Philip St. J. Russell

Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers [Invited]

We review the use of hollow-core photonic crystal fibers (PCFs) in the field of ultrafast gas-based nonlinear optics, including recent experiments, numerical modeling, and a discussion of future prospects. Concentrating on broadband guiding kagome-style hollow-core PCF, we describe its potential for moving conventional nonlinear fiber optics both into extreme regimes—such as few-cycle pulse compression and efficient deep ultraviolet wavelength generation—and into regimes hitherto inaccessible, such as single-mode guidance in a photoionized plasma and high-harmonic generation in fiber.

Photonic crystal fibres for chemical sensing and photochemistry

In this review, we introduce photonic crystal fibre as a novel optofluidic microdevice that can be employed as both a versatile chemical sensor and a highly efficient microreactor. We demonstrate that it provides an excellent platform in which light and chemical samples can strongly interact for quantitative spectroscopic analysis or photoactivation purposes. The use of photonic crystal fibre in photochemistry and sensing is discussed and recent results on gas and liquid sensing as well as on photochemical and catalytic reactions are reviewed. These developments demonstrate that the tight light confinement, enhanced light-matter interaction and reduced sample volume offered by photonic crystal fibre make it useful in a wide range of chemical applications.

Bandgap guidance in hybrid chalcogenide–silica photonic crystal fibers

We report a hybrid chalcogenide-silica photonic crystal fiber made by pressure-assisted melt-filling of molten glass. Photonic bandgap guidance is obtained at a silica core placed centrally in a hexagonal array of continuous centimeters-long chalcogenide strands with diameters of 1.45 μm. In the passbands of the cladding, when the transmission through the silica core is very weak, the chalcogenide strands light up with distinct modal patterns corresponding to Mie resonances. In the spectral regions between these passbands, strong bandgap guidance is observed, where the silica core transmission loss is 60 dB/cm lower. The pressure-assisted fabrication approach opens up new ways of integrating sophisticated glass-based devices into optical fiber circuitry with potential applications in supercontinuum generation, magneto-optics, wavelength selective devices, and rare-earth-doped amplifiers with high gain per unit length.

Spectrally smooth supercontinuum from 350 nm to 3 mum in sub-centimeter lengths of soft-glass photonic crystal fibers.

The conversion of light fields in photonic crystal fibers (PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced.

Raman-like light scattering from acoustic phonons in photonic crystal fiber

Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core.

All-solid bandgap guiding in tellurite-filled silica photonic crystal fibers

We report all-solid bandgap-guiding fibers formed by pumping molten tellurite glass into silica-air photonic crystal fiber at high pressure. The spectral positions of the guidance bands agree well with multipole simulations and bandgap calculations. The micrometer-diameter tellurite strands are found to contain microheterogeneities (most probably originating from devitrification), which increase the fiber attenuation, although no evidence of crystallization is seen in the bulk tellurite glass. The technique offers a potential route to employing difficult-to-handle glasses, or glasses unsuitable for fiber drawing, in fiber-based amplifiers, modulators, filters, and nonlinear devices.

Sonic band gaps in PCF preforms: enhancing the interaction of sound and light

We study the localisation and control of high frequency sound in a dual-core square-lattice photonic crystal fibre preform. The coupled states of two neighboring acoustic resonances are probed using an interferometric set up, and experimental evidence is obtained for odd and even symmetry trapped states. Full numerical solutions of the acoustic wave equation show the existence of a two-dimensional sonic band gap, and numerical modelling of the strain field at the defects gives results that agree well with the experimental observations. The results suggest that sonic band gaps can be used to manipulate sound with great precision and enhance its interaction with light.

Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF.

An efficient and tunable 176-550 nm source based on the emission of resonant dispersive radiation from ultrafast solitons at 800 nm is demonstrated in a gas-filled hollow-core photonic crystal fiber (PCF). By careful optimization and appropriate choice of gas, informed by detailed numerical simulations, we show that bright, high quality, localized bands of UV light (relative widths of a few percent) can be generated at all wavelengths across this range. Pulse energies of more than 75 nJ in the deep-UV, with relative bandwidths of ~3%, are generated from pump pulses of a few μJ. Excellent agreement is obtained between numerical and experimental results. The effects of positive and negative axial pressure gradients are also experimentally studied, and the coherence of the deep-UV dispersive wave radiation numerically investigated.

Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes.

We report a hollow-core photonic crystal fiber that is engineered so as to strongly suppress higher-order modes, i.e., to provide robust LP01 single-mode guidance in all the wavelength ranges where the fiber guides with low loss. Encircling the core is a single ring of nontouching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes. We show that the resulting modal filtering effect depends on only one dimensionless shape parameter, akin to the well-known d/Λ parameter for endlessly single-mode solid-core PCF. Fabricated fibers show higher-order mode losses some ∼100 higher than for the LP01 mode, with LP01 losses <0.2  dB/m in the near-infrared and a spectral flatness ∼1  dB over a >110  THz bandwidth.

Entangling different degrees of freedom by quadrature squeezing cylindrically polarized modes.

C. Gabriel1,2,∗, A. Aiello1,2, W. Zhong1,2, T. G. Euser1, N.Y. Joly2,1, P. Banzer1,2, M. Förtsch1,2, D. Elser1,2, U. L. Andersen1,2,3, Ch. Marquardt1,2, P. St.J. Russell1,2 and G. Leuchs1,2 1 Max Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1, D-91058 Erlangen, Germany 2 Institute for Optics, Information and Photonics, University Erlangen-Nuremberg, Staudtstr. 7/B2, D-91058 Erlangen, Germany 3 Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark ∗ Christian.Gabriel@mpl.mpg.de