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We report on an in-band pumped soft-aperture Kerr-lens mode-locked Ho:CALGO bulk laser at 2.1 um, generating 2 W of average power with 112 fs pulses at 91-MHz repetition rate. To the best of our knowledge, this is the highest average power from a 100-fs class mode-locked laser based on a Tm3+ or Ho3+ doped bulk material. We show that the laser has excellent noise properties with an integrated relative intensity noise of 0.02% and a timing jitter of 950 fs (RMS phase noise 0.543 mrad) in the integration interval from 10 Hz to 10 MHz. The demonstrated combination of high average power, short pulses, and low-noise make this an outstanding laser source for spectroscopy and many other applications at 2.1 um., Comment: 4 pages, 6 figures

Multi-pass cell (MPC) based temporal pulse compressors have emerged in the last years as a powerful and versatile solution to the intrinsic issue of long pulses from Yb-based high-power ultrafast lasers. However, the spectral broadening of high-energy (typically more than 100 uJ) pulses has only been realized in complex setups, i.e., in large and costly, pressure-controlled vacuum chambers to avoid strong focusing, ionization, and damage on the mirrors. Here, we present spectral broadening of 2 mJ pulses in a simple and compact (60 cm long) multi-pass cell operated in ambient air. Instead of the traditional Herriott cell with concave-concave (CC/CC) mirrors, we use a convex-concave (CX/CC) design, where the beam stays large at all times allowing both to minimize damage and operate in ambient air. We demonstrate spectral broadening of 2.1 mJ pulses at 100 kHz repetition rate (200 W of average power) from 2.1 nm (pulse duration of 670 fs) to a spectral bandwidth of 24.5 nm, supporting 133 fs pulses with 96% transmission efficiency. We show the compressibility of these pulses down to 134 fs, and verify that the spectral homogeneity of the beam is similar to previously reported CC/CC designs. To the best of our knowledge, this is the first report of a CX/CC MPC compressor, operated at high pulse energies in air. Because of its simplicity, small footprint and low cost, we believe this demonstration will have significant impact in the ultrafast laser community., The following article has been submitted to Optics Letters. 4 pages, 4 figures

We report on an in-band pumped SESAM mode-locked Ho:CALGO bulk laser with a record-high average power of 8.7 W and an optical-to-optical efficiency of 38.2% at a central wavelength of 2.1 um. At this power level, the bulk laser generates pulses with a duration of 369 fs at 84.4-MHz repetition rate, corresponding to a pulse energy of 103 nJ and a peak power of 246 kW. To the best of our knowledge, this is the highest average power and pulse energy directly generated from a mode-locked bulk laser in the 2-3 um wavelength region. Our current results indicate that Ho:CALGO is a competitive candidate for average power scaling of 2 um femtosecond lasers., 9 pages, 5 figures

The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-of-the-art commercial THz-TDS with less than $200~\mu \text{W}$ of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize.

We demonstrate pulse compressibility from 840 fs to 38 fs of 10 mJ pulses from a thin-disk amplifier at a repetition rate of 3 kHz after nonlinear broadening in a multipass cell. In addition, the temporal-intensity contrast is enhanced via nonlinear ellipse rotation of more than a factor 50 with an optical efficiency of 56%. We believe this is the first published experimental combination of multipass cell-based nonlinear compression and nonlinear ellipse rotation-based contrast enhancement preserving both pulse compressibility and beam quality.

We demonstrate THz generation with a low noise GHz repetition rate laser with state-of-the-art photoconductive emitters. We measure a maximum dynamic range of 73 dB and a bandwidth of more than 3 THz.

We present THz generation in a back-cooled spintronic THz emitter in reflection geometry, excited with a high-average power Yb-laser providing up to 18.5 W at 400 kHz repetition rate. This allows us to increase the THz power.

We demonstrate spectral broadening of 2.1-mJ pulses at 100-kHz repetition rate to a spectral bandwidth of 24.5 nm, supporting 133-fs pulses with 96 % efficiency, in a compact multipass cell operated in ambient air.

We present an investigation of an ErAs:InAlGaAs photoconductive antenna receiver optimized for 1030 nm probe pulses at 24 mW THz power and show a dynamic range of 92 dB in 10 s measurement time. The saturation behavior under high THz average power is studied as well.

We demonstrate an ultra-broadband, high dynamic range THz source using organic crystal BNA, excited with a high-power, ultrafast Yb laser operating at 540 kHz repetition rate, reaching a record-high THz average power of 5.6 mW.

We report on high-power mode-locked femtosecond Ho:CALGO laser at 2.1 µm. The laser generates a record-high average output power of 8.7 W (369 fs) with SESAM mode-locking and 1.2 W (152 fs) with Kerr-lens mode-locking.

We present experimental evidence of repetition-rate dependent thermal effects that limit THz generation efficiency in the tilted-pulse front geometry in lithium niobate at high average powers.

High average power femtosecond lasers have made spectacular progress in the last decades – moving from laboratory-based systems with maximum average powers of tens of watts to kilowatt-class mature industrial systems in a short time. The availability of such systems opens new possibilities in many fields; one of the most prominent ones that have driven many of these technological advances is precise high-speed material processing, where ultrashort pulses have long been recognized to provide highest precision processing of virtually any material, and high average power extends these capabilities to highest processing rates. Here, we focus our attention on one high-average power technology with large unexplored potential for this specific application: directly modelocked multi-MHz repetition frequency high-power thin-disk oscillators. We review their latest state-of-the-art and discuss future directions and challenges, specifically with this application field in mind.

We demonstrate THz generation with state-of-the-art iron doped InGaAs photoconductive emitters using a low-noise GHz repetition rate amplified femtosecond laser operating at a central wavelength of 1560 nm. We measure a maximum dynamic range of 73 dB and a bandwidth of more than 3 THz. This unique laser source combining low-noise and high repetition rate is a promising alternative to the frequently used commercial fiber systems at 100 MHz.

We demonstrate optical rectification in BNA organic crystals using bursts of pulses from a high-power thin-disk oscillator at 13.3 MHz repetition rate. We measure an average power of 0.95 mW at 10% duty cycle with a conversion efficiency of 4×10-4. To the best of our knowledge, this is the highest average power reported with commercially available BNA so far. The detected THz spectrum bandwidth spans up to 6 THz.

High power ultrafast lasers in the short-wave mid-IR wavelength region (2-3 µm) are of great interest for a large number of applications in science and technology. Finding paths to increase the average power of ultrafast laser systems directly emitting in this wavelength region has seen particularly strong interest, due to the potential of these sources as direct drivers for the generation of XUV, mid-IR and THz. Among different power-scalable technologies, the development of thin-disk lasers (TDLs) at 2 µm is very promising for power and energy scaling of ultrafast lasers in this wavelength range, but only very few results have so far conclusively shown this potential [1] , [2] . Recently, we have shown the potential of Ho:YAG for further power scaling by demonstrating a cw, single fundamental-mode 2-µm TDL with >100 W and 54% optical-to-optical efficiency, representing the highest power TDL around 2 µm [3] . In the present work, we report first high-power modelocking SESAM-modelocking of this system, reaching an average power of 40-W in ps-pulses, which respresents the highest average power from a modelocked oscillators in the wavelength region.

Ultrashort pulse 2-µm lasers are of great interest for various applications in science and technology, such as spectroscopy or medicine, but also in fundamental science as driving lasers for amplified secondary sources in the XUV, mid-IR and THz regions. During last decades, great progress has been made in development of various Tm and Ho doped gain media based modelocked lasers in the 2 µm wavelength range. Among these lasers, diode-pumped 2-µm femtosecond lasers are particularly interesting due to their low-cost and power scaling potential. Pulses as short as 170 fs has been demonstrated with Tm:LuScO 3 crystal at 2093 nm at 113 mW of average power [1] . Highest power of 500-mW was achieved at 242-fs pulses with Tm:Lu 2 O 3 ceramics [2] . Another approach to achieve shorter pulse duration is by co-doping with Ho. However, diode-pumped femtosecond laser with Tm,Ho-codoped materials has not been achieved so far. Here we demonstrate a 212-fs 200-mW diode-pumped modelocked disordered garnet Tm,Ho:CLNGG laser for the first time.

Optical rectification using tilted pulse fronts in lithium niobate (LN) is currently the method of choice for the generation of strong-field THz pulses for THz-TDS. The success of this technique owes mostly to the high conversion efficiencies achievable, reaching the percent level at room temperature [1] . So far, this method has been mostly used with amplified laser systems, with repetition rates up to the kHz range and corresponding pulse energies of more than 1 mJ. To increase the DR and SNR of today’s THz sources, there is an increasing demand for high average power, high repetition rate THz sources. Recently, we have demonstrated up to 66 mW THz average power, driven by a >100-W average power mode-locked thin-disk oscillator with 13.3 MHz repetition rate and pulse energies on the 10 µJ level [2] , enabling us to achieve the highest average power of a laser-driven THz source at MHz repetition rates. Despite this promising first achievement, the conversion efficiency of 6∙10 -4 was significantly lower than the record conversion efficiencies obtained with lower repetition rates and higher pulse energies. Furthermore, the scaling laws in this unusual excitation regime remain so far unexplored. Here we present an in-depth investigation of this excitation regime using a 2+1D model including pump beam depletion. It is shown that a combination of spatial walk-off and pump beam break-up is responsible for a reduction in efficiency at small beam sizes. Furthermore, we discuss possibilities to overcome the current limitations and predict that watt-level THz sources at MHz repetition rates will become available in the very near future.

Optical rectification in organic crystals have proven to be an excellent material to achieve ultra-broad bandwidth THz radiation with high optical-to-THz conversion efficiency up to the percent level in simple collinear setups. However, so far, most results using organic crystals were restricted to low repetition rates, because of the generally accepted idea that these crystals have poor thermal properties. Only very recently the organic crystal HMQ-TMS was explored at MHz repetition rate showing mW-level THz average power [1] , [2] . Another promising crystal is BNA (N-benzyl2-methyl-4-nitroaniline), which has recently been used with a conversion efficiency of 0.8 % albeit at moderate repetition rate of 100 kHz [3] . Here, we demonstrate mW-class, high dynamic range, broadband THz generation in BNA crystal (Swiss Terahertz GmbH), driven by an ultrafast laser with MHz repetition rate. We obtain 490 µW of THz average power with a conversion efficiency of 5×10 -4 . To the best of our knowledge, this is the first time this material is investigated at high (MHz) repetition rate. To reach this efficiency, we use bursts of MHz-repetition rate driving pulses with variable duty cycle which can decouple the average power scaling from the pulse energy and allows us to disentangle the influence of average power, thermal effects, and peak intensity.

We demonstrate single, solid-state pulse compression of a 49-W industrial fiber laser emitting 242-fs pulses, which can be conveniently adjusted to operate at different repetition rates to achieve > 1-GW peak power and sub-50 fs with high compression efficiency by simply translating a glass plate inside the compressor.

We demonstrate a SESAM, soliton-modelocked, Ho:YAG thin-disk oscillator with more than 40 W of average power, 1.66-ps pulses at energy of 0.78 µJ, representing the highest average power so far demonstrated from an oscillator in the 2 µm wavelength region. We discuss current limitations and paths to overcome them in the near future.

We demonstrate a 13.3 MHz repetition rate, broadband THz source with milliwatt-average power, obtained by collinear optical rectification of a high-power Yb-doped thin-disk laser in the organic crystal BNA (N-benzyl-2-methyl-4-nitroaniline). Our source reaches a maximum THz average power of 0.95 mW with an optical-to-THz efficiency of 4e-4 and a spectral bandwidth spanning up to 6 THz at -50 dB, driven by 2.4 W average power (after an optical chopper with duty cycle of 10%), 85 fs-pulses. This high average power excitation was possible without damaging the crystal by using a diamond-heatsinked crystal with significantly improved thermal properties. To the best of our knowledge, this result represents the highest THz average power reported so far using the commercially available organic crystal BNA, showing the potential of these crystals for high average power, high repetition rate femtosecond excitation. The combination of high power, high dynamic range, high repetition rate and broadband spectrum makes the demonstrated THz source highly attractive to improve various time-domain spectroscopy applications. Furthermore, we present a first exploration of the thermal behavior of BNA in this excitation regime, showing that thermal effects are the main limitation in average power scaling in these crystals., Comment: Submitted to Optics Express

We investigate THz generation using tilted pulse fronts with high power, high repetition rate driving lasers. It is shown that small beam sizes limit the maximum conversion efficiency due to spatial walk-off.

We investigate average power scaling at MHz repetition rate and thermal behavior of THz generation in the organic crystal BNA for the first time, reaching an average power of 380 µW at 10% duty cycle.

We demonstrate pulse compression of 310-fs, 150-MW peak power pulses at 400-kHz repetition rate down to 27 fs, 1.05 GW of peak power using a single, dispersion-optimized Herriott-type multipass cell compressor with 92 % overall efficiency.

We demonstrate a SESAM, soliton-modelocked, Ho:YAG thin-disk oscillator with 40.5 W of average power, 1.66 ps-long pulses at a pulse energy of 0.78 μJ, representing the highest average power so far demonstrated from an oscillator in the 2 μm wavelength region.

This Joint Issue of Optics Express and Optical Materials Express features 36 state-of-the art articles written by authors who participated in the international conference Advanced Solid State Lasers held online from October 3-7, 2021. This review provides a summary of these articles covering a wide spectrum of topics around solid-state lasers from materials research to sources and from design innovation to applications.

We demonstrate the potential of state-of-the-art high-average power THz-TDS for THz imaging applications, by performing lensless imaging of a 3D-printed mixed polymer probe using a migration algorithm. Images obtained with our 10-mW average power THz-TDS show a clear enhancement of dynamic range, which results in a significantly improved image contrast compared to the same measurements performed using a state-of-the-art commercial TDS system with less than $200-\mu\mathrm{W}$ of average power. Our result is an important first step towards reflection-mode imaging of dielectric objects with an internal structure, potentially at long distances.

We demonstrate a 1030 nm 520 fs-pulse-driven ErAs: InGaAs photoconductive receiver suitable for detecting 14 mW average THz power (~ 950 nJ pulse energy). To the knowledge of the authors, this is the highest detected pulsed THz power using photoconductive receivers reported so far. The current (field) responsivity of this receiver is in the range of $110\pm 25\mu A/\sqrt{W}$ yet with slight saturation.

We present the first temperature-dependent refractive index measurements of Gallium Phosphide (GaP) for a broad THz bandwidth extending to 4 THz and a wide temperature range from 77 K to 500 K. Our results show no significant degradation in velocity matching at cryogenic temperatures compared to room temperature. These measurements are critical for future average power and bandwidth scaling of optical rectification in GaP using kW-class excitation.

We explore GaP at cryogenic temperatures for optical rectification (OR) at 100 W average excitation power for the first time. Our results indicate this is a promising route for broadband THz radiation using kW-class excitation.

We explore power and bandwidth scaling for the generation of highly-temporally-confined THz transients from spintronic emitters, driven by the 250-fs and 20-fs pulses of a high-power 28-MHz Yb-based laser, spectrally centered at 1030 nm.

We demonstrate a THz source using the tilted pulse front technique in Lithium Niobate, reaching a record-high average power of 66mW at 13.3MHz repetition rate, driven by a high-power Yb-based modelocked thin-disk laser.

We present latest progress in the demonstration of single-cycle THz sources with high average power, based on excitation with MHz repetition rate, hundred-watt class modelocked thin-disk oscillators

We demonstrate a THz source based on optical rectification of a high-power thin-disk laser using the tilted pulse front technique in Lithium Niobate, reaching a record-high average power of 66mW at 13.3MHz repetition rate.

We demonstrate a Ho:YAG thin-disk oscillator operating in fundamental-mode continous-wave regime delivering a record output power of 96 W and present first GaSb-SESAM-modelocking results.

We demonstrate THz generation using the tilted pulse front method in Lithium Niobate, driven at unprecedented high average power of more than 100 W and at 13.3 MHz repetition rate, provided by a compact amplifier-free modelocked thin-disk oscillator. The conversion efficiency was optimized with respect to pump spot size and pump pulse duration, enabling us to generate a maximum THz average power of 66 mW, which is the highest reported to date from a laser-driven, few-cycle THz source. Furthermore, we identify beam walk-off as the main obstacle that currently limits the conversion efficiency in this excitation regime (with moderate pulse energies and small spot sizes). Further upscaling to the watt level and beyond is within reach, paving the way for linear and nonlinear high-average power THz spectroscopy experiments with exceptional signal-to-noise ratio at MHz repetition rates., Comment: Submitted to Optics Letters