Giant Enhancement of Air Lasing by Complete Population Inversion in N_{2}

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm^{-2} level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to control the extent of the population transfer. We deplete the population in the X^{2}Σ_{g}^{+}(v=0) state of N_{2}^{+} almost completely by focusing a dual-color (800 nm and 1.6 µm) intense femtosecond laser pulse in a nitrogen gas, and make the intensity of N_{2}^{+} lasing at 391 nm enhanced by 5-6 orders of magnitude. By solving a time-dependent Schrödinger equation describing the population transfer among the three lowest electronic states of N_{2}^{+}, we reveal that the X^{2}Σ_{g}^{+}(v=0) population is depleted by the vibrational Raman excitation followed by the electronic excitation A^{2}Π_{u}(v=2,3,4)←X^{2}Σ_{g}^{+}(v=1)←X^{2}Σ_{g}^{+}(v=0), resulting in the excessive population inversion between the B^{2}Σ_{u}^{+}(v=0) and X^{2}Σ_{g}^{+}(v=0) states. Our results offer a promising route to efficient population transfer among vibrational and electronic levels of molecules by a precisely designed intense laser field.

High-speed multi-pass tunable diode laser absorption spectrometer based on frequency-modulation spectroscopy.

We report a multi-pass tunable diode laser absorption spectrometer based on the frequency-modulation spectroscopy (FMS) technique. It has the advantage of high scan speed and is immune to the etalon effect. A multi-pass Herriott-type cell was used in the spectrometer to increase the effective optical length to 17.5 m and compact the physical dimensions of the spectrometer to 60×30×30 cm3. Noise due to low-frequency fluctuation of the laser power and the 1/f noise in the rapid detection are sufficiently reduced by FMS. Interference fringes are effectively suppressed when the modulation frequency equals to integer or half-integer times of their free spectral range (FSR). An absorption line of C2H2 around 1.51 µm was recorded with the spectrometer to demonstrate its capabilities. The response frequency of the spectrometer is up to 100 kHz (10 µs) thanks to the high modulation frequency of FMS. The detection sensitivity of the spectrometer is about 240 ppb (3σ) at 100 kHz measurement repetition rate. The amplitude of the absorption signal is highly linear to the C2H2 concentration in the range of 300 ppb -100 ppm. Based on the Allan variation, the detection limit was determined to be 18 ppb with a detection time of 166 s.

[Study on the Spiral-Torus Herriott Type Cell].

With the rapid development of social economy, the environmental pollution and the ecological destruction are continuously deteriorating while sudden environmental pollution incidents occur frequently. Real-time monitoring harmful gases of the air take advantages of spectroscopic techniques for concentration measurement. Multipass optical cells are -widely used in absorption spectrometry technique to improve gas detection sensitivity under the condition of weak absorption. This paper proposes a spiral-torus type multipass optical device base on the structure of Herriott type cell. The optical device consists of multiple torus concave mirrors in a spiral way. Incident light propagates along with radical and axial direction in winding staircase pattern. The faculae on the inner wall present a spiral-type. The entrance and exit apertures are separated due to the spiral trace of optical rays, which increases the accessible adjustment of the apparatus. The effective optical length can be adjusted based on the proportional relationship to the reflective times. This device is characterized with easy adjustment and excellent mechanical performance due to its cylindrical structure. Based on ABCD matrix, the stability of the system was analyzed and the relationship between the number of reflections and the incident angle were discussed. With optical simulation software, we designed a device for separating polarized light, and the characteristics of its rotation was studied.

A machine learning-assisted fluorescent sensor array utilizing silver nanoclusters for coffee discrimination.

Coffee is a globally consumed commodity of substantial commercial significance. In this study, we constructed a fluorescent sensor array based on two types of polymer templated silver nanoclusters (AgNCs) for the detection of organic acids and coffees. The nanoclusters exhibited different interactions with organic acids and generated unique fluorescence response patterns. By employing principal component analysis (PCA) and random forest (RF) algorithms, the sensor array exhibited good qualitative and quantitative capabilities for organic acids. Then the sensor array was used to distinguish coffees with different processing methods or roast degrees and the recognition accuracy achieved 100%. It could also successfully identify 40 coffee samples from 12 geographical origins. Moreover, it demonstrated another satisfactory performance for the classification of pure coffee samples with their binary and ternary mixtures or other beverages. In summary, we present a novel method for detecting and identifying multiple coffees, which has considerable potential in applications such as quality control and identification of fake blended coffees.

Generating a Sustained Oxygen-Stable Atomic Concentration in a High-Temperature Gas Effect Investigation.

Modulated laser absorption spectroscopy is an ideal technique for evaluating flow-field parameters and determining flow-field quality by measuring the atoms dissociated in high-temperature environments. However, to obtain the absolute number density of atoms in the flow field, it is necessary to compare the measured modulated absorption spectroscopy signal with a known atomic concentration and establish a quantitative relationship through concentration calibration. Nevertheless, it remains a challenging task to prepare transient atomic samples with known concentrations that meet the calibration requirements. This study utilized the alternating-current glow discharge technique to dissociate oxygen in the air flow, resulting in the continuous generation of oxygen atoms. The absolute number densities of the generated oxygen atoms were determined by measuring the direct absorption spectra of centered on 777 nm for oxygen atoms. The number densities of the generated atoms were finely tuned by adjusting the discharge parameters. Throughout the 120-min continuous operation of the discharge system, the concentration of excited-state oxygen atoms remained stable within the range of (2.51 ± 0.02) × 108 cm-3, demonstrating the remarkable stability of the transient atomic concentration generated by the glow discharge plasma. This observation suggests that the generated atoms can be utilized as a standardized atomic sample of known concentration for absolute concentration calibration purposes.

Volatile organic compound removal by post plasma-catalysis over porous TiO2 with enriched oxygen vacancies in a dielectric barrier discharge reactor.

Non-thermal plasma (NTP) degradation of volatile organic compounds (VOCs) into CO2 and H2O is a promising strategy for addressing ever-growing environment pollution. However, its practical implementation is hindered by low conversion efficiency and emissions of noxious by-products. Herein, an advanced low-oxygen-pressure calcination process is developed to fine-tune the oxygen vacancy concentration of MOF-derived TiO2 nanocrystals. Vo-poor and Vo-rich TiO2 catalysts were placed in the back of an NTP reactor to convert harmful ozone molecules into ROS that decompose VOCs via heterogeneous catalytic ozonation processes. The results indicate that Vo-TiO2-5/NTP with the highest Vo concentration exhibited superior catalytic activity in the degradation of toluene compared to NTP-only and TiO2/NTP, achieving a maximum 96% elimination efficiency and 76% COx selectivity at an SIE of 540 J L-1. Mechanistic analysis reveals that the 1O2, ˙O2- and ˙OH species derived from the activation of O3 molecules on Vo sites contribute to the decomposition of toluene over the Vo-rich TiO2 surface. With the aid of advanced characterization and density functional theory calculations, the roles of oxygen vacancies in manipulating the synergistic capability of post-NTP systems were explored, and were attributed to increased O3 adsorption ability and enhanced charge transfer dynamics. This work presents novel insights into the design of high-efficiency NTP catalysts structured with active Vo sites.

Dual-comb optical activity spectroscopy for the analysis of vibrational optical activity induced by external magnetic field.

Optical activity (OA) spectroscopy is a powerful tool to characterize molecular chirality, explore the stereo-specific structure and study the solution-state conformation of biomolecules, which is widely utilized in the fields of molecular chirality, pharmaceutics and analytical chemistry. Due to the considerably weak effect, OA spectral analysis has high demands on measurement speed and sensitivity, especially for organic biomolecules. Moreover, gas-phase OA measurements require higher resolution to resolve Doppler-limited profiles. Here, we show the unmatched potential of dual-comb spectroscopy (DCS) in magnetic optical activity spectroscopy (MOAS) of gas-phase molecules with the resolution of hundred-MHz level and the high-speed measurement of sub-millisecond level. As a demonstration, we achieved the rapid, high-precision and high-resolution MOAS measurement of the nitrogen dioxide [Formula: see text]+[Formula: see text] band and the nitric oxide overtone band, which can be used to analyze fine structure of molecules. Besides, the preliminary demonstration of liquid-phase chiroptical activity (as weak as 10-5) has been achieved with several seconds of sampling time, which could become a routine approach enabling ultrafast dynamics analysis of chiral structural conformations.

High-resolution observation and analysis of the I2+ A2Π(3/2,u)-X2Π(3/2,g) system.

Rotationally resolved absorption spectra of I(2)(+) were recorded in 12,065-13,062 cm(-1) region by employing optical heterodyne velocity modulation absorption spectroscopy. In total, 4054 lines were assigned to 24 bands in the A(2)Π(3/2,u)-X(2)Π(3/2,g) system spanning the vibrational levels υ'' = 1-4 and υ(n)' = 11-19. The assigned lines were globally fitted and an error of 0.003 cm(-1) was obtained. Rotational constants, B(υ), were used to derive equilibrium parameters B(e)'' = 0.03977725(77) cm(-1), a(e)'' = 1.1819(24)×10(-4) cm(-1), r(e)'' = 2.584386(25) Å of the X(2)Π(3/2,g) state, and B(e)' = 0.0305787(37) cm(-1), a(e)' = 1.2353(23)×10(-4) cm(-1), r(e)' = 2.94758(18) Å of the A(2)Π(3/2,u) state. Vibrational energies were used to derive ω(e)'' = 239.0397(55) cm(-1), ω(e)x(e)'' = 0.64951(87) cm(-1) of the X(2)Π(3/2,g) state and ω(e)' = 138.103(11) cm(-1), ω(e)x(e)' = 0.45027(34) cm(-1) of the A(2)Π(3/2,u) state. The A(2)Π(3/2,u) (υ(n) = 13) state was found to be rotationally perturbed by the a(4)Σ(1/2,u)(-) (υ(n) = 17) state through second-order spin-orbit coupling.

Perturbation analysis of the υ = 6 level in the d3Δ state of CS based on its near-infrared absorption spectrum.

The spectrum of CS was recorded in the region of 12,086-12,630 cm(-1) by employing optical heterodyne concentration modulation laser absorption spectroscopy. Nearly 350 transitions were assigned to the (6, 0) band in the d(3)Δ-a(3)Π system of CS. The overtone transitions of the (12, 0) band in the a(3)Π(2)-a(3)Π(0) transition were first observed due to the perturbation interaction between d(3)Δ(1) and a(3)Π(2). The Λ doubling in the a(3)Π(1) state was also resolved at high rotational levels. The molecular constants of the a(3)Π (υ = 0) and d(3)Δ (υ = 6) levels and the perturbation parameters of the d(3)Δ (υ = 6) level were determined through nonlinear least-squares fitting using effective hamiltonians. The calculations of mixing fractions of the perturbed states were performed in order to obtain precise information on the perturbations of the d(3)Δ (υ = 6) levels. The mechanisms for perturbations of d(3)Δ (υ = 6) with the a(3)Π (υ = 12) and A(1)Π (υ = 1) levels, especially for the second-order perturbation, were discussed and explained according to first-order nondegenerate perturbation theory.

[LIF spectroscopic study of OH radical].

Hydroxyl radical molecular beam was generated by DC pulsed high voltage discharge. The rotational resolution excitation fluorescence spectra of the OH A 2sigma+ -X 2pi (1, 0) band were observed by using second harmonious output of a nanosecond dye laser (around 282 nm). The rotational temperature of the (1, 0) band was determined to be (30 +/- 1) K based on the analysis of the intensity distribution of the OH spectra. Moreover, the fluorescence decay curves of the (1, 1) band and the (0, 0) band and were obtained and the fluorescence lifetimes of A 2sigma+ (v' = 1) state and the X 2pi (v" = 0) state were fitted to be (637 +/- 16) and (675 +/- 13) ns, respectively.

A difference frequency generation spectrometer and its detection of atmospheric N2O.

The paper reports the realization and characterization of a difference frequency generation spectrometer using periodically poled lithium niobate (PPLN) crystal. The pump and signal laser we used is a Ti:sapphire ring laser and a diode pumped monolithic Nd:YAG laser, respectively. The continuous wave (cw) infrared radiation from 2.8 to 4.8 microm has been generated. The idler radiation can be used to study fundamental absorption bands of molecules and trace gas detection. In this work, we report the detection of nitrous oxide (N(2)O) in atmosphere, the minimum detectable concentration of 10.9 ppbV was achieved using a Herriott cell with the optical path length of 100 m.

[Difference-frequency generation in PPLN and water vapor detection in air].

The continuously tunable laser source has been realized in a periodically poled LiNO3 crystal based on difference frequency generation and quasi-phase-matching technique. The pump laser is an 1 W tunable Ti: Sapphire laser with a tunable region from 770 to 870 nm. The signal laser is an 1 W diode-pumped monolithic Nd : YAG laser. When the grating period is 20 microm and the temperature is tuned between room temperature and 200 degrees C, the generated wavelength of idler laser is around 2. 8 microm with the general power of .1-2 microW. The direct absorption spectra of (001 <-- 000) band of water in laboratory air were measured based on the laser source. The concentration of water vapor in the laboratory air was estimated with an absorption optical path of 8. 5 cm in open air.

Pressure-Dependent Detection of Carbon Monoxide Employing Wavelength Modulation Spectroscopy Using a Herriott-Type Cell.

Wavelength modulation spectroscopy (WMS) combined with a multipass absorption cell has been used to measure a weak absorption line of carbon monoxide (CO) at 1.578 µm. A 0.95m Herriott-type cell provides an effective absorption path length of 55.1 m. The WMS signals from the first and second harmonic output of a lock-in amplifier (WMS-1 f and 2 f, respectively) agree with the Beer-Lambert law, especially at low concentrations. After boxcar averaging, the minimum detection limit achieved is 4.3 ppm for a measurement time of 0.125 s. The corresponding normalized detection limit is 84 ppm m Hz-1/2. If the integrated time is increased to 88 s, the minimum detectable limit of CO can reach to 0.29 ppm based on an Allan variation analysis. The pressure-dependent relationship is validated after accounting for the pressure factor in data processing. Finally, a linear correlation between the WMS-2 f amplitudes and gas concentrations is obtained at concentration ratios less than 15.5%, and the accuracy is better than 92% at total pressure less than 62.7 Torr.

Near-infrared diode laser wavelength modulation-based photoacoustic spectrometer.

An inexpensive resonant photoacoustic spectrometer based on a low-power distributed feedback diode laser and wavelength modulation spectroscopy is developed. This sensor has been applied to the detection of acetylene (C2H2) using a properly designed photoacoustic cell operating on its second longitudinal mode. The minimum detectable limit of about 10 parts-per-million volume (signal to noise ratio=1) is achieved at atmospheric pressure, and the pressure and laser power linear dependence of the photoacoustic signal is also investigated. Moreover, in this paper we also describe some basic theory of gas photoacoustic spectroscopy.

Ultrabroadband tunable continuous-wave difference-frequency generation in periodically poled lithium niobate waveguides.

We report, for what is the first time to our knowledge, widely tunable mid-IR light generation over a range of greater than 1000 nm in the 4 microm region by employing a single quasi-phase-matched periodically poled niobate waveguide at room temperature. The waveguide we used was fabricated by annealed proton exchange based on periodically poled lithium niobate. A peak conversion efficiency of 10%/W and a linewidth as small as 37 MHz were achieved. The developed mid-IR light generator may find wide applications in trace gas detection of multiple atmospheric species and high-resolution spectroscopy.