Showing papers by "Philip St. J. Russell published in 2018"

Direct characterisation of tuneable few-femtosecond dispersive-wave pulses in the deep UV.

Abstract: Dispersive wave emission (DWE) in gas-filled hollow-core dielectric waveguides is a promising source of tuneable coherent and broadband radiation, but so far the generation of few-femtosecond pulses using this technique has not been demonstrated. Using in-vacuum frequency-resolved optical gating, we directly characterise tuneable 3fs pulses in the deep ultraviolet generated via DWE. Through numerical simulations, we identify that the use of a pressure gradient in the waveguide is critical for the generation of short pulses.

Long-Lived Refractive-Index Changes Induced by Femtosecond Ionization in Gas-Filled Single-Ring Photonic-Crystal Fibers

Abstract: Plasma recombination always follows photoionization of gas by intense femtosecond laser pulses, causing refractive-index changes via thermal and hydrodynamic effects. In gas-filled hollow-core photonic-crystal fibers, these phenomena are induced by self-compressing pulses with $\ensuremath{\mu}$J energies. Probing from the side of the fiber, the authors see refractive-index changes lasting tens of $\ensuremath{\mu}$s, and plasma-driven acoustic waves that excite MHz vibrations in the microstructure of the fiber. These results are important for the development of high-intensity, high-repetition-rate lasers where, in addition to nonlinear optics, plasma physics and optoacoustics also become relevant.

Broadband and tunable time-resolved THz system using argon-filled hollow-core photonic crystal fiber

Abstract: We demonstrate broadband, frequency-tunable, phase-locked terahertz (THz) generation and detection based on difference frequency mixing of temporally and spectrally structured near-infrared (NIR) pulses. The pulses are prepared in a gas-filled hollow-core photonic crystal fiber (HC-PCF), whose linear and nonlinear optical properties can be adjusted by tuning the gas pressure. This permits optimization of both the spectral broadening of the pulses due to self-phase modulation (SPM) and the generated THz spectrum. The properties of the prepared pulses, measured at several different argon gas pressures, agree well with the results of numerical modeling. Using these pulses, we perform difference frequency generation in a standard time-resolved THz scheme. As the argon pressure is gradually increased from 0 to 10 bar, the NIR pulses spectrally broaden from 3.5 to 8.7 THz, while the measured THz bandwidth increases correspondingly from 2.3 to 4.5 THz. At 10 bar, the THz spectrum extends to 6 THz, limited only by the spectral bandwidth of our time-resolved detection scheme. Interestingly, SPM in the HC-PCF produces asymmetric spectral broadening that may be used to enhance the generation of selected THz frequencies. This scheme, based on a HC-PCF pulse shaper, holds great promise for broadband time-domain spectroscopy in the THz, enabling the use of compact and stable ultrafast laser sources with relatively narrow linewidths (<4 THz).

Long-lived refractive index changes induced by femtosecond ionization in gas-filled single-ring photonic crystal fibers

Abstract: We investigate refractive index changes caused by femtosecond photoionization in a gas-filled hollow-core photonic crystal fiber. Using spatially-resolved interferometric side-probing, we find that these changes live for tens of microseconds after the photoionization event - eight orders of magnitude longer than the pulse duration. Oscillations in the megahertz frequency range are simultaneously observed, caused by mechanical vibrations of the thin-walled capillaries surrounding the hollow core. These two non-local effects can affect the propagation of a second pulse that arrives within their lifetime, which works out to repetition rates of tens of kilohertz. Filling the fiber with an atomically lighter gas significantly reduces ionization, lessening the strength of the refractive index changes. The results will be important for understanding the dynamics of gas-based fiber systems operating at high intensities and high repetition rates, when temporally non-local interactions between successive laser pulses become relevant.

Excitation of higher-order modes in optofluidic photonic crystal fiber.

Abstract: Higher-order modes up to LP33 are controllably excited in water-filled kagome- and bandgap-style hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10-20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagome HC-PCF and match numerical simulations. Mode indices are obtained by launching plane-waves at specific angles onto the fiber input-face and comparing the resulting intensity pattern to that of a particular mode. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation.

Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers

Abstract: Highly sensitive, quantitative detection of singlet oxygen (O2) is required for the evaluation of newly developed photosensitizers and the elucidation of the mechanisms of many processes in which singlet oxygen is known, or believed, to be involved. The direct detection of O2, via its intrinsic phosphorescence at 1270 nm, is challenging because of the extremely low intensity of this emission, coupled with the low quantum efficiency of currently available photodetectors at this wavelength. We introduce hollowcore photonic crystal fibre (HC-PCF) as a novel optofluidic modality for photosensitization and detection of O2. We report the use of this approach to achieve highly sensitive detection of the luminescence decay of O2, produced using two common photosensitizers, Rose Bengal and Hypericin, within the 60-m diameter core of a 15-cm length of HC-PCF. We demonstrate the feasibility of directly detecting sub-picomole quantities of O2 using this methodology, and identify some aspects of the HC-PCF technology that can be improved to yield even higher detection sensitivity.

Optomechanical cooling of a glass-fibre nanospike evanescently coupled to a whispering-gallery-mode bottle resonator

Abstract: Laser cooling of mechanical degrees of freedom is one of the most significant achievements in the field of cavity optomechanics [1]. Mesoscopic mechanical oscillators with high resonant frequencies (MHz to GHz) are typically favoured for laser cooling because they allow easier access to the sideband-resolved regime. The extension of this technique to the macroscopic scale, which usually involves lower frequency resonances, is not straightforward and several schemes have been proposed over the past decade [2, 3, 4, 5]. Here we report efficient passive optomechanical cooling of the motion of a free-standing waveguide that is dissipatively coupled to a high-Q whispering-gallery-mode (WGM) bottle resonator. The waveguide is an 8 mm long glass-fibre nanospike [6], which has a fundamental mechanical resonance at {\Omega}/2{\pi} = 2.5 kHz. Upon launching ~250 {\mu}W laser power at an optical frequency close to the WGM resonant frequency, the optomechanical interaction between nanospike and WGM resonator causes the nanospike resonance to be cooled from room temperature down to 1.8 K. Simultaneous cooling of the first higher order mechanical mode is also observed, causing strong suppression of the Brownian motion of the nanospike, observed as an 11.6 dB reduction in its mean square displacement. This result sets a new benchmark on the lowest frequency mechanical motion that can be passively cooled, and represents the first practical application of dissipative optomechanics. The results are of direct relevance in the many applications of high-Q WGM resonators, including nonlinear optics [7], atom physics [8], optomechanics [9, 10], and sensing [11, 12].

Stable Immobilization of Size‐Controlled Bimetallic Nanoparticles in Photonic Crystal Fiber Microreactor

Abstract: 1Technische Universität Darmstadt, Ernst-Berl-Institut für Technische und Makromolekulare Chemie, 64287 Darmstadt, Germany; 2Max-Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1/Bldg. 24, 91058 Erlangen, Germany; 3NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom; 4Lehrstuhl für Chemische Reaktionstechnik, University of Erlangen-Nuremberg, 91058 Erlangen, Germany;

Excitation of higher-order modes in optofluidic photonic crystal fiber

Abstract: Higher-order modes up to LP$_{33}$ are controllably excited in water-filled kagome- and bandgap-style hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10-20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagome HC-PCF and match numerical simulations. Mode indices are obtained by launching plane-waves at specific angles onto the fiber input-face and comparing the resulting intensity pattern to that of a particular mode. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation.

Optomechanical cooling and self-stabilization of a waveguide coupled to a whispering-gallery-mode resonator

Abstract: Laser cooling of mechanical degrees of freedom is one of the most significant achievements in the field of opto-mechanics. Here, we report, for the first time to the best of our knowledge, efficient passive optomechanical cooling of the motion of a freestanding waveguide coupled to a whispering-gallery-mode (WGM) resonator. The waveguide is an 8 mm long glass-fiber nanospike, which has a fundamental flexural resonance at {\Omega}/2{\pi} = 2.5 kHz and a Q-factor of 1.2 10**5. Upon launching 250 {\mu}W laser power at an optical frequency close to the WGM resonant frequency, we observed cooling of the nanospike resonance from room temperature down to 1.8 K. Simultaneous cooling of the first higher-order mechanical mode is also observed. The strong suppression of the overall Brownian motion of the nanospike, observed as an 11.6 dB reduction in its mean square displacement, indicates strong optomechanical stabilization of linear coupling between the nanospike and the cavity mode. The cooling is caused predominantly by a combination of photothermal effects and optical forces between nanospike and WGM resonator. The results are of direct relevance in the many applications of WGM resonators, including atom physics, optomechanics, and sensing.

Frequency-Tunable THz Source Using Ar-Filled HC-PCF Pulse Shaper

Abstract: We demonstrate a frequency-tunable phase-locked terahertz (THz) source relying on nonlinear optical propagation of a femtosecond near-infrared pulse inside a gas-filled hollow-core photonic-crystal fiber (HC-PCF).

Excitation of higher-order modes in optofluidic hollow-core photonic crystal fiber

Abstract: Higher-order modes are controllably excited in water-filled kagome-, bandgap-style, and simplified hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagome HC-PCF and match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation.

Broadband multi-species CARS in gas-filled hollow-core photonic crystal fiber

Abstract: We report CARS in hollow-core gas-filled PCF. The results pave the way to single-shot broadband (>4000 cm−1) CARS under ambient conditions. A detection limit of 300 ppm was reached with 200 mW overall laser power.

Pulse fragmentation and multi-soliton states in mid-infrared mode-locked fiber laser

Abstract: We observe, in a highly-pumped 2.8 μm mode-locked fiber laser, a variety of stationary multi-soliton states, including phase-locked soliton-pair and soliton-triplet states and stable harmonic mode-locking at two and three times the round-trip frequency.

Holographic Optical Tweezers at the Tip of a Needle

Abstract: Fibre-based optical tweezers typically rely on engineered fibre terminations yielding limited flexibility in number and positioning of trap sites. Here, we demonstrate holographic optical tweezers delivered by a lensless, high-NA multimode fibre with full positioning control of multiple tweezers independently and in all directions.

Optomechanically coupled glass nanospike array on the endface of a multicore fiber

Abstract: We report the fabrication and characterization of an optomechanically coupled array of glass nanospikes on the endface of a germanate multicore fiber. Strong optical gradient forces drive the mechanical motion of the free-standing nanospikes.

Thresholdless deep and vacuum ultraviolet Raman frequency conversion in H$_2$-filled photonic crystal fiber

Abstract: Coherent ultraviolet (UV) light has many uses, for example in the study of molecular species relevant in biology and chemistry. Very few if any laser materials offer UV transparency along with damage-free operation at high photon energies and laser power. Here we report efficient generation of deep and vacuum UV light using hydrogen-filled hollow-core photonic crystal fiber (HC-PCF). Pumping above the stimulated Raman threshold at 532 nm, coherent molecular vibrations are excited in the gas, permitting highly efficient thresholdless wavelength conversion in the UV. The system is uniquely pressure-tunable, allows spatial structuring of the out-coupled radiation, and shows excellent performance in the vacuum UV. It can also in principle operate at the single-photon level, when all other approaches are extremely inefficient.

Noise-seeded Backward Stimulated Raman Scattering in Gas-filled Hollow-Core Fibers

Abstract: We study, experimentally and theoretically, noise-seeded backward SRS in gas-filled hollow-core fibers. The spatio-temporal dynamics of the Raman coherence makes the backward Stokes signal stronger than the forward, despite a low backward/forward gain ratio.