Pulse-periodic Laser Research Articles

Optically (solar) pumped oxygen-iodine lasers

We present the results of theoretical and experimental studies demonstrating the possibility of developing an oxygen-iodine laser (OIL) with direct optical pumping of molecular oxygen involving inter-molecular interaction with charge transfer from donor molecule (buffer gas) to acceptor molecule (oxygen). This interaction lifts degeneracy of the lower energy states of molecular oxygen and increases its absorption cross section in the visible spectral region and the UV Herzberg band, where high quantum yield of singlet oxygen is achieved (QY ∼ 1 and QY ∼ 2, respectively) at the same time. A pulse-periodic optical pump sources with pulse energy of ∼50 kJ, pulse duration of ∼25 μs, and repetition rate of ∼10 Hz, which are synchronized with the mechanism of singlet oxygen generation, are developed. This allows implementation of a pulse-periodic oxygen-iodine laser with an efficiency of ∼25%, optical efficiency of ∼40%, and parameter L/T ∼ 1/1.5, where T is the thermal energy released in the laser active medium upon generation of energy L. It is demonstrated that, under direct solar pumping of molecular oxygen, the efficiency parameter of the OIL can reach L/T ∼ 1/0.8 in a wide range of scaling factors.

Injection and acceleration of electron bunch in a plasma wakefield produced by a chirped laser pulse

An ultrashort laser pulse propagating in plasma can excite a nonlinear plasma wakefield which can trap and accelerate charged particles up to GeV. One-dimensional analysis of electron injection, trapping, and acceleration by different chirped pulses propagating in plasma is investigated numerically. In this paper, we inject electron bunches in front of the chirped pulses. It is indicated that periodical chirped laser pulse can trap electrons earlier than other pulses. It is shown that periodical chirped laser pulses lead to decrease the minimum momentum necessary to trap the electrons. This is due to the fact that periodical chirped laser pulses are globally much efficient than nonchirped pulses in the wakefield generation. It is found that chirped laser pulses could lead to much larger electron energy than that of nonchirped pulses. Relative energy spread has a lower value in the case of periodical chirped laser pulses.

Diffuse volume-discharges without pre-ionization formed in SF6 and C2H6 mixtures

It has been shown that a volume discharge is forming in non-uniform electric-field rough cathode and even anode without pre-ionization in SF6 and C2H6 mixtures. The discharge is presented with many diffuse channels attached to bright circular cathode spots that diverge towards the anode, with the channels overlapping, form a spatially uniform glow discharge. Self-Initiated Volume Discharge (SIVD) has been performed at a total mixture pressure up to 8 kPa and energy deposition up to 200 J/L. The experimental results indicate that Self-Sustained Volume Discharge (SSVD) in SF6 and C2H6 mixtures develops in the form of SIVD, which is promising for creation of high energy and pulse-periodic HF laser.

A compact frequency-stabilized pulse-periodic waveguide CO2 laser for calibration of wavelength meters

A compact pulse-periodic waveguide CO2 laser with high-frequency excitation with automatic assignment of the 10P (14) line and stabilization of the emission frequency in the middle of this line using a sealed-off optoacoustic cell, which is filled with a mixture of C2H4 (0.1%) and nitrogen, was developed. It is shown that the laser-frequency instability is within 3 MHz, thus meeting the requirements that are imposed on the calibration tools for high-resolution wavelength meters that are similar to WS-6IR.

Thermal self-action of high-power continuous and pulse-periodic CO2 laser radiation in air: II. Laboratory experiments

Results of laboratory experiments on nonlinear propagation of high-power pulse-periodic and continuous CO2 laser radiation in a gas medium are presented. The experiments were carried out in a cell filled with a mixture of air and carbon dioxide in different partial concentrations (∼1–4%) under strong absorption and thermal blooming of laser radiation. The experimental conditions simulated the atmospheric propagation of high-power laser radiation along a kilometer-long path. Sharply focused laser beams were used; the regions of heat release along the beam channel were recorded by the shadow imaging technique. We found that, despite increased intensity, the focal waist of a laser beam is characterized by reduced heat release as compared to that in the pre- and postfocal beam regions. The saturation effect of the resonance absorption coefficient of CO2 at high pulse peak intensity (blooming effect) is considered as the most probable physical cause of the above feature. For continuous radiation, this effect is much weaker.

Thermal self-action of high-power continuous and pulse-periodic CO2 laser radiation in air: I. Numerical simulation of propagation along an atmospheric path

A problem of transportation of high-power laser radiation energy at a wavelength of 10.6 μm in the atmosphere is theoretically considered in terms of its thermal and wind nonlinearities. The power characteristics of continuous and pulse-periodic radiation are studied during propagation along a surface kilometer-long atmospheric path with sharp focusing onto a receiver. Key factors most affecting high-power radiation propagation are found. It is ascertained that an undesirable effect of thermal defocusing can be reduced using a train of low-power short pulses with a quite high repetition rate due to the effect of light-induced “blooming” of the gas medium. The questions of scaling optical paths from the viewpoint of their equivalence in manifestation of thermal nonlinearity are discussed.

Spin mode locking in quantum dots revisited

A comprehensive overview of coherent spin manipulation in (In,Ga)As quantum dots, singly charged with resident electrons or holes, is given, from orientation to rotation of the spins. These operations are performed by excitation of the charged exciton complex with laser pulses. The specifics of the approach is performing the manipulations on dot ensembles, potentially giving robustness to the coherent spin dynamics. In particular, we focus on the spin mode‐locking regime, in which the precession of the spins about an external magnetic field is synchronized with the periodic pulsed laser excitation initiating the operations. The periodic excitation protocol introduces a frequency comb through which detrimental effects of the inhomogeneities in the spin system can be avoided.

Method of composite nanomaterials synthesis under metal/oxide pulse-periodic laser treatment

A method of composite nanomaterials synthesis under metal/oxide pulse-periodic laser treatment has been developed on the example of a metal-semiconductor nanocomposite ZnO/Cu. A new approach to the creation of structures of layered composite nanomaterials based on zinc oxide nanofibers is implemented. For the first time for such materials synthesis, heating laser controlled distribution of power density has been used. Air heating of foil brass by pulse-periodic laser treatment intensifies the oxidation of the surface material. Preferable formation of ZnO is caused by a higher oxidation speed of zinc than copper and the zinc diffusion to the surface, as well. The length of zinc oxide nanofibers in the implementation of the selected modes of the laser treatment is less than 5 micrometers.

Diffractive lenses for high-power terahertz radiation beams

Techniques for manufacturing silicon binary (two-level) diffractive lenses and polypropylene kinoform diffractive lenses for the terahertz spectrum range are described. The elements are 1 and 0.8 mm thick, respectively. The silicon lens is manufactured in two versions: with no coating and with a parylene C (polyparaxylylene) antireflection coating. Characteristics of the diffractive optical elements are studied in the beam of a pulse-periodic free electron laser at a radiation wavelength of 141 μm and a repetition rate of 5.6MHz. The radiation resistance of the parylene coating, tested on the Novosibirsk free electron laser, was not impaired when the coating was exposed to an average power density of 4 kW cm−2, the peak power in a 100-picosecond pulse being almost 8 MW cm−2.

Effects of laser-induced quenching and restoration of photoluminescence in hybrid Si/SiOx nanoparticles

We studied the processes of pulsed and continuous-wave (CW) laser excitation of photoluminescence (PL) in nc-Si/SiOx nanoparticles. CW laser irradiation of the nc-Si/SiOx sol in dimethylsulfoxide (DMSO) and in a thin film was found to quench PL with an essentially nonexponential dynamics. The laser-induced variation of the Rayleigh scattering signal from the nc-Si/SiOx sol in DMSO was found to replicate that of the PL quenching. The IR and Raman spectra of the laser-exposed nc-Si/SiOx exhibited no new bands that could be evidence of their chemical transformations. There were also no perceptible effects pointing to laser-induced aggregation of the nc-Si/SiOx particles in the sol. 355 nm pulse-periodic laser irradiation of the nc-Si/SiOx particles preliminarily exposed to a 405 nm CW laser (which causes a strong PL quenching effect) was found to result in a rapid restoration of the original PL signal, which means that the effect of laser quenching of PL can be entirely reversible. We considered a mechanism of the reversible photosensitivity of nc-Si/SiOx, based on the processes of ionization of the photoluminescent oxygen-deficient centers in their suboxide shell and electron capture by traps, followed by their laser-assisted photorecombination.

Thermomechanical effect of pulse-periodic laser radiation on cartilaginous and eye tissues

This paper is devoted to theoretical and experimental studies into the thermomechanical action of laser radiation on biological tissues. The thermal stresses and strains developing in biological tissues under the effect of pulse-periodic laser radiation are theoretically modeled for a wide range of laser pulse durations. The models constructed allow one to calculate the magnitude of pressures developing in cartilaginous and eye tissues exposed to laser radiation and predict the evolution of cavitation phenomena occurring therein. The calculation results agree well with experimental data on the growth of pressure and deformations, as well as the dynamics of formation of gas bubbles, in the laser-affected tissues. Experiments on the effect of laser radiation on the trabecular region of the eye in minipigs demonstrated that there existed optimal laser irradiation regimens causing a substantial increase in the hydraulic permeability of the radiation-exposed tissue, which can be used to develop a novel glaucoma treatment method.

Single-Photon Technique for the Detection of Periodic Extraterrestrial Laser Pulses

To draw humankind's attention to its existence, an extraterrestrial civilization could well direct periodic laser pulses toward Earth. We developed a technique capable of detecting a quasi-periodic light signal with an average of less than one photon per pulse within a measurement time of a few tens of milliseconds in the presence of the radiation emitted by an exoplanet's host star. Each of the electronic events produced by one or more single-photon avalanche detectors is tagged with precise time-of-arrival information and stored. From this we compute a histogram displaying the frequency of event-time differences in classes with bin widths on the order of a nanosecond. The existence of periodic laser pulses manifests itself in histogram peaks regularly spaced at multiples of the-a priori unknown-pulse repetition frequency. With laser sources simulating both the pulse source and the background radiation, we tested a detection system in the laboratory at a wavelength of 850 nm. We present histograms obtained from various recorded data sequences with the number of photons per pulse, the background photons per pulse period, and the recording time as main parameters. We then simulated a periodic signal hypothetically generated on a planet orbiting a G2V-type star (distance to Earth 500 light-years) and show that the technique is capable of detecting the signal even if the received pulses carry as little as one photon on average on top of the star's background light.

A scanning pulse-periodic laser based on heterostructures containing ZnSe with electron beam pumping

Characteristics of a scanning pulsed green laser based on heterostructures containing ZnSe pumped by an electron beam with 200-ns duration and 8-keV energy are studied. If the active element is at room temperature, the scanning frequency is 20 Hz, and the repetition rate is 3 kHz, the average power of laser radiation is 1.6 mW, provided that the pulsed power is 2.7 W.

Thermocycling with pulse-periodic laser action for formation of nanoporous structure in metal material

Conditions of pulse-periodic laser action with thermocycling for formation of a nanoporous structure consisting of principally channel-type nanosize pores in two-component Cu-Zn alloy “brass of 62%” are experimentally defined. The transformation of laser radiation into a light spot with uniform distribution of power density was carried out with the aid of an optical system containing an element of diffractive computer optics – radiation focusator. Depth and area locality of the physical processes in heat-affected zone with preservation of initial material properties and absence of significant deformation in the remaining volume is ensured in the implementation of the chosen conditions. The amount of structural defects – vacancies, dislocations and their clusters increases in two-component brass under the influence of thermal stress. Wedge-shaped submicrocavities that converge at their edges with formation of extensive nanosize channels with widths not more than 100 nm are formed in the material of 0.05 mm thickness.

SUPER LONG CONDUCTIVE CHANNEL PRODUCTION BY IMPULSAR

The attempts to create a super long conductive channels were taken in order to study the upper atmosphere and to settle special tasks, related to solar energy transmission, accumulated by orbital solar stations. There upon the program of creation of ”Impulsar” represents a great interest, as this program in a combination with high-voltage high repetition rate electrical source can be useful to solve the above mentioned problems. The principle of conductive channel production can be shortly described as follows. The ”Impulsar”- laser jet engine vehicle - propulsion take place under the influence of powerful high repetition rate pulse-periodic laser radiation. In the experiments the CO2- laser and solid state Nd: YAG laser systems had been used. Active impulse appears thanks to air breakdown (＜ 30 km) or to the breakdown of ablated material on the board (＞30km), placed in the vicinity of the focusing mirror-acceptor of the breakdown waves. With each pulse of powerful laser the device rises up, leaving a bright and dense trace of products with high degree of ionization and metallization by conductive nano-particles due to ablation. Conductive dust plasma properties investigation in our experiments was produced by two very effective approaches: high power laser controlled ablation and by explosion of wire. Experimental and theoretical results of conductive canal modeling will be presented. The estimations show that with already experimentally demonstrated figures of specific thrust impulse the lower layers of the Ionosphere can be reached in several ten seconds that is enough to keep the high level of channel conductivity and stability with the help of high repetition rate high voltage generator. Some possible applications for new technology are highlighted.

Plasma formation by high-repetition rate pulsed periodic laser irradiation of metals in air at atmospheric and reduced pressures

High-speed spectroscopy with spatial and temporal resolution is used to determine the basic plasma parameters and to examine the shift from an air plasma to an erosion plasma during multipulse laser irradiation of various metals at repetition rates ranging from 5 to 50 kHz with irradiances q > 2∙108 W/cm2. The formation of a distinctive surface plasma structure is observed which (a) facilitates the expulsion of the shielding layer of "air" plasma produced during the first of a series of laser pulses by vapor from the target and (b) inhibits the propagation of a metal vapor plasma front into the air during the subsequent pulses.

Onion-like carbon nanoparticles generated by multiple laser irradiations on laser-ablated particles

A one-step process to synthesize onion-like carbon nanoparticles at room temperature under atmospheric pressure was developed. Periodic pulsed laser irradiation of carbon nanoparticles confined in the cavity of a carbon target rod induces drastic changes to their structure, changing them from amorphous to ordered concentric graphitic shells. The size distribution of gas-borne nanoparticles measured by a scanning mobility particle sizer showed the mean mobility diameter decreased from 83 nm to 18 nm as a result of restructuring of carbon agglomerates. Polyhedral and unagglomerated nanoparticles with shell structures composed of multiple graphene sheets were observed by a transmission electron microscopy.

Stabilization of the waveguide CO2 laser line for a laser photoacoustic SF6-leak detector

A compact waveguide pulse-periodic HF-excited CO2 laser with automatic tuning and wavelength stabilization at the 10P(16) generation line was designed in order to increase the accuracy of SF6 concentration measurements in air with a photoacoustic (PA) detector. The laser uses a nonselective cavity. The cavity length is tuned by a PZT actuator. A sealed-off SF6-filled PA cell is used as the reference to identify the desired laser line. The algorithm for selection and stabilization of the laser generation line is developed. The influence of different factors on the laser operation parameters is determined. The error in SF6 concentration measurements in air is less than 3% when using the designed laser. The ability of using this algorithm to stabilize other CO2 laser lines is discussed, in particular, those necessary for detecting ethylene and acetone vapors.

Highly efficient pulse-periodic XeCl lasers

The parameters of electric-discharge pulse-periodic XeCl lasers with a pulse duration of 25 - 40 ns, an energy of 0.2 - 0.7 J, and a pulse repetition rate up to 100 Hz have been investigated. It is shown that the total laser efficiency of 2.6 % and the maximum efficiency with respect to the stored energy of 3.8 % are obtained at a specific pump power of 2.8 - 3.3 MW cm-3 and a discharge circuit inductance of 3.5 - 4 nH.

Spatiotemporal spectroscopic diagnostics of a pulse-periodic strontium vapor laser

Results of spectroscopic investigations into plasma of a pulse-periodic strontium vapor laser operating in the superradiance mode on the infrared transition at λ = 6.45 μm are presented. The method of determining the electron temperature and concentration as well as the gas temperature – T e , n e , and T g – based on measuring the absolute intensities of some SrI and SrII and buffer gas (helium or neon) spectral lines is used. Time dependences of the line intensities during a current pulse (τ = 150 ns) and near afterglow (up to 3 μs) are obtained under conditions of non-equilibrium plasma ionization and recombination. The optical system collects radiation from the entire length of the plasma column by means of separating radial volume zones, includingthe central zone and the zone closer to the walls, with the monochromator slit. The results obtained allow us not only to calculate T e , n e , and T g values, but also to trace the spatiotemporal plasma evolution.