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

Hybrid air-bulk multi-pass cell compressor for high pulse energies with full spatio-temporal characterization

Multi-pass cell (MPC) compressors have proven to be the method of choice for compression of high average power long-pulse Yb lasers. Yet, generating sub-30 fs pulses at high pulse energy with compact and simple components remains a challenge. This work demonstrates an efficient and cost-effective approach for nonlinear pulse compression at high pulse energy using a hybrid air-bulk MPC. By carefully balancing the relative nonlinear contributions of ambient air and fused silica, we achieve strong spectral broadening without dispersion engineering or pressure-control inside the cell at 400-µJ pulse energy. In this way, we compress pulses from 220 fs to 27 fs at 40.3 W of average power (100 kHz repetition rate), enhancing the peak power from 1.6 GW to 10.2 GW while maintaining 78% of the energy within the main pulse. Our approach combines the strengths of gas-filled and bulk compression schemes and exhibits excellent overall optical transmission (91%) and spectral uniformity. Moreover, we utilize the INSIGHT technique to investigate spatio-temporal couplings and geometrical aberrations of the compressed pulse. Our results demonstrate remarkable temporal homogeneity, with an average Strehl ratio of 0.97 consistently observed throughout the entire spectral profile. Additionally, all spectrally-integrated Zernike coefficients for geometrical aberrations maintain values below 0.02λ.

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.

Atmospheric effects on the laser-driven avalanche-based remote detection of radiation

The effect of realistic atmospheric conditions on mid-IR (λ = 3.9 µm) and long-wave-IR (λ = 10 µm) laser-induced avalanche breakdown for the remote detection of radioactive material is examined experimentally and with propagation simulations. Our short-range in-lab mid-IR laser experiments show a correlation between increasing turbulence level and a reduced number of breakdown sites associated with a reduction in the portion of the focal volume above the breakdown threshold. Simulations of propagation through turbulence are in excellent agreement with these measurements and provide code validation. We then simulate propagation through realistic atmospheric turbulence over a long range (0.1-1 km) in the long-wave-IR regime (λ = 10 µm). The avalanche threshold focal volume is found to be robust even in the presence of strong turbulence, only dropping by ∼50% over a propagation length of ∼0.6 km. We also experimentally assess the impact of aerosols on avalanche-based detection, finding that, while background counts increase, a useful signal is extractable even at aerosol concentrations 10⁵ times greater than what is typically observed in atmospheric conditions. Our results show promise for the long-range detection of radioactive sources under realistic atmospheric conditions.

Ionization rate and plasma dynamics at 3.9 micron femtosecond photoionization of air

The introduction of mid-IR optical parametric chirped pulse amplifiers has catalyzed interest in multimillijoule, infrared femtosecond pulse-based filamentation. As tunneling ionization is a fundamental first stage in these high-intensity laser-matter interactions, characterizing the process is critical to understand derivative topical studies on femtosecond filamentation and self-focusing. Here, we report direct nonintrusive measurements of total electron count and electron number densities generated at 3.9 μm femtosecond midinfrared tunneling ionization of atmospheric air using constructive-elastic microwave scattering. Subsequently, we determine photoionization rates to be in the range 5.0×10^{8}-6.1×10^{9}s^{-1} for radiation intensities of 1.3×10^{13}-1.9×10^{14}W/cm^{2}, respectively. The proposed approach paves the wave to precisely tabulate photoionization rates in mid-IR for a broad range of intensities and gas types and to study plasma dynamics at mid-IR filamentation.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 10¹³ cm^-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

Nonlinear localization of high energy long wave laser pulses in fully correlated 3D turbulence

We study the interplay between three-dimensional (3D) fully correlated optical turbulence and nonlinearity in time and 3D space resolved long-wavelength infrared pulsed beam propagation. Here the average self-trapped beam waist exceeds the inner scale in contrast to near-infrared filaments, and we find that their nonlinear self-channeling remains robust even in the presence of strong turbulence. More surprisingly, our simulation results invite a conjecture that in regimes where diffraction and nonlinearity are roughly balanced, turbulence can result in a tighter localization of the nonlinear beam core.

Reflection and absorption of electromagnetic radiation by inhomogeneous photoionized plasma, produced by multiphoton ionization of inert gas atoms

The interaction of electromagnetic radiation with inhomogeneous plasma formed by multiphoton ionization of inert gas atoms has been studied. In high-frequency and normal skin effects modes the field structure in plasma is described and reflection and absorption coefficients are found. It is shown that as the thickness of the plasma region, in which the photoelectron density grows linearly, increases, both the depth of field penetration and the absorption coefficient increase, too. It is found that, due to the Ramzauer-Townsend effect, there is a relative increase in the effective frequency of photoelectron collisions with atoms, which is accompanied by a significant increase in the absorption coefficient.

Powerful terahertz waves from long-wavelength infrared laser filaments

Strong terahertz (THz) electric and magnetic transients open up new horizons in science and applications. We review the most promising way of achieving sub-cycle THz pulses with extreme field strengths. During the nonlinear propagation of two-color mid-infrared and far-infrared ultrashort laser pulses, long, and thick plasma strings are produced, where strong photocurrents result in intense THz transients. The corresponding THz electric and magnetic field strengths can potentially reach the gigavolt per centimeter and kilotesla levels, respectively. The intensities of these THz fields enable extreme nonlinear optics and relativistic physics. We offer a comprehensive review, starting from the microscopic physical processes of light-matter interactions with mid-infrared and far-infrared ultrashort laser pulses, the theoretical and numerical advances in the nonlinear propagation of these laser fields, and the most important experimental demonstrations to date.

Self-Guiding of Long-Wave Infrared Laser Pulses Mediated by Avalanche Ionization

Nonlinear self-guided propagation of intense long-wave infrared (LWIR) laser pulses is of significant recent interest, as it promises high power transmission without beam breakup and multifilamentation. Central to self-guiding is the mechanism for the arrest of self-focusing collapse. Here, we show that discrete avalanche sites centered on submicron aerosols can arrest self-focusing, providing a new mechanism for self-guided propagation of moderate intensity LWIR pulses in outdoor environments. Our conclusions are supported by simulations of LWIR pulse propagation using an effective index approach that incorporates the time-resolved plasma dynamics of discrete avalanche breakdown sites.