[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Third Harmonic Generation with Ultrashort Pulses in a C <sub/>2</sub> Cl <sub/>4</sub> Filled Liquid Core Fiber

摘要： Currently third harmonic generation (THG) in fibers is in focus of research, due to the prospects of enabling efficient generation of entangled photon triplets via the inverse, but spontaneous process. To date, microstructured and tapered fibers are the most prominent platforms to provide the high index contrast between the core and cladding material necessary to enable phase-matching (PM) to higher order modes (HOMs). Here, we employ a liquid core fiber (LCF) combining a silica cladding and a highly nonlinear core material, which can be temperature-tuned during operation [1]. We report the first experimental demonstration of THG in a liquid-core step-index-fiber formed by a silica capillary and a C2Cl4 filled core (Fig. 1b). The core liquid provides an enhanced electronic nonlinear coefficient compared to silica [2] and the LCF was designed to be single mode at the pump wavelength (1.55 μm). The modal dispersion of the fiber is engineered such that two HOMs at the TH wavelength are phase-matched within the spectrum of the ultrashort pump pulses, see Fig. 1a. An optofluidic mount is used for retaining, filling and launching pulses into the LCF. The LCF is pumped with 30 fs (Fig.1c) and 100 fs (Fig.1d) pulses (Toptica FemtoFiber pro IR and IRS-II, both at 80 MHz repetition rate). After filtering residual pump light the TH spectra are recorded for increasing power using a spectrum analyzer (Ando AQ6315A). The experimental results are shown in Fig.1c and d. In either case two distinct spectral peaks are visible in the TH corresponding to the two phase-matched HOMs. The images above Fig. 1d show the imaged output employing narrow bandpass filters, confirming the existence of individual modes. Comparing the efficiency for both pump pulses, the THG onset occurs at higher powers and the overall conversion is reduced for the shorter pulses. The high normal dispersion at the pump limits the effective conversion length due to dispersive pulse broadening. In conclusion, the capability of LCF for THG was demonstrated and future research towards quantum light generation in LCF can be envisioned.