Hollow-core fiber traps photons to enable attosecond pulses

The paper (published recently in Science)¹ describes two breakthroughs in different major fields. The first one relates to photonics and photonic materials and the second to Attoscience. Indeed, we report on a new theory of photonic guidance in hollow-core photonic crystal fiber (HC-PCF). The guidance is different from the now conventional photonic bandgap guidance but it relates to the so-called Von Neumann-Wigner bound and quasi-bound states within a continuum. That is, the HC-PCF exhibits the counter-intuitive property of accommodating concurrently core-guided modes and cladding continuum modes (even with the same symmetry).

The profile of the transmitted light through the Kagome-lattice hollow-core PCF. The fiber can accommodate guidance broadband guidance in the core with a continuum of cladding modes without interaction.

Crucially, the developed fiber exhibits transmission bandwidths several times wider than the state-of-the-art photonic bandgap HC-PCF with relatively low loss. The unique properties of this fiber made it possible to observe the second breakthrough, which relates to the field of ultra-fast optics where we report on the generation of 3-octave spanning coherent spectral comb-like light generation, using transient stimulated Raman scattering (SRS) in hydrogen-filled HC-PCF. In contrast with previous equivalent techniques, the spectrum is generated, firstly, with power levels six orders of magnitude lower, and secondly with pulses five orders of magnitude longer in duration. The implications are wide-ranging: The new photonic guidance theory would not only create a new direction for the design of next-generation photonic materials (e.g. broadband HC-PCF and photonic-crystals). In addition, the similarity between this guidance and the Von Neumann-Wigner states would constitute a new conceptual bridge between photonics and quantum mechanics.

The diffracted spectrum generated in a Kagome-lattice hollow-core PC filled with hydrogen. The spectrum spans from the UV to mid-IR.

Furthermore, the topic of multi-line coherent stimulated Raman scattering is a long-standing basic problem in nonlinear optical physics, and our new experiments and theory are a large step forward in this area, stimulated by the availability of the new hollow-core fiber. Indeed, the availability of ultra-broad coherent comb-like spectra that could be generated with very simple and moderately powerful lasers and which covers several octaves at wavelengths spanning the UV-IR, means that synthesizing attosecond pulses could be achieved more readily and at more "tamed" wavelengths than those used currently which lie in XUV and soft X-ray regions. Our new techniques would then also enable low-cost compact systems for sub-femtosecond optical pulse generation, a development that would have huge impact in research in many areas, such as laser science, materials science, biological research, etc.

The Bath team: from left: Mr. Francois Couny, Dr. Fetah Benabid and Mr. Phil Light. Other team members (not pictured) are Dr John Roberts of the Technical University of Denmark and Dr Michael Raymer of the University of Oregon. Images courtesy University of Bath Centre for Photonics & Photonic Materials.  

Fetah Benabid
University of Bath
Bath, UK
http://www.bath.ac.uk/physics/groups/cppm/

Fetah Benabid is an EPSRC Advanced Fellow and Lecturer in the Centre for Photonics & Photonic Materials. His current research activities focus on nonlinear and coherent optics (e.g. EIT and molecular modulation) in gas-phase filled HC-PCFs, and the development of new types of HC-PCFs. In recognition of the significance of his work he was awarded the European Physical Society Fresnel Prize in 2005.