Generating tunable frequency upshifts in mid-infrared laser pulses using relativistic ionization fronts

Tunable lasers are a well-developed field of study with a broad range of applications, including coherent IR spectroscopy, high-harmonic generation, and single cycle and attosecond pulse generation. We present theoretical, computational, and experimental progress towards a novel method of frequency upshifting that allows us to continuously tune a CO₂ laser pulse from ω_o to 2ω_o - i.e. over a spectral region known as the molecular fingerprint region because many common molecules have vibrational-rotational transitions here. In our concept, a mid-infrared CO₂ laser pulse is sent into a gas or partially ionized plasma while a counter-propagating higher-intensity Ti:Sapphire laser drives a time-dependent change in refractive index by creating a step-like relativistic ionization front which passes through the CO₂ laser pulse. Simulations with the particle-in-cell (PIC) code OSIRIS show that the upshifted wavelength can be tuned in a broad range with high efficiency by changing the plasma density from a tenth to four times critical density of the CO₂ light in a stationary plasma. We are currently conducting an experiment at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory to demonstrate this new method. By using a 2-3 picosecond CO₂ laser pulse with intensities ranging from 140-250 TW/cm², we expect to measure single-shot tunable upshifts between 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 being performed in collaboration with Stony Brook University and ATF.