Francesco Tani

Multimode ultrafast nonlinear optics in optical waveguides: numerical modeling and experiments in kagomé photonic-crystal fiber

We introduce a general full-field propagation equation for optical waveguides, including both fundamental and higher order modes, and apply it to the investigation of spatial nonlinear effects of ultrafast and extremely broadband nonlinear processes in hollow-core optical fibers. The model is used to describe pulse propagation in gas-filled hollow-core waveguides including the full dispersion, Kerr, and ionization effects. We study third-harmonic generation into higher order modes, soliton emission of resonant dispersive waves into higher order modes, intermodal four-wave mixing, and Kerr-driven transverse self-focusing and plasma-defocusing, all in a gas-filled kagome photonic crystal fiber system. In the latter case a form of waveguide-based filamentation is numerically predicted.

As_2S_3–silica double-nanospike waveguide for mid-infrared supercontinuum generation

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2  dB at 1550 nm in the case of free-space in-coupling, which is ∼7  dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.

Single-mode hollow-core photonic crystal fiber made from soft glass

We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagomé-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter ~30 µm) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices.

Low loss hollow optical-waveguide connection from atmospheric pressure to ultra-high vacuum

A technique for optically accessing ultra-high vacuum environments, via a photonic-crystal fiber with a long small hollow core, is described. The small core and the long bore enable a pressure ratio of over 108 to be maintained between two environments, while permitting efficient and unimpeded delivery of light, including ultrashort optical pulses. This delivery can be either passive or can encompass nonlinear optical processes such as optical pulse compression, deep UV generation, supercontinuum generation, or other useful phenomena.

Continuously wavelength-tunable high harmonic generation via soliton dynamics

We report the generation of high harmonics in a gas jet pumped by pulses self-compressed in a He-filled hollow-core photonic crystal fiber through the soliton effect. The gas jet is placed directly at the fiber output. As the energy increases, the ionization-induced soliton blueshift is transferred to the high harmonics, leading to emission bands that are continuously tunable from 17 to 45 eV.

Generation of microjoule pulses in the deep ultraviolet at megahertz repetition rates

Although ultraviolet (UV) light is important in many areas of science and technology, there are very few if any lasers capable of delivering wavelength-tunable ultrashort UV pulses at MHz repetition rates. Here we report the generation of deep-UV laser pulses at MHz repetition rates and \mu J-energies by means of dispersive wave (DW) emission from self-compressed solitons in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF). Pulses from an ytterbium fiber laser (~300 fs) are first compressed to ~25 fs in a SR-PCF-based nonlinear compression stage, and subsequently used to pump a second SR-PCF stage for broadband DW generation in the deep UV. The UV wavelength is tunable by selecting the gas species and the pressure. At 100 kHz repetition rate, a pulse energy of 1.05 \mu J was obtained at 205 nm (average power 0.1 W), and at 1.92 MHz, a pulse energy of 0.54 \mu J was obtained at 275 nm (average power 1.03 W).

Interaction between Kerr and ionization induced nonlinear fiber optics

Light-plasma interactions are explored in gas-filled photonic crystal fibers through self-compression of few-μJ pulses. Here we study the interaction between ionization-driven soliton dynamics and Kerr-based deep-UV generation.

Modulation Instability in Xenon-Filled Hollow-Core Photonic Crystal Fiber

Abstract: We experimentally access the modulation instability regime in xenon-filled kagome PCF. Soliton orders ~100 are obtained with few-μJ, 490 fs pulses at 800 nm. Numerical simulations confirm pulse breakup into ultrashort solitons.

Broadband and tunable time-resolved THz system using argon-filled hollow-core photonic crystal fiber

We demonstrate broadband, frequency-tunable, phase-locked terahertz (THz) generation and detection based on difference frequency mixing of temporally and spectrally structured near-infrared (NIR) pulses. The pulses are prepared in a gas-filled hollow-core photonic crystal fiber (HC-PCF), whose linear and nonlinear optical properties can be adjusted by tuning the gas pressure. This permits optimization of both the spectral broadening of the pulses due to self-phase modulation (SPM) and the generated THz spectrum. The properties of the prepared pulses, measured at several different argon gas pressures, agree well with the results of numerical modeling. Using these pulses, we perform difference frequency generation in a standard time-resolved THz scheme. As the argon pressure is gradually increased from 0 to 10 bar, the NIR pulses spectrally broaden from 3.5 to 8.7 THz, while the measured THz bandwidth increases correspondingly from 2.3 to 4.5 THz. At 10 bar, the THz spectrum extends to 6 THz, limited only by the spectral bandwidth of our time-resolved detection scheme. Interestingly, SPM in the HC-PCF produces asymmetric spectral broadening that may be used to enhance the generation of selected THz frequencies. This scheme, based on a HC-PCF pulse shaper, holds great promise for broadband time-domain spectroscopy in the THz, enabling the use of compact and stable ultrafast laser sources with relatively narrow linewidths (<4 THz).We demonstrate broadband, frequency-tunable, phase-locked terahertz (THz) generation and detection based on difference frequency mixing of temporally and spectrally structured near-infrared (NIR) pulses. The pulses are prepared in a gas-filled hollow-core photonic crystal fiber (HC-PCF), whose linear and nonlinear optical properties can be adjusted by tuning the gas pressure. This permits optimization of both the spectral broadening of the pulses due to self-phase modulation (SPM) and the generated THz spectrum. The properties of the prepared pulses, measured at several different argon gas pressures, agree well with the results of numerical modeling. Using these pulses, we perform difference frequency generation in a standard time-resolved THz scheme. As the argon pressure is gradually increased from 0 to 10 bar, the NIR pulses spectrally broaden from 3.5 to 8.7 THz, while the measured THz bandwidth increases correspondingly from 2.3 to 4.5 THz. At 10 bar, the THz spectrum extends to 6 THz, limited only b...

UV Soliton Dynamics and Raman-Enhanced Supercontinuum Generation in Photonic Crystal Fiber

Ultrafast broadband ultraviolet radiation is of importance in spectroscopy and photochemistry, since high photon energies enable single-photon excitations and ultrashort pulses allow time-resolved studies. Here we report the use of gas-filled hollow-core photonic crystal fibers (HC-PCFs) for efficient ultrafast nonlinear optics in the ultraviolet. Soliton self-compression of 400 nm pulses of (unprecedentedly low) ~500 nJ energies down to sub-6-fs durations is achieved, as well as resonant emission of tunable dispersive waves from these solitons. In addition, we discuss the generation of a flat supercontinuum extending from the deep ultraviolet to the visible in a hydrogen-filled HC-PCF. Comparisons with argon-filled fibers show that the enhanced Raman gain at high frequencies makes the hydrogen system more efficient. As HC-PCF technology develops, we expect these fiber-based ultraviolet sources to lead to new applications.