Letters in 2008

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 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.

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.

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 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.

Here researchers report an integrated detection device for terahertz near-field imaging in which all the necessary detection components, that is, an aperture, a probe and a terahertz detector, are integrated on one cryogenically cooled, semiconductor chip. This scheme enables highly sensitive, high-resolution detection of the evanescent field and promises new capabilities for high-resolution terahertz imaging.

X-ray Fourier transform holography using free-electron lasers has the potential to enable nanoscale imaging on the timescale of atomic motion. A technique that dramatically increases the efficiency of this technique could move us a step towards such imaging.

Short-wavelength UV laser diodes are required for applications ranging from sensing, data storage and materials processing. Here, an electrically driven semiconductor laser that operates at 342.3 nm, the shortest wavelength so far, is reported. The device emits milliwatt-scale powers at room temperature when driven by pulsed current.

Frequency mixing the fundamental-and second-harmonic fields of an ultrafast laser in any one of a number of materials can generate radiation at terahertz frequencies. A better understanding of this process leads to a brighter source of light at these very useful wavelengths.

Free-electron lasers can produce powerful pulses of radiation at very short wavelengths, even in the hard-X-ray region. In general, however, they comprise facilities several kilometres in length. A 55-m-long laser could open up the technology to a broader range of researchers.

Several technologies have been invented as alternatives to the LCD, which transmits only a small portion of the backlight. Now researchers have come up with a display involving a telescopic pixel design, which can transmit 36% of the backlight. The eventual result could be large, bright displays that offer higher contrast at a low cost.

Xiang Zhang and colleagues from the University of California, Berkeley, propose a new approach for confining light on scales much smaller than the wavelength of light. Using hybrid waveguides that incorporate dielectric and plasmonic waveguiding techniques, they are able to confine surface plasmon polaritons very strongly over large distances. The advance could lead to truly nanoscale plasmonics and photonics.