Final Report Summary - CQPMAMP (Chirped quasi-phasematching gratings for optical parametric chirped pulse amplification: physics, devices, and applications)
The overall theme of the EU funded research project “CQPMAmp” was to explore spatially varying phase-matching media for the purpose of optical parametric chirped pulse amplification (OPCPA). OPCPA is a topic of great interest in the photonics community because it offers generation of high-power and high-energy laser pulses with few-optical-cycle pulse durations (just tens of femtoseconds or even shorter) across the near- and mid-infrared optical spectrum. Intense laser pulses find applications in industry, for example in micro-machining, and in fundamental science, where they enable the generation of attosecond pulse generation and the study of attosecond processes in physics, chemistry, and biology. In our labs, we have pursued high-repetition-rate mid-infrared OPCPA, because this spectral region promises insights into new phenomena, and opportunities for coherent soft x-ray attosecond pulse generation.
In the project, our goal was to explore new ways of performing OPCPA by utilizing the unique design degrees of freedom offered by quasi-phase-matching (QPM) gratings. In these QPM gratings, the nonlinear-optical coefficient changes sign periodically or aperiodically to allow coherent interaction between multiple optical frequencies. By using chirped QPM structures, we can support high-gain and high-efficiency amplification over broad optical bandwidths with high-power pump lasers. Such devices offer many interesting possibilities and challenges that have yet to be fully understood or exploited. A major part of the project was to understand these devices, and use what we learn to improve our OPCPA performance enough for strong-field physics experiments.
The first aspect of this work was a detailed analysis of the physics of strongly-nonlinear interactions in chirped QPM (CQPM) devices. These interactions must be understood and controlled to achieve a practical amplifier system. Therefore we performed theoretical, numerical, and experimental studies to better understand CQPM media. This work, published in C. R. Phillips, B. W. Mayer, L. Gallmann, M. M. Fejer, and U. Keller, Opt. Express 22, 9627 (2014), led to a comprehensive understanding of OPCPA systems based on CQPM devices. We used this understanding to develop new mid-infrared OPCPA system configurations. The first major upgrade was published in B. W. Mayer, C. R. Phillips, L. Gallmann, M. M. Fejer, and U. Keller, Optics Letters 38, 4265 (2013), which produced record-short mid-infrared pulses, and enabled mid-infrared strong-field experiments (i.e. those using extreme laser intensities of order 1014 W/cm^2) in our group for the first time.
Next, we explored novel QPM configurations to enable bandwidth and power scaling, leading to about twice as much pulse energy. This result was published in B. W. Mayer, C. R. Phillips, L. Gallmann, and U. Keller, Optics Express 22, 20798 (2014). This improvement made it relatively straightforward to perform strong-field experiments on a day-to-day basis. Using this capability, we studied ionization processes in noble gases, and could identify the breakdown of the widely-used dipole approximation in strong-field physics, which is an important milestone in extending intense light-matter interactions beyond those accessible with conventional lasers. These results were published in A. Ludwig, J. Maurer, B. W. Mayer, C. R. Phillips, L. Gallmann, and U. Keller, Physical Review Letters 113, 243001 (2014).
In parallel, we studied next-generation OPCPA systems, specifically, design and demonstration of new devices for scaling pulse energy, average power, and bandwidth. Our earlier studies indicated important issues in chirped QPM media that new devices would have to overcome. These issues would be particularly important for a thin disk laser pumped OPCPA system, which was one of the initial goals of the project: it turned out that new techniques were required to support such a system using chirped QPM media. These insights led us to pursue a different approach to OPCPA, based on a so-called frequency-domain optical parametric amplification (FOPA) configuration utilizing “fan-out” QPM technology. Here the light to be amplified was spatially dispersed, and the QPM grating period was varied transverse to the beam propagation direction. Our successful demonstration of this technique (manuscript in preparation) using a single carefully designed QPM device opens up a new platform for frequency conversion and amplification in the future. Importantly, the geometry of the FOPA concept is compatible with average power scaling, and is therefore ideal for future ultra-broadband, multi-megahertz repetition rate OPCPA systems, which could be driven by high-power thin disk modelocked oscillators.
As well as this new FOPA architecture, we explored “diffractive” QPM devices, combining the functionalities of diffraction gratings and quasi-phase-matching gratings. This work will form the basis for our next-generation OCPPA system to potentially support single-optical-cycle pulses with high average power. This work will be continued under a new Swiss national science foundation (SNSF) project grant which was awarded in April 2015 (title: “Diffractive quasi-phase-matching for high power generation of few-cycle pulses”; main applicant Chris Phillips). The pump laser for our new OPCPA system will be a state-of-the-art 300 W average power commercial laser very recently installed in our labs. The new OPCPA will be implemented in the coming months.
In addition to broadband OPCPA, understanding efficient nonlinear interactions in chirped QPM media led to insights into new ways to achieve femtosecond pulse formation in modelocked lasers. First, we explored the combination of cascaded quadratic nonlinearities with semiconductor saturable absorber mirrors to obtain femtosecond soliton modelocking in a 100 MHz laser operating the normal dispersion regime, published in C. R. Phillips, A. S. Mayer, A. Klenner, and U. Keller, Optics Express, 22, 6060 (2014). Following on from this promising result, a more ambitious approach using chirped QPM media was explored, whereby a soliton based on the second-order nonlinearity was adiabatically excited and subsequently de-excited in a single QPM device. A 100-fs, 500 MHz modelocked laser using this approach was successfully demonstrated experimentally (C. R. Phillips, A. S. Mayer, A. Klenner, and U. Keller, “Femtosecond modelocking based on adiabatic excitation of quadratic solitons”, submitted). The large and self-defocusing nonlinearities supported mean that the technique has great potential for lasers operating at multi-gigahertz repetition rates.
In conclusion, the fellowship enabled important advances in several areas of research of great interest to the photonics community, including mid-infrared and high-repetition-rate OPCPA, strong-field physics driven by long-wavelength lasers, and femtosecond pulse formation in modelocked lasers. The work already resulted in 5 published results, 1 currently under review, 2 manuscripts in preparation, and 17 accepted conference presentations. Many further results building on these advances are expected in the next year. We anticipate that a new, previously inaccessible regime of light-matter interactions using intense, long-wavelength driving fields will soon become accessible, as we push our OPCPA sources towards high pulse energies (hundreds of microjoules), high repetition rates (100 kilohertz), and extremely short pulse durations (towards a single optical cycle). Indeed, the coherent x-ray pulses supported by such a system will help unravel the dynamics of processes occurring on attosecond or even sub-attosecond timescales, with nanometer-scale spatial resolution. Further, these features will be provided by a table-top system, thereby overcoming the extremely high cost and low coherence of conventional ultrashort x-ray laser sources.
In the project, our goal was to explore new ways of performing OPCPA by utilizing the unique design degrees of freedom offered by quasi-phase-matching (QPM) gratings. In these QPM gratings, the nonlinear-optical coefficient changes sign periodically or aperiodically to allow coherent interaction between multiple optical frequencies. By using chirped QPM structures, we can support high-gain and high-efficiency amplification over broad optical bandwidths with high-power pump lasers. Such devices offer many interesting possibilities and challenges that have yet to be fully understood or exploited. A major part of the project was to understand these devices, and use what we learn to improve our OPCPA performance enough for strong-field physics experiments.
The first aspect of this work was a detailed analysis of the physics of strongly-nonlinear interactions in chirped QPM (CQPM) devices. These interactions must be understood and controlled to achieve a practical amplifier system. Therefore we performed theoretical, numerical, and experimental studies to better understand CQPM media. This work, published in C. R. Phillips, B. W. Mayer, L. Gallmann, M. M. Fejer, and U. Keller, Opt. Express 22, 9627 (2014), led to a comprehensive understanding of OPCPA systems based on CQPM devices. We used this understanding to develop new mid-infrared OPCPA system configurations. The first major upgrade was published in B. W. Mayer, C. R. Phillips, L. Gallmann, M. M. Fejer, and U. Keller, Optics Letters 38, 4265 (2013), which produced record-short mid-infrared pulses, and enabled mid-infrared strong-field experiments (i.e. those using extreme laser intensities of order 1014 W/cm^2) in our group for the first time.
Next, we explored novel QPM configurations to enable bandwidth and power scaling, leading to about twice as much pulse energy. This result was published in B. W. Mayer, C. R. Phillips, L. Gallmann, and U. Keller, Optics Express 22, 20798 (2014). This improvement made it relatively straightforward to perform strong-field experiments on a day-to-day basis. Using this capability, we studied ionization processes in noble gases, and could identify the breakdown of the widely-used dipole approximation in strong-field physics, which is an important milestone in extending intense light-matter interactions beyond those accessible with conventional lasers. These results were published in A. Ludwig, J. Maurer, B. W. Mayer, C. R. Phillips, L. Gallmann, and U. Keller, Physical Review Letters 113, 243001 (2014).
In parallel, we studied next-generation OPCPA systems, specifically, design and demonstration of new devices for scaling pulse energy, average power, and bandwidth. Our earlier studies indicated important issues in chirped QPM media that new devices would have to overcome. These issues would be particularly important for a thin disk laser pumped OPCPA system, which was one of the initial goals of the project: it turned out that new techniques were required to support such a system using chirped QPM media. These insights led us to pursue a different approach to OPCPA, based on a so-called frequency-domain optical parametric amplification (FOPA) configuration utilizing “fan-out” QPM technology. Here the light to be amplified was spatially dispersed, and the QPM grating period was varied transverse to the beam propagation direction. Our successful demonstration of this technique (manuscript in preparation) using a single carefully designed QPM device opens up a new platform for frequency conversion and amplification in the future. Importantly, the geometry of the FOPA concept is compatible with average power scaling, and is therefore ideal for future ultra-broadband, multi-megahertz repetition rate OPCPA systems, which could be driven by high-power thin disk modelocked oscillators.
As well as this new FOPA architecture, we explored “diffractive” QPM devices, combining the functionalities of diffraction gratings and quasi-phase-matching gratings. This work will form the basis for our next-generation OCPPA system to potentially support single-optical-cycle pulses with high average power. This work will be continued under a new Swiss national science foundation (SNSF) project grant which was awarded in April 2015 (title: “Diffractive quasi-phase-matching for high power generation of few-cycle pulses”; main applicant Chris Phillips). The pump laser for our new OPCPA system will be a state-of-the-art 300 W average power commercial laser very recently installed in our labs. The new OPCPA will be implemented in the coming months.
In addition to broadband OPCPA, understanding efficient nonlinear interactions in chirped QPM media led to insights into new ways to achieve femtosecond pulse formation in modelocked lasers. First, we explored the combination of cascaded quadratic nonlinearities with semiconductor saturable absorber mirrors to obtain femtosecond soliton modelocking in a 100 MHz laser operating the normal dispersion regime, published in C. R. Phillips, A. S. Mayer, A. Klenner, and U. Keller, Optics Express, 22, 6060 (2014). Following on from this promising result, a more ambitious approach using chirped QPM media was explored, whereby a soliton based on the second-order nonlinearity was adiabatically excited and subsequently de-excited in a single QPM device. A 100-fs, 500 MHz modelocked laser using this approach was successfully demonstrated experimentally (C. R. Phillips, A. S. Mayer, A. Klenner, and U. Keller, “Femtosecond modelocking based on adiabatic excitation of quadratic solitons”, submitted). The large and self-defocusing nonlinearities supported mean that the technique has great potential for lasers operating at multi-gigahertz repetition rates.
In conclusion, the fellowship enabled important advances in several areas of research of great interest to the photonics community, including mid-infrared and high-repetition-rate OPCPA, strong-field physics driven by long-wavelength lasers, and femtosecond pulse formation in modelocked lasers. The work already resulted in 5 published results, 1 currently under review, 2 manuscripts in preparation, and 17 accepted conference presentations. Many further results building on these advances are expected in the next year. We anticipate that a new, previously inaccessible regime of light-matter interactions using intense, long-wavelength driving fields will soon become accessible, as we push our OPCPA sources towards high pulse energies (hundreds of microjoules), high repetition rates (100 kilohertz), and extremely short pulse durations (towards a single optical cycle). Indeed, the coherent x-ray pulses supported by such a system will help unravel the dynamics of processes occurring on attosecond or even sub-attosecond timescales, with nanometer-scale spatial resolution. Further, these features will be provided by a table-top system, thereby overcoming the extremely high cost and low coherence of conventional ultrashort x-ray laser sources.