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Intracavity High Harmonic Generation

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A growing number of scientific applications are currently driving a strong research effort to develop novel ultrafast laser sources that enable high average powers, i.e., sources that combine high peak power and high repetition rate. Among many important examples, one prominent application is high-harmonic generation (HHG), where higher repetition rates are desired to obtain a higher flux in the UV, enabling faster measurements at higher signal-to-noise ratio.

SESAM modelocked thin disk laser (TDL) oscillators are outstanding candidates to drive these experiments directly from low-noise table-top oscillators. Nowadays, average powers > 270 W [1] and pulse energies > 80 µJ [2] can be reached at multi-megahertz repetition rates from such modelocked oscillators, which is comparable to state-of-the-art amplifier systems.

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Figure 1. Video of thin disk laser operation (Blender animation: martin.hoffmann@unine.ch)

In particular, making use of the high intracavity peak power levels inside a TDL to drive extreme nonlinear optics experiments is a promising approach that has not yet been demonstrated. In our project "Efficient megahertz XUV light source" , which was recently funded with an ERC starting grant, we want to make use of the high intracavity peak power available inside a TDL resonator to efficiently drive HHG.

 

Figure 2. State-of-the-art pulse duration and peak power of thin disk lasers, showing the potential of this technology for intracavity HHG

Passive enhancement cavities for femtosecond laser pulses have already proven to be excellent configurations for exploiting nonlinear processes between light pulses and matter, in particular targeting the widespread area of HHG at MHz repetition rates. Up-to-date, passive enhancement cavities can provide intracavity power of 18 kW at 78 MHz with 200 fs pulses and power levels > 200 µW in the UV [3], opening the door to exciting applications such as VUV/XUV precision spectroscopy on He + or even exploring nuclear transitions. Very recently, the first demonstration of direct XUV frequency comb spectroscopy of Argon was reported at MHz repetition rate using a UV frequency comb driven by an ultrafast high-power fiber laser system coupled to a passive enhancement cavity [4]. However, passive enhancement cavities require careful coherent coupling of femtosecond pulses from the driving laser into the resonator. The circulating pulse has to match the driving pulses, which becomes challenging in the presence of intracavity nonlinearities, phase distortions and dispersion. Furthermore, efficient extraction of the UV radiation from these very high finesse cavities is challenging.

Driving HHG inside TDLs would be substantially simpler. In this case, both pulse formation and laser amplification are achieved inside the cavity, where the nonlinear process takes place. In addition, there is no need for coherent coupling and the circulating pulse can simply adapt to the present nonlinearity. Another potential advantage is that different transverse mode profiles can be achieved, for example TEM 01 , for efficient output coupling of the high harmonics via a hole in a cavity mirror.

 

Figure 3. Schematics of proposed intralaser high-harmonics generation experiment


Driving HHG efficiently inside a TDL requires short pulse durations obtained directly from the oscillator (typically sub-100 fs). Until recently, the pulse duration available from TDLs was limited to > 200 fs [5]. However, this limit was recently overcome and pulses as short as 62 fs were demonstrated from a SESAM-modelocked TDL based on the novel broadband gain material Yb:CALGO [6], which represents an important step towards our targeted experiments of intracavity HHG.

In addition, another key point in particular for applications in spectroscopy and metrology, is the frequency stability of such high-power oscillators. Stabilization of the carrier-envelope offset (CEO) frequency is challenging, as it requires complex and sensitive detection schemes such as f-to-2f interferometers. Until very recently, it remained unclear whether the strongly multimode pumping scheme of TDLs could potentially increase the noise of the oscillator to the level where a stable frequency comb cannot be achieved. However, it was demonstrated that stabilization of the CEO frequency of a SESAM-modelocked TDL is possible [7], further confirming the potential for TDLs for intracavity HHG and the generation of UV-frequency combs. These two key milestone demonstrations increase our confidence that ultrafast TDLs are ideal sources for intralaser megahertz extreme nonlinear optics, in particular for driving HHG. The resulting high-repetition rate compact XUV source will be a unique tool for diverse applications in spectroscopy and metrology.

 

Relevant publications:

  1. C. J. Saraceno, F. Emaury, O. H. Heckl, C. R. E. Baer, M. Hoffmann, C. Schriber, M. Golling, T. Suedmeyer, and U. Keller, 275 W average output power from a femtosecond thin disk oscillator operated in a vacuum environment, Opt. Express 20, 23535 (2012) PDF
  2. C. J. Saraceno, F. Emaury, C. Schriber, M. Hoffmann, M. Golling, T. Südmeyer, and U. Keller, Ultrafast thin-disk laser with 80-μJ pulse energy and 242  W of average power, Opt. Lett. 39, 9 (2014) PDF
  3. I. Pupeza, T. Eidam, J. Rauschenberger, B. Bernhardt, A. Ozawa, E. Fill, A. Apolonski, T. Udem, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tunnermann, T. W. Hansch, and F. Krausz, Power scaling of a high-repetition-rate enhancement cavity, Opt. Lett. 35, 2052 (2010).
  4. A. Cingoz, D. C. Yost, T. K. Allison, A. Ruehl, M. E. Fermann, I. Hartl, and J. Ye, Direct frequency comb spectroscopy in the extreme ultraviolet, Nature 482, 68 (2012).
  5. T. Südmeyer, C. Kränkel, C. R. E. Baer, O. H. Heckl, C. J. Saraceno, M. Golling, R. Peters, K. Petermann, G. Huber, and U. Keller, High-power ultrafast thin disk laser oscillators and their potential for sub-100-femtosecond pulse generation, Appl. Phys. B 97, 281 (2009). PDF
  6. A. Diebold, F. Emaury, C. Schriber, M. Golling, C. J. Saraceno, T. Südmeyer, and U. Keller, Phase-stabilization of the carrier-envelope-offset frequency of a SESAM modelocked thin disk laser, Opt. Lett. 38, 1 (2013). PDF
  7. A. Klenner, F. Emaury, C. Schriber, A. Diebold, C. J. Saraceno, S. Schilt, U. Keller, and T. Südmeyer, Phase-stabilization of the carrier-envelope-offset frequency of a SESAM modelocked thin disk laser, Opt. Express 21, 24770 (2013). PDF