Laser sources and developments

Experimental facility

One of our three laboratories is dedicated to ultrafast research on the femtosecond and attosecond timescale. The available equipment covers intense, pulsed few-cycle sources in the Terahertz, infrared, mid-infrared and visible as well as intense attosecond pulses up to the water window (285-420 eV). These sources are currently used for XUV and attosecond metrology, for exploring ultrafast magnetization dynamics in ferromagnetic and exotic materials and for testing novel hard x-ray pulse characterization tools, such as an ultrafast Terahertz-based streak camera.
The laboratory is equipped with a state of the art, TW-class Ti:sapphire based CPA laser system (Pulsar,Amplitude Technologies) installed in 2012. The laser system has three outputs (1*40 mJ and 2*20 mJ) delivering transform-limited 15-80 fs FWHM pulses at 800 nm. The system is designed to support CEP stabilized operation which opens the way to field-sensitive applications. We also operate a high energy, optical parametric amplifier (twin-TOPAS-HE, Light Conversion) delivering multi-mJ, sub-60 fs, phase-stable pulses with a central wavelength tunable between 1200 and 15'000 nm. Downstream, a THz pulse generation setup based on organic crystals (DAST,OH1,DSTMS) delivers ultra-intense single-cycle THz pulses in the 1-20 THz frequency range.
A high harmonic generation (HHG) beamline equipped with XUV spectrometer and tight focusing optics offers cutting edge technology for investigating ultrafast processes of e.g. ultrafast magnetization dynamics based on the transverse magnetic optical Kerr effect (T-MOKE) and other techniques. The multi-beam Ti:sapphire laser system with its secondary laser sources (THz,IR, VIS, XUV) offers versatile pump-probe configurations with a lot of possibilities for cutting-edge, multi-color experiments at excellent temporal and spatial resolution.

Laser-driven THz source

Our group is pioneering the production of ultra-intense, single-cycle Terahertz pulses based on optical rectification of intense mid-infrared pulses in organic crystals, such as DSTMS, OH1, DAST, HMQ-TMS and others.
Terahertz radiation located between the optical and the microwave spectral region (i.e. 0.1-10 THz) is well suited to explore fundamental physical phenomena and to drive new applications in material science, biology, medicine and many others. Ultra-strong THz fields offer in particular new opportunities in femtomagnetism, for investigating collective effects in gases or solids, for charged particle manipulations and for THz-assisted high harmonic generation. Strong THz transients are also a versatile tool at X-ray free electron laser facilities for novel pump-probe experiments and for the temporal characterization of the femtosecond X-ray pulses.

Our recent Terahertz source development based on organic crystals is extremely appealing as the source offers
  • high nIR-to-THz conversion efficiency (1-2%)
  • multi octave-spanning spectra across the entire THz gap (1-15 THz)
  • collinear pump beam configuration (i.e. no sophisticated pump pulse shaping like pulse front tilting and imaging is needed)
  • naturally collimated and aberration-free THz radiation. This makes focusing by a single optics to diffraction-limited spot-size and thus the realization of highest field strength feasible.
  • a stable absolute phase, which is essential for field-sensitive experiments.

The optical rectification in organic materials permits the realization of extremely short and intense THz electro-magnetic field transients (up to GV/m, several Tesla). These materials provide low THz absorption and a non-linear coefficient one order of magnitude larger than conventionally used LiNbO3. Velocity matching is achieved in collinear geometry for pump wavelengths between 1.35 and 1.5 µm. A precise control of the absolute phase of the THz pulse has been recently demonstrated by our group by combining dispersion properties of different transparent plastics.

Parameters of the PSI THz laser-based source

Sources LiNbO3 [1] Organic crystals [2-7] Air plasma [8]
Pump wavelength [nm] 800-1040 800;1500 800 + 400
Pulse energy [µJ] <45 <50 <1
Laser to THz conversion efficiency [%] <0.4 2 0.01
Electric field strength MV/cm <1 <6 0.6
Focus size [mm] <1 <0.3 <0.3
Emission frequency range <1 THz 1-15 THz 1-20 THz

References:
  1. C. Vicario, B. Monoszlai, C. Lombosi, A. Mareczko, A. Courjaud, J. A. Fülöp, and C.P. Hauri
    Pump-pulse width and temperature effects in lithium niobate for efficient THz generation
    Opt. Lett. 38,5373 (2013).
  2. C. Vicario, C. Ruchert and C.P. Hauri
    High field THz generation in organic materials
    Journal of Modern Optics ( 2013).
  3. C. Ruchert, C. Vicario and C.P. Hauri
    Spatiotemporal Focusing Dynamics of Intense Supercontinuum THz Pulses
    Phys. Rev. Lett. 110, 123902 (2013).
  4. C. Ruchert, C. Vicario and C.P. Hauri
    Scaling Sub-mm single-cycle transients towards MV/cm fields via optical rectification in organic crystal OH1
    Opt. Lett. 37,899 (2012).
  5. C.P. Hauri, C. Ruchert, F. Ardana and C. Vicario
    Strong-field single-cycle THz pulse generated in organic crystal
    Appl. Phys. Lett. 99,161116 (2011).
  6. B. Monoszlai, C. Vicario, and C.P. Hauri
    High energy terahertz pulses from organic crystals: DAST and DSTMS pumped at Ti:sapphire wavelength
    Opt. Lett. 38,5106 (2013).
  7. C. Vicario, B. Monoszlai and C.P. Hauri
    GV/m single-cycle terahertz fields from a laser-driven large-size partitioned organic crystal
    Phys. Rev. Lett. 112,213901 (2014).
  8. C. Ruchert, F. Ardana, A. Trisorio, C. Vicario, and C.P. Hauri
    Towards high power single-cycle THz laser pulses for initiating high-field sensitive phenomena
    Chimia 65 (2011) 320–322.

UV pulse shaping for electron bunch tailoring



High-brightness electron beams are essential to efficiently drive compact x-ray free electron lasers (FELs), such as the SwissFEL. The production of such electron beam is challenging as it poses high demands on the driving laser pulse applied to a copper photocathode. For state of the art electron guns used at FELs picosecond, flat-top, pulses in the range of 250-280 nm are required. Typically the third harmonic of a Ti:sapphire laser is used as it allows to reach several tens of microJoule pulse energy in the deep UV. But shaping of deep-UV pulses with fast rise- and fall-time and flat-top temporal and spatial profile is a great challenge. Since the photon quantum energy is high (4-5 eV) it is difficult to transfer well-established shaping technology developed for near infrared pulses to the deep UV range as the quantum photon energy is close to the ionization threshold of transparent optical materials. We are exploring novel techniques to perform temporal pulse shaping in the deep UV.

* Indirect pulse shaping: UV temporal pulse shaping has been explored by applying pulse shaping to the near infrared pulse prior to frequency tripling. A versatile ultrabroadband tunable Ti:sapphire chirped pulse amplification system allows temporal shaping via the spectral domain [1]. In our system a Mazzler located in the regenerative cavity is used to control the spectral bandwidth of the amplified pulse. A Dazzler allows for wavelength pre-selection with a continuous variation of the central wavelength within a range of 755-845 nm with a bandwidth of 110nm<Δλ<16 nm. It allows powerful pulse shaping in the near infrared. Unfortunately, due to the nonlinear conversion process the UV pulse shape gets significantly distorted.

* Direct pulse shaping in the deep UV by AOPDF: In order to overcome the drawbacks of indirect UV pulse shaping we developed an amplitude and phase-shaping scheme directly in the UV. It consists of an efficient prism stretcher and a acousto-optic programmable dispersive filter (AOPDF) applied directly in the UV. This scheme allows direct manipulation of the pulses in deep ultraviolet spectral range (270 nm) due to arbitrary phase and amplitude control [3].

* Direct pulse shaping by Chirped-Matched Sum Frequency Generation:

We developed a novel approach for the production of flat-top deep UV pulses by mixing two stretched Gaussian pulses in a BBO crystal. Usually the acceptance bandwidth of the harmonic generation crystals limits the minimum rise time of the flat-top profile. We explored a novel scheme which relies on chirp-matched sum frequency generation, which overcomes this limitation. This scheme combined with IR spectral manipulation is a novel approach for deep-UV pulse shaping [2] allows for the generation of few-hundred microjoule, picosecond, deep-UV pulses with a flattop like shape.
* Direct pulse shaping by pulse stacking: Another technique for flattop pulse formation relies on stacking several pulse replicas in time. For this scheme to work up to six birefringent alpha-BBOs are used in sequence, which produce up to 64 individual pulses, each with a different delay, given by the thickness of the crystals. The ensemble of those pulses sums up to a quasi-flattop pulse in the temporal domain. As the scheme has only passive elements it is expected to be robust and well suited for long-term operation [4]. Unfortunately the flatness of the pulse is dominated by the individual pulse duration, and shows a typical modulation depth of 10-20 percent. This may cause problems in the linear accelerator as the temporal laser intensity modulation turns into an electron density modulation which can lead to unwanted effects in the accelerator, such as filamentation in phase-space due to micro-bunching.

References:
  1. A. Trisorio, P. M. Paul, F. Ple, C. Ruchert, C. Vicario, C.P. Hauri
    Ultrabroadband TW-class Ti:sapphire laser system with adjustable central wavelength, bandwidth and multi-color operation Opt. Express 19, 20128 (2011)
  2. C. Vicario, A. Trisorio, G. Arisholm, C. P. Hauri
    Deep-ultraviolet picosecond flat-top pulses by chirp-matched sum frequency generation
    Opt. Lett. 37, 1619 (2012)
  3. A. Trisorio, C. Ruchert, C. P. Hauri
    Direct shaping of picosecond high energy deep ultraviolet pulses
    Appl. Phys. B 105, 255 (2011)
  4. A. Trisorio, M. Divall, C. Vicario, A. Courjaud and C. P. Hauri
    New concept for the SwissFEL gun laser
    TUPSO88 Proceedings of FEL2013, New York, NY, USA (2013)

Few Cycle pulse generation

Intense few-cycle femtosecond laser pulses in the infrared (700-1000 nm) and short-wavelength mid-infrared spectral region (1000-3000 nm) have found numerous applications. In our laboratory, we use them for

  • intense attosecond pulse generation in the XUV [1],
  • production of single-cycle pulses in the terahertz (THz) spectral region (1-20 THz) via optical rectification in organic crystals [2]
  • for time-resolved investigations in solids.

We apply various techniques, such as gas filled hollow core fiber [3], filamentation in noble gases [4] and nonlinear frequency conversion in nonlinear organic crystals [5] depending on the pulse energy and wavelength required by the application. These pulse shortening techniques are based on the nonlinear interaction of a femtosecond laser pulse with a χ2 or χ3 media (gas, crystal) leading to a significant pulse spectral broadening and eventually to temporal self-compression down to a few optical cycles. We have established R&D partnerships with industry to develop and validate an accurate and robust device for few-cycle mid-infrared pulse temporal characterization [6], for example. Further activities are ongoing in order to temporally shape and stabilize the carrier-envelope phase (CEP) of these few-cycle pulses.
In the future these short pulses will be combined with the femtosecond hard x-ray SwissFEL laser for time-resolved investigations.

References:
  1. F. Ardana-Lamas, G. Lambert, A. Trisorio, B. Vodungbo, V. Malka, P. Zeitoun and C.P. Hauri
    Spectral characterization of fully phase-matched high harmonics generated in a hollow waveguide for free-electron laser seeding
    New J. Phys. 15 073040 (2013)
  2. C.P. Hauri, C. Ruchert, C. Vicario, F. Ardana
    Strong-field single-cycle THz pulses generated in an organic crystal
    Appl. Phys. Lett. 99, 161116 (2011)
  3. M. Nisoli, S. De Silvestri and O. Svelto
    Generation of high energy 10 fs pulses by a new pulse compression technique
    Appl. Phys. Lett. 68, 2793 (1996)
  4. A. Braun, G. Korn, X. Liu, D. Du, J. Squier and G. Mourou
    Self-channeling of high-peak-power femtosecond laser pulses in air
    Opt. Lett. 20, 73 (1995)
  5. A. Trisorio, M. Divall, B. Monoszlai, C. Vicario and C.P. Hauri
    Intense sub-2 cycle infrared pulse generation via phase mismatched cascaded nonlinear interaction in DAST crystal
    Opt. Lett. 39, 2660 (2014)
  6. A. Trisorio, S. Grabielle, M. Divall, N. Forget, and C.P. Hauri
    Self-referenced spectral interferometry for ultra-short infrared pulse characterization
    Opt. Lett. 37, 2892 (2012)

New schemes for intense mid-IR femtosecond sources

Intense laser pulses in the mid infrared wavelength range (3 to 20 μm) are important for accessing rotational-vibration modes in solids and moleculs, for investigating semiconductors, superconductors and magnetic materials [1,2] as well as for EXPLORING novel schemes to scale of table-top soft x-ray sources towards the water window and beyond [3–5].

Currently this spectral range is typically accessed with a multistage optical parametric amplifier system. As this technology has certain drawbacks we are investigating alternative approaches. Our research is based on the careful analysis and understanding of the nonlinear amplification process employzing simulations. Through this approach we could already demonstrate efficient broadband UV generation [6] as it offers insight into the mixing process. This helps to understand the energy transfer and to identify critical experimental parameters for optimizing the nonlinear mixing process.
An example of a recent study is shown next. We have investigated the direct mid-IR generation using the bi-color output of our cutting-edge Ti:Sapphire amplifier system [7]. Through the integration of an intra-cavity acousto-optic pulse shaping [8] in the amplifier system allows for simultaneous amplification of two wavelengths in the amplifier change. This output is well suited for direct difference frequency generation to the mid-IR.

Unfortunately most materials show strong two photon absorption (TPA) for this mixing process. Through our numerical analysis we have made an analysis of the mixing process by looking at the energy transfer inside the nonlinear crystal (Figure 1).

The use of compressed pulses let the nonlinear process terminate after 300 μm and the achieved idler energy is limited by TPA and group velocity mismatch. We could show that a much more efficient mixing process can be achieved by using chirped input pulses since then the full crystal length is employed for the mixing process (Figure 2). Indeed, in the chirped-pulse configuration the pump beam penetrates much deeper into the nonlinear crystal and the energy conversion to the idler takes place over a longer interaction range. Our illustration also visualizes that the energy flow is behaving differently in the chirped approach. While in Figure 1 all spectral components are affected equally, in the chirped case (Figure 2) it is the central part of the pump spectrum which is contributing most to the mixing process. After 400 μm depletion sets in, but there is still sufficient energy in the spectral wings of the pump pulse to drive the conversion process further. The long wavelength part of the spectrum is now interacting with the long wavelength part of the signal spectrum and can thus also contribute to the energy transfer into the idler. Our approach predicts a significantly improved efficiency for the mixing process and provides a compact, phase-stable, intense infrared source for high-field applications.

References:
  1. W. Kuehn et al. Phys. Rev. Lett. 107, 067401 (2011).
  2. F. Junginger et al. Phys. Rev. Lett. 109, 147403 (2012).
  3. C.P. Hauri et al. Opt. Lett. 32, 868 (2007).
  4. T. Popmintchev et al. Science 336, 1287 (2012).
  5. M.-C Shen et al. ArXiv14010240 Phys. (2013).
  6. C. Vicario, A. Trisorio, G. Arisholm and C.P. Hauri, Opt. Lett. 37, 1619 (2012).
  7. C. Erny and C.P. Hauri, Appl. Phys. B DOI 10.1007/s00340-014-5846-6
  8. T. Oksenhendler et al. Appl. Phys. B 83, 491 (2006).

Photocathode development

Electron beams in modern linear accelerators, which preserve emittance, are limited in brightness by the intrinsic emittance of the photocathode electron source. Therefore it becomes important for large scale facilities such as the Swiss free electron laser (SwissFEL) to reduce this fundamental limit.
Our group has carried out quantum efficiency studies for various laser wavelengths (from 261 to 282 nm) and various technical metal surfaces (Mo, Nb, Al, Cu) in a dedicated unbaked vacuum chamber in the absence of a significant electrical field. Cu and Cs2Te were also studied under high RF fields. We have demonstrated significant reduction of thermal emittance, by tuning the laser photon energy closer to the effective workfunction of the material.
Another ongoing investigation is focusing on single tip cathodes with a micro-structured surface (multifilamentary cylindric Nb3Sn tip). This surface consists of more than 10’000 tips per mm2, each tip of 2-7 micrometer in diameter. Such novel cathodes offer the potential for generating electron beams at ultra-low emittance, as the emittance scales linearly with the radius of the emitting surface area.





References:
  1. C.P. Hauri, R. Ganter, F. Le Pimpec, A. Trisorio, C. Ruchert, H.H. Braun
    Intrinsic emittance reduction of an electron beam from metal photocathodes
    Phys. Rev. Lett. 104, 234802 (2010).
  2. F. Le Pimpec, F. Ardana-Lamas, C.P. Hauri, C. Milne
    Quantum efficiency of technical metal photocathodes under laser irradiation of various wavelength
    Appl. Phys. A 112, 647 (2013).
  3. F. Ardana-Lamas, F. Le Pimpec, A. Anghel, C.P. Hauri
    Towards high brightness electron beams from multifilamentary Nb3Sn wire photocathode
    Phys. Rev. ST Accel. Beams 16, 043401 (2013).
  4. A. Anghel, F. Ardana-Lamas, F. Le Pimpec, C.P. Hauri
    Large charge extraction from metallic multifilamentary Nb3Sn photocathode
    Phys. Rev. Lett. 108, 194801 (2012).