Federico Belli

Raman-induced temporal condensed matter physics in gas-filled photonic crystal fibers.

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means.

Angle-resolved photoemission spectroscopy with 9-eV photon-energy pulses generated in a gas-filled hollow-core photonic crystal fiber

A recently developed source of ultraviolet radiation, based on optical soliton propagation in a gas-filled hollow-core photonic crystal fiber, is applied here to angle-resolved photoemission spectroscopy (ARPES). Near-infrared femtosecond pulses of only few μJ energy generate vacuum ultraviolet radiation between 5.5 and 9 eV inside the gas-filled fiber. These pulses are used to measure the band structure of the topological insulator Bi2Se3 with a signal to noise ratio comparable to that obtained with high order harmonics from a gas jet. The two-order-of-magnitude gain in efficiency promises time-resolved ARPES measurements at repetition rates of hundreds of kHz or even MHz, with photon energies that cover the first Brillouin zone of most materials.

Vacuum UV to IR supercontinuum generation by impulsive Raman self-scattering in hydrogen-filled PCF

A supercontinuum extending from 125nm to 1200nm is generated in hydrogen-filled kagomé-PCF by means of impulsive Raman self-scattering of few-μJ ultrashort pulses at 805nm. The source shows no optical damage and is stable over time.

Control of ultrafast pulses in a hydrogen-filled hollow-core photonic-crystal fiber by Raman coherence

We present the results of an experimental and numerical investigation into temporally non-local coherent interactions between ultrashort pulses, mediated by Raman coherence, in gas-filled kagome-style hollow-core photonic crystal fiber. A pump pulse first set up the Raman coherence, creating a refractive index grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second probe pulse allows high degree of control over its evolution as it propagates along the fiber, in particular soliton self-compression and dispersive wave (DW) emission. In the experiments reported, a DW is emitted at ~300 nm, with a central frequency that oscillates with the pump-probe delay. The results demonstrate that strong Raman coherence created in broadband guiding gas-filled kagome-PCF can be used to control the dynamics of ultrashort probe pulses, even in difficult-to-access spectral regions such as the deep and vacuum ultraviolet.

Extremely broadband single-shot cross-correlation frequency-resolved optical gating using a transient grating as gate and dispersive element.

A cross-correlation frequency-resolved optical gating (FROG) concept, potentially suitable for characterizing few or sub-cycle pulses in a single shot, is described in which a counter-propagating transient grating is used as both the gate and the dispersive element in a FROG spectrometer. An all-reflective setup, which can operate over the whole transmission range of the nonlinear medium, within the sensitivity range of the matrix sensor, is also proposed, and proof-of-principle experiments for the ultraviolet and visible-to-near-infrared spectral ranges are reported.

Non-local Soliton Interactions in Raman-gas Photonic Crystal Fibers

Slow Raman response in gas-filled photonic crystal fibers enables non-local interactions between two successive ultrashort solitons. Different spatiotemporal modulation of the medium refractive index can be obtained by varying time delay between the solitons.

Generation of three-octave-spanning transient Raman frequency comb in hydrogen-filled hollow-core PCF

A noise-seeded transient Raman frequency comb spanning three octaves from 180 to 2400 nm is generated by pumping a hydrogen-filled hollow-core photonic crystal with 300 fs pulses of energy 26 μJ and wavelength 800 nm.

Efficient Broadband Vacuum-Ultraviolet Generation in Gas-Filled Hollow-Core Photonic Crystal Fibers

We report two techniques for the efficient generation of tunable ultrafast pulses in the vacuum-ultraviolet, covering at least 117-200 nm, by pumping gas-filled kagome-style photonic crystal fibers with few-µJ, 35 fs, 800 nm laser pulses.

Raman-induced soliton oscillations and tunneling in gas-filled photonic crystal fibers

We have developed an analytical model to study pulse propagation in photonic crystal fibers filled by Raman-active gases. Solitons can show oscillations and tunneling inside the Raman-induced sinusoidal temporal potential in these fibers.

Spatiotemporal Nonlinear Dynamics in Gas-Filled Photonic-Crystal Fibers

We numerically and experimentally explore spatiotemporal effects during ultrashort pulse propagation in gas-filled kagome-PCF. These include self-focusing, intermodal dispersive-wave emission, and multi-mode transient Raman frequency-comb generation.