Semiconductor lasers

Lasers speed up

Nature Phys. 2, 484–488 (2006)

Cavity quantum electrodynamics (CQED) — controlling the spontaneous emission from a light source by changing its environment — has previously been used to increase the efficiency of LEDs and single-photon sources. Now, researchers at Stanford University have shown that CQED can also boost the modulation speed of semiconductor lasers.

By placing an InGaAs multiple-quantum-well laser inside a photonic crystal cavity, Altug and colleagues have achieved direct modulation of the laser at frequencies greater than 100 GHz, at an emission wavelength of 940 nm. The speed increase is due to the nanocavity, which enhances the spontaneous emission rate by a factor of 75 through a phenomenon known as the Purcell effect. This approach does away with the need to increase the modulation rate by pumping the laser harder, which causes heating and a drop in device reliability. The result could be a new breed of ultrafast, compact lasers harnessed in next-generation telecommunication systems and on-chip optical interconnects.

Metamaterials

Telecom triumph

Opt. Lett. 31, 1800–1802 (2006)

UNIVERSITÄT KARLSRUHE/OSA

2005 was the year that witnessed the birth of materials possessing a negative refractive index at optical wavelengths. Now, researchers have moved towards the telecommunications window and fabricated a metamaterial — an artificially engineered structure with a negative index of refraction — for use at a wavelength of 1.5 m.

Gold-coated nanostructures were used in earlier work, but Gunnar Dolling and co-workers have turned to silver, which offers lower losses than gold in the telecommunication wavelength regime. Their metamaterial consists of a glass lattice riddled with tiny rectangular holes approximately 500 nm long and 284 nm wide. The lattice is covered with layers of indium tin oxide, a spacer layer of magnesium fluoride and a silver top coating. The refractive index of the lattice is determined by the geometry of the structure and the thickness of the metal and spacer layers.

By tuning these parameters, the team obtained a negative refractive index with a real part of -2 at a wavelength of 1.5 m. The hope is that such developments will lead to photonic devices that can manipulate optical data in new ways.

Attosecond light

Mirror magic

Appl. Opt. 45, 4147–4156 (2006)

The generation and control of ultrashort light pulses may become easier thanks to a clever mirror design from a collaboration of German researchers. Attosecond pulses (10^-18 seconds in duration) could offer a useful new tool for probing the fast electronic processes that occur in atoms and solids, but they are notoriously hard to create and control. The problem is that the pulses have a large bandwidth and exist in the extreme ultraviolet part of the electromagnetic spectrum, where few materials reflect light well.

Wonisch and co-workers have now produced chirped (aperiodic) broadband mirrors capable of reflecting attosecond pulses. Using electron-beam evaporation in an ultrahigh vacuum along with ion-beam polishing, they deposited ultrathin layers of molybdenum and silicon onto fused silica. By numerically modelling the pulse reflections from the multilayers they were able to optimize their mirror design. The resulting structure consists of ten layers, and each layer thickness must be controlled with subnanometre precision. The mirror offers a reflectivity of about 3%, enabling the production of pulses as short as 150 attoseconds, the shortest reported to date.

Fundamental optics

Holey puzzle

Phys. Rev. Lett. 96, 213901 (2006)

Shining light on holey plates produces strange effects. If a metal plate contains very small holes — with a diameter smaller than the wavelength of the light — hardly any radiation is expected to pass through. But experiments show that periodic arrays of holes give rise to strong, localized radiation called 'extraordinary optical transmission'. Scientists have interpreted this phenomenon in terms of surface plasmons — oscillating electromagnetic fields at the metal surface.

Now a team of French researchers has explored another theory: that the transmission is related to the diffraction of evanescent waves (which decay exponentially with distance from the surface). Gay and colleagues deposited thin silver films on silica, then etched one-dimensional structures into the metal — 100-nm-wide slits next to subwavelength grooves. By shining coherent light at the arrays and measuring the far-field intensity pattern produced, they found that the transmitted light exhibits both plasmon-like properties and diffracted evanescent wave characteristics. The debate continues.

Sensors

An eye on the weather

Meas. Sci. Technol. 17, 1715–1722 (2006)

Lasers could soon allow us to analyse the weather more accurately. R. A. Ellis and colleagues have built laser instrumentation that is able to not only distinguish between rain, hail, sleet and snow, but also to measure the speed and size of precipitation particles.

The instrument is based upon the interference of light scattered from precipitation particles that are moving. A transmitter fires light pulses into the open air and two detector units separated by 20° receive the Doppler-shifted light. The detected signals are then analysed and cross-correlated to yield measurements of droplet size and velocity. A modem link can be used to conveniently download the data to a central office for analysis. Field trials performed over ten months in the UK and USA demonstrate the sensor's accuracy and suggest that it could be ideal for roadside, meteorological and aviation applications where measurements of visibility are critical.