Laser-gas Interaction Research Articles

Mid-IR parts-per-billion level nitric oxide detection using solid-state laser intracavity photothermal sensor (SLIPS)

Highly sensitive, laser-based sensors often utilize extended gas interaction paths (e.g., multi-pass cells) that impact their size, weight, and susceptibility to misalignment, limiting their use to laboratory applications. This study reports the first demonstration of photothermal nitric oxide (NO) detection in the mid-infrared range at 5.26 µm utilizing a unique and miniature Solid-state Laser Intracavity Photothermal Sensor (SLIPS) with an ultra-short laser-gas interaction path length of 1 mm. The gas sample detection is realized inside the resonator of a monolithic diode-pumped solid-state laser (DPSSL) emitting at 1064 nm. Modulated mid-infrared radiation absorbed by NO inside the laser cavity causes local changes in photothermally-induced gas refractive index (RI). This effect results in the DPSSL frequency shifts proportional to gas concentration, which are being detected. A minimum detection limit of 50 ppbv (parts per billion of volume) and a noise equivalent absorption (NEA) coefficient of 7.9 × 10−7 cm−1 for 10 s integration time have been achieved in an ultra-compact sensing volume (4 µl) and ultra-short interaction length of the sensor as a result of its superb RI sensitivity of 1.8 × 10−11. Furthermore, the SLIPS is not limited in gas excitation laser wavelength because only a near-infrared detector is needed for spectroscopic signal readout.

Broadband Terahertz Detection by Laser Plasma with Balanced Optical Bias.

Using a controlled optical bias and balanced geometry, we propose a new scheme for broadband terahertz detection by laser-gas interaction without high-voltage manipulation. Compared to the conventional optical bias scheme, the common noise is reduced and the dynamic range as well as the signal-to-noise ratio are doubled. It provides a simple alternative for coherent broadband terahertz detection. The influence of optical bias on terahertz waveform is also investigated, and the evolution of the terahertz-induced second harmonic with probe delay is further revealed. This new detection scheme for broadband terahertz will boost the application of terahertz time-domain spectroscopy for its miniaturization and integrability.

Inhibition of electron refluxing in laser-gas interactions for enhanced positron generation

We report an experimental investigation of a laser-gas-converter approach for generating high-yield ultrashort MeV positrons. We observe that MeV electrons with a high charge of several tens of nC can be well generated by a , laser interacting with a high-density gas jet. However, it is shown that the propagation of the highly charged electron beam is significantly inhibited because the electrons are reflected by the sheath potential in the density decreasing region of the gas target, thus leading to a low positron yield. Consequently, by using an integrated nozzle-converter design to eliminate the density falling ramp of the gas target such that the electron refluxing is inhibited, we observe a significant enhancement of positron yield (up to a factor of 15), finally reaching a positron yield of . This high-yield ultrashort MeV positron may have great potential toward the simulation of astrophysical pair plasma.

Performance enhancement of a near-infrared NH3 sensor based on PSO-LSSVM for denitrification industrial process

A near-infrared NH3 sensor based on a 1512 nm distributed feedback (DFB) laser was designed to perform real-time and online detection of NH3 in the industrial denitrification. A miniaturized multi-pass gas cell (MPGC) is utilized as a site for infrared laser-gas interaction, in which the infrared light is reflected back and forth 76 times within the MPGC, with an effective optical path length of 20 m. High-precision concentration inversion of the detected second harmonic (2f) signals at both high and low concentrations was performed by the particle swarm optimization-least squares support vector machine (PSO-LSSVM) algorithm, and the linear correlation coefficient R2 between the standard and measured concentrations could be as high as 0.9999 or later in both cases. The sensor was tested for stability for 1 h and its Allan -Werle deviation was analyzed. The minimum limit of detection (LoD) of the sensor is 3.95 ppb at an averaging time of 38 s. An outdoors deployment of the sensor was reported for detection of the leaking NH3 in Laboratory Building.

Multiparameter-controlled laser ionization within a plasma wave for wakefield acceleration

We present an optical setup for well-defined ionization inside a plasma such that precisely controlled spots of high electron density can be generated. We propose to use the setup for Trojan Horse Injection (or Plasma Photocathode Emission) where a collinear laser beam is needed to release electrons inside a plasma wakefield. The reflection-based setup allows a suitable manipulation of the laser near field without disturbing the spectral phase of the laser pulses. A required ionization state and volume can be reached by tuning the beam size, pulse duration and pulse energy. The ionization simulations enable a prediction of the ionization spot and are in good agreement with dedicated experiments which measured the number of electrons created during the laser–gas interaction.

Hazardous Gas Detection by Cavity-Enhanced Raman Spectroscopy for Environmental Safety Monitoring.

We demonstrate the practicability of cavity-enhanced Raman spectroscopy (CERS) with a folded multipass cavity as a unique tool for the detection of hazardous gases in the atmosphere. A four-mirror Z-sharped multipass cavity results in a greatly extended laser-gas interaction length to improve the Raman signal intensity of gases. For Raman intensity maximization, the optimal number of intracavity beams of a single reflection cycle is calculated and then the cavity parameters are designed. A total of 360 intracavity beams are realized, which are circulated four times in the cavity based on the polarization. ppb-Level Raman gas sensing at atmospheric pressure for several typical explosive gases and toxic gases in ambient air, including hydrogen (H2), methane (CH4), carbon monoxide (CO), hydrogen sulfide (H2S), and chlorine (Cl2), is achieved at 300 s exposure time. Our CERS apparatus, which can detect multiple gases simultaneously with ultrahigh sensitivity and high selectivity, is powerful for detecting hazardous gases in the atmosphere, and it has excellent potential for environmental safety monitoring.

Research progress of high-order harmonics and attosecond radiation driven by interaction between intense lasers and plasma

The realizing of the detection and control of ultrafast process conduces to understanding and remoulding the physical world at a microcosm level. The attosecond light source with attosecond temporal resolution and nanometer spatial resolution can realize real-time detection and manipulation of the atomic-scale electronic dynamics and relevant effects of the substances. Therefore, attosecond science is considered as one of the most important milestones in the history of laser science. and has been listed as an important scientific and technological development direction in the coming 10 years. High-order harmonic generation (HHG) from intense laser-matter interaction is one of the most important routes to breaking through the femtosecond limit and achieving brilliant attosecond pulse radiations, and thus having aroused great interest in recent years. After more than 20-year development, the research about attosecond pulse generation by laser-gas interaction has reached a mature stage. This method produces the shortest isolated pulse in the world to date, with a pulse width being only 43 as. However, this method based on ionization-acceleration-combination encounters inevitable difficulties in pursuing the relativistically intense attosecond pulses and the highest possible photon energy. Quite a lot of studies have proved that the HHG efficiency from laser-plasma interaction can be a few orders of magnitude higher than that in gaseous media, which makes it possible to produce pulses with shorter pulse width and higher photon energy. In this article, we introduce the main generation mechanisms, research progress and frontier applications of HHG through the laser-plasma interaction process. In Section 2, we introduce the HHG generation mechanisms, including coherent wake emission, which is used to describe the HHG process driven by a nonrelativistic laser; relativistic oscillating mirror, which can well explain most of HHG processes generated from plasma-vacuum interface in relativistic regime; coherent synchrotron emission, which is suited to explain the HHG synchronously emitted from isolated electron sheets. The research progress is summarized in Section 3 from the aspects of radiation efficiency, polarization characteristics, phase characteristics, generation and diagnosis of isolated attosecond pulses, etc. Frontier applications of these ultra-broadband intense attosecond pulses are presented in the last section, such as the study of electronic dynamics, process, coherent diffraction imaging, diagnosis of extreme states of matter, the generation of extremely intense fields, etc. Finally, an outlook on the future development trends and innovation breakthroughs is also presented.

Antiresonant Hollow-Core Fiber-Based Dual Gas Sensor for Detection of Methane and Carbon Dioxide in the Near- and Mid-Infrared Regions.

In this work, we present for the first time a laser-based dual gas sensor utilizing a silica-based Antiresonant Hollow-Core Fiber (ARHCF) operating in the Near- and Mid-Infrared spectral region. A 1-m-long fiber with an 84-µm diameter air-core was implemented as a low-volume absorption cell in a sensor configuration utilizing the simple and well-known Wavelength Modulation Spectroscopy (WMS) method. The fiber was filled with a mixture of methane (CH4) and carbon dioxide (CO2), and a simultaneous detection of both gases was demonstrated targeting their transitions at 3.334 µm and 1.574 µm, respectively. Due to excellent guidance properties of the fiber and low background noise, the proposed sensor reached a detection limit down to 24 parts-per-billion by volume for CH4 and 144 parts-per-million by volume for CO2. The obtained results confirm the suitability of ARHCF for efficient use in gas sensing applications for over a broad spectral range. Thanks to the demonstrated low loss, such fibers with lengths of over one meter can be used for increasing the laser-gas molecules interaction path, substituting bulk optics-based multipass cells, while delivering required flexibility, compactness, reliability and enhancement in the sensor’s sensitivity.

A Calibration-Free Methodology for Resonantly Enhanced Photoacoustic Spectroscopy Using Quantum Cascade Lasers

Photoacoustic spectroscopy (PAS) is a highly sensitive technique for trace gas sensing, which requires frequent re-calibration for changing environmental influences and input light power fluctuation. This is a major drawback against its deployment for on-site, long-term remote applications. To address this drawback here we present the theory and application of a Calibration-Free Wavelength Modulation Photoacoustic Spectroscopy (CF-WM-PAS) technique. It is applied for measurements of CH4 gas concentration in the mid-infrared at $8.6~\mu {m}$ wavelength using a Quantum Cascade Laser (QCL). The method normalizes the second harmonic ( ${R}_{2{f}}$ ) component, dominated by laser-gas interaction and optical intensity, by the first harmonic ( ${R}_{1{f}}$ ) component dominated by Residual Amplitude Modulation (RAM) DC offset, to isolate the output from changes in the gas matrix, optical intensity and electrical gain. This normalization technique removes influences from changes in the resonant frequency, gas concentration and incident optical power. It is confirmed using a ±1600 Hz change in modulation frequency around the resonance, a 1% to 10% change in gas concentration and an up to 78.3% attenuation in input light intensity with a custom built miniaturized 3D-printed sensor. A Normalized Noise Equivalent Absorption (NNEA) of $4.85\times 10^{-9}$ Wcm−1Hz−1/2 for calibration-free ${R}_{2{f}}/{R}_{1{f}}$ measurements is demonstrated.

Absolute Measurement of Laser Ionization Yield in Atmospheric Pressure Range Gases over 14 Decades.

Strong-field ionization is central to intense laser-matter interactions. However, standard ionization measurements have been limited to extremely low density gas samples, ignoring potential high density effects. Here, we measure strong-field ionization in atmospheric pressure range air, N_{2}, and Ar over 14decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons. Our results are consistent with theoretical rates for isolated atoms and molecules and quantify the ubiquitous presence of ultralow concentration gas contaminants that can significantly affect laser-gas interactions.

Controlling terahertz radiation in gas plasma with time-delayed laser pulses by the pulsing shaping technology

We theoretically study the terahertz generation in laser-gas interaction with time-delayed laser pulses by using the femtosecond pulsing shaping technology. We show that the terahertz spectrum can be periodically modulated and effectively tuned in the frequency domain with varying the phase-control parameters. We also show that the terahertz pulse is split into multiple subpulses in the time domain, and that the relative time interval and pulse intensity of these terahertz subpulses can be effectively modulated. Finally, our analysis illustrates that the generation of shaped terahertz radiation is determined by the character of the time-delayed laser pulses.

A nozzle for high-density supersonic gas jets at elevated temperatures.

We present the development of a gas nozzle providing high-density gas at elevated temperatures inside a vacuum environment. Fused silica is used as the nozzle material to allow the placement of the nozzle tip in close proximity to an intense, high-power laser beam, while minimizing the risk of sputtering nozzle tip material into the vacuum chamber. Elevating the gas temperature increases the gas-jet forward velocity, allowing us to replenish the gas volume in the laser-gas interaction region between consecutive laser shots. The nozzle accommodates a 50 μm opening hole from which a supersonic gas jet emerges. Heater wires are used to bring the nozzle temperature up to 730 °C, while a cooling unit ensures that the nozzle mount and the glued nozzle-to-mount connection is kept at a temperature below 50 °C. The presented nozzle design is used for high-order harmonic generation in hot gases using gas backing pressures of up to 124 bars.

The SNR improvement for quartz-enhanced photoacoustic spectroscopy using wavelength calibration and fiber reflector

Wavelength calibration technique combined with a fiber reflector was used to improve the signal to noise ratio (SNR) of quartz-enhanced photoacoustic spectroscopy (QEPAS). A distributed feedback laser diode (DFB-LD), driven by sawtooth wave and high frequency sinusoidal wave, was used to excite the second harmonic signal of a quartz tuning fork (QTF) through laser-gas molecular interaction. Two collimators conducted the laser alignment through the spacing gap of QTF forks. Central wavelength of the DFB-LD was locked to the target gas absorption center by identifying the second harmonic signal maximum and applying calibration feedback on the driving current. The gas absorption center calibration and gas concentration measurements are conducted at a specific interval. The SNR of the photoacoustic signal was further acoustically enhanced by using a pair of on-beam acoustic resonators through increasing the photo-acoustic conversion efficient, and optically enhanced by using a fiber reflector to improve the laser power for photoacoustic signal excitation. The experimental results show that the SNR in wavelength calibration mode is 15 times higher than the conventional wavelength scanning mode and QEPAS signal with fiber reflector is 1.37 times stronger compared with that without a fiber reflector.

Modulation of terahertz-spectrum generation from an air plasma by tunable three-color laser pulses

We theoretically demonstrate that the terahertz-spectrum generation in three-color laser-gas interactions can be effectively manipulated by tuning the laser wavelengths. We show that by positive and negative detuning of one of the color laser wavelengths in the three-color laser pulse, the terahertz spectral range can be greatly expanded, and in particular, the high spectral frequency components emerge. However, the total terahertz energy is maintained at a relatively high level and can even significantly increase. Moreover, we also show that the middle and high spectral frequency components of the terahertz spectrum can be effectively tuned by modulating the relative wavelength of the three-color laser pulses. Finally, our analysis shows that the ionization events play an important role in the intense and shaped terahertz generation.

Highly accurate intensity factors of pure CO2 lines near 2 μm.

Line intensities for carbon dioxide are measured with a novel spectroscopic approach, assisted by an optical frequency comb synthesizer for frequency calibration purposes. The main feature of the spectrometer consists in the exploitation of optical feedback from a V-shaped high-finesse optical resonator to effectively narrow a distributed feedback diode laser at the wavelength of 2 μm. Laser-gas interaction takes place inside an isothermal cell, which is placed on the transmission from the cavity. High quality, self-calibrated, absorption spectra are observed in pure CO2 samples at different gas pressures, in coincidence with three lines of the R-branch of the ν1 + 2ν2 + ν3 band. Line intensities are determined using a global fitting approach in which a manifold of spectra are simultaneously analyzed across the range of pressures between 5 and 100 Torr, sharing a restricted number of unknown parameters. Various sources of uncertainty have been identified and carefully quantified, thus leading to an overall uncertainty ranging between 0.17% and 0.23%. The measured values are in a very good agreement with recent ab initio predictions.

Fluid dynamics analysis of a gas attenuator for X-ray FELs under high-repetition-rate operation.

Newtonian fluid dynamics simulations were performed using the Navier-Stokes-Fourier formulations to elucidate the short time-scale (µs and longer) evolution of the density and temperature distributions in an argon-gas-filled attenuator for an X-ray free-electron laser under high-repetition-rate operation. Both hydrodynamic motions of the gas molecules and thermal conductions were included in a finite-volume calculation. It was found that the hydrodynamic wave motions play the primary role in creating a density depression (also known as a filament) by advectively transporting gas particles away from the X-ray laser-gas interaction region, where large pressure and temperature gradients have been built upon the initial energy deposition via X-ray photoelectric absorption and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on an initially slower time scale. If the inter-pulse separation is sufficiently short so the filament cannot recover, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle. By performing simulations at higher repetition rates but lower per pulse energies while maintaining a constant time-averaged power, the amplitude of the hydrodynamic motion per pulse becomes smaller, and the evolution of the temperature and density distributions approach asymptotically towards, as expected, those calculated for a continuous-wave input of the equivalent power.

Dual-laser absorption spectroscopy ofC2H2at1.4μm

Spectroscopic parameters (line intensity factor, pressure self-broadening, and shifting coefficients) of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ at $1.4\ensuremath{\mu}\mathrm{m}$ were accurately measured using a dual-laser approach, based upon the technique of optical phase locking. This generated an absolute frequency scale underneath the absorption spectra. A pair of extended-cavity diode lasers was used. One of them, the probe laser, is forced to maintain a precise frequency offset from a reference laser, which is an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Laser-gas interaction takes place inside an isothermal multipass cell that is stabilized at the temperature of the triple point of water. The fidelity in the observation of the shape associated to the ${P}_{e}$(14) line of the $2{\ensuremath{\nu}}_{3}+{\ensuremath{\nu}}_{5}$ band allowed us to measure the spectroscopic parameters, with a global uncertainty for the line strength of 0.22%.

Effect of two-color laser pulse duration on intense terahertz generation at different laser intensities

We study theoretically terahertz generation in two-color laser-gas interactions based on a transient photocurrent model. We show that terahertz generation depends on the laser pulse duration and intensity of the two-color laser field. Furthermore, the terahertz amplitude increases with an increase of the laser pulse duration at low laser intensity, but decreases with an increase of the laser pulse duration at high laser intensity. Our analysis shows that the ionization events play an important role in terahertz generation and the terahertz amplitude is determined by the superposition of contributions from individual ionization events.

The extreme nonlinear optics of gases and femtosecond optical filamentation

Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas interactions.

Analytical model for THz emissions induced by laser-gas interaction.

We develop a one-dimensional model of THz emissions induced by laser-driven, time-asymmetric ionization and current oscillations in a hydrogen gas. Our model highlights complex scalings of the THz fields with respect to the laser and gas parameters, in particular, a non-monotonic behavior against the laser parameters. Analytical expressions of the transmitted and reflected fields are presented, explaining the THz spectra observed in particle-in-cell and forward-pulse propagation codes. The backward-propagating THz wave is mainly driven by the electron current oscillations at the plasma frequency, and its resulting spectrum operates below the plasma frequency. The transmitted THz wave is emitted from both plasma current oscillations and photo-ionization. Their respective signal presents a contribution below and around the plasma frequency, plus a contribution at higher frequencies associated to the photo-induced current. The interplay between these two mechanisms relies on the ratio between the propagation length and the plasma skin depth.