Guangyu Fan

A strong-field driver in the single-cycle regime based on self-compression in a kagome fibre

Over the past decade intense laser fields with a single-cycle duration and even shorter, subcycle multicolour field transients have been generated and applied to drive attosecond phenomena in strong-field physics. Because of their extensive bandwidth, single-cycle fields cannot be emitted or amplified by laser sources directly and, as a rule, are produced by external pulse compression—a combination of nonlinear optical spectral broadening followed up by dispersion compensation. Here we demonstrate a simple robust driver for high-field applications based on this Kagome fibre approach that ensures pulse self-compression down to the ultimate single-cycle limit and provides phase-controlled pulses with up to a 100 μJ energy level, depending on the filling gas, pressure and the waveguide length.

Hollow-core-waveguide compression of multi-millijoule CEP-stable 3.2 μm pulses

We present the extension of high-power pulse compression deeper into the challenging IR spectral range around 3.2 μm wavelength, where the effects of material absorption, dispersion, and free electron disturbance on nonlinear propagation become increasingly limiting parameters. 5 mJ, 80 fs pulses from a KTA parametric amplifier were spectrally broadened in a large-core hollow fiber with argon as the nonlinear medium. Subsequent compression through anomalous dispersion in CaF2 yielded 2.5 mJ close-to-transform-limited two-cycle pulses exhibiting a passively stabilized carrier envelope phase (CEP). Furthermore, we outline the feasibility of generating sub-two-cycle pulses with good spatial and temporal characteristics.

Enhanced multi-colour gating for the generation of high-power isolated attosecond pulses

Isolated attosecond pulses (IAP) generated by high-order harmonic generation are valuable tools that enable dynamics to be studied on the attosecond time scale. The applicability of these IAP would be widened drastically by increasing their energy. Here we analyze the potential of using multi-colour driving pulses for temporally gating the attosecond pulse generation process. We devise how this approach can enable the generation of IAP with the available high-energy kHz-repetition-rate Ytterbium-based laser amplifiers (delivering 180-fs, 1030-nm pulses). We show theoretically that this requires a three-colour field composed of the fundamental and its second harmonic as well as a lower-frequency auxiliary component. We present pulse characterization measurements of such auxiliary pulses generated directly by white-light seeded OPA with the required significantly shorter pulse duration than that of the fundamental. This, combined with our recent experimental results on three-colour waveform synthesis, proves that the theoretically considered multi-colour drivers for IAP generation can be realized with existing high-power laser technology. The high-energy driver pulses, combined with the strongly enhanced single-atom-level conversion efficiency we observe in our calculations, thus make multi-colour drivers prime candidates for the development of unprecedented high-energy IAP sources in the near future.

High-power top-hat pulses from a Yb master oscillator power amplifier for efficient optical parametric amplifier pumping

We demonstrate shaping of high-energy broadband Yb amplifier pulses for the generation of a (sub)picosecond top-hat temporal pulse profile that significantly improves pumping efficiency of an optical parametric amplifier (OPA). Phase-only modulation is applied by an acousto-optic programmable dispersion filter. This simple scheme is scalable to a high average power due to a relatively broad bandwidth of the Yb:CaF(2) gain medium used in the amplifier that supports a sub-150-fs transform-limited pulse duration. Additionally we show that OPA seeding with supercontinuum remains possible because top-hat-shaped pulses passed through a glass block recompress to ≈200 fs with minimum satellite production.

Soliton-based ultrafast multi-wavelength nonlinear switching in dual-core photonic crystal fibre

Systematic experimental study of ultrafast multi-wavelength all-optical switching performance in a dual-core photonic crystal fibre is presented. The focus is on nonlinearly induced switching between the two output ports at non-excitation wavelengths, which are generated during nonlinear propagation of femtosecond pulses in the anomalous dispersion region of a dual-core photonic crystal fibre made of multicomponent glass. Spatial and spectral characteristics of the fibre output radiation were measured separately for both fibre cores under various polarization and intensity conditions upon selective, individual excitation of each fibre core. Polarization-controlled nonlinear switching performance at multiple non-excitation wavelengths was demonstrated in the long-wavelength optical communication bands and beyond. Depending on the input pulse polarization, narrowband switching operation at 1560 nm and 1730 nm takes place with double core extinction ratio contrasts of 9 dB and 14.5 dB, respectively. Moreover, our approach allows switching with simultaneous wavelength shift from 1650 to 1775 nm with extinction ratio contrast larger than 18 dB. In addition, non-reciprocal behaviour of the soliton fission process under different fibre core excitations was observed and its effect on the multi-wavelength nonlinear switching performance was explained, taking into account the slight dual-core structure asymmetry. The obtained results represent ultrafast all-optical switching with an extended dimension of wavelength shift, controllable with both the input radiation intensity and the polarization by simple propagation along a 14 mm long fibre.

X-SEA-F-SPIDER characterization of over octave spanning pulses in the infrared range.

We show a practical implementation of a pulse characterization method for sub-cycle pulse measurements in the infrared spectral range based on spectral shearing interferometry. We employ spatially-encoded arrangement filter-based spectral phase interferometry for direct electric field reconstruction with external ancila pulses (X-SEA-F-SPIDER). We show merits and limitations of the setup and an in-depth comparison to another widely used temporal characterization technique - Second-Harmonic Generation Frequency Resolved Optical Gating (SHG-FROG). The X-SEA-F-SPIDER implementation presented in this paper allows measurement of sub-cycle pulses with over one octave wide spectrum spanning the 900-2400 nm range without adding any extra dispersion due to the pulse characterization apparatus.

Multi-millijoule Few-Optical-Cycle Pulses in Mid-IR: Scaling Power, Energy and Wavelength

We discuss prospects of the generation of high power mid-IR pulses by analyzing soliton self-compression of multi-mJ 4-μm pulses and the development of few-optical-cycle 6-μm optical parametric amplifier. Scalability of pump lasers is addressed.

Ultrafast multi-wavelength switch based on dynamics of spectrally-shifted solitons in a dual‑core photonic crystal fiber.

Nonlinear propagation of ultrafast near infrared pulses in anomalous dispersion region of dual-core photonic crystal fiber was studied. Polarization tunable soliton-based nonlinear switching at multiple non-excitation wavelengths was demonstrated experimentally for fiber excitation by 100 fs pulses at 1650 nm. The highest-contrast switching was obtained with the fiber length of just 14 mm, which is significantly shorter compared to the conventional non-solitonic in-fiber switching based on nonlinear optical loop mirror. Advanced numerical simulations show good agreement with the experimental results, suggesting that the underlying dual-core soliton fission process supports nonlinear optical switching and simultaneous pulse compression to few-cycle durations at the level of 20 fs.

Strong field applications of Gigawatt self-compressed pulses from a Kagome fiber

Nonlinear self-compression of 1.7-µm pulses in a gas-filled Kagome fiber down to a single cycle duration and pulse energies up to 100 µJ provides a uniquely simple driver source for high-harmonic generation and above-threshold ionization experiments.

13mJ sub-three optical cycle 3.9-um pulses through hollow-core-waveguide compression

Few-cycle high peak power mid-IR sources are required for strong field applications, such as driving HHG in the keV range [1] and high efficiency THz generation [2]. In order to expose target to a peak field without causing excessive ionization, ultrashort pulses consisting of a few-cycles are needed. Parametric sources based on KTA crystals have been successfully used for the generation of multi-mJ pulses in the 3–4 μm range [3], however, the duration is typically limited to sub-100 fs duration due to phase matching bandwidth limitations. A robust and popular scheme for the generation of few-cycle pulses is via post-compression in a hollow core fiber (HCF) filled with noble gas [4] that yields uniform pulse compression across the beam and the perfect beam profile at the output due to spatial filtering effects.