Reversible Mode Switching in Y coupled Terahertz Lasers

Electrically independent terahertz (THz) quantum cascade lasers (QCLs) are optically coupled in a Y configuration. Dual frequency, electronically switchable emission is achieved in one QCL using an aperiodic grating, designed using computer-generated hologram techniques, incorporated directly into the QCL waveguide by focussed ion beam milling. Multi-moded emission around 2.9 THz is inhibited, lasing instead occurring at switchable grating-selected frequencies of 2.88 and 2.92 THz. This photonic control and switching behaviour is selectively and reversibly transferred to the second, unmodified QCL via evanescent mode coupling, without the transfer of the inherent grating losses

Y coupled terahertz quantum cascade lasers

Here we demonstrate a Y coupled terahertz (THz) quantum cascade laser (QCL) system. The two THz QCLs working around 2.85 THz are driven by independent electrical pulsers. Total peak THz output power of the Y system, with both arms being driven synchronously, is found to be more than the linear sum of the peak powers from the individual arms; 10.4 mW compared with 9.6 mW (4.7 mW + 4.9 mW). Furthermore, we demonstrate that the emission spectra of this coupled system are significantly different to that of either arm alone, or to the linear combination of their individual spectra.Comment: 9 pages, 3 figure

On the shape of barchan dunes

Barchans are crescent-shaped sand dunes forming in aride regions with unidirectional wind and limited sand supply. We report analytical and numerical results for dune shapes under different environmental conditions as obtained from the so-called `minimal model' of aeolian sand dunes. The profiles of longitudinal vertical slices (i.e. along the wind direction) are analyzed as a function of wind speed and sand supply. Shape transitions can be induced by changes of mass, wind speed and sand supply. Within a minimal extension of the model to the transverse direction the scale-invariant profile of transverse vertical cuts can be derived analytically.Comment: to appear in J. Phys.: Condens. Matter 17 (2005

Longitudinal Computer Generated Holograms for Digital Frequency Control in Electronically Tunable Terahertz Lasers

A transverse computer-generated hologram (CGH) diffracts and provides flexible control of incident light by steering it to any point in the projected image plane - i.e. CGHs are able to direct the light to where it is needed and away from where it is not. In addition, the number of resolvable points in the image projection plane is a function of the CGH's pixel count. Here we report a longitudinal CGH (LCGH), a photonic structure, which swaps the ability to steer light toward fixed spatial points for digital control in the frequency domain. This is of particular interest in the context of tunable lasers. In this regard, an LCGH offers two important degrees-of-freedom (DOFs): 1) provides high-resolution wavevector or k space resolution within the Brillouin zone; 2) enables full control to define or modify the reflectivity at each resolvable k point, so attaining a target spectral response. We demonstrate the flexibility of our LCGH approach by achieving purely electronic tuning between six digitally-selected operating frequencies in a single section terahertz (THz) quantum cascade laser (QCL). These switchable single-frequency devices will simplify combining the power and flexibility of THz QCLs with spectroscopic applications, such as remote sensing, spectral analysis, and both security and medical imaging.Comment: 17 pages, 4 Figures, submitted to Nature Photonic

Differential rotation of nonlinear r-modes

Differential rotation of r-modes is investigated within the nonlinear theory up to second order in the mode amplitude in the case of a slowly-rotating, Newtonian, barotropic, perfect-fluid star. We find a nonlinear extension of the linear r-mode, which represents differential rotation that produces large scale drifts of fluid elements along stellar latitudes. This solution includes a piece induced by first-order quantities and another one which is a pure second-order effect. Since the latter is stratified on cylinders, it cannot cancel differential rotation induced by first-order quantities, which is not stratified on cylinders. It is shown that, unlikely the situation in the linearized theory, r-modes do not preserve vorticity of fluid elements at second-order. It is also shown that the physical angular momentum and energy of the perturbation are, in general, different from the corresponding canonical quantities.Comment: 9 pages, revtex4; section III revised, comments added in Introduction and Conclusions, references updated; to appear in Phys. Rev.

Chandra observations of the hybrid morphology radio sources 3C 433 and 4C 65.15: FR IIs with asymmetric environments

We present Chandra observations of the hybrid morphology radio sources 3C 433 and 4C 65.15, two members of the rare class of objects possessing an FR I jet on one side of the core and an FR II lobe on the other. The X-ray spectrum of 3C 433 shows intrinsic absorption (with a column density of N_H=8e22 cm-2), such as is typical of FR II narrow-line radio galaxies. There is excess X-ray emission below 2 keV containing contributions from diffuse soft X-ray emission (likely hot gas with kT~1.2 keV) as well as from the nucleus. The core of 3C 433 is extended in hard X-rays, presumably due to X-ray emission from the inner-jet knot on the FR I side that is apparent in the radio map. It is possible that the X-ray emission from this inner-jet knot is absorbed by the dust known to be present in the host galaxy. The spectrum of 4C 65.15 can be modeled with a simple power law with perhaps mild intrinsic absorption (N_H=1.3e21 cm-2). X-ray emission is detected at the bend in the FR I jet. This X-ray jet emission lies above the extrapolation from the high-frequency radio synchrotron emission and has a spectral slope flatter than alpha_rx, indicating that the jet spectral energy distribution is concave as with other FR II quasar jets. Both 3C 433 and 4C 65.15 have unabsorbed X-ray luminosities, radio luminosities, and optical spectra typically seen in comparable sources with FR II morphologies. Presumably the FR I structure seen on one side in these hybrid sources is generated by a powerful jet interacting with a relatively dense environment.Comment: 26 pages, 7 figures (2 color), accepted by Ap