Research round-up

02 Sep 2004

A look at some of the innovations in optics that have been reported this month.

OLEDs
Blending dendrimers is a simple way to tune the emission properties of organic light-emitting diodes, according to Jonathan Markham and colleagues from the UK universities of St Andrews and Oxford. The team says it has achieved an efficiency of 1.5 lm/W and tuned the emission from 401 nm to 477 nm by varying the blend composition of a fluorine-thiophene dendrimer. (Applied Physics Letters 85 1463)

Solar cells
Researchers from the TITK Institute in Rudolstadt, Germany have fabricated polymer solar cells on flexible 5 x 5 cm ITO-coated polyester films. The white-light power conversion efficiency is said to be 3%. The team has made active areas of up to 175 mm² to show the reproducibility of its technique. "These results are encouraging steps on the way to device up-scaling and developing process technologies suitable for a future reel-to-reel production of flexible organic photovoltaic modules," conclude the authors. (Applied Physics Letters 85 1481)

Semiconductor lasers
Short-wavelength quantum cascade lasers (QCLs) emitting between 3.7 and 4.2 microns have been made by M. P. Semtsiv and colleagues from the Humboldt University and Forschungszentrum Rossendorf in Germany. Their QCL is based on strain-compensated InGaAs-AlAs on InP and operates in pulsed mode up to a temperature of 330 K with maximum single-facet output peak powers of 6 W at 8 K and 240 mW at 296 K. (Applied Physics Letters 85 1478)

Extended cavity lasers
Rafael Aldaz and co-workers from Stanford University, US, unveil a monolithically-integrated long vertical-cavity surface-emitting laser in the current issue of Optics Express. The cavity is formed between an AlGaAs/GaAs DBR and a concave mirror on a glass substrate which acts as the output coupler. "The structure is a promising way of achieving the performance of a vertical extended cavity surface emitting laser (VECSEL) with the fabrication advantages of a VCSEL," report Aldaz and colleagues. "The lasing wavelength is 980 nm with a threshold of 20 mA for a 52 micron mesa and a maximum output power of 39 mW." (Optics Express 12 3967)

Tunable sources
Stanford University scientists have also teamed up with Thales Research and Technology in France to develop an optical parametric oscillator (OPO) based on GaAs. The OPO uses an all-epitaxially-grown orientation patterned GaAs crystal that is 0.5 mm thick, 5 mm wide and 11 mm long. The team says that tuning either the near-infrared pump wavelength between 1.8 and 2 microns or the temperature of the crystal allows the OPO to tune between 2.28 and 9.14 microns. The pump threshold of the singly resonant OPO was said to be 16 µJ for 6-ns pulses and the photon conversion slope efficiency was said to reach 54%. (Optics Letters 29 1912).