Broadband terahertz solid-state emitter driven by Yb:YAG thin-disk oscillator

We report on a table-top, high-power, terahertz (THz) solid-state emitter driven by few-cycle near-infrared pulses at 16 MHz repetition rate in gallium phosphide (GaP) crystals. Two external nonlinear multi-pass cells are used to shorten the output of a home-built, 100 W, 265 fs, 6.2 μJ Yb:YAG thin-disk oscillator, operating at 1030 nm, to 18 fs with 3.78 μJ pulse energy. The broadband spectrum of the THz driver allowed for the extension of the THz cutoff frequency to 5.7 THz at the dynamic range of 104. By employing the high-power Yb:YAG thin-disk oscillator, the low efficiency of the THz generation is circumvented, resulting in the generation of up to 100 μW, multi-octave THz pulses at 5 THz cutoff frequency in a 2 mm thick GaP crystal.


Introduction
Sources of intense, broadband, and coherent light in the terahertz (THz) spectral region (between 0.1 THz and 30 THz) operating at high average power and high repetition rates are seeing increasing interest in the ultrafast community as they bene t various applications, from non-destructive timeresolved molecular spectroscopy and imaging to THz communication and remote sensing [1].
Over the last 30 years, availability of the intense laser pulses based on chirped-pulse ampli cation [2] in solidstate lasers, in particular Ti:sapphire-based systems [3], accelerated the progress in the generation of ultra-broadband 5 The authors current addresses are listed above.
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intense THz pulses based on ionizing gases [4]. Here the generated THz pulses do not suffer from phonon resonances or echoes unlike THz generation and detection in solids. However, the kilohertz (kHz) operation of these sources limits the measurement signal to noise ratio and increases the acquisition time. Therefore, novel, intense, broadband and coherent THz sources operating at a higher repetition rate, ideally in the megahertz (MHz) regime are required to decrease the measurement acquisition time and to enable resolving subtle processes and couplings that would never rise above the noise oor of current experiments.
Alternatively, THz pulses can be generated by optical recti cation of few-cycle pulses in solids. The solid emitters have large gaps in their spectra due to phonon absorption and dispersion. However, they require driving pulses with lower peak intensities, in the order of tens of GW/cm 2 , making them suitable candidates for the available laser technology. Diodepumped Yb-doped oscillators have the promise to play an important role in advancing solid-state THz emitters and detectors due to their higher average and peak power compared to other solid-state oscillators. Nowadays, they deliver pulses with tens of micro-joules of energy and hundreds of watts of average power [5,6], reaching up to 25 MW peak power at 16.3 MHz repetition rate [7].
Yb-doped ber ampli ers with hundreds of femtoseconds pulse duration and tens of watts of average power have been used to generate THz pulses with a cutoff frequency as high as 3 THz at 6.5 µW average power or 0.3 mW narrowband pulses [8][9][10][11][12]. However, in order to increase the bandwidth of the generated THz pulses, shorter pump pulses are desired. In 2018, Xu et al demonstrated THz pulses with negligible average power and 4 THz cutoff frequency by using 20 fs, 5.5 W pulses from an Yb ber ampli er [13]. Later on, Paradis et al extended the bandwidth to 5 THz by using 50 fs, 4 W pulses from an Yb-doped thin-disk oscillator [14]. Meyer et al demonstrated the average power scaling of THz pulses to 78 µW at 2 THz cutoff frequency by employing 120 W, 580 fs pulses of an Yb-doped thin-disk oscillator [15]. Recently, Drs et al extended the amplitude of the THz spectrum to 5 THz at 0.3 mW average power by using a 95 fs Yb:YAG thin-disk oscillator [16].
In what follows we demonstrate scaling in average power and spectral bandwidth of THz pulses driven by 18 fs pulses via optical recti cation in gallium phosphide (GaP) crystal. In our scheme, the output pulses of the oscillator are externally shortened to allow for extending the THz bandwidth, while the low ef ciency of the processes is circumvented by employing a high-power Yb:YAG thin-disk oscillator operating at 16 MHz repetition rate. We demonstrate an increase in the spectral bandwidth of the generated broadband THz pulses to 5.7 THz cutoff frequency at 10 µW of average power, in a 0.2 mm thick GaP crystal, and to 5 THz cutoff frequency at 100 µW of average power, in a 2 mm thick GaP crystal.

Experimental setup
A home-built Kerr lens mode-locked Yb:YAG thin-disk oscillator is used to generate THz pulses. A 6 mm thick crystalline quartz plate is included in the laser cavity to achieve Kerr lens mode-locking, resulting in 265 fs pulses at 1030 nm. The oscillator operates in a reduced atmosphere (160 mbar of pressure) and delivers 6.2 µJ, 100 W pulses at 16 MHz repetition rates [17].
Two external nonlinear spectral broadening stages are developed to extend the spectral bandwidth of the oscillator's output and to satisfy the desired conditions for broadband THz generation. It is known that the interaction of intense laser pulses with materials with cubic nonlinearity results in self-phase modulation and generation of new spectral components. However, because of the uneven distribution of energy in a Gaussian beam pro le, the beam quality after the selfphase modulation is severely degraded, limiting the achievable throughput ef ciency [18]. To overcome this limitation, the B-integral per pass through the nonlinear medium should be kept below π while the small broadening factor can be circumvented by increasing the passes through the nonlinear medium in a wave-guide like multi-pass geometry [19][20][21][22]. In this scheme, the nonlinear medium is placed in the focal plane of a Herriott-type imaging cell (HC) with resonator stability. The beam is re-imaged after each pass to the medium and the induced temporal phase is compensated after each pass by using dispersive mirrors. Figure 1(a) shows a schematic of the two consecutive HCs used in our system. In the rst stage, 6.2 µJ pulses of the oscillator were sent to an HC containing two curved dispersive mirrors with radius of curvature of 300 mm, and group delay dispersion (GDD) of −160 fs 2 for 350 nm spectral bandwidth centered at 1030 nm. A 6.35 mm thick, anti-re ection-coated fused silica (FS) plate was used as the nonlinear medium. The output spectrum of the rst HC after 38 passes through the medium is shown in gure 1(b) (blue curve) corresponding to an 11.3-fold spectral broadening and 82% optical ef ciency. Increasing the number of passes at this stage does not lead to a higher factor of spectral broadening, due to the limited bandwidth of the dispersive mirrors of the HC and the accumulated residual linear and nonlinear phase from each round trip. The output pulses of the rst stage were coupled out and compressed to 46 fs by using seven additional dispersive mirrors, compensating the total residual dispersion of 840 fs 2 around 1030 nm.
Afterwards, the compressed pulses were sent to a second HC containing two concave dispersive mirrors with radius of curvature 250 mm, and a 6.35 mm thick, anti-re ection-coated FS plate as the nonlinear medium. The concave mirrors at this stage have a GDD of −120 fs 2 for 600 nm spectral bandwidth. The output spectrum after six passes through the nonlinear medium is shown in gure 1(b) (red curve). At this stage the residual accumulated linear and nonlinear phase is negligible as the number of passes is small. The output pulses were sent to a compressor containing three dispersive mirrors with −120 fs 2 GDD per bounce and were compressed to 18 fs. A home-built, frequency-resolved optical gating based on second harmonic generation (SHG-FROG) employing a 10 µm β-barium borate crystal was used to characterize the pulses ( gure 1(c)).
The described frontend delivers 18 fs pulses with a total energy of 3.8 µJ, corresponding to 60% optical ef ciency.
THz transients were generated via optical recti cation of the femtosecond near-infrared pulses in an uncoated 110-cut GaP crystal in a collinear phase-matching geometry, where n g (pump) = n(THz), i.e. the group velocity of the pump pulse is equal to the phase velocity of the emitted THz wave [23].
The 3.8 µJ, 18 fs pulses at 1030 nm were focused using a convex lens with 250 mm focal length. The GaP was placed before the focus of the laser at a beam diameter of 700 µm (1/e 2 ), corresponding to 97 GW/cm 2 peak intensity and a uence of 2 mJ/cm 2 at an average power of 60.5 W for THz generation.
The generated THz beam was then collimated using a 2 ′′ off-axis bare-gold-coated, pierced parabolic mirror with a focal length of 101.6 mm. The pump beam was transmitted through the hole in the center of the parabolic mirror and the re ected THz beam was passed through a Te on lter to block the residual of the pump beam, and afterwards was redirected towards an electro-optic sampling (EOS) setup for detection.
Less than 1% of the fundamental beam before the THz generation stage was separated by a calcium uoride plate and used as the probe beam for the EOS. The 18 fs pulses offer suf cient temporal resolution to resolve the highest theoretically predicted frequency of the THz pulses. A second 2 ′′ off-axis bare-gold-coated, pierced, parabolic mirror with a focal length of 50.8 mm was used to recombine the probe and the THz beams and also to focus the THz beam into the EOS nonlinear crystal. The two beams were overlapped and focused into a 0.15 mm thick, 110-cut GaP crystal, while their relative delay was controlled by a motorized linear stage (Physik Instrumente Q-545). Due to the Pockels effect, the refractive index of the GaP is modi ed in proportion to the strength of the THz eld, which can be resolved by detecting the changes in the polarization state of the probe beam.
The polarization was then evaluated by using an ellipsometer consisting of a quarter-wave plate (Thorlabs AQWP10M-980), Wollaston prism (Thorlabs WP10), and a balanced photodiode (Thorlabs PDB210A). The THz pulses were mechanically chopped at a frequency of 1 kHz. The mechanical chopper was placed at a focus of THz beam generated using a second pair of bare-gold-coated parabolic mirrors prior to the EOS setup. A Stanford SR830 lock-in ampli er was used to read the output of the balanced photodiode. In addition to the electric eld of the THz pulses, the THz power was measured by a commercial Golay cell detector (Tydex GC-1P) at the position of the EOS crystal. Figure 2 shows the measured average power and the retrieved spectra of the generated THz pulses in 2 mm, 1 mm, and 0.2 mm thick GaP crystal. In the power calculation, power losses caused by the transmission of the Te on lter and the re ections of the parabolic mirrors are taken into account. For the 2 mm GaP and at pump peak intensities beyond 70 GW/cm 2 , the dispersion for the few-cycle pump pulses and the temporal walk-off between the generated THz pulses and the pump pulses become more prominent, resulting in a different behavior ( gure 2(a)) [24,25].

Conclusions
The normalized spectra for three different thicknesses of GaP are compared in gure 2(b). As shown, the cutoff frequency of THz pulses for the 0.2 mm GaP extends to 5.7 THz, taking the dynamic range of 10 4 into account. It is noticeable that at the maximum pump power, no extra cooling of the GaP crystal was required even after hours of radiation. Figure 3 shows the resolved electric eld of the THz transient and its corresponding retrieved spectrum for the 0.2 mm thick GaP crystal, delivering the broadest THz spectrum in our study. Each data set is acquired in a 10 min scan with a 30 ms integration constant. Due to the relatively high humidity of the laboratory (≈ 46%), absorption of the THz pulses by water vapor molecules [26] in air can be resolved in the trailing edge of the THz transient in the time domain and appears as several dips in the retrieved spectrum, which are in good agreement with the water absorption lines from the HITRAN database (red curve in gure 3(b)) [27]. As can be seen, the water vapor lines are reliably detected with 10 GHz resolution. The large amplitude of the free induction decay at the trailing edge of the pulse suggests high absorption of the generated THz pulses by water molecules. Therefore, decreasing vapor water molecules in the system would lead to higher output power of the generated THz transients.
In conclusion, driving THz solid-state emitters by fewcycle pulses generated externally from high repetition rate, Yb:YAG, thin-disk oscillators is a promising method for producing broadband THz pulses with wide application in THz imaging and spectroscopy at rapid acquisition time. In this letter, we demonstrated the pulse shortening of 6 µJ, 265 fs pulses from a Kerr lens, mode-locked Yb:YAG oscillator in an external, cascaded, nonlinear stage based on self-phase modulation to 3.8 µJ, 18 fs pulses at 16 MHz repetition rate. By using the few-cycle pulses to pump GaP as a solid-state emitter, a broadband THz spectrum with a cutoff frequency as high as 5.7 THz and average power of 10 µW is generated. Scaling the thickness of the crystal to 2 mm results in the generation of 100 µW THz pulses with a lower cutoff frequency at 5 THz, corresponding to 6 pJ pulse energy. The observed cutoff in this experiment is in good agreement with the calculation in [28]. It can be seen that at higher conversion ef ciencies the effects of dispersion of pump pulses, temporal walkoff between the interacting beams, and spectral red-shifting become prominent, and slow down the slope of the ef ciency curve. The observed strong absorption of THz pulses with water vapor molecules suggests that higher output power can be achieved by reducing the humidity in the system. Moreover, the resolved absorption lines of water prove the dynamic range, robustness, and stability of the system as a spectroscopy module.