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.

Spectrum at the fibre output measured using a triple grating spectrometer. A Si detector was used over the range covered by the blue curve (~0.27 to ~1.66 PHz) and a PbS detector for frequencies below ~0.27 PHz (the red curve). The dashed lines indicate the expected positions of the Stokes and anti-Stokes Raman bands.

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)].

Frequency up-conversion of a broadband input signal M00 by a Raman coherence wave generated by the beat note of the pump (P) and Stokes (S1, S2) signals. The bandwidth is replicated and up-shifted to the vicinity of the first (M1) and second (M2) mixing anti-Stokes bands by the fundamental vibrational excitation of hydrogen. The quasi-monochromatic pump pulse is centred at 532 nm.

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.

CARS spectrum of a sample containing nitrogen, hydrogen and methane. The central pump peaks at 532 nm, the supercontinuum Stokes seed is located at the long wavelength side, and anti-Stokes lines are generated below 532 nm.

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.

Experimental data (symbols) and numerical simulations (solid lines) showing parametric Raman gain suppression in a H2-filled kagomé HC-PCF. The fundamental LP01 mode Raman signals are completely suppressed in the vicinity of the zero dispersion point (at 24 bar), revealing the presence of weak higher-order mode signal (II).

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. 

(a) Spectrum of the Raman comb generated in a H2-D2 mixture. (b) Spectrum after addition of 4 bar of Xe. Inset shows the same spectrum recorded with a higher resolution. Top panel shows the dispersed spectrum in (a) captured on a screen.