1. Introduction

During the last decades, broadband terahertz (THz) generation driven by ultrafast lasers has opened up access to numerous applications such as THz time-domain spectroscopy or spectral imaging [1]. However, the conversion efficiency between the driving laser and the THz radiation is typically relatively low, ranging between a few parts per million up to a few percent. Although several microwatts of THz average power are sufficient for some applications such as terahertz time-domain spectroscopy, many others such as THz imaging or spectroscopy of highly absorptive samples would strongly benefit from higher THz average powers [2]. In recent years, we have witnessed a considerable progress in the development of high-power THz systems. New THz sources based on plasmonic-enhanced photoconductive antennas [3,4] and organic nonlinear crystals [5,6] achieve milliwatt-class broadband THz generation from moderate pump power levels of only a few watts. Another highly promising direction for increased THz power is the use of well-established techniques such as optical rectification (OR) in nonlinear crystals or two-color laser filamentation at much higher pump power levels from novel ultrafast lasers. Up to 1.35 mW of THz average power with a spectrum reaching up to 6 THz have been generated by driving OR in gallium phosphide (GaP) with a 100-W nonlinearly compressed thin-disk laser (TDL) oscillator [7]. Later on, driving OR in lithium niobite instead of GaP, 66 mW of THz average power with a spectrum extending up to 2 THz were generated using the same driving laser [8]. Finally, 50 mW of THz average power with a spectrum expected to extend up to 30 THz was demonstrated using two-color laser filamentation in a neon gas target driven by a 160-W chirped-pulse fiber-amplifier system at 100 kHz repetition rate [9]. However, the complexity, the size and the cost of these systems due to, e.g., the required diode pump power or the vacuum chamber might still be a limiting factor for many applications requiring small and compact sources.

Driving a nonlinear frequency conversion process inside a laser cavity has proven to be a successful approach for reducing the complexity of the system and increasing the overall efficiency [10–12]. Similarly, placing the THz generation crystal in a laser cavity allows for recycling of the unconverted pulse energy in each cavity roundtrip. This strongly increases the available driving power in comparison to the outcoupled beam. However, previous demonstrations of intra-cavity THz generation were so far strongly restricted in the demonstrated power levels. In [13], intra-oscillator THz generation reaching up to 7 µW of THz average power was achieved by a transient photocurrent at the surface of a semiconductor saturable absorber mirror, which also initiated and stabilized the pulses in a femtosecond Ti:sapphire oscillator. In 2008, this intra-oscillator technique was applied in a soliton fiber laser [14]. Here a THz yield of 4 µW and a conversion efficiency of 3.1×10⁻⁵ were achieved. In the same year, passive enhancement cavities were used for the generation of THz radiation using OR [15] and THz generation inside a dual frequency optical parametric oscillator via difference frequency generation in DAST was achieved [16]. The first optical rectification inside a femtosecond oscillator was demonstrated in 2010 using a Ti:sapphire laser oscillator [17]. However, the THz average power was not measured and the THz spectrum was restricted to 2.5 THz due to the 1-mm-thick zinc telluride crystal used for OR.

In the last decade, ultrafast technology strongly progressed [18–20]. Today, ultrafast solid-state laser oscillators can operate at several hundred watts of intra-oscillator average power with sub-100-fs pulses [21–25] which is required for powerful and broadband THz generation [7,26–28].

In our experiment, we drive OR in a GaP crystal which is placed inside the cavity of a Kerr-lens mode-locked (KLM) Yb:CALGO bulk laser oscillator. Yb:CALGO is an outstanding gain material for ultrafast lasers thanks to its broadband gain emission spectrum combined with a low quantum defect and relatively high thermal conductivity [29–31], making it particularly attractive for compact and efficient ultrafast DPSSLs [32]. For example, 30-fs pulses at optical-to-optical efficiencies of ∼30% were recently achieved [33]. Moreover, it can be pumped by low cost 980 nm multimode pump diodes. In our intra-oscillator THz system, OR was driven in a GaP crystal at 22 W of intra-cavity laser power, 105 fs pulse duration, and 75 MHz repetition rate. We achieve up to 150 µW of THz average power with a spectrum extending to 5.5 THz using only 7 W of diode pump power. Compared to a single-pass system with similar performance [27], the intra-cavity approach requires 20 times less pump power. We also demonstrate that the THz spectrum can be extended up to ∼7 THz at the expense of THz average power using a thinner GaP crystal combined with shorter pulse duration. These results show that intra-oscillator THz generation via OR is a promising way to simplify and reduce the size of the current state-of-the-art THz sources. GaP can support more than hundred watts of driving power [34], and we expect that milliwatt THz power levels are within reach of this technology.

2. Experimental setup

The experimental setup is shown in Fig. 1. The laser oscillator is based on a 3-mm-long, antireflection-coated (AR) Yb(3 at.%):CALGO crystal. The crystal is mounted in a water-cooled copper holder and optically pumped at 980 nm by a commercially available 10-W multimode fiber coupled laser diode module. The positive group delay dispersion (GDD) introduced by the gain medium, the GaP crystal, and the self-phase modulation is compensated by several dispersive mirrors (DM). A 2.7-mm diameter hard aperture enables stable KLM operation. An output coupler (OC) with a transmission of 1.5% is placed as a folding mirror, thus providing two out-coupled beams. One beam is used to characterize the driving laser while the other beam is used for electro-optic sampling (EOS). The GaP crystal is placed close to the end mirror of the laser cavity without any active cooling. Both faces of the GaP crystal have an AR coating which was designed and grown in our own ion beam sputtering coating facility. The THz radiation is generated in two directions inside the GaP crystal, however, only one direction is characterized. The strongly diverging THz beam is directed by two off-axis parabolic mirrors (OAPM) onto a second 0.15-mm-thick GaP crystal for EOS. The THz beam is mechanically chopped at around 75 Hz for subsequent lock-in detection. The THz average power is measured by placing a pyroelectric power meter at the position of the GaP crystal used for the detection. Three layers of black plastic foil were inserted between the two OAPMs in order to prevent any of the parasitic scattered infrared light from reaching the detector. Their individual THz transmission of 46% has been determined in a previous study [27] and the here stated measured THz average powers were corrected by this transmission factor. The system was purged by dry air to decrease the relative humidity to below 20% in order to reduce the water vapour absorption of the THz radiation.

 

Fig. 1. Experimental setup for intra-oscillator THz generation and detection by electro-optic sampling. OC, 1.5% transmission output coupler; DM, dichroic mirror; HA, hard aperture; OAPM, off-axis parabolic mirror; ChW, chopper wheel; QWP, quarter waveplate; WP, Wollaston prism; BPD, balanced photodetector. The footprint of the overall system is 80 cm × 40 cm.

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3. Experimental results

We characterized the system in two different configurations optimized for either high THz average power or for a broader THz spectrum. Table 1 summarizes the characteristics of the driving laser and the resulting THz performance. The first configuration optimized for high THz average power uses a 0.3-mm-thick GaP crystal. OR in GaP was driven at 22 W of intra-cavity average power with 105 fs pulse duration corresponding to 12.2 nm of full width at half maximum (FWHM) spectral bandwidth [Figs. 2(a) and 2(b)] and 75 MHz repetition rate.

 

Fig. 2. a) Autocorrelation trace and b) optical spectrum of the driving pulse for the configuration 1 (using a 0.3-mm-thick GaP) and the configuration 2 (using a 0.1-mm-thick GaP). c) Normalized THz E-field in the frequency domain detected by electro-optic sampling (plain line) and the corresponding theoretical phase matching curves (dashed lines) for both configurations. τ, FWHM pulse duration; Δλ, FWHM spectral bandwidth.

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Table 1. Driving laser parameters and resulting THz performance corresponding to the configuration 1 using a 0.3-mm-thick GaP crystal and the configuration 2 using a 0.1-mm-thick crystal.^a

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The pulse duration was adapted to the phase matched window of the 0.3-mm GaP crystal according to [27]. The beam diameter inside the crystal was ∼400 µm, corresponding to a peak intensity of ∼4 GW/cm², significantly below the damage threshold of 60 GW/cm² [35]. Figure 2(c) shows the generated THz spectrum (blue line) retrieved from the EOS measurement in the frequency domain. The THz spectrum is centred at 2 THz and extends up to 5.5 THz, slightly below the limit imposed by the phase matching condition. We measured up to 150 µW of THz average power leading to an optical-to-optical efficiency of 7×10⁻⁶ and 2×10⁻⁵ with respect to the driving laser and the multimode pump diode average power, respectively.

The second configuration aims to further exploit the potential of the ultrafast driving source for broadband THz generation. It utilizes a 0.1-mm GaP crystal allowing for a phase-matched window extending up to ∼8 THz. However, the broader phase-matched window of the thinner GaP crystal comes at the expense of a reduced THz average power due to the shorter interaction length. The pulse duration was adjusted to 48 fs corresponding to 12.2 nm of FWHM spectral bandwidth by reducing the negative GDD introduced by the DMs. The laser beam diameter inside the GaP crystal was decreased to ∼350 µm by slightly adjusting the cavity length. The decrease of the pulse duration and beam diameter led to a higher peak intensity inside the GaP crystal estimated to be ∼9 GW/cm² in this configuration. Driving OR at 20 W of intra-cavity average power and 80 MHz repetition rate resulted in the generation of 35 µW of THz average power with a spectrum centred at 3 THz and extending up to ∼7 THz. The drop of the average THz power is consistent with the change in the crystal thickness and the driving laser peak intensity. The measured spectrum does not reach the 8-THz phase-matching limit mostly due to the thicker GaP crystal of 0.15 mm used for EOS, restricting the measurement range to approximately 7 THz.

In the current setup we were limited to intra-cavity average powers of ∼20 W by the onset of a continuous-wave lasing breakthrough at higher pumping powers. We attribute this effect to the thermal lens and multiphoton absorption inside the GaP crystal disrupting the KLM operation. We expect that optimization of the cavity layout to accommodate the thermal lens combined with higher repetition rate will allow for significant increase of the intra-cavity driving power. Further advances are expected by implementing efficient cooling of the GaP crystal.

4. Conclusion

In this proof-of-principle experiment, we have demonstrated efficient and broadband THz generation based on OR in GaP in a simple collinear geometry directly inside the cavity of an ultrafast laser oscillator. In a first configuration, the system generated up to 150 µW of THz average power with an optical spectrum extending up to 5.5 THz and 35 µW with a spectrum extending up to 7 THz in a second configuration. This simple concept of intra-oscillator THz generation allows for high average driving powers in combination with short pulse duration which are otherwise available only from more complex laser systems requiring diode pump powers exceeding the 100 W range. For instance, in comparison to a single-pass THz-source based on OR in GaP driven by a high-power TDL oscillator utilizing the same GaP thickness, our intra-oscillator approach requires 20 times lower pump power at a fraction of the component cost [27]. We also expect a further improvement of the technology is easily within reach. Since up to 1.35 mW of THz average power has already been generated by single-pass OR in GaP [7], we believe that the intra-oscillator approach will soon reach a comparable performance.