Amplification of multi-gigawatt 3 ps pulses in an atmospheric CO2 laser using ac Stark effect

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.

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.

Broadband (8.5-13.5 µm) intra-pulse difference frequency generation in a LiGaS2 crystal of a 90 fs 744 nm laser pulse after its continuous redshift in air

Broadband frequency downconversion of a 90 fs 744 nm Ti:sapphire laser pulse into the mid-infrared (IR) was demonstrated via its filamentation-induced self-frequency shift in air and subsequent intra-pulse difference frequency generation in a LiGaS₂ crystal. The filamentation of the laser pulse in air provided its continuous spectral broadening to the Stokes wing with spectral humps separated by ∼1000cm^-1 that was appropriate for the laser pulse difference frequency conversion into the mid-IR. The difference frequency emission spectrum spanned from 8.5 to 13.5 µm at the e^-2 level. The transform limited pulse duration of the mid-IR pulse was 47 fs, which corresponded to a 1.3-cycle laser pulse. Energy conversion efficiency was up to 10^-4 and 5⋅10^-4 without and with chirp compensation, respectively.

Hybrid CO2-Ti:sapphire laser with tunable pulse duration for mid-infrared-pump terahertz-probe spectroscopy

We describe a mid-infrared pump - terahertz-probe setup based on a CO₂ laser seeded with 10.6 μm wavelength pulses from an optical parametric amplifier, itself pumped by a Ti:Al₂O₃ laser. The output of the seeded CO₂ laser produces high power pulses of nanosecond duration, which are synchronized to the femtosecond laser. These pulses can be tuned in pulse duration by slicing their front and back edges with semiconductor-plasma mirrors irradiated by replicas of the femtosecond seed laser pulses. Variable pulse lengths from 5 ps to 1.3 ns are achieved, and used in mid-infrared pump, terahertz-probe experiments with probe pulses generated and electro-optically sampled by the femtosecond laser.

Modeling of a picosecond CO2 amplifier with electrical and optical pumping

As a long-wavelength laser with strong energy storage capacity and large scale of amplification, the CO₂ laser is considered an effective amplifier in the next-generation picosecond terawatt infrared laser system, but the pulse splitting effect caused by its discrete gain spectrum limits its behavior. In this paper, we have developed a specific model of a CO₂ amplifier, which is optically and electrically pumped at the same time. The model is based on gas discharge that is combined with photon absorption, temperature, and wave equation. The proposed hybrid pumped CO₂ amplifier scheme can increase the gain proportion of the sequence band transition (00⁰2-10⁰1, 00⁰3-10⁰2, etc.) from 12% to nearly 50% of the regular band and broaden the bandwidth of each line by over 15.8% by the overlap of the sequence band and regular band. The relative energy concentration of the first subpulse can be increased by up to 190% when the amplification factor reaches 10³. The study on the model of a picosecond CO₂ amplifier with electrical and optical pumping may contribute to the amplification of an ultrafast mid-infrared pulse to the terawatt or higher region.

Measurements of nonlinear absorption of intense 10 µm laser pulses in n-Ge, GaAs, and ZnSe

We study the nonlinear absorption of high-power 10 µm radiation propagating through n-Ge, GaAs, and ZnSe materials widely used in the long-wave infrared range. Measurements are performed using both nanosecond and picosecond CO₂ laser pulses providing intensities up to 100MW/cm² and 5GW/cm², respectively. Nonlinear absorption coefficients in optical quality n-Ge are found to be the same for both pulse durations. The nonlinear absorption coefficient of these materials scales inverse proportionally to their bandgap energies, indicating increased photoionization in the smaller bandgap materials.

Control of the nonlinear response of bulk GaAs induced by long-wavelength infrared pulses

The nonlinear optical response of GaAs is studied using extremely nonresonant 10 μm laser pulses with peak intensities greater than 2 GW/cm ². We observe over an order of magnitude enhancement in the four-wave mixing efficiency by decreasing the CO ₂ laser beat-wave frequency. This enhancement is attributed to currents of photoexcited unbound carriers modulated at the beat frequency, confirmed by measurements of nonlinear absorption at this long wavelength as well as a fully microscopic analysis of the excitation dynamics. Modeling of such nonperturbative semiconductor-laser interactions predicts that further decreasing the beat frequency can increase the nonlinear response and allow for its control over two orders of magnitude.

Ultrashort infrared 2.5-11 μm pulses: spatiotemporal profiles and absolute nonlinear response of air constituents

We measure the detailed spatiotemporal profiles of femtosecond laser pulses in the infrared wavelength range of λ=2.5-11  μm and the absolute nonlinear response of major air constituents (N₂, O₂, and Ar) over this range. The spatiotemporal measurements reveal wavelength-dependent pulse front tilt and temporal stretching in the infrared pulses.

Measurements of the nonlinear refractive index of air, N2, and O2 at 10 μm using four-wave mixing

We report on measurements of the nonlinear index of refraction of air, N₂, and O₂ at a wavelength close to 10 μm by collinear four-wave mixing of a 200 MW CO₂ laser beat-wave. The use of a 200 ps long beat-wave comprising radiation amplified on the 10P20 and 10R16 lines of the CO₂ laser provides a sensitive method to measure the small nonlinearities characteristic of the gas phase in a spectral region where no such data exists.

High-power, mid-infrared, picosecond pulses generated by compression of a CO₂ laser beat-wave in GaAs

We report on the generation of a train of ∼2  ps, 10 μm laser pulses via multiple four-wave mixing and compression of an infrared laser beat-wave propagating in the negative group velocity dispersion region of bulk GaAs and a combination of GaAs and NaCl crystals. The use of a 200 ps, 106 GHz beat-wave, produced by combining laser pulses amplified on the 10P(20) and 10P(16) transition of a CO₂ laser, provides a novel method for generating high-power, picosecond, mid-IR laser pulses at a high repetition rate. By using 165 and 882 GHz beat-waves, we show that cascaded phase-mismatched difference frequency generation plays a significant role in the four-wave mixing process in GaAs.

Supercontinuum generation from 2 to 20 μm in GaAs pumped by picosecond CO₂ laser pulses

We report on the generation of supercontinuum radiation from 2 to 20 μm in a 67 mm long GaAs crystal pumped by a train of 3 ps CO2 laser pulses. Temporal measurements indicate that sub-picosecond pulse splitting is involved in the production of such wide-bandwidth radiation in GaAs. The results show that the observed spectral broadening is heavily influenced by four-wave mixing and stimulated Raman scattering.