Research round-up

21 Jan 2005

A look back at some of the optical innovations unveiled in the last month.

Optical trapping
Researchers from the Universities of Southampton in the UK and Tromsø in Norway have used a Y-branched optical waveguide to sort polystyrene microspheres. A fiber laser emitting at 1066 nm is butt-coupled to the waveguide. The 6-micron diameter particles are diluted in de-ionized water and sit in a cell on top of the waveguide. The particles are optically guided in the waveguide's evanescent field and exit through one of its Y-branches. "In the majority of cases, particles follow the branch that is illuminated with the higher power," say the authors. (Optics Express 13 1)

Spectroscopy
A lightweight spectrometer which measures water vapor and methane in the stratosphere has been unveiled by a team from Germany. The compact high-altitude in-situ laser diode (CHILD) spectrometer has been designed for balloon-borne experiments. Based on a near-infrared tunable diode laser, the authors explain that free-air absorption measurements are made using an open-path Herriott cell. "CHILD has successfully measured stratospheric methane and water vapour profiles on high-altitude balloons on four campaigns," say the authors. "In situ spectra were recorded with a sensitivity of 0.1 ppm ground to 0.4 ppm at 32000 m for methane and 0.15 - 0.5 ppm for water." (Applied Optics 44 91)

Random lasers
Ken-ichi Ueda and his colleagues from the University of Electro-Communications in Tokyo, Japan, have developed a random microchip laser. The random laser consists of a transparent ceramic Nd:YAG microchip and an Nd:YAG powder tablet. Multiple scattering from the powder provides the necessary feedback. Pumped by a laser diode array emitting at 805 nm, the random laser is said to operate in a quasi-continuous regime. (Optics Express 13 121)

Microscopy
A laser terahertz emission microscope (LTEM) could be used to locate electrical failures in integrated circuits, according to a team of scientists from RIKEN and the Universities of Okayama and Osaka in Japan. The LTEM uses a mode-locked Er-doped fiber laser emitting 10 fs pulses at 790 nm to irradiate the circuit. It also collects terahertz emissions from the sample, which are proportional to the local electric field. The authors say their system has a spatial resolution of below 3 microns and could be used to examine large-scale integrated circuits. (Optics Express 13 115)

Fluorescence lifetime imaging
Researchers in France have developed a time-domain fluorescence lifetime (FLIM) microscope based on a custom-built picosecond laser. Specifically, the excitation source is a low-repetition rate (3.7 MHz) passively modelocked diode-pumped laser based on Nd:YVO₄. "By frequency doubling and tripling the laser, excitation at either 532 nm or 355 nm can be easily produced with an average power of several tens of milliwatts," say the authors. "These two wavelengths allow us to cover a wide range of intrinsic fluorophores and fluorescent probes with an incident power far above the 1 mW typically used." They add that the measurable lifetime has also been increased, as a fluorescence intensity measurement can occur up to 250 ns after the excitation pulse. (Optics Letters 30 168)