Letters in 2008

Before the practical implementation of quantum information schemes, there is a need to reduce loss, both in terms of photons and the information they carry. A robust scheme now experimentally demonstrated tackles this problem using so-called decoherence-free subspace.

Extracting light from organic LEDs is difficult owing to the refractive index of the materials used, and the output efficiency is typically limited to around 15–20%. By embedding a grid with a low refractive index into the organic layers and using a microlens array researchers have now managed to increase this figure to 34%, representing an improvement by factor of 2.3 over a conventional device.

High-speed imaging gives us a fascinating insight into ultrafast changes in materials. By combining the speed of optical pulses and the short wavelength of X-ray pulses, imaging with 50-nm spatial and 10-ps temporal resolution is possible, with scope to go much further.

In a random laser, the conventional optical cavity is replaced by light scattering from many particles. The random arrangement of the particles makes it difficult to tune the lasing to a chosen wavelength. However, tuning is possible by controlling the size of the particles.

Determining the exact number of photons in a weak light pulse is an important requirement for many applications in quantum optics. Now, contrary to popular belief, Andrew Shields and colleagues have demonstrated that an avalanche-photodiode detector can perform the task.

A waveguide–integrated GeSi electro-absorption modulator on silicon with an ultra-low energy consumption of 50 fJ^–1bit is presented. Operating in the spectral range of 1539—1553 nm, the CMOS–compatible device has an active area of 30 µm² and is anticipated to be useful for future communication systems based on large–scale electronic–photonic integration on silicon.

Two-photon excitation is attractive for photodynamic therapy as it potentially allows deeper penetration within biological tissue and targeting with better precision. However, two-photon cross-sections of light-sensitive drugs are typically small, which has until now limited their practical utility. Now Anderson and colleagues have come up with a new family of light-sensitive drugs that are designed for efficient two-photon excitation. They demonstrate selective closure of blood vessels in mice using one of their new drugs.

The ‘spaser’ (surface plasmon amplification by stimulated emission of radiation) is a relatively new and exciting concept analogous to the laser. It involves amplifying specific surface plasmon modes using a nanoscale device. Zheludev and co-workers extend this concept by suggesting that metamaterials could be used to create a lasing spaser, that is, a spaser that can emit light with high spatial coherence.

Mode–locked fibre lasers enable high–power yet very stable optical frequency combs, paving the way towards higher resolution spectroscopy. The power scalability of such fibre–based systems opens the possibility of frequency combs operating with average powers in excess of 10 kW.

We report the experimental observation of one- and two-dimensional grating patterns formed in a disordered metal-nanoparticle layer by a single light pulse. The phenomenon is attributed to interference effects between the incident light and waveguided modes. Such self-patterning behaviour could be useful for the fabrication of complex nanostructures and advanced photonic devices.

Light absorbers are not 100% efficient, and it is a challenge to absorb light completely for any direction of incidence. Using nanostructured metal surfaces, de Abajo and colleagues show that such omnidirectional absorption is now possible, potentially leading to more efficient solar cells.

Optical-frequency antennas efficiently couple light into very small volumes. Introducing an important concept from radiofrequency antenna design, that of loading with so-called lumped circuit elements, may provide a way of tuning the frequency response of optical nanoantennas.

Laser-generated plasmas are important for the creation of X-ray lasers and attosecond light pulses, but observing the internal dynamics of a plasma is difficult. This paper reports a method for real-time imaging of the electric-field distribution in such plasmas with ultrahigh temporal resolution, yielding a new insight into their behaviour

Metamaterials that possess frequency tunability enable new device functions. By external optical control through the incorporation of semiconductors in metallic split-ring resonators, the researchers provide an elegant solution to frequency-agile terahertz metamaterials.

The drive to develop detectors capable of counting the number of photons in a weak optical pulse is motivated by potential applications in quantum computing. Superconducting nanostructures are one exciting approach: offering high sensitivity and operate at repetition rates up to 80 MHz.

Silicon photonics is deemed to be the solution for dense on-chip optical networks. Now, by using cascaded silicon microring resonators, scientists demonstrate an ultracompact switch that is insensitive to wavelength and temperature. The switch also has fast error-free operation in multiple 40-Gbit s⁻¹ optical channels and is suitable for scalable networks.

By scaling down device size, the principles of radio antennas can be used in the optical regime. These optical antennas act as a bridge between optics and electronics, collecting and enhancing light to enable the creation of tiny semiconductor photodetectors.

Antennas are used to direct the propagation of radio waves. However, this directionality is not so easy to achieve at optical frequencies. Optical antennas that can direct the emission from single fluorescent molecules represent an intriguing route to single-photon sources.

The authors show that metal oxide and colloidal quantum dots can be combined to fabricate monochrome LEDs with a brightness that matches that of the best organic-based quantum-dot LEDs, but with the advantage of improved shelf-life robustness. The reported maximum external electroluminescence efficiency is nearly 0.1%, which represents a 100-fold improvement over previously reported structures

It has been known for many decades that tightly focusing light introduces asymmetry. The impact of this on imaging, as is now demonstrated using solid immersion lenses, is that resolution becomes dependent on the polarization of the light.