UV Pulse Compression in Hollow Core Fibers Using XPM

Ultrashort UV pulses are of great interest due to their wide ranging applications such as biosensing [5], high harmonic generation [3], and time resolved spectroscopy [4]. Hollow Core Fibers (HCF) play a critical role for ultrashort UV pulse generation through soliton dynamics [6], four wave mixing [2], or cross-phase modulation (XPM) [1]. We are developing a novel approach, first introduced at CLEO US 2022 [1], of generating few-cycle pulses in the 266 nm region using XPM which promises to deliver higher pulse energies than other approaches. Here XPM is used to broaden and compress third-harmonic pulses, with $\sim 10\mu \mathrm{J}$ pulse energy, of an 800 nm Ti:Sapphire amplifier, see fig. 1a for a schematic overview of the process [1]. When characterized via 2DSI, the pulse duration retrieved at the end of the fiber assembly was 10 fs at FWHM, with a 6 fs transform limited pulse, as shown by the red and blue traces in fig. 1c. The retrieved phase indicates that the pulse self-compresses to 6.8 fs 20 cm after the fiber assembly output (due to propagation through air), see the black trace in fig. 1c. Current efforts are focused on cleaning up the mode generated by the 3 ^rd harmonic arm, fig. 1a inset, to improve HCF coupling efficiency. The characterization of even shorter UV pulses with transform-limited duration below 2 fs is on-going, as well as the effort to scale up the pulse energy to few tens of $\mu \mathrm{J}$ .