A. Nazarkin

Synthesis of periodic femtosecond pulse trains in the ultraviolet by phase-locked Raman sideband generation

We demonstrate a new technique for femtosecond-pulse generation that employs ultrafast modulation of a laser field phase by impulsively excited molecular rotational or vibrational motion with subsequent temporal compression. An ultrashort pump pulse at 800 nm performs impulsive excitation of a molecular gas in a hollow waveguide, and a weak delayed probe pulse at 400 nm is scattered on the temporal oscillations of its dielectric index. The resultant sinusoidal phase modulation of the probe pulse permits probe pulse temporal compression by use of both positively and negatively dispersive elements. The potential of this new method is demonstrated by the generation of a periodic train of 5.8-fs pulses at 400 nm with positive group-delay dispersion compensation.

Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber

We report tunable third-harmonic generation (THG) in an Ar-filled hollow-core photonic crystal fiber, pumped by broadband <2 microJ, 30 fs pulses from an amplified Ti:sapphire laser system. The overall dispersion is precisely controlled by balancing the negative dielectric susceptibility of the waveguide against the positive susceptibility of the gas. We demonstrate THG to a higher-order guided mode and show that the phase-matched UV wavelength is tunable by adjusting the gas pressure.

Influence of ionization on ultrafast gas-based nonlinear fiber optics

We numerically investigate the effect of ionization on ultrashort high-energy pulses propagating in gas-filled kagomé-lattice hollow-core photonic crystal fibers by solving an established uni-directional field equation. We consider the dynamics of two distinct regimes: ionization induced blue-shift and resonant dispersive wave emission in the deep-UV. We illustrate how the system evolves between these regimes and the changing influence of ionization. Finally, we consider the effect of higher ionization stages.

Manipulation of coherent Stokes light by transient stimulated Raman scattering in gas filled hollow-core PCF

Transient stimulated Raman scattering is investigated in methane-filled hollow-core photonic crystal fiber. Using frequency-chirped ps-pulses at 1.06 microm as pump and tunable CW-radiation as Stokes seed, the vibrational excitation of the CH(4) molecules can be controlled on the sub T(2) time-scale. In this way the generated Stokes pulse can be phase-locked to the pump pulse and its spectrum manipulated.

Phase-matching and Gain of deep-UV dispersive-wave generation

Gas-filled hollow-core photonic crystal fibres enable nonlinear fibre optics in parameter regimes hitherto impossible with glass-core fibres, such as: ultra-violet transmission, pressure tuneable zero dispersion wavelengths down to 200 nm, and ultra-short duration soliton energies at the microjoule level. Two techniques for ultra-violet light generation in such fibres have utilised these properties: third harmonic generation [1], and dispersive wave generation [2]. In this work we study the phase-matching and gain landscapes of the latter case.

Nonlinear optics in gas-filled HC-PCF in the plasma regime

Laser-driven ionization in Ar-filled HC-PCF is accessed through self-compression of few-microjoule pulses. Modeling confirms that the observed blue-shifted spectral bands are caused by light-plasma interactions over an extended length in the fiber.

Generation of self-compressed laser pulses under the condition of two-photon resonant difference-frequency mixing in gases

Abstract Two-photon resonant difference-frequency generation is investigated for laser pulses short compared to the medium polarization relaxation time T 2 taking into account coherent propagation effects. On the basis of numerical solution of the Maxwell–Bloch equation we demonstrate that the process can be accompanied by temporal self-compression of the generated field leading to the formation of pulses at the difference-frequency with duration significantly shorter and intensity higher than those of the input pump and injection pulse.

Coherent multi-order stimulated Raman generated by two-frequency pumping of hydrogen-filled hollow core PCF

The development of hollow-core photonic crystal fiber (HC-PCF) has led to a number of interesting experiments in the gas-phase nonlinear optics, an example being stimulated Raman scattering (SRS) in hydrogen gas [1, 2]. Multi-octave Raman frequency comb generation has been observed in H2-filled HC-PCF using a single-line pump laser [3]. If such frequency combs could be made coherent, they would find a wide range of practical uses, ranging from sub-fs pulse synthesis to optical clocks and control of carrier-envelope phase [4]. Although the results of theoretical study of higher order SRS, under conditions similar to those reported in [3], suggest that Stokes noise can be filtered out if the Raman gain is high enough, so far no direct evidence of mutual coherence between the individual comb lines has been reported. Here we describe an effective two-frequency-pumping technique for producing mutually-coherent multi-order Raman sidebands in hydrogen-filled HC-PCF.

Theoretical study of dispersive wave generation in ar-filled hollow-core PCF above the plasma threshold

Hollow-core photonic-crystal fibre (HC-PCF) [1] provides a highly efficient means for investigating light-matter interactions at sustained intensity levels inaccessible to both traditional bulk setups (due to limited interaction lengths) or conventional optical fibres (due to the low damage threshold of glass). Recent experiments have shown that strong UV pulses can be generated, through emission of dispersive radiation, by launching near-IR femtosecond pulses into a gas-filled kagomé-lattice PCF [2]. Phase-matching to the UV can be accounted for through the special dispersion characteristics of the gas-filled HC-PCF [3]. Shorter UV wavelengths require higher energy pulses, causing the intensity to enter the ionisation regime when pulse propagation will be influenced by the presence of free electrons. The transition from the “traditional” regime where the Kerr effect dominates to the plasma regime where ionisation becomes important (novel in the context of photonic crystal fibres) is studied numerically in this work.

Three-wave stimulated raman scattering in hydrogen-filled photonic crystal fiber

Gas-filled hollow-core photonic crystal fiber provides convenient conditions for studying stimulated Raman scattering in gases [1–3]. In this kind of fiber, the transmitted mode propagates almost entirely in the gas-filled core. High light intensities are maintained along the fiber, leading to a much longer effective interaction lengths compared to free space systems. As a result, strong nonlinear effects may be seen at relatively low pump pulse energy. In the present study, we use a hydrogen-filled hollow-core photonic bandgap fiber (HC-PBG) with a carefully engineered guidance band extending from 990 to 1230 nm. For the rotational Raman transition S0(1), with a Raman shift of 18 THz, and a pump laser line at 1064 nm, just two Stokes lines (1134 nm and 1215 nm) and one anti-Stokes line (1002 nm) are situated inside this band. Whereas signals at these wavelengths are guided in the fiber core and experience transmission losses below 0.8 dB/m, higher order Raman lines do not lie within the transmission band and are strongly suppressed.