Research articles

Scientists report a dual-wavelength quantum cascade laser that lases at wave factors k ≈ 0 and k ≈ 3.6 × 10⁸ m⁻¹. The finding may change the conventional idea that population inversion of lasing occurs only at k ≈ 0 and give ways on designing intersub-band devices with high k-space.

The application of a very strong magnetic field is experimentally demonstrated to enable operation of terahertz quantum cascade lasers at much higher temperatures than usual. Lasing at a frequency of 3 THz is reported at up to 225 K when a field of 19.3 T is applied. The results validate theoretical predictions that quantum confinement is a route towards room temperature operation.

The ability to modulate optical plasmons, propagating along a metal–dielectric waveguide, on the femtosecond time scale suggests that plasmons may be a suitable data carrier for future ultrasfast communication applications.

A monolithically grown Ge/Si avalanche photodetectors (APD) with a gain–bandwidth product of 340 GHz, the highest value for any APDs operating at 1,300 nm, and a sensitivity equivalent to commercially available III-V compound APDs is reported. The excellent performance paves the way to achieving low-cost, CMOS-based, Ge/Si APDs operating at data rates of 40 Gb s⁻¹ or higher, where the performance of III-V APDs is severely limited.

Applications of microdisk lasers are intrinsically limited by their planar and isotropic emission. Now, by implementing appropriate diffraction gratings along the disk circumference, scientists present a vertically emitting terahertz quantum-cascade microdisk laser, shedding light on the fabrication of arrays of single-mode, highly collimated and powerful terahertz sources.

Random-number generators are important in digital information systems. However, the speed at which current sources operate is much slower than the typical data rates used in communication and computing. Chaos in semiconductor lasers might help to bridge the gap.

The spin Hall effect, an interaction between particles because of their intrinsic spin, is a central tenet in the field of spintronics. The direct observation of an optical equivalent of the spin Hall effect is now reported.

Coupled optical resonators are one approach to slowing the propagation of light. An array of more than 100 such resonators has now been demonstrated using a photonic crystal. Such a structure can slow light down to below 1% of its speed in a vacuum.

The ability to perform low-power, continuous-wave nonlinear optics, in particular four-wave mixing, is demonstrated in doped-silica-glass waveguide ring resonators. The device's low loss and ease of manufacture may make the approach suitable for nonlinear all-optical photonic integrated circuits.

Femtosecond-scale synchronization using mode-locked lasers has been limited to periods of just a few minutes. Now it is shown that, by combining a number of laser techniques, sub-10-fs-precision synchronization of remote lasers and microwave sources is possible for more than 10 hours.

Low-cost, efficient solar cells are sought as an alternative to silicon photovoltaics. Here a dye-based bifacial solar cell that is capable of efficient generation of electricity for light incident on either its front or rear face is demonstrated.

Hollow-core photonic-crystal fibres enable confinement of light on a much tighter scale than is possible with conventional fibre. But dispersion makes it difficult to transmit very short, sub 100 fs, pulses over long distances. A chirped structure could offer a solution.

Tiny optical cavities can influence spontaneous emission of light from atoms and their artificial equivalent, quantum dots. In the past, two–dimensional photonic crystals have been used to create such cavities for quantum dots, now a three–dimensional structure enables full confinement of light in all directions.

An organic LED that acts as an electrically driven source of surface plasmons is reported. The device generates a freely propagating beam of surface plasmons and has potential applications in integrated organic photonics and sensing.

Scientists exploit the use of Airy beams — an unusual class of optical waves — in optical manipulation. The beam can be used to transport particles along curved paths without moving the light beam, a technique that seems poised for many microfluidic applications especially in the biological sciences.

A design of on-chip optomechanical resonator that simultaneously maximizes a high mechanical Q-factor in the megahertz range and an ultrahigh optical finesse is reported. Studies of the mechanical properties of the cavity achieve the first direct observation of mechanical normal-mode coupling in a micromechanical system.

A millimetre-scale liquid lens that is harmonically driven and thus has an oscillating shape is demonstrated. By synchronizing the electronic timing of the image capture with the oscillations, a variable focus lens with a response time of 100 Hz is achieved. Simulations suggest that a faster response is possible for smaller lenses based on the same design.

Metamaterials, based on split-ring resonators, for example, enable complete control over electromagnetic waves in terms of both the electric and magnetic vector components. Measuring the absolute extinction cross-section of a single split-ring resonator advances our understanding of these useful materials.

The ability to efficiently transfer photons from a light source to an optical circuit is crucial, and requires efficient coupling of light to optical fibres and waveguides. Using state-of-the-art fabrication techniques, Hong-Gyu Park and colleagues create a device that uses nanowires to inject light into photonic-crystal waveguides in an efficient way. The structure could become an important part of the nanophotonics toolbox.

Table-top laser-driven plasma accelerators have the potential advantages of being ultracompact and powerful. Electron beams can be created by irradiating gas jets with intense laser light, however, until now it has proved difficult to achieve stable, high-energy beams. Jongmin Lee and colleagues report the first generation of stable gigaelectronvolt-class electron beams using a laser-based accelerator, and make an important step along the road to future particle accelerators.