Stimulated Raman scattering
Hollow-core photonic crystal fibre (HC-PCF) filled with molecular gases is an ideal vehicle for studying gas-phase stimulated Raman scattering (SRS) [Benabid (2002)]. In contrast to free-space arrangements, they offer long collinear path lengths, tight confinement of light and gas within the hollow core and pressure-adjustable dispersion. The Division's activities in this area have mainly focused on efficient generation of Raman sidebands via SRS, in particular through the controllable molecular modulation induced by the travelling coherence waves excited in the medium. There is also a link to soliton self-compression, which can result in impulsive excitation of Raman coherence if the pulse duration becomes shorter than half a cycle of the Raman oscillation.
Impulsive Raman self-scattering
Hollow-core photonic crystal fibre (HC-PCF) filled with molecular gases offers excellent performance as an ultra-low-threshold modulator and frequency shifter for ns and ps laser pulses. Motivated by this, we have studied experimentally and numerically the propagation of a self-compressed 40 fs laser pulse in a hydrogen-filled HC-PCF. Since the pulse duration is much shorter than the phase relaxation time T2 of the molecular coherence, the Raman coherence is impulsively and efficiently excited, resulting in phase-modulation of a dispersive wave in the VUV, generating in this way a supercontinuum from the NIR well into the VUV (>125 nm) [Belli (2015)]. We have also explored Raman scattering in hydrogen-filled kagomé-PCF by pumping with pulses of 300 fs duration and 20 µJ energy. This generates a pure vibrational, noise-seeded frequency comb spanning more than three octaves, from 180 nm in the deep UV to 2400 nm in the mid-IR. All the anti-Stokes lines were as broad as the pump (in some cases even slightly broader) so that with an average bandwidth of ~52 THz (FWHM) they filled the spectral gaps between the lines [Tani (2015)]. This line-broadening is a direct consequence of operating in the transient regime, when the shapes of the Raman lines are expected to replicate the pump. As a result, SPM-induced broadening of the pump is transferred to all the Raman lines.
Broadband all-LP01 frequency conversion
Raman coherence waves excited in a gas by the interference of pump and Stokes signals can be used to frequency shift a third "mixing" signal, provided phase-matching is satisfied (it acts as a kind of transient nonlinear hologram). Conventionally, this can be arranged using non-collinear beams or higher-order waveguide modes. In this work we reported the collinear phase-matched frequency shifting of an arbitrary mixing signal using only the fundamental LP01 modes of a hydrogen-filled HC-PCF. This is made possible by the S-shaped dispersion curve that occurs around the (pressure-tunable) zero dispersion point. Broadband phase-matched frequency up-shifting by 125 THz is then possible from the UV to the near IR. Long interaction lengths and tight modal confinement reduce the peak intensities required, allowing record conversion efficiencies in excess of 70%. The system is of great interest in coherent anti-Stokes Raman spectroscopy and for wavelength conversion of broadband laser sources [Bauerschmidt (2015)].
Multi-gas coherent anti-Stokes Raman spectroscopy
In this work we performed coherent anti-Stokes Raman spectroscopy (CARS) in a HC-PCF filled with gaseous samples [Hupfer (2016)]. Analysing a multi-component gas mixture in one single, fast measurement requires (1) a broadband seed covering the Stokes lines of all involved gas species, (2) a broadband-guiding HC-PCF, and (3) simultaneous phase-matching for all Raman transitions. For these reasons we use a kagomé-PCF, which enables broadband phase-matching, together with a broad-band Stokes seed signal. This enables simultaneous detection of multiple trace gases, including also measurements of their concentrations. The fibre can in principle increase the CARS signal level by 60 dB compared to free-space arrangements.
Intramodal gain suppression in the vicinity of the zero dispersion point
In 1964 Bloembergen and Shen [Bloembergen (2015)] predicted that gain in Raman-active media could be coherently suppressed if the rates of phonon creation and annihilation exactly balance. This only occurs if synchronous, collective molecular oscillations (or Raman coherence waves) created by pump-to-Stokes scattering are identical to those annihilated in pump-to-anti-Stokes scattering. In free space, this can only be accomplished over limited interaction lengths in non-collinear geometries. In contrast, using a gas-filled kagomé HC-PCF, we have demonstrated for the first time dramatic suppression of the Raman gain over long collinear path-lengths in hydrogen [Bauerschmidt (2015)]. This is achieved by operating close to the pressure-tunable zero dispersion point of the fibre, where the conditions for coherent gain suppression are fulfilled. At a precise pressure, generation of Stokes and anti-Stokes bands in the fundamental mode is entirely suppressed. In this regime, other processes such as spontaneous Raman scattering may be investigated under conditions of high luminosity. This study could have implications in quantum information and particle micromanipulation.
Dense Raman combs using gas mixtures
We have recently reported a novel scheme for generating dense clusters of Raman sidebands. We use a broadband-guiding HC-PCF filled with a mixture of H2, D2, and Xe for efficient interactions between the gas mixture and pump pulses at only 5 μJ energy. This results in the generation from noise of more than 135 ro-vibrational Raman sidebands, covering the visible spectral region with an average spacing of only 2.2 THz [Hosseini (2016)]. Such a spectrally dense and compact fibre-based source is ideal for applications requiring closely-spaced narrowband laser lines with high spectral power density, such as spectroscopy and sensing.
