Correlative imaging allows the investigation of biological samples under various aspects. We are working towards a correlated extreme ultraviolet (EUV), infrared (IR) and visible (VIS) fluorescence microscope in a single integrated setup. With ptychography, which is a lensless imaging technique, quantitative amplitude and phase information are obtained. EUV ptychography provides resolution in the nanometer scale and an excellent element contrast, but suffers from low throughput due to limited coherent photon flux. Using longer wavelengths such as UV/visible and Near-IR enables high-speed imaging with sub micrometer resolution. Further, the combination with fluorescence detection adds functional contrast with micrometer-scale resolution. Here, we demonstrate in a proof-of-concept experiment correlated ptychography-fluorescence microscopy in the visible range. By using the reconstructed beam from the ptychography measurement the fluorescence scanning map can be deconvolved, which significantly improves the resolution.
EUV ptychography combines high resolution, strong material contrast, reasonable penetration depth, and easy sample preparation. It thus has the potential to bridge the gap between visible light and electron microscopy. In this contribution, we present recent results on table-top ptychographic EUV microscopy. The experimental setup relies on a high photon flux 13.5 nm wavelength high-order harmonic source. A simple amplitude mask upstream of the sample tailors the illumination. The sample is scanned by piezo-driven stages. Finally, far-field diffraction patterns are recorded by an EUV detector and fed into a ptychographic iterative engine to retrieve both the illumination and the sample transmission in amplitude and phase. The resolution of the EUV microscope (16 nm) was characterized using a Siemens star test sample. In the next step, a thin lamella of an integrated circuit was investigated. The resulting EUV images exhibit a rich diversity in amplitude and phase. We analyzed that the relative amplitude precision is better than 4% and the absolute phase precision as good as ~ 20 mrad. In this way, the microscope provides excellent input data for further analysis of the material composition. For this purpose, the projected scattering quotient is calculated from the reconstructed phase and amplitude and compared to tabulated material parameters. Here, materials like Al, Si3N4, and SiO2 were determined with high sensitivity. Finally, biological samples, namely germlings of the filamentous fungus Aspergillus nidulans were investigated. A similar scattering quotient analysis allowed us to clearly distinguish regions with low- and high lipid and phospholipid concentrations.
In this contribution, we present a scanning coherent diffractive imaging (i.e. ptychography) microscope operating in the EUV. Coherent EUV radiation at 13.5 nm is generated by high-order harmonic generation using a high-power fiber laser system. Utilizing structured illumination, a highly stable EUV source and ptychography setup sub 20 nm half-pitch resolution is demonstrated on a resolution test chart. Moreover, the lamella of an integrated structure is investigated and its contained materials are identified using the measured quantitative amplitude and phase.
We present an ultrafast high-power coherently combined thulium-based fiber-CPA. The laser system aims to deliver 400 µJ pulse energy at a pulse repetition frequency of 500 kHz. The simultaneous availability of 200 W-class average power and 1 GW-class peak power in the 2 µm wavelength regime will not only be unique in the world of science but also will pave the way to more compact, cost-effective lasers which are ideally suited for high-harmonic generation into the water-window and nonlinear-frequency conversion to the mid-IR and THz spectral range.
We present the first high-power coherently combined thulium-doped fiber-CPA. The laser system delivers >228 µJ pulse energy with <120 fs pulse duration at a pulse repetition frequency of 500 kHz. Excellent long-term stability is achieved with an average power fluctuation of <0.5% RMS over >48 hours of operation even at an average-power >120 W. The simultaneous availability of 100 W class average power and 1 GW-class peak power in the 2 µm wavelength regime is not only unique in the world of science but also comes in a fully engineered and commercially available system design optimized for long-term operation..
High harmonic generation at high repetition rate is realized with a high average power 100W, 600kHz fiber laser system. Optimization is done for two different operation regimes. At 69-75eV the source delivers a world-record photon flux of >10^11photons/s/harmonic when using argon gas jets. The use of neon gas allows for operation at significantly shorter wavelength. The important 93eV harmonic can be generated at 5·10^9 photons/s/(1% bandwidth), while even higher values of >10^10 photons/s/(1% bandwidth) are achieved between 115-140eV. The HHG source provides excellent long-term power stability of ~1% RMS for each of the operation regimes.
Intense, ultrafast laser sources with an operation wavelength beyond the well-established near-IR are valuable tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. Such laser systems enable the scaling of the phase matching photon energy cut-off in high-order harmonic generation, which allows for the generation of coherent soft X-ray radiation up to, and even beyond, the water window. Such laser-driven sources enable a plethora of subsequent applications. A number of these applications can significantly benefit from an increase in repetition rate. In that regard, ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept for driving high-order harmonic generation. These lasers are capable of delivering ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with HHG in an antiresonant hollow-core fiber is a promising approach to realize high photon energy cut-off HHG from a compact setup. The realization is based on combining nonlinear pulse self-compression (leading to strong-field waveforms) and phase-matched high-order harmonic generation in a single antiresonant hollow-core fiber. In this demonstration, a photon energy cut-off of approximately 330 eV has been achieved, together with a photon flux >106 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subjects of our future work.
In recent years coherent diffraction imaging (CDI) has evolved into a mature technology. Thanks to its lensless nature, it allowed to bypass the limitations of X-ray optics. At the same time, laser development in combination with high harmonic generation (HHG) has pushed the coherent XUV photon flux to values comparable to 3rd generation synchrotron facilities, which enables lensless imaging experiments that were previously only possible at large-scale facilities. Furthermore, the intrinsic short pulse duration of HHG radiation has potential for imaging experiments down to attosecond time scales. In this contribution, we present our latest results on lensless imaging using a fiber laser driven HHG source at 92 eV. A high photon flux source is used for scanning coherent diffractive imaging (ptychography) demonstrating sub-50 nm resolution. Further, an extension to Fourier transform holography is shown, which enables to increase the useable bandwidth by a factor of five without sacrificing spatial resolution. This paves the way for combing high-resolution table-top lensless imaging with attosecond pump-probe experiments.
Intense, ultrafast laser sources with an emission wavelength beyond the well-established near-IR are important tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. In particular, such laser systems enable high photon energy cut-off HHG up to, and even beyond, the water window thus enabling a plethora of subsequent experiments. Ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept in this regard. These lasers already deliver ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with hollow-core fiber based nonlinear pulse compression is a promising approach to realize high photon energy cut-off HHG drivers. Herein, we show that thulium-doped, fiber-laser-driven HHG in argon can access the highly interesting spectral region around 90 eV. Additionally, we show the first water window high-order harmonic generation experiment driven by a high repetition rate, thulium-doped fiber laser system. In this proof of principle demonstration, a photon energy cut-off of approximately 400 eV has been achieved, together with a photon flux <105 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subject of our future work.
We report on the generation of 102 μJ-, sub-2 cycle pulses with several GW of peak power centered at 1.73 μm wavelength. The intense few-cycle source features 8.8 fs pulse duration (full-width at half maximum, FWHM). It is operated at 196 kHz pulse repetition rate and provides a record average power of 20 W. This result is enabled by the combination of two average power scalable concepts: direct emission from an ultrafast Tm-doped fiber laser system and nonlinear pulse compression in a gas-filled antiresonant hollow-core fiber. Ultrafast Tm-doped fiber lasers are scalable to kW-level average power, which emphasizes the great potential of this approach for driving high cut-off HHG at high repetition rates.
This contribution reports on the recent advances in high harmonic generation (HHG) with high power femtosecond fiber lasers at high repetition rates. The capabilities of high power fiber lasers, the challenges of phase matching in the tight-focusing regime and recent experimental results will be discussed. In particular, post compressed pules as short as 30 fs, with ~150 μJ pulse energy at 0.6 MHz repetition rate have been used for efficient HHG into the XUV. Despite the tight focusing phase matching is ensured by providing the target gas with adequately high density. A conversion efficiency in excess of 10-6 at ~30 eV has been achieved in xenon gas. This resulted in more than 100μW of average power (>1013 photons per second), which represents the highest photon flux achieved by any HHG source in this spectral region so far. In addition, further pulse compression yielded few-cycle pulses at high average power that have enabled efficient soft Xray generation in neon and helium. HHG in neon provided more than 3·109 photons/s within a 1% bandwidth at 120 eV and helium allowed for HHG up to the water window spectral region beyond 283 eV. These compact sources provide highest photon flux on a table-top and will enable exciting applications such as nanometer-resolution imaging or coincidence spectroscopy in the near future.
Over the last decade, the performance of femtosecond fiber laser systems has been rapidly improved. However, further improvements might be held back due to different physical limitations such as nonlinearities or optically induced damage. We demonstrate that with the coherent combination of four parallel fiber amplifiers record pulse energies and peak-powers of 5.7 mJ and 22 GW, respectively, could be achieved. These values could be realized with a chirped-pulse-amplification (CPA) laser system running at a repetition rate of 40 kHz and delivering a compressed average power of 230 W. A high combination efficiency of 89% was achieved demonstrating the scalability of the combining approach to a larger number of channels.
KEYWORDS: Crystals, Sapphire, Nonlinear crystals, Laser crystals, Frequency conversion, High power lasers, Harmonic generation, Finite element methods, Atomic force microscopy, Laser systems engineering
Lasers used for diverse applications from industry to fundamental science tend to increasing output powers. Some
applications require frequency conversion via nonlinear optical crystals, which suffer from the formation of temperature
gradients at high power operation which causes thermal lensing or destruction of the crystal due to tensile stresses. To
avoid these unwanted effects we joined a beta barium borate (BBO) crystal with sapphire disks serving as effective heat
spreaders due to their high thermal conductivity (thermal conductivity κ = 42 W/Km). Therefore, smooth and flat crystal
surfaces were joined by plasma-activated bonding. The joining relies on covalent bonds, which are formed via a
condensation reaction of the surfaces which are first connected by Van der Waals forces. The cleaned surfaces are
activated by plasma and brought into contact, pressed together and heat treated at a temperature of about 100°C. Special
attention has been paid to the cleaning of the surfaces. Therefor the surfaces have been evaluated before and after
treatment by means of atomic force microscopy. A stable connection has been formed successfully, which has been
tested in a proof of principle experiment and demonstrated efficient second harmonic generation at up to 253 W of input
power. Compared to a bare single BBO crystal it could be shown that the temperature within the crystal compound is
significantly reduced. Such hybrid structures pave the way for frequency conversion at kilowatts of average power for
future high power lasers.
Coherent combination applied to state-of-the-art femtosecond fiber CPA systems has recently opened the route towards high energy and average power laser systems. The ~2 mJ, 340 fs, 196 W (100 kHz) pulses of such a system are coupled to an argon-filled hollow-core fiber for spectral broadening via self-phase modulation. Subsequent compression in a chirped mirror compressor leads to ~1 mJ, 45 fs, 96 W pulses. Under different conditions 580 μJ, 26 fs, 11.6 GW, 135 W (250 kHz) pulses are achieved. This is an unprecedented combination of average power and pulse energy.
The process of high harmonic generation allows for up-conversion of infrared laser light towards the EUV or soft X-ray
region. If very short (few-cycle) laser pulses are employed and their carrier envelope phase (CEP) is well controlled the
generation of so-called isolated attosecond pulses becomes feasible. Today’s few-cycle laser technology relies on Ti:Sapphire laser systems and hollow fiber based post-compression. The output power of such lasers is typically below
1 W and the repetition rate is limited to a few kilohertz due to thermo-optical limitations of the Ti:Sapphire amplifiers.
In this contribution we present a different approach combining the advantages of fiber laser technology with nonlinear frequency conversion. A high power femtosecond fiber laser system serves as pump laser for an ultrabroadband optical parametric amplifier. As a result we are able to generate intense CEP-stable pulses with only two optical cycles duration at repetition rates up to 0.6 MHz. The excellent beam quality ensured by the fiber based pump laser enables focusing of these pulses to high intensities, thus, allowing for the generation of high harmonics and attosecond pulses at exceptionally high repetition rates. We will present the design of the laser system and discuss specific challenges such as the broadband signal generation, the temporal synchronization of the pump laser and the carrier envelop phase stabilization. In addition, experimental results on high repetition rate XUV continuum generation will be presented, demonstrating the feasibility of our approach.
The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems
emitting ultrashort pulses. In this contribution the experimental results of a coherently combined femtosecond fiber CPA
laser system consisting of 2 amplifiers is presented. Combining efficiencies as high as 89% and compressed pulse
energies of up to 3mJ were achieved. Additionally, the system showed excellent long-term stability.
Numerical and analytical considerations of nonlinear compression in noble-gas-filled hollow core fiber pulse
compressors are shown. Experiments are presented that enhance the peak power of existing fiber CPA architectures from
1.8 GW to 7.4 GW in a single stage. The pulses are shortened to 40 fs, which corresponds to a pulse shortening of more
than one order of magnitude. Sending these pulses into a second hollow fiber further shortens the pulses to 25 fs at a
pulse energy of 430 μJ at 30 kHz equaling an average power of 13 W. The peak power after the second stage is higher
than 10 GW. In addition, we present very recent experiments on average power scaling of hollow fiber pulse
compressors to the 100 W level. We compressed 200 W, 200 μJ, 500 fs pulses from a fiber CPA to 93 W, 93 μJ, 81 fs.
These unique pulse parameters should be very well suited for high harmonic generation.
Few-cycle pulses offer a wide range of interesting applications, for example in time-resolved studies of ultra-fast
phenomena in physics, chemistry and biology. Nonlinear spectral broadening in photonic crystal fibers (PCFs) followed
by dispersive compression allows for the generation of extremely short optical pulses. By employing this technique pulse
durations of only 5.5 fs (2.4 optical cycles) have been achieved so far. In this contribution we take advantage of SC
generation in all-normal dispersion PCF (ANDi PCF), which features only positive group-velocity dispersion across the
spectral region of interest. Spectral broadening therefore is dominated by self-phase modulation and optical wave
breaking, leading to smooth and highly coherent SC spectra. We show generation of SC spectra covering more than one
optical octave around 810 nm central wavelength. Active phase control and spectral shaping were employed to compress
the pulses to 3.64 fs (1.3 optical cycles), which is the shortest pulse duration achieved from SC compression in solid core
fibers to date. In contrast to other approaches, the presented concept delivers pulses with an excellent temporal pulse
quality and can be extended to even larger bandwidths to reach the sub-cycle regime, provided an adequate compressor
is employed.
We present second harmonic generation of a high average power, high energy femtosecond Yb doped fiber chirped pulse
amplifier. This system is operated at various repetition rates at a central wavelength of 1040 nm. After two pre-amplification
stages a main amplifier is used to achieve the required pulse energy for efficient second harmonic
generation. It is comprised of a 1.2 m long photonic crystal fiber with a mode field diameter of 45 μm. A dielectric
grating based compressor is used for compression to a pulse duration of about 406 fs. Second harmonic generation is
then achieved in a 500 μm thick BBO crystal. The conversion efficiency of the second harmonic generation remained
almost constant at >60 % for all repetition rates and average power levels. At 5.25 MHz the highest average power of
135 W at 520 nm was achieved. In addition this comes with an excellent beam quality which is validated by a measured
M2 < 1.2.
We present a high peak and average power laser system with ultrashort pulses at high repetition rates. Pulse shortening
and peak power enhancement of a state-of-the-art fiber laser system is achieved by utilizing nonlinearity, namely selfphase
modulation and subsequent compression in a chirped mirror compressor. The nonlinear interaction is achieved by
propagation in a noble gas filled hollow core fiber with an inner diameter of 200 μm and a length of 0.5 m. A total
second dispersion of -7000 fs2 is applied by a chirped mirror compressor resulting in ultrashort pulses of 71 fs duration.
This is achieved by coupling 400 MW, 800 fs pulses from the CPA system to the Xenon filled hollow core fiber. The
average power at the output of the compressor is measured to be 10 W at 50 kHz repetition rate resulting in 200 μJ pulse
energy. Hence, the compressed pulses have a peak power of more than 2 GW. Consequently, the pulses of the CPA
system are shortened by a factor of ten and the peak power is enhanced by a factor of 5. In addition this approach offers
further peak and average power scalability.
In this contribution we present a simple and robust pulse shaping device based on coherent pulse stacking. The device is
based on fiber Bragg gratings written in a polarisation maintaining step index fiber and a fiber optical circulator. Up to
four pulse replicas are reflected by fiber Bragg gratings and interfere at the output of the device. Temperature control
allows tuning of the relative pulse amplitudes and phases of the pulse replicas. We experimentally demonstrated 235 ps
and 416 ps long flattop pulses with rising and falling edges shorter than 100 ps. In contrast to other pulse shaping
techniques the presented setup is robust, alignment free, provides excellent beam quality and is also suitable for pulse
durations up to several nanoseconds.
We report on an ytterbium-doped fiber CPA system delivering 325 W of average power at 40 MHz repetition rate
corresponding to 8.2 μJ pulse energy. The pulse duration is as short as 375 fs resulting in 22 MW of peak power.
We present a degenerated-parametric amplifier with gigawatt peak power operating at 1030 nm and 30 kHz repetition
rate. Pulses of a fiber chirped pulse amplification (FCPA) system with 650 fs pulse duration and 1 mJ pulse energy are
frequency doubled and used as pump source for a two stage optical parametric amplifier. Both the FCPA and the optical
parametric amplifier (OPA) are seeded by the same YB:KGW oscillator. Spectral broadening of the OPA seed signal in a
short-polarization-maintaining-step-index fiber creates enough bandwidth for sub 30 fs pulse generation, while temporal
synchronization of pump and signal is realized by means of a multipass cell in the OPA signal beam path. Parametric
amplification of the broadband signal takes place in two 1 mm BBO crystals. Pulse compression via chirped mirrors
yields 81 μJ pulses as short as 29 fs. The corresponding pulse peak power is estimated to be as large as 2 GW. Together
with the good beam quality (measured M2<1.8) this device enables high intensity experiments at high repetition rates.
We present a Q-switched microchip laser emitting 1064nm pulses as short as 100ps synchronized to a cavity dumped
femtosecond laser emitting 800nm pulses as short as 80fs. The synchronization is achieved by presaturating the
saturable absorber of the microchip laser with femtosecond pulses even though both lasers emit at different
wavelengths. The mean timing jitter is 40ps and thus considerably shorter than the pulse duration of the microchip laser.
We report on a high repetition rate noncollinear optical parametric amplifier system (NOPA) seeded by a cavity
dumped Ti:Sapphire oscillator. The pump pulses for parametric amplification are generated via soliton generation in a
highly nonlinear photonic crystal fiber with a subsequent fiber-based amplification stage and are therefore
synchronized. The system is capable of producing high energy ultra-short pulses at repetition rates up to 2 MHz.
We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation.
Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal
generation is a promising alternative to white light generation in bulky glass or sapphire plates. As pump source we
propose the use of a high repetition rate ytterbium-doped fiber chirped pulse amplification system.
We report on the generation of 50 fs pulses with an average output power of more than 50W. This is done by combining a high average power fiber CPA system with a microstructured large-mode-area fiber for nonlinear compression. The fiber CPA system delivers 300 fs pulses with a repetition rate of 73MHz in a linearly polarized beam with diffraction-limited quality. The average output power can exceed 100W. Nonlinear compression of these pulses is done by launching the light into a very short piece of a microstructured fiber and then removing the phase with a pair of chirped mirrors.
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