Generation of high repetition rate THz radiation at the mill-watt-level via optical rectification in an enhancement cavity

. We propose an experimental method for the generation of coherent terahertz radiation in the spectral region between 0.2 THz and 2 THz, with a high repetition rate of nearly 100 MHz, and with an average power at the milliwatt level. An Ytterbium-doped mode locking laser is amplified to 60 W, and pulses are stacked into an optical cavity up to 750 W. There, they interact with a Gallium Phosphide crystal producing THz radiation via optical rectification. With the cavity enhanced configuration, we show that more than one order of magnitude can be gained with respect to simply focalize the 60 W beam into the GaP crystal.


Introduction
Terahertz (THz) radiation, which commonly refers to optical waves lying in the frequency range of 0.1-10 THz, has emerged as a promising area of research in recent years due to its unique properties and potential applications in various fields, such as spectroscopy [1], imaging [2], and tomography [3].However, generating and detecting THz radiation is still a challenging task due to the lack of efficient and compact sources and detectors.At present, different approaches for THz generation have been explored, such as optically pumped molecular gas terahertz laser [4], quantum cascade laser [5], photoconduction [6], and free-electron lasers [7].Each of those methods presents limitations mainly related to costs, dimensions, environmental working parameters, and thus in general low applicability.A valid alternative for THz generation is represented by frequency down-conversion of near infrared radiation in a nonlinear crystal, such as difference frequency generation (DFG) [8] and optical rectification [9] schemes.This technology allows to generate THz radiation at room-temperature in a wider frequency range, together with a smaller structure and lower costs compared to other methods.
In the framework of high repetition rate and high average power THz radiation, we propose a simple experimental setup based on intracavity optical rectification in a Gallium Phosphide (GaP) crystal [9,10].In particular, we use a commercial Ytterbium-doped mode locking laser with a repetition rate of nearly 100 MHz, later amplified to 60 W of average power with a fiber amplifier, and finally boosted to nearly 750 W with the use of an enhancement cavity, where optical rectification occurs in a GaP crystal.There, pulses have a temporal length of 370 fs.Here we present our simulations of the process, showing the possibility of producing a THz comb with an average power at the milliwatt level.Since this is an ongoing experiment, the first measurements will be presented later at the EOSAM conference.The high repetition rate and the milliwatt level average power open the possibility of promising implementations of efficient hyperspectral imaging architectures in the THz range.

Experimental setup and simulations
The proposed method for the generation of THz radiation is based on the setup shown in Figure 1.A commercial Menlo Orange Yb mode-locked laser (MLL) produces pulses with a repetition rate of 92.857 MHz at 1035 nm.Then, a high-power fiber amplifier from NKT Photonics (HPA) allows the amplification of the pulses up to an average power of 60 W, via a standard chirped pulse amplification approach: a chirped volume Bragg grating (CVBG) serves as a stretcher for the pulses to a length of nearly 300 ps, while an identical crystal is used as a compressor, to reduce pulse length to 370 fs.Notice that the CVBG crystals impose a super-gaussian spectrum of 7 nm FWHM.Later, laser pulses are coupled into an enhancement cavity (EC), where they interact with a 2 mm-long antireflection coated GaP crystal, and produce THz radiation via optical rectification.The cavity is designed to maximize the gain factor to 12.5, taking into account the residual losses of the AR coating of the GaP crystal (6%, single pass) and its group delay dispersion.The four cavity mirrors have the following reflectivity: RIC = 84% for the plane input mirror and R i > 99.999% for the other mirrors (one plane mirror and two curved mirrors with 750 mm radius of curvature).The intracavity beam in the GaP crystal has a waist of 1.00 mm keeping the laser intensity (assuming an intracavity power of 750 W) just below the damage threshold of the GaP [11].
, 08021 (2023) Figure 2a shows the simulations of the THz power as a function of the IR pulse length, for GaP crystals of different thickness of 0.5 mm, 1.0 mm, and 2.0 mm.For comparison, we also show the THz power without the use of an enhancement cavity, rather by focusing the 60 W beam directly in the GaP crystal with a waist of 300 μm, near its damage threshold.In these conditions, the use of the enhancement cavity is clearly advantageous in terms of THz power.In particular, the pulses of our setup would produce a THz comb with an average power of 1 mW with the 2 mm-long crystal, increasing the power of a factor 14 with respect to the single pass configuration.Figure 2b shows the THz spectrum (THz normalized intensity) generated by the intracavity method for the same GaP crystal thicknesses of Fig. 2a, assuming a pulse duration of 370 fs.It results in a spectrum centred at nearly 1 THz with a bandwidth of 1.2 THz FWHM.

Conclusions and future perspectives
We proposed and simulated a simple experimental setup for the generation of a THz pulses with high repetition rate and mW-level average power via optical rectification of a Yb mode-locked laser in an enhancement cavity using a GaP crystal.We presented some simulations that shows a milliwatt of average power in our conditions of 370 fs IR pulses, mainly limited by the damage threshold of the GaP crystal.As an ongoing experiment, results and measurements will be presented at the EOSAM 2023 conference.Besides this, in the future other nonlinear crystals can be investigated, for example with higher damage threshold, so that a higher THz power can be reached.This simple approach to generate high power and high repetition rate THz radiation will allow the application of fast hyperspectral imaging in the THz frequency range

Fig. 2 .
Fig. 2. a) Simulations of intracavity THz generation as a function of the IR pulse duration τ, for different thickness of the GaP crystal, for an IR beam waist of 1 mm.Red line is the THz power for a 2-mm GaP without the enhancement cavity and with a beam waist of 300 μm, for comparison.b) Simulations of the expected intracavity THz spectrum (THz normalized intensity) for a pulse duration of 370 fs and for different GaP thickness.Spectra are centered around 1 THz, with a FWHM of nearly 1.2 THz.