Frequency upshifting a CO2 laser pulse by collision with a relativistic ionization front

Research into tunable lasers is a well-developed field that has a broad range of applications including coherent IR spectroscopy, high-harmonic generation, and single cycle and attosecond pulse generation. We show simulation and experimental results of a novel method of frequency upshifting a 9.2 μm, two picosecond, CO₂ laser pulse of intensity I_o = 140 TW/cm² by colliding it with a relativistic ionization front. A high-intensity Ti:Sapphire laser driver propagates through a hydrogen gas jet and is used to create a step-like ionization front that propagates close to the speed of light. As observed by the CO₂ pulse, the driver induces a temporal change to the index of refraction, which causes frequency upshift and pulse duration compression of the CO₂ laser pulse as it collides with the ionization front. Particle-in-Cell (PIC) simulations show that the wavelength of the upshifted pulse can be tuned in a broad range with high efficiency by changing the plasma density. For experimental demonstration of this effect, we are presently conducting an experiment at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory where we expect to continuously upshift the frequency of the incident CO₂ laser in the wavelength range from 9.2 μm to 4.6 μm. We have designed a single-shot spectrometer for the transmitted radiation that is capable of characterizing the energy, frequency, and bandwidth of the upshifted pulse. This experimental work is performed in collaboration with Stony Brook University and the ATF at Brookhaven National Laboratory.