Ultra fast nonlinear optical technology for 40 high frequency high energy THz light source generation

Laser-based intense field terahertz (THz) pulses are a very useful tool for direct excitation of specific low frequency modes to better understand the structure and electron dynamics of various materials, such as terahertz angular resolution light emission spectroscopy and THz nanoimaging. These applications require high average power THz pulse light sources with repetition rates above 100kHz. In 2020, scientists from Accelerator National Laboratory in the United States [1] and Jena, Germany [2] demonstrated two technological routes based on optical rectification and two-color field laser targeting of gas targets, respectively.

FIG. 1 Experimental device [1]

The main work of the Accelerator National Laboratory is to generate high refrequency and high power THz pulses by optical rectification of near kilowatt laser sources based on multi-cavity compression. The experimental device is shown in Figure 1. The output pulse of the oscillator is amplified to 6W by a fiber amplifier, and then amplified to 1.39kW by Yb: YAG slats. The light beam is reduced and incident into the multipass cavity for spectrum broadening. 99% of the compression pulse is used as the pump pulse for subsequent optical rectification, and 1% is used as the probe pulse to measure the THz pulse generated by electro-optical sampling. The pump pulse goes through the grating to realize the wave front tilt, and then through two cylindrical lenses with focal lengths of 151mm and 80mm respectively to adjust to the correct Angle and incident into the lithium niobate crystal. The resulting THz pulse goes through three off-axis objective lenses to expand, collimate and focus to the GaP for electro-optical sampling to measure the THz electric field.

FIG. 2 THz spectra at different pump powers [1]

THz spectra under different pump powers are shown in FIG. 2: As part of the optical path is in the air during electro-optical sampling measurement, there are some spectral absorption caused by water absorption and pulse oscillations in the time domain. In addition, with the increase of pump power and temperature, the absorption of high frequency components of lithium niobate is increased, resulting in the weakening of high frequency components of THz spectra.

FIG. 3 Output THz pulse power and crystal holder temperature change curve with pump power [1]

In Figure 3, the left vertical axis is the generated THz power, and the right vertical axis is the temperature of the crystal support. When the pump power increases to nearly 400W, the THz pulse power increases first and then stagnates, and the temperature of the support also increases gradually. Limited by temperature, the maximum THz power generated by the device is 144mW, corresponding pulse energy is 1.44uJ, corresponding energy conversion rate is 0.042%, and photon conversion efficiency is 19%.

FIG. 4 Experimental device for THz generation based on two-color field gas target shooting [2]

The Jena research group in Germany used two-color field laser to hit gas targets to generate high-refrequency and high-power THz pulses [2]. The experimental device is shown in Figure 4. The front end is a 16-way ytterbium-doped CPA system (only 2 of which are used), with a refrequency of 100kHz, pulse energy of 1.6mJ, and pulse width of 230fs. Then the spectrum is widened by using aerated hollow core capillary with a transmittance of 75%, compressed to 30fs by chirp mirror and divided into two beams. One beam is used as the detection light of electro-optic sampling, and the other beam is focused on the gas target through BBO frequency doubling, frequency doubling and fundamental frequency light, and produces THz when the plasma is produced, and then the thermal power meter is used to measure its power. The time domain electric field was measured by electro-optical sampling.

FIG. 5 THz electric field, spectrum, output power and conversion efficiency generated by different gas targets [2]

The time-domain electric field of THz pulse measured under different gas targets and the spectra after inversion are shown in Figure 5. The spectra under different gas targets have similar shapes. The widest spectrum is generated in the Ne gas target, and THz pulses with the highest average power of 50mW are obtained.

The above high frequency and high energy THz light source is helpful to develop the next generation of X-ray free electron lasers.

References:

[1] Patrick L. Kramer, Matthew K. R. Windeler, Katalin Mecseki, Elio G. Champenois, Matthias C. Hoffmann, and Franz Tavella, "Enabling high repetition rate nonlinear THz science with a kilowatt-class sub-100 fs laser source," Opt. Express 28, 16951-16967 (2020)

[2] Buldt, J., Mueller, M., Stark, H. et al. Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power. Appl. Phys. B 126, 2 (2020).