Wideband-tuneable, nanotube mode-locked, fibre laser.

Ultrashort-pulse lasers with spectral tuning capability have widespread applications in fields such as spectroscopy, biomedical research and telecommunications. Mode-locked fibre lasers are convenient and powerful sources of ultrashort pulses, and the inclusion of a broadband saturable absorber as a passive optical switch inside the laser cavity may offer tuneability over a range of wavelengths. Semiconductor saturable absorber mirrors are widely used in fibre lasers, but their operating range is typically limited to a few tens of nanometres, and their fabrication can be challenging in the 1.3-1.5 microm wavelength region used for optical communications. Single-walled carbon nanotubes are excellent saturable absorbers because of their subpicosecond recovery time, low saturation intensity, polarization insensitivity, and mechanical and environmental robustness. Here, we engineer a nanotube-polycarbonate film with a wide bandwidth (>300 nm) around 1.55 microm, and then use it to demonstrate a 2.4 ps Er(3+)-doped fibre laser that is tuneable from 1,518 to 1,558 nm. In principle, different diameters and chiralities of nanotubes could be combined to enable compact, mode-locked fibre lasers that are tuneable over a much broader range of wavelengths than other systems.

Electrically switchable random to photonic band-edge laser emission in chiral nematic liquid crystals

Using a chiral nematic liquid crystal with a negative dielectric anisotropy, it is possible to switch between band-edge laser emission and random laser emission with an electric field. At low frequencies (1 kHz), random laser emission is observed as a result of scattering due to electro-hydrodynamic instabilities. However, band-edge laser emission is found to occur at higher frequencies (5 kHz), where the helix is stabilized due to dielectric coupling. These results demonstrate a method by which the linewidth of the laser source can be readily controlled externally (from 4 nm to 0.5 nm) using electric fields. © 2012 American Institute of Physics.

Digital data transmission using electro-optically modulated vertical-cavity surface-emitting laser with saturable absorber

An electro-optically (EO) modulated oxide-confined vertical-cavity surface-emitting laser (VCSEL) containing a saturable absorber in the VCSEL cavity is studied. The device contains an EO modulator section that is resonant with the VCSEL cavity. A type-II EO superlattice medium is employed in the modulator section and shown to result in a strong negative EO effect in weak electric fields. Applying the reverse bias voltages to the EO section allows triggering of short pulses in the device. Digital data transmission (return-to-zero pseudo-random bit sequence, 27-1) at 10Gb/s at bit-error-rates well below 10-9 is demonstrated. © 2014 AIP Publishing LLC.

Pulsed laser writing of holographic nanosensors

Tuneable optical sensors have been developed to sense chemical stimuli for a range of applications from bioprocess and environmental monitoring to medical diagnostics. Here, we present a porphyrin-functionalised optical sensor based on a holographic grating. The holographic sensor fulfils two key sensing functions simultaneously: it responds to external stimuli and serves as an optical transducer in the visible region of the spectrum. The sensor was fabricated via a 6 nanosecond-pulsed laser (350 mJ, λ = 532 nm) photochemical patterning process that enabled a facile fabrication. A novel porphyrin derivative was synthesised to function as the crosslinker of a polymer matrix, the light-absorbing material, the component of a diffraction grating, as well as the cation chelating agent in the sensor. The use of this multifunctional porphyrin permitted two-step fabrication of a narrow-band light diffracting photonic sensing structure. The resulting structure can be tuned finely to diffract narrow-band light based on the changes in the fringe spacing within the polymer and the system's overall index of refraction. We show the utility of the sensor by demonstrating its reversible colorimetric tuneability in response to variation in concentrations of organic solvents and metal cations (Cu 2+ and Fe2+) in the visible region of the spectrum (λmax ≈ 520-680 nm) with a response time within 50 s. Porphyrin-functionalised optical sensors offer great promise in fields varying from environmental monitoring to biochemical sensing to printable optical devices. This journal is © the Partner Organisations 2014.

Laser microstructuring of Si surfaces for low-threshold field-electron emission

Dense arrays of high aspect ratio Si micro-pyramids have been formed by cumulative high intensity laser irradiation of doped Si wafers in an SF6 environment. A comparative study using nanosecond (XeCl, 308 nm) and femtosecond (Ti: Sapphire, 800 nm and KrF, 248 nm) laser pulses has been performed in this work. The influence of pulse duration and ambient gas pressure (SF6) is also presented. Scanning electron microscopy has shown that upon laser irradiation conical features appear on the Si surface in a rather homogenous distribution and with a spontaneous self alignment into arrays. Their lowest tip diameter is 800 nm; while their height reaches up to 90 mum. Secondary tip decoration appears on the surface of the formed spikes. Areas of 2 X 2 mm(2) covered with Si cones have been tested as cold cathode field emitters. After several conditioning cycles, the field emission threshold for the studied Si tips is as low as 2 V/mum, with an emission current of 10(-3) A/cm(2) at 4 V/mum. Even though these structures have smaller aspect ratios than good quality carbon nanotubes, their field emission properties are similar. The simple and direct formation of field emission Si arrays over small pre-selected areas by laser irradiation could lead to a novel approach for the development of electron sources. (C) 2003 Elsevier B.V. All rights reserved.