Resolved Photothermal Common-Path Interferometry under Conditions of Developed Heat Diffusion

We present a study of the problem of measuring ultra-low absorption in quartz materials using the time-resolved photothermal common-path interferometry (TPCI) method, which we proposed and elaborated, with allowance for developed heat diffusion in the samples. This task is related to the implementation of one of the ways of increasing the sensitivity of measurements, namely, increasing the energy of the heating pulse by increasing its duration. Analytical formulas for the time dependence of the power of the time-varying component of the probe radiation are obtained in the Gaussian approximation for the laser beams. A correction factor that takes into account the heat diffusion effect is calculated theoretically and used during calibration. The rate at which the power of the time-varying component decreases when the sample is cooled after the end of the heating pulse is calculated. When measuring in crystalline quartz, quartz glass, and also in air, the calculated rate coincided with the experimental one, which is additional evidence for the reliability of the calculations of the correction factor for calibrating the measurements. When the duration of the heating pulse is increased to 5 ms, the calculated sensitivity of the scheme for measuring absorption in quartz glasses is 2 × 10−9 cm−1.

Cleaning up Operando Battery X-Ray Diffraction with Total Scattering Computed Tomography

Operando X-ray diffraction (XRD) and total scattering investigations of battery electrode materials allow us to relate structure and electrochemical properties down to the atomic level. By studying the materials under real working conditions without allowing the materials to relax or react outside the battery, we obtain the most reliable possible structural information. Unfortunately, in most operando experiments, even when we use heavily modified cells to remove barriers to our probe radiation, the signal from the material of interest is significantly obscured by contributions from other battery components. The problems are increased when we seek to study electrode materials which either are, or become, amorphous during battery cycling: removing non-sample contributions is an essential part of pre-processing total scattering data for pair distribution function (PDF) analysis, which can reveal the local structures of non-crystalline materials.To address these problems, we have developed operando total scattering computed tomography (TSCT) as a method for obtaining clean, 5-dimensional (3-D space, time, and chemical structure) information on working batteries. The method allows us to select specific areas of the active electrode and obtain XRD and PDF information without any contributions from other components of the battery. The high-quality of the resulting data has allowed us to understand the cycling mechanisms of poorly scattering, amorphous, alloying anode materials (sodium cycling in phosphorus)[1], and complex, multi component, conversion-alloying systems (lithium cycling in bismuth vanadate)[2]. While the first example mainly uses the focusing power of TSCT to obtain the best possible data, the latter case uses the spatial resolution of the data to identify signs of a longer-term deactivation mechanism based on agglomeration of the nanoclusters of active material during repeated cycling. Further studies are currently in progress to apply the method to other difficult anode systems including silicon.[1] J. Sottmann, M. Di Michiel, H. Fjellvåg, L. Malavasi, S. Margadonna, P. Vajeeston, G.B.M. Vaughan, D.S. Wragg, Chemical Structures of Specific Sodium Ion Battery Components Determined by Operando Pair Distribution Function and X-ray Diffraction Computed Tomography, Angew. Chem. Int. Ed. 56, pp. 11385-11389, 2017[2] J. Sottmann, A. Ruud, Ø.S. Fjellvåg, G.B.M. Vaughan, M. Di Michel, H. Fjellvåg, O.I. Lebedev, P. Vajeeston, D.S. Wragg, 5D total scattering computed tomography reveals the full reaction mechanism of a bismuth vanadate lithium ion battery anode, Phys. Chem. Chem. Phys. 24, pp.27075-27085, 2022 Figure 1

Characterization and De-Embedding of Wafer Probe Radiation for Integrated Antenna Measurements

Characterization and De-Embedding of Wafer Probe Radiation for Integrated Antenna Measurements

Polarization-dependent intensity ratios in double resonance spectroscopy.

Double Resonance is a powerful spectroscopic method that unambiguously assigns the rigorous quantum numbers of one state of a transition. However, there is often ambiguity as to the branch (ΔJ) of that transition. Spectroscopists have resolved this ambiguity by using the dependence of the double resonance intensity on the relative polarization directions of pump and probe radiation. However, published theoretical predictions for this ratio are based upon a weak (i.e., non-saturating) field approximation. This paper presents theoretical predictions for these intensity ratios for cases where the pump field is strongly saturating in the two limits of transitions dominated by homogeneous or of inhomogeneous broadening. Saturation reduces but does not eliminate the magnitude of the polarization effect (driving the intensity ratio closer to unity) even with strong pump saturation. For the case of an inhomogeneously broadened line, such as when Doppler broadened linewidth dominates over the power-broadened homogeneous line width, a large fraction of the low pump power polarization anisotropy remains. This paper reports predicted polarization ratios for both linear and circular pump and probe field polarizations. The present predictions are compared with experimental measurements on CH4 ground state → ν3 → 3ν3 transitions recently reported by de Oliveira et al.63 and these are in better agreement than with the weak field predictions.

Two-color four-wave mixing contribution to an electrostrictive laser-induced grating signal in CO2-N2 mixtures and gas diagnostics.

Multiparameter determination in the gas phase using the versatile laser-induced grating (LIG) technique is a challenging task due to interdependence of observables on multiple thermodynamic parameters. In C O 2-N 2 mixtures, simultaneous determination of species concentration and gas temperature can be achieved by using an additional C O 2 concentration-dependent contribution to the LIG signal, which appears if 1064 nm pump pulses are employed. This contribution can be attributed to a direct, quasi-resonant two-color four-wave mixing (TCFWM) of the pump and probe radiations in C O 2. A detailed study of the laser power and beam polarization, as well as mixture composition, pressure, and temperature dependencies of the TCFWM intensity relative to that of the LIG signal, allowed for the formulation of analytical relations enabling simultaneous mixture composition and temperature determination.

Precision Spectroscopy of Radiation Transitions between Singlet Rydberg States of the Group IIb and Yb Atoms

The measurements of microwave (μw) and radio-frequency (RF) radiation quantitative parameters may be based on the quantum–optical approach to determine the spectral characteristics of radiation transitions between the Rydberg states of atoms. Frequencies and matrix elements are calculated for dipole transitions between opposite-parity Rydberg states nL 1L and n′L±1 1L±1 (where n′= n,n±1,n±2) of the singlet series in the alkaline–earth–metal-like atoms of group IIb (Zn, Cd, Hg) and Yb. The matrix elements determine the shifts of Rydberg-state energy levels in the field of resonance μw or RF radiation, splitting the resonance of electromagnetically induced transparency (EIT) for intensely absorbed probe radiation. Numerical computations based on the single-electron quantum defect method (QDM) and the Fues’ model potential (FMP) approach with the use of the most reliable data from the current literature on quantum defect values are performed for frequencies and matrix elements of transitions between singlet Rydberg states of 1S0-, 1P1-, 1D2-, and 1F3-series in Zn, Cd, Hg, and Yb atoms. The calculated data are approximated by polynomials in the powers of the principal quantum numbers. The polynomial coefficients are determined with the use of a standard curve-fitting interpolation polynomial procedure for numerically calculated functions. These approximation expressions provide new possibilities for accurately evaluating the frequencies and matrix elements of dipole transitions between Rydberg states over a wide range of quantum numbers n &gt;&gt; 1, accompanied by the emission and absorption of μw and RF photons.

Narrow intensity range optical anisotropy in air induced by a femtosecond laser breakdown.

In this paper, we study the optical anisotropy induced by femtosecond laser radiation in air during an optical breakdown. Using a transverse pump-probe technique, we demonstrate that this anisotropy appears in a narrow range of pump intensities, which are close to the optical breakdown threshold in air and lead to a phase shift of probe radiation, polarized collinear to the pump. The intensity range where an induced intense anisotropy occurs makes it possible to estimate the magnitude of the 5th-order Kerr nonlinear refractive index component in air.

Thermo-Optical Measurements and Simulation in a Fibre-Optic Circuit Using an Extrinsic Fabry–Pérot Interferometer under Pulsed Laser Heating

Advantages of using an external Fabry–Pérot interferometer (EFPI) as a high-speed local temperature deformation sensor are demonstrated for the fibre-optic circuit combining a powerful laser beam for surface heating with a low-power probing radiation. The difference in the formation of the heating and probing radiation provides a simple basis for varying the gap between the fibre end and the surface in order to change the ratio between the heating and EFPI measuring areas. Using an example of modelling the laser heating by radiation from a standard single-mode fibre, we demonstrate the possibility of employing the EFPI to measure the temperature deformation of the surface on a quasi-isothermal area with the temperature close to the maximum at gap values of more than 100 μm. With the condition of preliminary calibration, the proposed scheme can be used to evaluate the heat treatment of the surface with the speed of the applied photodetector. The practical possibilities of the method are demonstrated on examples of heating some metal and semiconductor samples by laser pulses of microsecond duration.

Generation and detection of sound at the effect of femtosecond pulses on a metal film on a dielectric substrate

The excitation of sound by a femtosecond laser pulse in a metal layer on a dielectric substrate has been studied. The modulations of the reflection coefficient of the metal, which arise due to the propagation of sound in it, are described in detail. It is shown that, in addition to oscillations corresponding to odd harmonics of sound waves, oscillations corresponding to even harmonics can be present on the profile of the Fourier image of the reflection coefficient change. The efficiency of even harmonic generation depends on the substrate material and the metal film thickness. The dependence of the reflection coefficient change on time has been studied. It has been established that if the electron heat flux reaches the metal–dielectric interface before the energy of the electrons is transferred to the lattice, then modulations are present both due to sound that occurs at the metal–vacuum interface and at the metal–dielectric interface. The wavelength of probe radiation also affects the reflection coefficient change. If real and imaginary parts of the permittivity at the wavelength of probing radiation are comparable in magnitude, then the Fourier image of the reflection coefficient change contains a smaller number of maxima, and the change in the reflection coefficient over time is accompanied by splitting of peaks.

Damage-free X-ray spectroscopy characterization of oxide thin films

Thin polymer or oxide films are ubiquitous components in many devices, including membranes for filtration or electrodes in electrochemical energy storage. High energy electron or x-ray probes for microscopy and spectroscopy are useful to characterize and understand these materials. However, irreversible damage by the probe radiation remains a challenge. Here, we show that graphene serves as an x-ray and electron transparent substrate that substantially reduces radiation damage of an oxide thin film. We demonstrate this using highly focused x-ray beams, which show that compared to oxide thin films supported on a substrate, graphene-supported regions show minimal changes in the x-ray spectra as a function of x-ray dose. These results pave the way for the development of experimental setups that allow for long exposure time measurements with limited sample damage and substrate-directed radiation patterning.

Application of magnetically induced [formula omitted] atomic transitions of cesium atoms in strong magnetic fields

In this work, magnetically induced (MI) transitions of the Cs atoms in strong magnetic fields up to 6 kG, and their applications in spectroscopy are studied experimentally and theoretically. An attractive feature of these transitions is their large frequency shift slope in a magnetic field (−4 MHz/G). Therefore, in strong magnetic fields the frequency shift of MI transitions reaches several tens of GHz and importantly, these transitions do not overlap with others, which is of practical interest for possible use in new frequency ranges. To implement the process of electromagnetically induced transparency (EIT) in a strong magnetic field (1–2 kG), the MI Fg=4→Fe=2 transitions were first used at the frequency of probe radiation, while the frequency of coupling radiation is resonant the Fg=3→Fe=2 transitions. To form the EIT resonance, a cell of sub-micron thickness filled with Cs atom vapors was used, along with strong permanent magnets.

Formation of strongly shifted EIT resonances using “forbidden” transitions of Cesium

Atomic transitions satisfying Fe−Fg=ΔF=±2 (where Fe stands for excited and Fg stands for ground state) of alkali atoms have zero probability in zero magnetic field (they are so-called ”forbidden” transitions) but experience a large probabilty increase in an external magnetic field. These transitions are called magnetically induced (MI) transitions. In this paper, we use for the first time the σ+ (ΔmF=+1) MI transitions Fg=3→Fe=5 of Cesium as probe radiation to form EIT resonances in strong magnetic fields (1 - 3 kG) while the coupling radiation frequency is resonant with Fg=4→Fe=5σ+ transitions. The experiment is performed using a nanometric-thin cell filled with Cs vapor and a strong permanent magnet. The thickness of the vapor column is 852 nm, corresponding to the Cs D2 line transition wavelength. Due to the large frequency shift slope of the MI transitions (∼ 4 MHz/G), it is possible to form contrasted and strongly frequency-shifted EIT resonances. Particularly, a strong 12 GHz frequency shift is observed when applying an external magnetic field of ∼ 3 kG. Preliminary calculations performed considering Doppler-broadened three level systems in a nanocell are in reasonable agreement with the experimental measurements.

Электромагнитно-индуцированная прозрачность в ячейках конечных размеров с антирелаксационным покрытием стенок

In this paper, we propose an approach that makes it possible to obtain an analytical expression of the spectral dependence of the resonance of electromagnetically induced transparency detected by bichromatic radiation in a cell of finite size in the longitudinal direction of the laser beam. Specular and diffuse reflections of atoms from walls are considered. It was found that a significant difference between these types of reflections takes place only for cells of small longitudinal dimensions compared to the splitting wavelength of the ground state of atoms. A physical explanation is proposed for the difference between the Stokes and anti-Stokes scattering channels of probe radiation in terms of dressed states.