Picosecond CO2 Laser Research Articles

Self-channeling of a multi-Joule 10 µm picosecond pulse train through long distances in air.

In the long-wave infrared (LWIR) range, where, due to wavelength scaling, the critical power of Kerr self-focusing Pcr in air increases to 300-400 GW, we demonstrate that without external focusing a train of picosecond CO2 laser pulses can propagate in the form of a single several-centimeter diameter channel over hundreds of meters. The train of 10 µm pulses, for which the total energy ≥20 J is distributed over several near-terawatt picosecond pulses with a maximum power ≤2Pcr, is generated naturally during short pulse amplification in a CO2 laser. It is observed that the high-power 10 µm beam forms a large diameter "hot gas" channel in the ambient air with a ≥ 50 ms lifetime. Simulations of the experiment show that such filamentation-free self-channeling regime has low propagation losses and can deliver multi-Joule/TW-power LWIR pulses over km-scale distances.

Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO2 laser

Understanding the formation and long-timescale evolution of atmospheric plasmas produced by ultrashort, long-wavelength infrared (LWIR) pulses is an important but partially understood problem. Of particular interest are plasmas produced in air with a peak laser intensity ∼1012 W/cm2, the so-called clamping intensity observed in LWIR atmospheric guiding experiments where tunneling and multi-photon ionization operative at near-IR or shorter wavelengths are inoperative. We find that avalanche breakdown on the surface of aerosol (dust) particles can act to seed the breakdown of air observed above the 200 GW/cm2 threshold when a train of 3 ps 10.6 μm laser pulses separated by 18 ps is used. The breakdown first appears at the best focus but propagates backward toward the focusing optic as the plasma density approaches critical density and makes forward propagation impossible. The velocity of the backward propagating breakdown can be as high as 109 cm/s, an order of magnitude greater than measured with ns pulse-produced breakdown, and can be explained rather well by the so-called breakdown wave mechanism. Transverse plasma expansion with a similar velocity is assisted by UV photoionization and is observed as a secondary longitudinal breakdown mechanism in roughly 10% of the shots. When a cm-size, TW power beam is propagated, interception of aerosol particles is guaranteed and several (40 cm−3) breakdown sites appear, each initially producing a near-critical density plasma. On a 10 ns–1 μs timescale, shockwaves from each site expand radially and coalesce to produce a large hot gas channel. The radial velocity of the expansion agrees well with the prediction of the blast wave theory developed for ultrafast atmospheric detonations.

Efficient second harmonic generation of a high-power picosecond CO2 laser.

A comparative analysis of AgGaSe2, GaSe, CdGeAs2, and Te for second harmonic generation (SHG) of a picosecond CO2 laser at intensities up to 50 GW/cm2 is presented. We demonstrate external energy conversion efficiency of >20% in AgGaSe2. Conversion efficiency >5% is measured in GaSe and CdGeAs2. Self-focusing and multifilamentation are found to severely limit the SHG process in CdGeAs2 and Te at such high fields. Demonstration of ≥150 MW SH pulses for a 10 μm picosecond pump, in combination with femtosecond CO2 laser development, will open new strong-field applications in the 4.5-5.5 μm range.

Ultra-Fast Laser Fabrication of Alumina Micro-Sample Array and High-Throughput Characterization of Microstructure and Hardness

In the light of recent advances in material informatics, there is a great demand for high-throughput approaches of sample fabrication and property characterization. Currently, no high-throughput approach has been demonstrated for the fast sampling of the microstructure and the correlated properties. In this paper, we demonstrate the ultra-fast fabrication of an alumina sample array and the high-throughput hardness characterization of these sample units. The alumina sample array was fabricated using picosecond (PS) laser micromachining and CO2 laser sintering within a short time (i.e., less than a few minutes). After laser sintering, the hardness of these sample units was characterized using micro-indentation, and the microstructure was observed using scanning electron microscopy (SEM). In each sample unit, the microstructure was uniform for the entire top surface and within about 20 µm depth from the top surface. The relative density (RD) and corresponding micro-hardness of the sample units was found to continuously vary over a wide range from 89% RD with 600 kgf/mm2 hardness to 99% RD with 1609 kgf/mm2 hardness. For these laser-sintered samples, the correlation of hardness and relative density of the alumina matched well with the literature reports on sintered alumina obtained using conventional low-throughput furnace sintering experiments.

Single-shot measurement of the nonlinear refractive index of air at 9.2 µm with a picosecond terawatt CO2 laser.

We developed a simple, accurate single-shot method to determine the nonlinear refractive index of air by measuring the evolution of the spatial shape of a laser beam propagating through the atmosphere. A distinctive feature of this new method, which relies on a modified Fresnel propagation model for data analysis, is the use of a hard aperture for producing a well-defined, high-quality beam from a comparatively non-uniform quasi-flat-top beam, which is typical for high-peak-power lasers. The nonlinear refractive index of air for a very short (2 ps) long-wave infrared (LWIR) laser pulse was measured for the first time, to the best of our knowledge, yielding n2=3.0×10-23m2/W at 9.2 µm. This result is 40% lower than a corresponding measurement with longer (200 ps) LWIR pulses at a similar wavelength.

Megafilament in air formed by self-guided terawatt long-wavelength infrared laser

The diffraction-compensated propagation of high-power laser beams in air could open up new opportunities for atmospheric applications such as remote stand-off detection, long-range projection of high-energy laser pulses and free-space communications. Here, we experimentally demonstrate that a self-guided terawatt picosecond CO2 laser beam forms in air a single centimetre-scale-diameter megafilament that, in comparison with a short-wavelength laser filament, has four orders of magnitude larger cross-section and guides many joules of pulse energy over multiple Rayleigh distances at a clamped intensity of ~1012 W cm–2. We discover that this megafilament arises from the balance between self-focusing, diffraction and defocusing caused by free carriers generated via many-body Coulomb-induced ionization that effectively decrease the molecular polarizability during the long-wavelength laser pulse. Modelling reveals that this guiding scheme may enable transport of high-power picosecond infrared pulses over many kilometres in the 8–14 μm atmospheric transmission window. A terawatt picosecond CO2 laser beam is shown to form a centimetre-scale-diameter filament in air that is capable of carrying several joules of energy.

High duty cycle inverse Compton scattering X-ray source

Inverse Compton Scattering (ICS) is an emerging compact X-ray source technology, where the small source size and high spectral brightness are of interest for multitude of applications. However, to satisfy the practical flux requirements, a high-repetition-rate ICS system needs to be developed. To this end, this paper reports the experimental demonstration of a high peak brightness ICS source operating in a burst mode at 40 MHz. A pulse train interaction has been achieved by recirculating a picosecond CO2 laser pulse inside an active optical cavity synchronized to the electron beam. The pulse train ICS performance has been characterized at 5- and 15- pulses per train and compared to a single pulse operation under the same operating conditions. With the observed near-linear X-ray photon yield gain due to recirculation, as well as noticeably higher operational reliability, the burst-mode ICS offers a great potential for practical scalability towards high duty cycles.

Brookhaven National Laboratory's Accelerator Test Facility: research highlights and plans

The Accelerator Test Facility (ATF) at Brookhaven National Laboratory has served as a user facility for accelerator science for over a quarter of a century. In fulfilling this mission, the ATF offers the unique combination of a high-brightness 80 MeV electron beam that is synchronized to a 1 TW picosecond CO2 laser. We unveil herein our plan to considerably expand the ATF's floor space with an upgrade of the electron beam's energy to 300 MeV and the CO2 laser's peak power to 100 TW. This upgrade will propel the ATF even further to the forefront of research on advanced accelerators and radiation sources, supporting the most innovative ideas in this field. We discuss emerging opportunities for scientific breakthroughs, including the following: plasma wakefield acceleration studies in research directions already active at the ATF; laser wakefield acceleration (LWFA), where the longer laser wavelengths are expected to engender a proportional increase in the beam's charge while our linac will assure, for the first time, the opportunity to undertake detailed studies of seeding and staging of the LWFA; proton acceleration to the 100–200 MeV level, which is essential for medical applications; and others.

Laser-induced cavities and solitons in overcritical hydrogen plasma

A picosecond CO2 laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼1021 cm−3, corresponding to laser wavelength λ = 1 μm. CO2 laser offers a solution. The CO2 laser’s 10 μm wavelength shifts the critical plasma density to 1019 cm−3, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO2 laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.

A picosecond IR laser system with tunable wavelength, based on a hybrid CO_2 laser

This paper describes a picosecond laser system based on a hybrid CO2 laser, using optically controlled semiconductor switches with a pulse width from 100 ps to 300 ns, followed by conversion of the radiation in nonlinear crystals by means of second-harmonic generation. Silver selenogallate AgGaSe2 and zinc germanium diphosphide ZnGeP2 are used as nonlinear crystals. Two-stage conversion of the radiation of a picosecond CO2 laser with wavelength 9.4-10.6 μm into laser radiation with wavelengths 4.7-5.3 μm is carried out in the apparatus with an efficiency of 0.3%-0.5%, while 4.7-5.3-μm radiation is converted into 2.4-2.65-μm radiation with an efficiency of 8%. The overall conversion efficiency is 4×10−4 or 0.04% for an output pulse energy up to 35 μJ.

Registration of picosecond CO2 laser pulses by means of two-stage parametric transformation in nonlinear crystals

A technique for registering the temporal structure of picosecond pulses of CO2 laser radiation with an energy of 1.5–4.5 μJ at a wavelength of 10.27 μm using two-stage parametric transformation of IR radiation frequency into visible light under pumping of nonlinear crystals by Nd:YAG-laser radiation in a Q-switched mode is described. A GaSe nonlinear crystal was used at the first stage of transformation (10.27 μm + 1.064 μm → 0.964 μm). Radiation was further transformed (1.064 μm + 0.960 μm → 0.506 μm) by using the same pumping in an α-HIO3 nonlinear crystal. For the first time, no additional optical elements were present between the stages of the frequency transformer in the proposed optical scheme. The transformed radiation was registered with a Hamamatsu Temporal Disperser C1587 streak camera in a region of the photocathode maximum spectral sensitivity of ∼0.5 μm with a temporal resolution of up to 2 ps. The minimum recorded pulse duration of the CO2 laser was ∼45 ps.

Laser amplifier for picosecond CO2facilities of terawatt power level

In this paper the design features and the results of studying an amplifier for a picosecond terawatt CO2-laser facility are given. The active medium of the amplifier (a CO2–N2–He gas mixture) is excited by a self-sustained volume discharge. The amplifier with an aperture of 0.1 m×0.1 m and with an active medium volume of 10 l provides the possibility for amplifying the 10 μ m radiation of a picosecond laser up to the terawatt power level. A volume discharge at a pressure of 10 atm and at a molecular gas content of 5% with a power input level of 80 J l−1 atm−1 was initiated. The optical gain constant of a small signal of the amplifier active medium was 0.027 cm−1 at a pressure of 8 atm, which confirms the possibility of high-pressure amplifier use as a final amplifier in picosecond CO2-laser facilities with a power output level of 1–10 TW.

Nonlinear refraction and optical limiting in [quotation mark]thick[quotation mark] media

We experimentally and theoretically investigate optical beam propagation in nonlinear refractive materials having a thickness greater than the depth of focus of the input beam (i.e., internal self-action). A simple model based on the constant shape approximation is adequate for analyzing the propagation of laser beams within such media under most conditions. In a tight focus geometry, we find that the position of the sample with respect to the focal plane, z, is an important parameter in the fluence limiting characteristics of the output. The behavior with z allows us to perform a sample Z-scan from which we can determine the sign and magnitude of the nonlinear refraction index. In CS2, we have used this method to independently measure the negative thermally induced index change and the positive Kerr nonlinearity with nanosecond and picosecond CO2 laser pulses, respectively. We have experimentally examined the limiting characteristics of thick CS2 samples that qualitatively agree with our analysis for both positive and negative nonlinear refraction. This analysis is useful in optimizing the limiting behavior of devices based on self-action.

Dynamical temporal evolution of molecular IR absorption spectra observed with picosecond CO2 laser pulses

The variation of the collisionless absorption spectral shapes for vibrationally excited SF6 and C2F5Cl molecules as a function of the picosecond probe pulse duration was determined using a two-IR frequency pump–probe scheme. The experiments revealed a spectral narrowing with decreasing pulse duration, similar to a recent observation in C3F7I with a single laser pulse. For C2F5Cl, the vibrational temperature dependence of the spectral narrowing was also studied. The observed experimental results are shown to be consistent with a dynamic multitier classification of energy levels in the QC of polyatomic molecules. An empirical calculation was used to evaluate the relevant parameters associated with the intramolecular coupling. Specifically, the time constants for such a coupling are evaluated to be >20 ps for all the molecules studied. The effect of variable density of states on the time constant and spectral width is also discussed on the basis of the C2F5Cl data. It was found that both the redshift and the degree of spectral broadening increased with the internal energy of the molecule. The redshift was also observed to saturate at 〈n〉∼13. This dynamical spectral measurement is distinguished from the usual static spectral measurements in that the laser–molecule interaction time is used as a parameter, thus circumventing the usual pitfalls of inferring dynamical information from spectral measurements.

Observation of anomalous pulse duration dependent IR absorption spectrum in C3F7I

The collisionless infrared absorption spectrum of C3F7I was measured with picosecond and nanosecond CO2 laser pulses. The spectral width was found to decrease significantly as the measuring pulse duration was reduced. This data was interpreted with the multitier level scheme of the vibrational quasicontinuum. It was concluded that the measured absorption spectrum for any polyatomic molecule in a dense vibrational quasicontinuum should be dynamic in general and should depend on the interaction time. This is because the number of states interacting with the laser field indirectly via intramolecular coupling is time dependent. Therefore, the essentially static spectral measurement can yield dynamical information on the interaction of energy levels within the molecule.

High-intensity CO_2 laser interactions with indium antimonide

The interaction of picosecond CO2 laser pulses with intrinsic and doped indium antimonide at high intensities is investigated both experimentally and theoretically. A consistent theory is presented that explains the phenomena of high-density carrier generation, transient reflectivity, and melting in the semiconductor. All physical parameters become highly nonlinear at high laser intensities and carrier densities. Account was taken of the dynamic energy band-gap shift, changes in the multiphoton absorption cross section and the effective mass of the electron, dynamic reflectivity, and transmission. Keldysh nonlinear carrier generation is found to be essential to explain the experimental observations. The spark formation threshold of both intrinsic and doped InSb was measured to be 4.3 GW/cm2, corresponding to an energy fluence threshold of 0.26 J/cm2, at a laser pulse duration of 45 psec. The pulse duration scaling of the damage threshold is also discussed.

Two-photon and three-photon absorption coefficients of InSb

The two-photon absorption coefficient β of intrinsic InSb and the three-photon absorption coefficient γ of doped InSb were measured with high-intensity picosecond CO2 laser pulses. The picosecond duration permitted the determination of the multiphoton absorption coefficients without interference from carrier recombination and diffusion. Moreover, free-carrier absorption was made use of explicitly to enhance the absorption of the laser beam, permitting a more accurate measurement of these coefficients. The values of β and γ were found to be 2.5 cm/MW and 0.2 cm3/MW2, respectively. These values are compared with theoretical calculations in the literature.

Picosecond laser study of the quasicontinuum of C2F5Cl

The quasicontinuum of the vibrationally hot (∼10 CO2 laser photons) C2F5Cl molecule was investigated using a two-laser pulse arrangement. The first laser pulse preheated the molecules into the QC (quasicontinuum) while the second picosecond CO2 laser was employed to probe the absorption characteristics of the excited molecules. The second laser could be tuned either in frequency or in pulse duration. Several experiments were performed. In the first experiment, the small signal absorption spectrum of C2F5Cl was measured and compared with the dissociation spectrum obtained by Borsella et al.1 The same ν6+ν10 hot band was observed with a considerably narrower linewidth. In the second experiment, the saturation properties of the hot band were investigated. It was found that the absorption line could be characterized as inhomogeneously broadened with no evidence for any broadening due to intramolecular damping. In a third series of experiments, the absorption of C2F5Cl at low as well as high laser energy fluences was measured as a function of the laser pulse duration between 22 and 120 ps. It was found that (i) the absorption can be characterized as predominantly intensity dependent and (ii) the small signal absorption cross section increased considerably as the pulse duration was decreased. These observations were discussed using the multitier energy level classification scheme in the QC. It was concluded that within the laser pulse duration, individual states within the first tier which interacted with the laser were not coupled to each other. The change in the absorption cross section as a function of the pulse duration is consistent with the picture that under laser irradiation, a rapid equilibrium was achieved with the final state populations remaining relatively constant in time. The final populations were dependent almost entirely on the laser intensity. Increasing the laser pulse duration had little effect on the final state distribution and the total energy absorbed by the molecule.

Material Considerations in the Microfabrication of Grating Microstructures for use in a Laser-Powered Linear Accelerator

ABSTRACTCollaborative work between Brookhaven and Los Alamos National Laboratories is developing a new type of linear accelerator that uses a high-power, picosecond pulse CO2 laser to irradiate a specialized form of grating with a pitch of 10.6 microns. The electromagnetic field that results can be used to accelerate electrons at field gradients of several GeV/m with potential efficiencies much better than current accelerators. The grating must be conductive to minimize resistive losses, be able to withstand high fields without damage, and requires dimensional tolerances in the sub-micron range. These requirements focus attention on grating material selection, microfabrication methods, and metrological methods used for quality control. At present, several types of gratings have been manufactured by reactive ion etching of fused silica in CHF 3/Ar or etching silicon with KOH/H 2O or ethylenediamine-pyrocatechol solutions. Metrological analysis of the gratings has begun with a Tracor Northern 5700 digital image analyzer.

Characterization of a picosecond CO_2 laser system

The pulse duration of an optical free induction decay picosecond CO2 laser system was modeled and compared to autocorrelation measurements. Satisfactory agreement was obtained indicating that pulses as short as 20 psec were produced. The pulse duration can be continuously adjusted up to ∼200 psec by changing the CO2 gas pressure.