Femtosecond damage experiments and modeling of broadband mid-infrared dielectric diffraction gratings

Abstract

High peak and average power lasers with high wall-plug efficiency, like the Big Aperture Thulium (BAT) laser, have garnered tremendous attention in laser technology. To meet the requirements of the BAT laser, we have developed low-dispersion reflection multilayer dielectric (MLD) gratings suitable for compression of high-energy pulses for operations at 2 micron wavelength. We carried out 10000-on-1 damage tests to investigate the fluence damage thresholds of the designed MLD gratings and mirrors, which were found between 100-230 mJ/cm2. An ultrashort pulsed laser (FWHM = 53 fs, λ = 1.9 μm) operating at 500 Hz was used in the serpentine raster scans. The atomic force microscope images of the damage sites show blister formation of the underlying layers at lower fluences but ablation of the grating pillars at higher fluences. We simulated the dynamic electronic excitation in the MLD optics with a finite-difference in the time domain approach in 2D. The simulation results agree well with the LIDT measurements and the observed blister formation. This model is able to evaluate the absolute LIDT of MLD gratings.

Development of high peak and average power Tm:YLF lasers

“…The pulses were measured using a second-order autocorrelation, which revealed sub-300 fs FWHM pulse durations. Ongoing work will include implementing new in-house developed multilayer dielectric gratings [16] to improve the compressor throughput, enabling pulse compression at higher energies.…”

Nanoscale reshaping of resonant dielectric microstructures by light-driven explosions

“…Much higher laser intensities would be required for near-IR laser pulses to impart the same kinetic energy to the tunneled electrons. Third, because mid-IR photons with energies much smaller than the bandgap (ℏ𝜔 𝐿 ≪ 𝐸 𝑔 ) satisfy the Keldysh adiabatic tunneling condition [39][40][41] 𝛾 K < 1 (where 𝛾 K = 𝜔 𝐿 √ 𝑚 eff 𝐸 𝑔 /𝑞 𝑒 𝐸 hs is the Keldysh adiabaticity parameter and 𝐸 hs ∝ √𝐼 hs is the peak optical field at the hot spot), the volume of the high-density hot plasma produced by the laser pulse is much smaller than that of the optical energy itselfhence the cascading of the localization scales from hundreds of nanometers (the size of the photonic hot spot) to tens of nanometers (the size of the ablated region).…”

“…The above scaling offers further justification for using a longer-wavelength laser for FLANEM, while the numerical estimate is fully consistent with our experimentally measured ablation rate of 𝑣 abl ≈ 30 nm per pulse. A 3D simulation based on the EPOCH (Extendable PIC Open Collaboration) code 42 incorporated with the home-built refractive index and dynamic Keldysh photoionization modules [39][40][41] was performed to elucidate the nano-trenching process.…”

High average power ultrafast laser technologies for driving future advanced accelerators

“…Furthermore, LLNL is currently further scaling the gas-cooling technique for this material system that will enable high average power required for LPA applications, as well as improved understanding of material properties under use conditions. Working in collaboration with OSU, multilayer dielectric gratings (MLD) suitable for pulse compression of high energy, high average power lasers near at wavelength near 1.9 μm [49] were developed. These gratings were demonstrated to have femtosecond damage thresholds greater than 100 mJ/cm2 for ∼ 50 fs duration pulses.…”

“…The other factor is field intensification inside the structure due to standing wave modes inside the multi-layered structures, which are affected simultaneously by ionization due to the strong field of the laser. Until recently, such processes were never modeled dynamically [79,80], and recent modeling development shows that even though the materials consisting of individual layers may have higher LIDT, the intensification inside gratings allow significant free carrier generation, thereby significantly lowering the LIDT of these grating systems [80]. A sustained modeling and optimization coupled with grating fabrication and LIDT testing for at least a five year period is necessary to explore how far this di-electric approach can be pushed for PW class laser operations.…”