Few-cycle broadband mid-IR pulse generation via optical parametric down-conversion

• Main Author: Liu, Kun
• Other Authors: Wang Qijie
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/146230
• Institution: Nanyang Technological University

Broadband few-cycle mid-infrared (mid-IR) sources in the wavelength range of 2-50 μm are attracting extensive attention from researchers for their various applications. In contrast with traditional incoherent thermal mid-IR sources that can only provide some frequency-resolved applications, broadband few-cycle mid-IR sources enable more advanced applications which require time-resolved characterizations, such as molecular transition tracing, ultrafast phonon coupling and charge interaction studies under ultrafast excitation in semiconductors. In addition, combined with high peak power and high repetition rate, few-cycle mid-IR sources are also promising tools for high-flux coherent soft X-ray generation with 1 keV photon energy, high-flux incoherent femtosecond hard X-ray generation with 100 keV photon energy and attosecond pulse generation. The recent advances of the optical parametric down-conversion techniques such as optical parametric oscillator (OPO) , optical parametric amplification (OPA), optical parametric chirped-pulse amplification (OPCPA) and difference frequency generation (DFG) provide a pathway of generating broadband few-cycle mid-IR radiations. In this thesis, we focus on the generation of few-cycle, broadband, >3 μm mid-IR pulses with a kHz repetition rate and > 1 μJ pulse energy based on OPCPA, intrapulse DFG (IPDFG) and OPA. Moreover, some efforts have also been made in nonlinear pulse compression, high harmonic generation (HHG) and ultrabroadband supercontinuum generation (SCG) using the developed high-energy, few-cycle mid-IR sources. We summarize our contributions as follows. 1. We design and present a carrier-envelope-phase (CEP) stable 3 μm mid-IR OPCPA system based on PPLN and PPSLT crystals, pumped by a 1030 nm, 1 ps, 10 kHz, Yb: YAG Innoslab laser. The OPCPA system can deliver up to 3 W, 300 μJ, 61 fs pulses centered at 3 μm with a CEP fluctuation of 391 mrad for a 100 s duration. A YAG-based nonlinear pulse compression stage further compresses the 3 μm pulses to 21 fs corresponding to two-cycle pulse width, with a 255 μJ pulse energy. Driven by 3 μm, 21 fs pulses after the nonlinear pulse compression, HHG with up to the 15th order is generated in a 0.5 mm thick ZnO crystal. Compared with HHG driven by 3 μm pulses before the nonlinear pulse compression, one order of magnitude enhancement of the harmonic intensity is demonstrated, revealing the significance of the intensity enhancement of the driving source. 2. To further extend the spectrum to the deeper mid-IR region, we utilize the self-phase modulation (SPM) effect in GaSe to produce an internal signal of IPDFG and report a broadband, > 6 μm, SPM-assisted mid-IR IPDFG source directly driven by 3 μm pulses from the OPCPA system we developed, without the use of the extra nonlinear spectral broadening stage. The IPDFG pulses with a 7-15 μm spectral coverage, a 0.91 μJ pulse energy, and a 9.1 mW output power are generated. The contribution of the SPM effect in the process of IPDFG is confirmed experimentally. The generated IPDFG pulse is compressed to 60fs (1.8-cycle) by adding extra bulk materials. The good compressibility directly verifies the good spectral coherence of IPDFG pulses. 3. By nonlinear pulse compression of 3 μm OPCPA pulses in YAG, the spectrum of 3 μm pulses is broadened to 4.5 μm and an external signal of IPDFG is thus produced. Using such pulses, we boost the efficiency of IPDFG to a record value of up to 5.3%. 5 μJ, 50 mW, 68 fs (2.1 cycles centered at 9.7 μm) IPDFG pulses spanning from 6 to 13.2 μm are obtained. With a suitable focal spot, the field strength of the IPDFG pulses can exceed 0.27 V/Å, enabling some applications in nonlinear optics such as HHG and SCG. As a demonstration, pumped by the IPDFG pulses, a 2.4 μJ, 24 mW SC source with a 3-octave bandwidth covering 2 to 16 μm and a 2.7 μJ, 27 mW SC source with a 2.3-octave bandwidth covering 3 to 14.5 μm are generated in a KRS-5 crystal and a ZnSe crystal, respectively. Two SC spectra both support sub-cycle pulse widths. 4. In the abovementioned works, we have obtained two-cycle mid-IR pulses with a broadband spectrum from 6 to 15 μm and a uJ-level pulse energy based on the IPDFG method. Further, we explore the mid-infrared pulse generation with a broader spectrum supporting sub-cycle pulse width using an OPA method. We present a multi-microjoule, ultra-broadband mid-IR OPA source based on a GaSe nonlinear crystal and a driving source at ~2 μm, which can deliver idler pulses with a spectrum covering 4.5 to 13.3 μm at -3 dB and 4.2 to 16 μm in the full spectral range and a pulse energy of ~3.4 μJ, centered at 8.8 μm. The spectrum supports a sub-cycle transform-limited pulse width of ~19 fs. To our best knowledge, this is the broadest spectrum ever obtained by OPA systems in this spectral region.