Franz Kärtner

• Professor (Full) at Center for Free-Electron Laser Science

Terahertz-driven (THz) accelerators and manipulators promise to yield short femtosecond electron bunches of high brightness with intrinsic synchronization to the driving laser at a compact and economic footprint. However, development of practical devices requires THz sources that reliably provide pulse energies in the sub-mJ to mJ regime, which in...

Here, we present on the progress of developing a multi-layered accelerator structure designed to boost the 50 keV output of a DC electron gun to energies of ~ 400 keV powered by a single high-energy terahertz ("THz") pulse. An integrated piezo-actuated mirror inside the matchbox sized structure enables fine-tuning of the electric field in the inter...

We have investigated spectroscopic properties of Tm:YLF material around 1900 nm in detail to understand its amplification performance at cryogenic temperatures. Fluorescence lifetime and emission cross section (ECS) measurements are performed in the 78-300 K range using crystals with Tm doping levels of 0.5% and 2.5. The radiative lifetime of Tm:YL...

Development of THz-powered photoguns promises to bring benefits to many applications by scaling down the size and cost of these devices as well as creating new capabilities. In particular, THz photoguns producing MeV beams are desired as the front end for THz-powered LINACs. Generating sufficient THz energy, however, is challenging, as is transport...

The availability of high-energy pulses with durations shorter than the period of their carrier frequency (sub-cycle) will reveal new regimes of strong-field light–matter interactions. Parametric waveform synthesis (that is, the coherent combination of carrier-envelope-phase-stable pulses that emerge from different optical parametric amplifiers) is...

We have investigated temperature and doping dependence of fluorescence lifetime in YLF crystals in the 78-300 K range, for samples with Yb-doping levels of 0.5, 1 and 25%. Radiation trapping prolonged fluorescence lifetimes are first measured to understand the pros and cons of this inevitable process on fractional thermal load and laser gain. Later...

Oral presentation prepared for crygenic Yb:YLF mode-lcoking results for the Europhoton 2020 conference

Integrated modelocked lasers with high power are of utmost importance for next generation optical systems that can be field-deployable and mass produced. Here we study fully integrated modelocked laser designs that have the potential to generate ultrashort, high power, and high quality pulses. We explore a large mode area laser for high power pulse...

The knowledge of the exact frequency of an optical source has always been one of the ultimate goals in optics. Since the discovery of the laser, complex systems have been developed to address this challenge. That effort reached a significant milestone with the advent of the femtosecond laser frequency comb that reduced the system size from an entir...

Integrated modelocked lasers with high power are of utmost importance for next generation optical systems that can be field-deployable and mass produced. Here we study fully integrated modelocked laser designs that have the potential to generate ultrashort, high power, and high quality pulses. We explore a large mode area laser for high power pulse...

We numerically investigate the power and energy scaling potential of cryogenic Yb:YLF regenerative amplifiers in rod geometry. Our approach is based on solving the coupled set of equations describing thermal behavior of the material and its effect on spectroscopic properties, gain, and overall amplification. The approach is first benchmarked with e...

Coherent, broadband pulses of extreme ultraviolet (XUV) light provide a new and exciting tool for exploring attosecond electron dynamics. Using photoelectron streaking, interferometric spectrograms can be generated that contain a wealth of information about the phase properties of the photoionization process. If properly retrieved, this phase infor...

We report, to the best of our knowledge, the first mode-locked operation of Yb:YLF gain media at cryogenic temperatures. A saturable Bragg reflector was used for initiating and sustaining mode locking. Once aligned, the system was self-starting and quite robust. Using output couplers in the 10–40% range, 3–5 ps long pulses with an average power as...

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with millimeter-scale, segmented int...

Coherently-pumped (Kerr) solitons in an ideal optical microcavity are expected to undergo random quantum motion that determines fundamental performance limits in applications of soliton microcombs. Here, this diffusive motion and its impact on Kerr soliton timing jitter is studied experimentally. Typically hidden below technical noise contributions...

Coherently-pumped (Kerr) solitons in an ideal optical microcavity are expected to undergo random quantum motion that determines fundamental performance limits in applications of soliton microcombs. Here, this diffusive motion and its impact on Kerr soliton timing jitter is studied experimentally. Typically hidden below technical noise contributions...

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase-matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especiall...

Despite the popularity and ubiquitousness of the tilted-pulse-front technique for single-cycle terahertz (THz) generation, optimization of the experimental setup remains complex and difficult due to the sensitive dependence on and coupling between the optical pulse parameters, including fluence, beam size, angular dispersion and temporal compressio...

Both the group velocity and phase velocity of two solitons can be synchronized by a Kerr-effect mediated interaction, causing what is known as soliton trapping. Trapping can occur when solitons travel through single-pass optical fibers or when circulating in optical resonators. Here, we demonstrate and theoretically explain a new manifestation of s...

Coherent, broadband pulses of extreme ultraviolet (XUV) light provide a new and exciting tool for exploring attosecond electron dynamics. Using photoelectron streaking, interferometric spectrograms can be generated that contain a wealth of information about the phase properties of the photoionization process. If properly retrieved, this phase infor...

We report, what is to our knowledge, the first mode-locked operation of Yb:YLF gain media at cryogenic temperatures. A saturable Bragg reflector (SBR) was used for initiating and sustaining mode-locking. Once aligned, the system was self-starting and quite robust. Using output couplers in the 10-40% range, 3 to 5 ps long pulses with an average powe...

We numerically investigate the power and energy scaling potential of cryogenic Yb:YLF regenerative amplifiers in rod geometry. Our approach is based on solving the coupled set of equations describing thermal behavior of the material and its effect on spectroscopic properties, gain and overall amplification. The approach has been first benchmarked w...

Yb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak power levels, and has become the workhorse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and the search for alternative broadband TD gain me...

High energy single- to few-cycle terahertz pulses enable the exploration of the frontier of science, such as electron acceleration, strong-field physics, and spectroscopy. One important method of generating such terahertz pulses is to use the tilted-pulse-front (TPF) technique. However, due to the non-collinear phase-matching, the large angular dis...

We demonstrate a novel, energy-efficient, cost-effective simple method for seeding CEP-stable OPCPAs. We couple the CEP-stable idler of a broadband OPCPA into a hollow core Kagome fiber thus compensating for the angular chirp. We obtain either relatively narrow bandwidths with ∼36% coupling efficiency or quarter-octave spanning bandwidths with ∼2.2...

We report, what is to our knowledge, the highest average power obtained directly from a Yb:YLF regenerative amplifier to date. A fiber front-end provided seed pulses with an energy of 10 nJ and stretched pulsewidth of around 1 ns. The bow-tie type Yb:YLF ring amplifier was pulse pumped by a kW power 960 nm fiber coupled diode-module. By employing a...

Frequency combs in the mid-IR wavelength are usually implemented by difference-frequency generation (DFG) that mixes pump pulses and signal pulses. Different from most optical parametric amplifiers that operate at a typical low repetition rate of <0.1 MHz, mid-IR frequency combs require that pump/signal pulse repetition rate must be at least as hig...

We introduce a new regime of cascaded quadratic nonlinearities which result in a continuous red shift of the optical pump, analogous to a Raman shifting process rather than self-phase modulation. This is particularly relevant to terahertz generation, where a continuous red shift of the pump can resolve current issues such as dispersion management a...

We report a cross-correlation technique based on perturbation of local electron field emission rates that allows for the full characterization of arbitrary electric fields down to 4 femtojoules using plasmonic nanoantennas.

Ultrafast light-matter interactions lead to optical-field-driven photocurrents with an attosecond-level temporal response. These photocurrents can be used to detect the carrier-envelope-phase (CEP) of short optical pulses, and could be utilized to create optical-frequency, petahertz (PHz) electronics for information processing. Despite recent repor...

We report 190-mJ pulses with spectral content supporting sub-ps pulse-duration at a 10 Hz repetition rate generated by a cryogenically-cooled, bulk Yb:YLF laser amplifier system. The amplifier system relies on a chirped pulse amplification architecture and consists of a fiber front-end, a regenerative amplifier, and two 4-pass amplifiers. The fiber...

We present, what is to our knowledge, the first detailed lasing investigation of cryogenic Yb:YLF gain media in the E//a-axis. Compared to the usually employed E//c-axis, the a-axis of Yb:YLF provides a much broader and smooth gain profile, but this comes at the expense of reduced gain product. We have shown that, despite the lower gain, which (i)...

Optical frequency synthesizers have widespread applications in optical spectroscopy, frequency metrology, and many other fields. However, their applicability is currently limited by size, cost, and power consumption. Silicon photonics technology, which is compatible with complementary-metal-oxide-semiconductor fabrication processes, provides a low-...

Dielectric Laser Acceleration (DLA) achieves the highest gradients among structure-based electron accelerators. The use of dielectrics increases the breakdown field limit, and thus the achievable gradient, by a factor of at least 10 in comparison to metals. Experimental demonstrations of DLA in 2013 led to the Accelerator on a Chip International Pr...

High-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undes...

At the surfaces of nanostructures, enhanced electric fields can drive optical-field photoemission and thereby generate and control electrical currents at frequencies exceeding 100 THz (refs. 1–11). A hallmark of such optical-field photoemission is the sensitivity of the total emitted current to the carrier-envelope phase (CEP)1–3,7,11–17. Here, we...

Ability to selectively enhance the amplitude and maintain high coherence of the supercontinuum signal with long pulses is gaining significance. In this work, an extra degree of freedom afforded by varying the dispersion profile of a waveguide is utilized to selectively enhance supercontinuum. As much as 16 dB signal enhancement in the telecom windo...

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with mm-scale, segmented interaction...

In our recent work [1], in search for a thermo mechanically strong broadband thin-disk (TD) laser gain material, we have investigated continuous-wave (cw) laser performance of Ti:Sapphire, Cr:LiSAF, Cr:LiCAF and Alexandrite under identical conditions, and show that among the investigated transition metal doped gain media, Alexandrite is the best al...

Yb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak powers, and became the work-horse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and search for alternative broadband TD gain media, with sui...

A theory of terahertz generation using a superposition of beamlets is developed. It is shown how such an arrangement produces a distortion-free tilted pulse front. We analytically show how a superposition of beamlets produces terahertz radiation with greater efficiency and spatial homogeneity compared to tilted pulse fronts generated by diffraction...

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especiall...

We present detailed measurements of effective emission cross section spectra of the Alexandrite gain medium in the 25-450 °C temperature range and provide analytic formulas that can be used to match the measured spectra. The measurement results have been used to investigate the wavelength and temperature dependence of small signal gain, as well as...

Terahertz-based electron acceleration has recently emerged as a promising candidate for driving next-generation high-brightness electron sources. Although initial demonstrations have proven the feasibility of this technology for accelerating and manipulating the phase space of electrons, further demonstrations of exquisite timing control are requir...

We explore the dynamics of a system where input spectra in the optical domain with very disparate center frequencies are strongly coupled via highly phase-matched, cascaded second-order nonlinear processes driven by terahertz radiation. The only requirement is that one of the input spectra contain sufficient bandwidth to generate the phase-matched...

Highly-efficient optical generation of narrowband terahertz radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation driven by temporal beating of chirped pulses but remain, however...

Ability to selectively enhance the amplitude and maintain high coherence of the supercontinuum signal with long pulses is gaining significance. In this work an extra degree of freedom afforded by varying the dispersion profile of a waveguide is utilized to selectively enhance supercontinuum. As much as 16 dB signal enhancement in the telecom window...

Intraoperative margin assessment is clinically important, especially for tissue conserving surgery like Mohs micrographic surgery in which minimization of the surgical area is crucial. Instead of the complex frozen pathology protocol, slide-free histopathological imaging of hematoxylin-eosin stained whole-mount skin tissues is demonstrated by using...

Attosecond metrology sensitive to sub-optical-cycle electronic and structural dynamics is opening up new avenues for ultrafast spectroscopy of condensed matter. Using intense lightwaves to precisely control the fast carrier dynamics in crystals holds great promise for next-generation petahertz electronics and devices. The carrier dynamics can produ...

High-repetition-rate, high-power, few-cycle mid-infrared lasers with carrier-envelope phase (CEP) stabilization are ideal driving sources for studying strong-field nonlinear processes, such as strong-field driven electron emission, solid-state high-harmonic generation, and nonlinear microscopy. Here, we report on a 1-MHz, 1-μJ, femtosecond, 2.1-µm...

We explore the dynamics of a system where input spectra in the optical domain with very disparate center frequencies are strongly coupled via highly phase-matched, cascaded second-order nonlinear processes driven by terahertz radiation. The only requirement is that one of the input spectra contain sufficient bandwidth to generate the phase-matched...

We introduce a technique to generate compressed broadband terahertz pulses based on cascaded difference-frequency generation. The approach employs a non-uniform sequence of pump pulses in aperiodically poled crystals. The pump-pulse format and poling of crystals conceived are such that the emergent terahertz pulse is already compressed. The method...

A spatio-temporal analysis of terahertz generation by optical rectification of tilted pulse fronts is presented. Closed-form expressions of terahertz transients and spectra in two spatial dimensions are furnished in the undepleted limit. Importantly, the analysis incorporates spatio-temporal distortions of the optical pump pulse such as angular dis...

We present a CMOS-compatible, Q-switched mode-locked integrated laser operating at 1.9 µm with a compact footprint of 23.6 × 0.6 × 0.78mm. The Q-switching rate is 720 kHz, the mode-locking rate is 1.2 GHz, and the optical bandwidth is 17nm, which is sufficient to support pulses as short as 215 fs. The laser is fabricated using a silicon nitride on...

Intraoperative margin assessment of surgical tissues during cancer surgery is clinically important, especially in the case of tissue conserving surgery like Mohs micrographic surgery in which minimization of the surgical area is considered crucial. Frozen pathology is the gold standard of assessing excised tissues for signs of remaining cancerous l...

We demonstrate multimodal label-free nonlinear optical microscopy in human skin enabled by a fiber-based two-color ultrafast source. Energetic femtosecond pulses at 775 nm and 1250 nm are simultaneously generated by an Er-fiber laser source employing frequency doubling and self-phase modulation enabled spectral selection. The integrated nonlinear o...

We report on a few-cycle, carrier–envelope-phase-stable laser source based on supercontinuum generation driven by an amplified Er:fiber-based system. Laser pulses from an Er:fiber oscillator are amplified, and these amplified pulses generate a stable supercontinuum in a highly nonlinear optical fiber. The short- and long-wavelength tails of this co...

The polarization states of high-harmonics generated in silicon with elliptical excitation are studies. Circularly polarized harmonics are demonstrated with both circular and non-circular excitation, determined by crystal symmetry and the dynamical response of the system.

We present a segmented THz based device (STEAM) capable of performing multiple high-field operations on the 6D-phase-space of ultrashort electron bunches. With this single device, powered by few-micro-Joule, single-cycle, 0.3 THz pulses, we demonstrate record THz-acceleration of >30 keV, streaking with <10 fs resolution, focusing with >2 kT/m stren...

We report on an optical synthesis of two compressed channels from our parametric waveform synthesizer, leading to a 0.6 mJ 3.4 fs pulse (3.2 fs transform limited) with a central wavelength of 1.8 /an, corresponding to 0.6 optical cycles.

The dynamically resolved response of the canted-antiferromagnet FeBO 3 excited near a magnon resonance shows fast oscillations after THz-excitation’ followed by the magnons’ intrinsic relaxation’ enabling to probe transient magnetic relaxation dynamics over large frequency range.

Circularly polarized high-harmonics can be generated from solids with elliptical and circular driver polarization. We investigate the temporal coherence properties of both cases and compare them to those of linearly polarized high-harmonics.

We present HHG driven with a sub-cycle mJ-level parametric waveform synthesizer. The variation of the HHG spectral shape and yield as a function of the relative phase between the synthesizer channels is shown.

We present results from a pulsed-optical timing link using single-mode fiber components installed in SwissFEL. The system shows 2.6-fs RMS timing jitter in [1 MHz - 20 µHz] and presents a versatile alternative to polarization-maintaining version.

We use arrays of electrically connected bowtie nanoantennas to detect the carrier-envelope phase of few-cycle optical pulses with noise performance close to the shot-noise limit. Our results pave the way towards low-cost, low-profile CEP monitoring and tagging.

We have investigated photoemission from an emitter array of Au nanospirals on an ultrafast time scale via velocity-map-imaging (VMI) spectroscopy. Circular dichroism of the velocity distribution of the emitted electrons is observed.

We demonstrate a silicon photonics optical frequency synthesizer (SPOFS). The frequency instability obtained in the telecom band is 1×10−12 at 1s level, comparable to a bench-top commercial optical frequency synthesizer system.

We demonstrate label-free multiphoton microscopy of human brain and skin tissues with an Er:fiber-laser-based ultrafast source that emits femtosecond pulses at 775 nm and 1250 nm.

We demonstrate a passively CEP-stable, 1-MHz, 2-µm dispersion managed optical parametric amplifier (OPA) with a chirped-pulse DFG front-end, pumped by an all fiber 1-µm source generating compressed µJ-level output pulses with 94.5 fs pulse duration.

We compare two different methods for relative phase synchronization among two channels of a parallel parametric waveform synthesizer. The achieved stability allows for reproducible high-energy sub-cycle pulses capable to generate isolated attosecond pulses without gating.

We demonstrate a self-calibrated optical frequency synthesizer using a fully-integrated erbium-doped tunable laser. A 20 nm tuning range from 1544 nm to 1564 nm is achieved with ~10−13 frequency instability at 10s averaging time.

A cryogenic Yb:YAG composite-thin-disk laser driver has demonstrated stable operation at 500 Hz with 1-joule 20-ns pulses. Results with chirped pulses will be presented. Joule-level pulses at 500 Hz compressible to 5 ps are expected.

We develop a Terahertz driven accelerator technology with the goal to construct a compact, fully coherent, attosecond X-ray source suitable to outrun radiation damage effects due to high X-ray irradiance required for serial X-ray crystallography.

We demonstrate fiber-based two-color sources that produce femtosecond pulses in two biomedical transmission windows (800 and 1300 nm). This powerful source enables multiphoton microscopy for both virtual skin biopsy and protein nano-crystal scoring.

We demonstrate a novel energy-efficient method for seeding CEP-stable OPCPAs. We couple the CEP-stable idler of a broadband OPCPA into a Kagome fiber thus compensating for its angular chirp. We show the pulse compressibility.

The analytical formalism for the reconstruction of terahertz pulses measured via electro-optic sampling is generalized for multilayer crystals.

We develop a Terahertz driven accelerator technology with the goal to construct compact electron and X-ray sources. Here, we use a two-stage segmented-terahertz-electron-accelerator-and-manipulator (STEAM) setup to demonstrate greatly improved electron beam energy and emittance.

We report on strong-field, ultrabroadband THz radiation generation for magneto-optic experiment, demonstrated with the resonant excitation and dynamically resolved relaxation measurement of FeBO3. Tunability of the central frequency of the THz pulses is demonstrated experimentally.

Ultra-precise timing has become a prerequisite for many modern large-scale scientific instruments, and timing precision is a crucial enabling factor to achieve the ultimate goals of those instruments. Here, we review the recent progress in timing technologies, including timing characterization methods among different kinds of sources (optical laser...