Barry Luther-Davies

Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics

The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulas for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.

Generation of 5-fs pulses and octave-spanning spectra directly from a Ti:sapphire laser

Spectra extending from 600 to 1200 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser producing 5-fs pulses. Specially designed double-chirped mirror pairs provide broadband controlled dispersion, and a second intracavity focus in a glass plate provides additional spectral broadening. These spectra are to our knowledge the broadest ever generated directly from a laser oscillator.

Ultrafast all-optical chalcogenide glass photonic circuits

Chalcogenide glasses offer large ultrafast third-order nonlinearities, combined with low two-photon absorption and absence of free carrier absorption in a photosensitivity medium. We review the key properties of these materials, including the strong photosensitivity and focus on several recent demonstrations of ultra-fast all-optical signal processing: optical time division multiplexing, all-optical signal regeneration and wavelength conversion.

Supercontinuum generation in dispersion engineered highly nonlinear (y=10/W/m) As2S3 chalcogenide planar waveguide

We demonstrate supercontinuum generation in a highly nonlinear As(2)S(3) chalcogenide planar waveguide which is dispersion engineered to have anomalous dispersion at near-infrared wavelengths. This waveguide is 60 mm long with a cross-section of 2 mum by 870 nm, resulting in a nonlinear parameter of 10 /W/m and a dispersion of +29 ps/nm/km. Using pulses with a width of 610 fs and peak power of 68 W, we generate supercontinuum with a 30 dB bandwidth of 750 nm, in good agreement with theory.

Long, low loss etched As(2)S(3) chalcogenide waveguides for all-optical signal regeneration.

We report on the fabrication and optical properties of etched highly nonlinear As(2)S(3) chalcogenide planar rib waveguides with lengths up to 22.5 cm and optical losses as low as 0.05 dB/cm at 1550 nm - the lowest ever reported. We demonstrate strong spectral broadening of 1.2 ps pulses, in good agreement with simulations, and find that the ratio of nonlinearity and dispersion linearizes the pulse chirp, reducing the spectral oscillations caused by self-phase modulation alone. When combined with a spectrally offset band-pass filter, this gives rise to a nonlinear transfer function suitable for all-optical regeneration of high data rate signals.

Fabrication and characterization of low loss rib chalcogenide waveguides made by dry etching

We report the fabrication and characterization of rib chalcogenide waveguides produced by dry etching with CF4 and O2. The high index contrast waveguides (Deltan ~1) show a minimum propagation loss of 0.25 dB/cm. The high refractive nonlinearity of 100 times silica in As2S3 allowed observation of a pi phase shift due to self-phase modulation of an 8 ps duration 1573 nm pulse in a 5 cm long waveguide.

Ultrafast ablation with high-pulse-rate lasers. Part I: Theoretical considerations

We propose a novel ultrafast pulsed laser deposition (PLD) technique, which eliminates the well-known problem of contamination of the films produced by PLD with particulates ejected from the target. The method uses low energy, picosecond duration laser pulses delivered onto a target at rates of several tens of MHz and thus differs from conventional the PLD method which utilizes high energy, nanosecond duration pulses delivered at low (≈10 Hz) repetition rates. In this article we present the theoretical background justifying the method and define the optimal conditions for efficient evaporation of a target with given thermodynamic properties. By reducing the laser pulse energy while maintaining optimum evaporation, the number of atoms evaporated by each pulse is reduced to the point where it becomes impossible for macroscopic lumps of material to be ejected with the available laser energy, thus preventing the source of particle contamination in the film. To achieve high evaporation rate, the laser pulse re...

Waveguides and Y junctions formed in bulk media by using dark spatial solitons

Dark spatial solitons are created when intense quasi-plane waves containing amplitude or phase discontinuities propagate through self-defocusing, Kerr-like media. We show that dark solitons form not only stable waveguides but also structures equivalent to Y-junction splitters and other devices.

Producing air-stable monolayers of phosphorene and their defect engineering

It has been a long-standing challenge to produce air-stable few- or monolayer samples of phosphorene because thin phosphorene films degrade rapidly in ambient conditions. Here we demonstrate a new highly controllable method for fabricating high quality, air-stable phosphorene films with a designated number of layers ranging from a few down to monolayer. Our approach involves the use of oxygen plasma dry etching to thin down thick-exfoliated phosphorene flakes, layer by layer with atomic precision. Moreover, in a stabilized phosphorene monolayer, we were able to precisely engineer defects for the first time, which led to efficient emission of photons at new frequencies in the near infrared at room temperature. In addition, we demonstrate the use of an electrostatic gate to tune the photon emission from the defects in a monolayer phosphorene. This could lead to new electronic and optoelectronic devices, such as electrically tunable, broadband near infrared lighting devices operating at room temperature.

Breakthrough switching speed with an all-optical chalcogenide glass chip: 640 Gbit/s demultiplexing

We report the first demonstration of error-free 640 Gbit/s demultiplexing using the Kerr non-linearity of an only 5 cm long chalcogenide glass waveguide chip. Our approach exploits four-wave mixing by the instantaneous nonlinear response of chalcogenide. Excellent performance is achieved with only 2 dB average power penalty and no indication of error-floor. Characterisation of the FWM efficiency for the chalcogenide waveguide is given and confirms the good performance of the device.