Probe Laser Pulse Research Articles

Measurement of temporal evolution of antiferromagnetic domain change in nickel oxide driven by 2TO phonon mode excitation via pump-probe experiment

The temporal evolution of antiferromagnetic domain pattern changes under resonant excitation of the second-order transverse optical phonon mode in nickel oxide was investigated using mid-infrared free electron laser (MIR-FEL) pulses and visible nanosecond probe laser pulses at room temperature. We visualized the domain patterns through magneto-optical polarized microscopy in transmission and observed their temporal variation after the phonon excitation via MIR-FEL. We evaluated the differences throughout the timeline using the structural similarity index measure technique from domain images with and without MIR-FEL irradiation. We found that the MIR-FEL irradiation induces significant structural changes in the domain patterns. The maximum difference was observed at the timing of the MIR-FEL irradiation and exponentially recovered with the time constant of 1.4 ± 0.2 ms. This rather long-time constant could be owing to the spin relaxation, whilst further investigation is needed to confirm the underlying mechanism.

Laser induced breakdown spectroscopy for composition monitoring of graded Al[sbnd]Cu alloy surface

Laser-induced breakdown spectroscopy (LIBS) is a powerful method for non-invasive elemental composition monitoring which can be applied for processing graded metal alloys, provided that challenges with reliable and rapid spectral data processing are overcome to enable composition analysis in real-time. In this study, we describe an integrated experimental-computational methodology for analyzing LIBS spectra data obtained from graded composition metal alloy surfaces with a mean rate of composition analysis up to 15.7 Hz. The accuracy and speed of this method was assessed via analysis of a graded AlCu film prepared by magnetron sputtering representing a gradient alloy processed by additive manufacturing. Using a pulsed femtosecond laser and gated sCMOS detector, a series of LIBS measurements were taken across the composition gradient of an AlCu sputtered alloy. Energy Dispersive Spectroscopy (EDS) was used both to calibrate composition measurements and quantify the error of the LIBS-based rapid analysis method demonstrated in this paper, which yielded a mean percent error relative to the EDS data in the composition measurement along the gradient as low as 1.4 % when optimized for the cumulative number of probe laser pulses on a single measurement spot. The femtosecond probe pulse method described in this paper allows for the alloy composition to be accurately measured in hundredths of a second, potentially allowing for real-time composition monitoring in gradient alloy processing, including thin film sputtering, additive manufacturing, and other rapid processing technologies for graded metal alloys.

Time-resolved imaging of GHz acoustic vibrations at arbitrary frequencies using asynchronous periodic probe laser pulses

We present a technique for imaging gigahertz surface acoustic waves in the time domain at arbitrary frequencies, in which acoustic waves are excited asynchronously to the optical probe pulse train used to detect them. We apply this approach to the two-dimensional imaging of electrically excited single-frequency surface acoustic waves on a GaAs substrate with a deposited interdigital transducer. This technique significantly increases the possible fields of application for gigahertz acoustic wave device imaging.

Phase sensitive slow light in semiconductor quantum dots

This article presents a theoretical investigation of phase-controlled propagation of slow light under an electromagnetically induced transparency window in a multilayered one-dimensional photonic crystal with semiconductor quantum dot nanostructures as defects. To achieve the electromagnetically induced transparency window, a setup utilizes a faint probe laser pulse alongside two powerful control laser beams arranged in a closed-loop cascade configuration. In this arrangement, a significant Kerr nonlinearity, approximately on the scale of ∼10−13m2/V2, is discerned within the quantum dot nanostructures at a probe wavelength of 1.55 μm. It is observed that both the amplitude and the phase relationship of the control Rabi frequencies play crucial roles in governing the dynamics of the group velocity of the probe field in the quantum dot nanostructures. Under the regime of electromagnetically induced transparency, the group velocity of the probe field is significantly reduced, approximately by a factor of 103 times as compared to the light speed in free space. These findings hold promise for potential applications in nonlinear optics.

New Instrument for Time-Resolved OH and HO2 Quantification in High-Pressure Laboratory Kinetics Studies.

We have constructed a new time-resolved high-pressure fluorescence assay by gas expansion (HP-FAGE) apparatus, optimized for the detection of OH and HO2 radicals in complex gas-phase reactions. The new instrument fills a gap in the existing experimental toolkit for chemical kinetics by enabling the quantification of two key reactive species with microsecond time resolution from high-pressure sources, which was previously not attainable. The HP-FAGE is interfaced with a flow reactor, designed for pressures up to 100 bar and temperatures up to 1000 K, in which reactions are initiated by laser photolysis of radical precursors at repetition rates of 1-10 Hz. The HP-FAGE samples gas out of the reactor into a miniature FAGE chamber, where OH is detected by resonant laser-induced fluorescence using a time-delayed probe laser pulse. HO2 is converted to OH via reaction with NO and then detected by OH fluorescence. The novel FAGE design places the probe region very close to the gas expansion, minimizing the transport time of sampled molecules and resulting in time resolution better than 20 μs for both OH and HO2. We calibrate the sensitivity of HP-FAGE, validate its performance with measurements of well-known reaction kinetics (OH + CH4, OH + OH, OH + HO2, and HO2 + HO2), and discuss prospects for its future use.

Toward Direct Exploration of the Few-Femtosecond Dynamics of Electronic Coherence and Correlation in Quantum Materials Using Time- and Angle-Resolved Photoemission Spectroscopy

Over the last two decades, time- and angle-resolved photoemission spectroscopy (trARPES) has become a mature and established experimental technique for the study of ultrafast electronic and structural dynamics in materials. To date, most trARPES investigations have focused on the investigation of processes occurring on time scales of ≳30 fs, in particular, relaxation and thermalization, and have therefore been blind to the initial sub-10 fs dynamics related to electronic coherence and correlation effects. In this article, we illustrate how current trARPES setups reach their limits when it comes to addressing such extraordinarily short time scales and present an experimental configuration that provides the time, energy, and momentum resolutions required to monitor few-femtosecond dynamics on the relevant energy and momentum scales. We discuss the potential capabilities of such an experiment to study the electronic response of materials in the strong-field interaction regime at PHz frequencies and finally review a theoretical concept that may in the future even overcome the competing resolution limitations of trARPES experiments, as imposed by the time–bandwidth product of the probing laser pulse. Our roadmap for ultrafast trARPES indicates a path to break new experimental ground in quantum nonequilibrium electronic dynamics, from which new possibilities for ultrafast control of optical and electronic signals in quantum materials can be explored.

Raman Spectroscopy of Conical Intersections Using Entangled Photons.

Ultrafast Raman spectroscopy with attosecond pulses in the extreme ultraviolet and X-ray regime has been proposed theoretically for tracking the non-adiabatic dynamics of molecules in great detail. The large bandwidth of these pulses, which span several electronvolts within a couple of femtoseconds, provides a unique tool for tracking non-adiabatic phenomena. However, spectroscopy with classical light is limited by the time-bandwidth product of the probe laser pulse. In this work, we theoretically investigate an ultrafast Raman spectroscopy scheme that utilizes pairs of entangled photons. Our model simulations demonstrate that the dynamics in the vicinity of a conical intersection can be resolved with unprecedented resolution in the time and frequency domain.

Attosecond delay locking of large arm pump-probe system

With the development of ultrafast science and attosecond laser technology, the pump-probe system based on isolated attosecond laser pulses is a key to attosecond science, which will be used to study electronic dynamics on an attosecond time-scale. To obtain stable and reliable signals, it is necessary to ensure ultra-stable and ultra-accurate synchronization. Any timing jitter can cause signal to disperse or get buryied in noise, making it impossible to obtain the true physical mechanism. Based on the above, the delay between pump laser pulse and probe laser pulse must be controlled with an attosecond time resolution. In this work, a dual-layer system is developed to achieve high-precision synchronization locking. To ensure that both layers have the same time jitter, we design an adapter to secure the elements placed during installation. Timing jitter is obtained by shaking interference fringes through fast Fourier transformation, and can be calculated in several ms. Then error signals are fed back to the PZT stage in order to compensate for real-time optical path drift. Through such a design, a time-delay accuracy of 7.64 as to 15.53 as is realized, which is linearly related to the interferometer arm length ranging from 1 m to 5 m, with an <i>R</i><sup>2</sup> of 0.96. Moreover, the error between the experimental result of arm length of 8 m and 10 m and the result fitted with the above data is less than 3 as. These results show that using a small interferometer can achieve the fast detection of the time-delay accuracy of long-arm attosecond pump-probe detection system in large scientific instrument, which is of great significance in guiding ther applications such as in non-collinear attosecond streaking spectroscopy, time-resolved photoelectron spectroscopy, and coherent synthesis.

ẢNH HƯỞNG CỦA ĐỘ KẾT HỢP ĐƯỢC TẠO BỞI PHÁT XẠ TỰ PHÁT VÀ BƠM KHÔNG KẾT HỢP LÊN CHUYỂN MẠCH TOÀN QUANG TRONG MÔI TRƯỜNG NGUYÊN TỬ BA MỨC BẬC THANG KHI CÓ MẶT CỦA PHA TƯƠNG ĐỐI

This paper investigates the impact of spontaneously generated coherence (SGC) and relative phase on the propagation of a probe laser pulse in the presence of an incoherent pump field. It was discovered that when the control laser field is turned on, the medium forms an electromagnetically induced transparency (EIT) effect. Hence, the propagation of the probe laser pulse has a like-soliton form/shape. On the contrary, the probe laser field is rapidly attenuated when the control laser field is turned off. This feature allows the creation of a mechanism to turn “off” and “on” the probe laser field through “turning off” and “on” the control laser field, i.e., an all-optical switching mechanism based on EIT. Besides intensity and frequency, the laser field is also characterized by polarization and phase. The EIT has the nature of quantum interference, so it is very sensitive to changes in polarization, especially in the presence of relative phase between laser fields. Therefore, they also significantly affect pulse propagation and optical switching performance. The research results could be helpful for experimental observations or applications in photonic devices such as logic gates, quantum optical information processors, and quantum computers.

Singlet-triplet dephasing in radical pairs in avian cryptochromes leads to time-dependent magnetic field effects.

Cryptochrome 4a (Cry4a) has been proposed as the sensor at the heart of the magnetic compass in migratory songbirds. Blue-light excitation of this protein produces magnetically sensitive flavin-tryptophan radical pairs whose properties suggest that Cry4a could indeed be suitable as a magnetoreceptor. Here, we use cavity ring-down spectroscopy to measure magnetic field effects on the kinetics of these radical pairs in modified Cry4a proteins from the migratory European robin and from nonmigratory pigeon and chicken. B1/2, a parameter that characterizes the magnetic field-dependence of the reactions, was found to be larger than expected on the basis of hyperfine interactions and to increase with the delay between pump and probe laser pulses. Semiclassical spin dynamics simulations show that this behavior is consistent with a singlet-triplet dephasing (STD) relaxation mechanism. Analysis of the experimental data gives dephasing rate constants, rSTD, in the range 3-6 × 107s-1. A simple "toy" model due to Maeda, Miura, and Arai [Mol. Phys. 104, 1779-1788 (2006)] is used to shed light on the origin of the time-dependence and the nature of the STD mechanism. Under the conditions of the experiments, STD results in an exponential approach to spin equilibrium at a rate considerably slower than rSTD. We attribute the loss of singlet-triplet coherence to electron hopping between the second and third tryptophans of the electron transfer chain and comment on whether this process could explain differences in the magnetic sensitivity of robin, chicken, and pigeon Cry4a's.

Nonlinear optical properties and transient absorption in Hibiscus Sabdariffa dye solution probed with femtosecond pulses

Highly conjugated organic molecules are attractive chromophores for nonlinear optics due to their significant third-order nonlinear response near the absorption bands located in the broad spectral range from 200 to 900 nm. In this work, we report on the wavelength-dependence of one- and two-photon absorption processes in Hibiscus sabdariffa organic dye excited by femtosecond (fs) laser pulses centered at 1030 nm and 515 nm. Z-scan and transient absorption (TA) techniques have been employed to investigate the absorption and relaxation processes in the target dye. The dye is found to have a significant negative nonlinear absorption coefficient (β) of value ∼ −2.7 × 10−11 cm/W at 515 nm probing laser pulses, while it has a positive value equal to +2.1 × 10−13 cm/W when 1030 nm laser pulses were employed. The TA decay measurements using 40 fs laser pulses, centered at 515 nm wavelength, reveal that the dye exhibits a short relaxation time of ∼3 ps attributed to the involvement of the excited state proton transfer (ESPT) process in the Hibiscus dye solution. Moreover, our time-integrated fluorescence measurements revealed that no fluorescence emission could be detected. The application of ultrashort pulses of ∼40 fs duration enabled the tracking of ESPT in the Hibiscus dye, which was not detected in previous studies that employed laser pulses of picoseconds and nanoseconds duration.

Simultaneous quantum squeezing of light polarizations and atomic spins in a cold atomic gas

We present a scheme to realize simultaneous quantum squeezing of light polarizations and atomic spins via a perturbed double electromagnetically induced transparency (DEIT) in a cold four-level atomic ensemble coupled with a probe laser pulse of two polarization components. We derive two coupled quantum nonlinear Schr\"odinger equations from Maxwell-Heisenberg-Langevin equations describing the quantum dynamics of the atoms and the probe pulse and develop a quantum theory of vector optical soliton (VOS), which have ultraslow propagation velocity and extremely low generation power. We solve the non-Hermitian eigenvalue problem describing the quantum fluctuations on the background of the VOS and rigorously prove that all fluctuation eigenmodes (including continuous modes and four zero modes) obtained constitute a biorthonormal and complete set. We find that, due to the giant self- and cross-Kerr nonlinearities contributed by the perturbed DEIT, a large polarization squeezing of the probe pulse can be realized. We also find that, together with the polarization squeezing of the probe pulses, a significant squeezing of atomic spins also occurs simultaneously. The results of the simultaneous squeezing of light polarizations and atomic spins by using only a coherent probe pulse reported here opens a route for uncovering the unique property of the quantum interface between light and atomic ensembles and also for applications in quantum information and precision measurement.

Modulation of non-adiabatic alignment of N<sub>2</sub> molecule by femtosecond laser pulses

The alignment of molecules in the femtosecond laser field induces the refractive index of the propagation medium to change, resulting in the spectral modulation effect. In this paper, the time evolution of the wavelength offset of a probe laser pulse under the influence of the non-adiabatic molecular alignment and Kerr effect is measured experimentally by the pump-probe method in nitrogen medium, and the alignment degree of N<sub>2</sub> molecules is obtained. By solving the time-dependent Schrödinger equation theoretically, the expression of the degree of molecular non-adiabatic alignment is obtained, and the time evolution of non-adiabatic alignment of N<sub>2</sub> molecules is calculated. Taking into account the combined influence of the non-adiabatic molecular alignment and Kerr effect on the change of refractive index of the propagation medium, the modulation effect of birefringence on the spectrum of the probe pulse is achieved. The experimental result accords well with the theoretical calculation, demonstrating that the spectral modulation results obtained by experimental measurement can be used to characterize the alignment degree. Further, the double pulse pump method is used to control the degree of molecular alignment. It is found that the degree of molecular alignment can be enhanced by the double pulse pump method. Moreover, by adjusting the delay time between the two pump laser pulses, that is, adding the second pump laser pulse at one rotational period and half rotational period, respectively, the enhancement and loss of the alignment of N<sub>2</sub> molecules can be achieved, which is named the “alignment switch” effect. The molecular alignment control induced by the double pulse pump method can also be applied to the other molecular systems with different alignment and anti-alignment times, such as CO<sub>2</sub> molecules and O<sub>2</sub> molecules, indicating that the double pulse pump method can be used universally.

Interference patterns in ionization of Kramers–Henneberger atom

We combine IR pump and XUV probe laser pulses to visualize the Kramers–Henneberger (KH) state of the potassium atom. We demonstrate that ionization of such an atom exhibits some molecular-like features such as low order interference maxima in photoelectron momentum spectra. The locations of these maxima allow to estimate spatial dimensions of the KH atom and can be used for accurate calibration of high intensity laser fields. At the same time, we show that an analogy between the KH atom and a homo-nuclear diatomic molecule cannot be extended too far. In particular, higher order interference maxima are very difficult to observe in the case of the KH state. We attribute this to a particular structure of the KH potential which does not confine electron motion to a well-defined potential well unlike in real diatomic molecules.

Low-order nonlinearities and high-order harmonics generation in Zn and ZnSe nanoparticles synthesized during femtosecond laser ablation at a 50 kHz pulse repetition rate

We synthesize Zn and ZnSe nanoparticles during laser ablation of bulk material in ethanol and analyze them using the Z-scan technique and the generation of high harmonics using a 50 kHz, 40 fs, and 1030 nm laser. The low-order nonlinear absorption and nonlinear refraction coefficients in the colloidal suspensions of the formed nanoparticles were measured using 1030 nm and 515 nm probing laser pulses, respectively. The values of the nonlinear refractive index and nonlinear absorption coefficient of Zn NPs at 1030 nm wavelength were calculated to be γ = − 1.4 × 10 − 13 c m 2 W − 1 and β = 2.7 × 10 − 11 c m W − 1 , respectively. Similarly, in the case of ZnSe NPs, γ = − 4.9 × 10 − 14 c m 2 W − 1 and β = 0.5 × 10 − 11 c m W − 1 , respectively. In the case of 515 nm probing pulses, the corresponding nonlinear optical parameters of Zn NP suspension were calculated to be γ = − 1.7 × 10 − 14 c m 2 W − 1 and β = 1.0 × 10 − 11 c m W − 1 , and for ZnSe NP suspension, the value of β was calculated to be 2.1 × 10 − 11 c m W − 1 . The nonlinear refractive index of this suspension was γ = − 2.4 × 10 − 14 c m 2 W − 1 . We also report the studies of high-order harmonics generation in plasmas containing Zn and ZnSe NPs using single- (1030 nm) and two-color (1030 nm + 515 nm) laser fields. The maximal generated extreme ultraviolet (XUV) harmonics were 33rd (Zn NPs) and 27th (ZnSe NPs) orders. The decrease of the maximal generating harmonic orders was explained using the three-step model. The high brightness of the ninth and 10th harmonics generated in the two-color laser field was obtained due to the resonance properties of Zn NPs. The maximal generated XUV harmonics were the 33rd (Zn nanoparticles) and 27th (ZnSe nanoparticles) orders.

Plasma Dynamics Characterization for Improvement of Resonantly Enhanced Harmonics Generation in Indium and Tin Laser-Produced Plasmas

In this study, we characterize the properties of indium and tin laser-induced plasmas responsible for efficient high-order harmonics generation of the ultrashort pulses propagating through these media. The optimally formed plasma was determined using the analysis of the time-resolved variations in the spectral and morphological features of spreading indium and tin plasma components under different regimes of laser ablation. We report the measurements of plasma velocities under different regimes of ablation and correlate them with the optimal delay between the heating and probe laser pulses for the generation of harmonics with the highest yield. Electron temperatures and densities are determined using the integrated and time-resolved spectral measurements of plasmas. The resonance-enhanced harmonics are compared with other harmonics from the point of view of the modulation of plasma characteristics. The harmonics of 800 and 1200–2200 nm lasers and their second-harmonic fields were analyzed at optimal conditions of Sn and In plasma formation. The novelty of this work is the implementation of the diagnostics of the dynamics of plasma characteristics for the determination of the optimal plasma formation for harmonics generation. Such an approach allows for the demonstration of the maximal harmonic yield from the studied plasma and the definition of the various resonance-induced harmonic generation conditions.

Realizing fast temperature measurement and simulating Maxwell’s demon with nearly nondestructive detection in cold atoms

Optical detection and manipulation of the thermal properties is an essential subject of cold atoms in the quantum era. For laser cooled alkali atoms, we have experimentally realized deterministic temperature measurement with time cost below 1 ms and effective filtering of colder atoms with temperature less than 1 μK, with the help of nearly nondestructive detection. The quick temperature measurement is accomplished by carefully resolving the diffusion dynamics of atoms with the information provided by a single probe laser pulse in the form of bucket detection, while suppressing the amplitude and phase noises of probe laser. The separation of colder atoms is attainable as the velocity differences of atoms translate into nontrivial position differences, when the diffusion sustains for a few tens of milliseconds. In particular, these efforts are based on a labeling process that distinguishes the cold atoms under study from the others by specific internal states, while the nearly nondestructive detection is implemented via driving a cycling transition with continuous optical pulses. Moreover, such a position-dependent labeling process can be further modified to become velocity-dependent, with which we have demonstrated a Maxwell’s demon-type operation on cold atoms, as Maxwell’s demon’s intricate abilities can be understood as measuring the velocity of an individual particle and then performing feedback according to a straightforward dichotomy of the velocity value.

High-order above-threshold ionization from a coherent superposition of states

Quantum theory of high-order above-threshold ionization (HATI) of atoms, prepared in a coherent superposition of bound states, by a strong laser field is formulated. Numerical results are obtained using the solutions of the time-dependent Schr\"odinger equation and the improved strong-field approximation. As an example, coherent superposition of the excited $2s$ and $2p$ states of the He atom is used. Applying a two-level model, it is shown how the relative amplitude and phase of the states in this coherent superposition can be controlled with a weak resonant laser pulse. Numerical results presented for HATI by a strong nonresonant probe laser pulse confirm that the photoelectron spectra and the momentum distributions strongly depend on the relative phase of the states in the prepared coherent superposition. For the case of HATI by a strong resonant laser pulse, we modify our strong-field-approximation theory by applying the two-level model to describe the time evolution of the atomic bound state before the instant of ionization. Contrary to the HATI from a single state by a long laser pulse, the corresponding photoelectron spectra of HATI from the coherent superposition of states depend on the carrier-envelope phase of the resonant ionizing pulse. Therefore, the strong-field ionization from a coherent superposition of states by a resonant laser pulse can be used to determine the carrier-envelope phase for long pulses for which the standard methods such as stereo--above-threshold ionization technique fail. The results strongly depend on the relative amplitude and phase of states in the coherent superposition of states, which can be controlled with a weak resonant laser pulse.

In-Situ Imaging of a Light-Induced Modification Process in Organo-Silica Films via Time-Domain Brillouin Scattering.

We applied time-domain Brillouin scattering (TDBS) for the characterization of porogen-based organosilicate glass (OGS) films deposited by spin-on-glass technology and cured under different conditions. Although the chemical composition and porosity measured by Fourier-transform infrared (FTIR) spectroscopy and ellipsometric porosimetry (EP) did not show significant differences between the films, remarkable differences between them were revealed by the temporal evolution of the Brillouin frequency (BF) shift of the probe light in the TDBS. The observed modification of the BF was a signature of the light-induced modification of the films in the process of the TDBS experiments. It correlated to the different amount of carbon residue in the samples, the use of ultraviolet (UV) femtosecond probe laser pulses in our optical setup, and their intensity. In fact, probe radiation with an optical wavelength of 356 nm appeared to be effective in removing carbon residue through single-photon absorption processes, while its two-photon absorption might have led to the breaking of Si-CH3 bonds in the OSG matrix. The quantum chemical calculations confirmed the latter possibility. This discovery demonstrates the possibility of local modifications of OSG films with a nanometric resolution via nonlinear optical processes, which could be important, among other applications, for the creation of active surface sites in the area-selective deposition of atomic layers.

FORMATION OF THE OBJECT IDENTIFICATION ZONE WITH LASER INFORMATION-MEASURING SYSTEMS AT SHORT DISTANCES

The optimization of the methods for the formation of the spatial-energy distribution of the probing radiation power and the processing the receiving signal by the locating laser information-measuring systems (LIMS), taking into account the spatial-temporal structure, is carried out, and the analysis of the existing methods of their processing is carried out too. An assessment of the integral criteria for the LIMS functioning when operating in conditions of interference has been made. The calculation of the parameters of the LIMS main links was carried out, taking into account the correlation between the resolution of the optical system and the capabilities of object detection, recognition and classification. A method was developed for the formation of the probing radiation density distribution and the receiving signal processing, taking into account its space-time structure, which made it possible to determine the optimal duration of the laser probe pulse. The determined duration makes it possible to eliminate errors in measuring the parameters of an object's movement under the influence of a combination of destabilizing factors and a lack of signal processing time, which will ensure the accuracy of the target detection and recognition.