We report on the clear difference in the behavior of laser-matter interaction of tightly focused high-intensity pulses with the bulk of solids when chirping and tuning the wavelength of the laser driver from near-IR to mid-IR. In transparent dielectrics, laser wavelength scaling leads to the change in the heating mechanism which, in turn, leads to a weak dependence of the plasma formation threshold on the pulse duration in mid-IR (4.6 μm) and a significant impact in the case of the near-IR (1.24 μm) excitation. In the case of semiconductors, the ionization mechanism remains within the framework of the multiphoton approximation, but an increase in the photon order for excitation by mid-IR pulses leads to a decrease in delocalization processes and losses in the prefocal region.
We demonstrate a novel approach of femtosecond laser pulse spectral-temporal control under filamentation in a wide range of pressures from 5 bar up to supercritical state. We showed that laser-induced processes such as supercontinuum generation and pulse self-compression could be tuned both by pressure and by energy adjustment due to the nonlinear refractive index alteration via pressure change. Thanks to the high pressure-controlled nonlinearity, a broadband (from 250 to 2500nm) supercontinuum with total efficiency more than 50% of energy can be generated in supercritical Xe and CO2. It was also demonstrated that the femtosecond laser pulse can be compressed in Xe in the femtosecond monofilamentation mode by 3.5 times with efficiency as high as 60%.
The deposited energy density (DED) serves as a key parameter in the process of the femtosecond laser pulse energy delivery into the bulk of transparent dielectrics. The laser-induced micromodification can be created if the value of DED exceeds a certain threshold, which is specific for each material and does not depend on the laser wavelength. In this contribution, we present a comprehensive study of the DED evolution with the driving pulse energy and wavelength under femtosecond microstructuring of transparent dielectrics. To precisely determine the laser impact area we applied for the first time a real-time diagnostic of microplasma based on third harmonic generation. This technique gives submicron spatial resolution and is extremely sensitive to the free electron density (about 10-5 of the critical electron density). We found out that the threshold DED equals to approximately 2.5 kJ/cm3 for fused silica and roughly corresponds to excess of glass transition temperature. The highest DED is achieved for the shortest wavelength (620 nm) and equals to 16 kJ/cm3.
We report the first-of-its-kind compact and robust coherent source operating in mid-IR based on Fe:ZnSe chalcogenide gain medium optically pumped by Er:ZBLAN fiber laser. In the research, we study the CW operation of cryogenically cooled laser based on Fe:ZnSe single crystals with different doping level grown from the vapor phase on a single-crystal seed by using the concurrent-doping technology. The maximal output power achieved is 2.1 W with 59% slope efficiency with respect to absorbed pump power, which is close to the Stokes shift limit. Measured Fe:ZnSe output spectra indicate a significant influence of re-absorption on generation wavelength. For high doping levels and output powers, spectrum shifts to the red wing, which makes possible continuous tuning from 4012 to 4198 nm. As well, tunability of the laser in a wide range of temperature is investigated.
We demonstrate a first-of-its-kind efficient chirped pulse amplification of broadband mid-IR (4-5 μm) femtosecond seed pulse (230 ps, 4μJ) generated in AgGaS2 based OPA driven by Cr:forsterite laser in multi-pass Fe2+:ZnSe amplifier optically pumped by solid-state Q-switched Cr:Yb:Ho:YSGG laser (2.85 μm, 30mJ, 5Hz, 0.6 J/cm2). The system delivers 1.2 mJ at pulse duration of 230 ps. Straightforward compression to 150 fs pulse is achievable with 70% efficiency using diffraction grating pair with peak power of about 6 GW. Further non-linear compression in a bulk CaF2 due to the SPM and anomalous GVD should provide the enhancement of peak power up to 20 GW. Possible routes to reach sub-TW and even TW power level in mid-IR are discussed.
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