High-energy mid-infrared sub-cycle pulse synthesis from a parametric amplifier

ARTICLE DOI: 10.1038/s41467-017-00193-4 OPEN High-energy mid-infrared sub-cycle pulse synthesis from a parametric amplifier Houkun Liang1,2, Peter Krogen1, Zhou Wang3, Hyunwook Park3, Tobias Kroh1,4, Kevin Zawilski5, Peter Schunemann5, Jeffrey Moses1,6, Louis F. DiMauro3, Franz X. Kärtner1,4,7 & Kyung-Han Hong1 High-energy phase-stable sub-cycle mid-infrared pulses can provide unique opportunities to explore phase-sensitive strong-field light–matter interactions in atoms, molecules and solids. At the mid-infrared wavelength, the Keldysh parameter could be much smaller than unity even at relatively modest laser intensities, enabling the study of the strong-field sub-cycle electron dynamics in solids without damage. Here we report a high-energy sub-cycle pulse synthesiser based on a mid-infrared optical parametric amplifier and its application to high-harmonic generation in solids. The signal and idler combined spectrum spans from 2.5 to 9.0 µm. We coherently synthesise the passively carrier-envelope phase-stable signal and idler pulses to generate 33 μJ, 0.88-cycle, multi-gigawatt pulses centred at ~4.2 μm, which is further energy scalable. The mid-infrared sub-cycle pulse is used for driving highharmonic generation in thin silicon samples, producing harmonics up to ~19th order with a continuous spectral coverage due to the isolated emission by the sub-cycle driver. 1 Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, USA. 2 Singapore Institute of Manufacturing Technology, 2 Fusionopolis Way, Singapore 138634, Singapore. 3 Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA. 4 Center for Free-Electron Laser Science, DESY and Department of Physics, University of Hamburg, 22607 Hamburg, Germany. 5 BAE System, MER15-1813, P.O. Box 868, Nashua, New Hampshire 03061, USA. 6 School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA. 7 The Hamburg Center for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany. Correspondence and requests for materials should be addressed to K.-H.H. (email: ) NATURE COMMUNICATIONS | 8: 141 | DOI: 10.1038/s41467-017-00193-4 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00193-4 G eneration of high-energy, few-cycle mid-infrared (mid-IR) pulses has progressed markedly over the last decade, driven by a number of applications, such as coherent soft X-ray high-harmonic generation (HHG)1–3, incoherent hard X-ray generation in laser-induced plasmas4, subfemtosecond electron emission5, two-dimensional infrared spectroscopy6 and time-resolved imaging of molecular structures7. The use of a carrier-envelope phase (CEP)-stable singlecycle or even sub-cycle pulse can inherently isolate the electron dynamics in the strong-field interactions8, 9. Therefore, highenergy, CEP-stable, sub-cycle mid-IR pulses can be a very unique tool for investigating ultrafast dynamics of strong-field interactions in solids and gases. Some examples are the sub-cycle control of electron motions via HHG in solids10–13, the sub-cycle electron tunnelling in nano-devices14, 15, the generation of isolated attosecond16 or even zeptosecond X-ray pulses17, controlling strongfield molecular ionisation and dissociation18, steering the atomicscale motion of electrons8, and sub-femtosecond control and metrology of bound-electron dynamics in atoms9. In particular, the studies on the strong-field interactions in solids and nanostructures are opening a great opportunity towards petahertz electronics19, 20. Recently, intensive effort has been made to generate few-cycle mid-IR pulses using several techniques, such as optical parametric amplifier (OPA)21, 22, laser filamentation23 and difference-frequency generation (DFG)24. Adiabatic difference frequency generation has produced few-μJ, shapeable, single-cycle mid-IR pulses25, though the technique’s scalability to the sub-mJ level is not obvious and CEP stability has not yet been demonstrated. Self-compression after spectral broadening in dielectric materials26–29 has also been used for generating sub-two-cycle mid-IR pulses. Regarding to the generation of mid-IR sub-cycle pulses, four-wave mixing through filamentation in a gas30 and a technique that cascades DFG, spectral broadening, and chirp-compensation31 have been Signal pulse demonstrated. However, the former gives low pulse energy (~0.5 μJ)32 and limited energy scalability as well as conical emission, while the latter has unknown CEP stability due to the complex nonlinear processes involved besides the low energy (~1 μJ). These impose limitations in the applications to strongfield light–matter interactions. On the other hand, in the visible to short-wavelength IR range, high-energy sub-cycle pulses have been demonstrated using pulse synthesisers8, 9, 33, 34. Greaterthan-octave-spanning spectra are generated and amplified in different spectral bands. After the phase management of individual bands, the multi-colour pulses are coherently combined to form a sub-cycle pulse. The main challenge of this approach is the complexity of the system because sophisticated phase control and stabilisation have to be implemented for eliminating the relative timing and phase jitters from the individual bands. Coherent synthesis between the signal and idler of a type-I collinear OPA in a passive way is an intriguing alternative for sub-cycle pulse generation. With near-degenerate signal and idler wavelengths, the total OPA bandwidth can be huge in a collinear geometry when the group velocity dispersion at the degenerate wavelength is small35. The signal and idler pulses are tightly synchronised in an OPA by nature. However, this method is challenging in the visible/near-IR range for several reasons. First, the dispersion over multi-octave bandwidths is very large, such that the signal and idler pulses will require post-compression. Second, the signal and idler need to have a stable relative CEP, which can only be achieved when both the pump and signal pulses are CEP stable, as their phase difference is transferred to the idler CEP. In the visible and near-IR ranges, such schemes usually require active CEP stabilisation. In contrast, in the mid-IR, the dispersion of OPA media can be very low within the transmission window, which enables the direct synthesis of the signal and idler pulses without post-compression. Idler pulse Synthesised pulse Nd:YLF CPA & Cryo-cooled Yb:YAG CPA CEP measurement BaF2 1 µm 1 µm Octave-spanning Ti:sapphire oscillator Intrapulse DFG 0.65–1.1 µm 2.1 µm seed 2.5–4.4 µm, 50 nJ Three-stage 2.1 µm OPCPA Si CSP 2.5–9 µm, 33 µJ Si Residual 2.1 µm 2.1 µm, 0.8 mJ 2.1 µm, 10 µJ GaSe Polariser Spectrometer Fig. 1 Schematic of the high-energy phase-stable sub-cycle mid-infrared optical parametric amplifier. CEP carrier-envelope phase, C (...truncated)