Few-Cycle Parametric Amplifiers and Sub-Cycle Waveform Synthesizers
Over the last decade, optical parametric chirped-pulse amplification (OPCPA)  has been developed to directly generate few-cycle high-energy pulses as an alternative method to chirped-pulse amplification (CPA) based on a conventional laser amplification that is limited by the gain bandwidth of the laser material. In OPCPA a femtosecond pulse comprising only a few cycles of light is stretched to picosecond or even nanosecond durations for high-energy amplification using narrowband high-energy pump lasers and then recompressed to the original pulse duration, which enables the generation of high-intensity laser pulses for strong-field physics experiments. Parametric amplification has several distinct advantages over conventional laser amplification: ultrabroadband phase matching can be achieved with reduced gain narrowing, it is wavelength agile, and it leads to low accumulated nonlinear phase and high contrast ratio. For these reasons, OPCPA has been intensively investigated over the past 10 years.
The advent of intense few-cycle laser pulses has started the research fields of ‘extreme nonlinear optics’ and attoscience, in which the light-matter interaction is governed directly by the laser electric field E(t), and not merely by the laser intensity I(t). In parallel, the past decade has also witnessed tremendous progress in our capability to coherently synthesize few- and even sub-cycle optical waveforms E(t) from multiple wavelength regions. This multi-color waveform synthesis has recently attracted enormous attention in the attoscience and strong-field physics community, as novel capabilities to create cycle-engineered driver waveforms open up unprecedented prospects, e.g., for precision control of strong-field interactions in atoms, molecules and solids, for optimized high-harmonic generation, for the generation of intense isolated attosecond pulses, and for attosecond pump-probe spectroscopy employing ultrashort pulses in the VIS/IR and XUV/soft-X-ray regions.
The Ultrafast Optics and X-Rays group is developing few-cycle parametric amplifiers and high-energy parametric waveform synthesizers for attoscience pursuing various approaches based on ytterbium and Ti:sapphire pump-laser technology.
Parametric waveform synthesizer based on OPCPA at 800 nm and 2 µm and ytterbium pump technology
Our group in collaboration with the group of Giulio Cerullo at Politecnico Milano has demonstrated the generation of three cycle optical pulses at 800 nm and 2 μm wavelengths using OPCPA [2,3,4]. Most recently, we have coherently combined the output of these two OPCPAs when seeded with CEP-controlled pulses to generate sub-cycle optical waveforms later to be used for high-order harmonic generation and attosecond pulse generation . Fig. 1 shows the schematic layout and Fig. 2 a photo of the high-energy parametric waveform synthesizer.
The OPCPAs are seeded from a common octave-spanning, carrier-envelope phase (CEP)-controlled, 5-fs Ti:sapphire laser. The combined spectrum is shown in Fig. 3 (A) and spans over 1.8 octaves, and has the capability of producing 2.5-fs, 0.6-cycle pulses. The phases of each spectral component is controlled by the AOPDFs, such that any waveform supported by the spectrum can be generated. A feedback loop based on a balanced optical cross-correlator (BOC)  is implemented to synchronize the two pulses, allowing attosecond-precision relative timing stability. With the feedback control on over a bandwidth of 30 Hz, the relative timing drift between the two output pulses is reduced to 250 as, less than 5% of the oscillation period of the SWIR-OPCPA (7.2 fs). Fig. 3B and 3C show the group delay over each pulse measured by two dimensional spectral shearing (2DSI) . The 2DSI measurement shows that the two OPCPA pulses are temporally overlapped and each is well compressed to within 10% of its transform-limited pulse duration. Fig. 3D and 3E demonstrate the CEP stability of the two individual pulses, with r.m.s. fluctuations as low as 135 mrad and 127 mrad, respectively. Fig. 3F plots a synthesized electric-field waveform and intensity profile assuming the CEPs (φ1=650mrad, φ2=-750mrad) optimal for achieving the shortest high-field transient, which lasts only 0.8 cycles (amplitude FWHM) of the carrier (centroid) frequency (λc = 1.26 μm). The lower inset of Fig. 3F clearly shows that the synthesized electric-field waveform is non-sinusoidal and the main feature lasts less than an optical cycle. Once the output of each of the OPCPAs is scaled to mJ energy, the combined pulse can be used for isolated attosecond pulse generation in the XUV range.
Ongoing work aims to further increase the pulse energy and to extend the bandwidth of this OPCPA waveform synthesizer.
Ti:sapphire-driven multi-mJ parametric synthesizer generating two-octave-wide waveforms
In this project we pursue the development of a novel multi-mJ 3-channel parametric synthesizer shown in Fig. 4 for generating a 2-octave-wide spectrum (~0.5-2.5 µm) with ~2-fs pulse duration [7,8,9,10,11,12,13]. The synthesizer is driven by a commercial 18-mJ cryogenically cooled Ti:sapphire laser and features self-CEP-stabilization . We foresee that this synthesizer will become a versatile tool for controlling strong-field interactions in atoms, molecules and solids and for attosecond pump-probe spectroscopy employing ultrashort pulses in the VIS/IR and XUV/soft-X-ray regions
1. Dubietis, R. Butkus, and A. Piskarskas, “Trends in chirped pulse optical parametric amplification,” IEEE J. Sel. Top. Quantum Electron. 12, 163 (2006).
2. J. Moses, S.-W. Huang, K.-H. Hong, O. D. Mücke, E. L. Falcão-Filho, A. Benedick, F. Ö. Ilday, A. Dergachev, J. A. Bolger, B. J. Eggleton, and F. X. Kärtner, “Highly stable ultrabroadband mid-infrared optical parametric chirped pulse amplifier optimized for superfluorescence suppression,” Opt. Lett. 34, 1639 (2009).
3. J. Moses, C. Manzoni, S-W Huang, G.Cerullo, and F. X. Kärtner, “Temporal Optimization of Ultrabroadband High-Energy OPCPA,” Opt. Express 17, 5540 (2009).
4. A. Siddiqui, G. Cirmi, D. Brida, F. X. Kärtner and G. Cerullo, “Generation of <7fs pulses at 800nm from a blue-pumped optical parametric amplifier at degeneraty,” Opt. Lett. 34, 3592 (2009).
5. S.-W. Huang, G. Cirmi, J. Moses, K-H. Hong, S. Bhardwaj, J. R. Birge, L-J. Chen, E. Li, B. J. Eggleton, G. Cerullo, and F. X. Kärtner, “ High-energy pulse synthesis with sub-cycle waveform control for strong-field physics,” Nat. Photonics 5, 477, (2011).
6. J. R. Birge, R. Ell and F. X. Kärtner, “Two-dimensional spectral shearing interferometry for few-cycle pulse characterization,” Opt. Lett. 31, 2063 (2006).
7. G. Cirmi, S. Fang, S.-H. Chia, O. D. Mücke, F. X. Kärtner, C. Manzoni, P. Farinello, and G. Cerullo, “High-energy pulse synthesis of optical parametric amplifiers,” talk CG-4.6 THU, CLEO/Europe-IQEC 2013, Munich, Germany, May 12-16, 2013.
8. O. D. Mücke, S. Fang, G. Cirmi, S.-H. Chia, F. X. Kärtner, C. Manzoni, P. Farinello, and G. Cerullo, “Millijoule-Level Parametric Synthesizer Generating Two-Octave-Wide Optical Waveforms for Strong-Field Experiments,” talk CTh3H.3, CLEO, San Jose, CA, June 9-14, 2013.
9. S. Fang, G. Cirmi, S.-H. Chia, O. D. Mücke, F. X. Kärtner, C. Manzoni, P. Farinello, and G. Cerullo, “Multi-mJ Parametric Synthesizer Generating Two-Octave-Wide Optical Waveforms,” invited talk WB3-1, CLEO Pacific Rim, Kyoto, Japan, June 30-July 4, 2013. [CLEO-PR 2013 Best Paper Award]
10. G. M. Rossi, G. Cirmi, S. Fang, S.-H. Chia, O. D. Mücke, F. X. Kärtner, C. Manzoni, P. Farinello, and G. Cerullo, "Spectro-Temporal Characterization of All Channels in a Sub-Optical-Cycle Parametric Waveform Synthesizer," invited talk SF1E.3, CLEO, San Jose, CA, June 8-13, 2014.
11. O. D. Mücke, S. Fang, G. Cirmi, G. M. Rossi, S.-H. Chia, H. Ye, Y. Yang, R. Mainz, C. Manzoni, P. Farinello, G. Cerullo, and F. X. Kärtner, "Toward Waveform Nonlinear Optics Using Multimillijoule Sub-Cycle Waveform Synthesizers," IEEE J. Sel. Top. Quantum Electron. 21, 8700712 (2015).
12. S.-H. Chia, G. Cirmi, S. Fang, G. M. Rossi, O. D. Mücke, F. X. Kärtner, "Two-octave-spanning dispersion-controlled precision optics for sub-optical-cycle waveform synthesizers," Optica 1, 315 (2014).
13. C. Manzoni, O. D. Mücke, G. Cirmi, S. Fang, J. Moses, S.-W. Huang, K.-H. Hong, G. Cerullo, and F. X. Kärtner, "Coherent pulse synthesis: towards sub-cycle optical waveforms," Laser & Photonics Rev. 9, 129 (2015).
14. G. Cerullo, A. Baltuška, O. D. Mücke, and C. Vozzi, “Few-optical-cycle light pulses with passive carrier-envelope phase stabilization,” Laser & Photonics Rev. 5, 323 (2011).