www.bwt-bj.com

BWT BEIJING LTD.

5*",No.4A hangfeng Rd.,Fangtai District,Beijing,100070,P.R.ChinaPhone:+ 86 (10} 83681053Fax:+ 86 (10) 83681077Email: sales@bwt-bj.com

optoelectronicworld neii;̂ ^

use in the medical field, for example infinding the extent of brain tumors withill-defined boundaries, and in the detec-tion of skin cancer. In fact, Mahadevan-[ansen sees the eventual development ofa handheld "Star Trek tricorder-like" Ra-man device that can be passed across theskin to find cancer.

A talk given by Robert Steele of Strat-egies Unlimited (Mountain View, CA)covered the prospects for solid-stateligbting, which has enjoyed a compoundannual grov^th rate from 1995 to 2005 of42%, although growth has slowed since(see "High-brightness LED market coolsafter three years of stellar growth," p. 57).While about half of all high-brightness

LEDs are used in mobile appliances, oth-er applications, such as illumination anddisplays, will see more future growth. In-novations such as color mixing of whitelight (see www.laserfocusworld.com/articles/255497) will aid this growth.Barbara Paldus, CEO of Einesse (Irvine,CA) described the use of photonics sen-sors for bioprocess measurement andcontrol, in particular monitoring cell per-formance in bioreactors (used to makebioproducts or for research). Suzanne Ar-ney of Lucent Technologies (Murray Hill,Nl) discussed the work in MEMS (micro-electromechanical systems) that is goingon at Lucent.

]ohn Wallace

QUANTUM-DOT LASERS

'Paint-on' laser could easesilicon interconnectsAlthough optical gain in infrared colloi-dal quantum dots was observed previ-ously, researchers at the University of To-ronto (Toronto, ON, Canada) have nowdeveloped the first quantum-dot IR laserfrom solution-processed materials.' This

"paint-on" laser—referring to the abilityto apply it as a thin-film layer to solidmaterials—could conceivably be usedto connect microprocessors on a siliconcomputer chip and enable the monolith-ic integration of electronic and photoniccomponents.

Because quantum-dot structures canconfine electrons and holes in threedimensions (compared to semiconduc-tor lasers, which confine carriers alongone axis in quantum wells), the densityof states at the band edge is better con-centrated and atomic-like states canbe spaced much farther apart than thethermal energy kT (k = Boltzmann'sconstant, T - temperature). As a re-sult, a quantum-dot laser can produceemission with reduced temperaturesensitivity. However, temperature sen-sitivity is also dependent on quan-tum-dot diameter: epitaxially grownquantum dots that use III-V semicon-ductor materials have large diameterscompared to the Bohr-excitation di-ameter, causing temperature sensitiv-ity. In addition, they require a caustic.

high-temperature growth process thatis highly restrictive for direct growthon silicon.

Small colloids,strong confinementConversely, small-radius colloidal quan-tum dots exhibit very strong quantumconfinement—much greater than kT atroom temperature. For example, varia-tion in the average diameter of colloi-dal lead sulfide (PbS) quantum dotsfrom 1 to 10 nm can produce excitonicabsorption peaks tunable between 850

THESE COLLOIDALNANOCRYSTALS ARESOLUTION-PROCESS-ABLE AND COMPATIBLEWITH A WIDE RANGEOF SUBSTRATES.

and 1800 nm. In addition, these colloi-dal nanocrystals are solution-processable(the growth is in solution) and compat-ible with a wide range of substrates, in-cluding crystalline silicon.

To build their quantum-dot laser, theresearchers first optimized the processconditions that resulted in optical gain

3 O June 2006 www.laserfocusworld.com Laser Focus World

The first "paint-on" laser has been fabricat-ed by creating a soiid nanocrystal thin film

from solution on the inner wall of a fusedsilica microcapillary tube (top). When ex-

ternally pumped by an 800 nm pulsed laser,waveguiding modes (WMs) and whisper-

ing-gallery modes (WGMs) are supportedalong the capillary; however, only those

WGMs that experience feedback producea lasing action at 1530 to1560 nm, depend-ing on film thickness along the capillary, A

cross-section optical microscope image ofthe tube (bottom) barely reveals the 1 mm

thick quantum-dot layer.

for colloidal quantum dots. In the courseof their analysis, they learned that maxi-mizing the density of quantum dots perunit volume also maximized the mate-rial gain of the system for a given level ofpopulation inversion. Rather than usinglong oleic-acid ligands in solution withthe PbS quantum dots {producing a pack-ing fraction of less than 20%), they usedshort butylamine ligands (to increase thepacking fraction to greater than 30%).And, to reduce scattering losses, varioussolvents were explored to optimize sur-face smoothness of solid thin films cre-ated from solution.

To monitor scattering losses and opti-mize film thickness, optical waveguideswere fabricated from solid thin films ofquantum dots, revealing that scatteringlosses from the nanocrystal-air inter-face and the nanocrystal oxide substratewere dominant over Rayleigh-scatteringlosses from the quantum dots.

Coated capillaryEinally, to create the laser, a solution con-taining 5.5 nm diameter PbS colloidalquantum dots was drawn into a 75 pm in-ner diameter fused-silica microcapillarytube and the solvent was forced to evapo-rate. The resulting solid thin film on theinner wall of the tube had a thickness ofless than 1 |jm and thickness uniformityof 10% over a 300 ^m length. The capil-lary was optically pumped with 800 nm,2 ps pulses at a 1 kHz repetition rate andcooled to a temperature of 80 K. At apump threshold of 177 \i]lcm^, whisper-ing-gallery-mode lasing oscillation com-menced, evidenced by the appearance of anarrow emission peak at 1532 nm.

Although increasing the temperatureof the capillary caused the lasing thresh-old to increase, the wavelength shiftedat an average rate of only 0.03 nm/K, 10

- -WMs

times smaller than the spectral shift inquantum-well lasers and twice as lowas epitaxial quantum-dot lasers—con-firming the low temperature sensitivityof this new colloidal quantum-dot thin-film laser.

"Our aim was to create a laser thatemits in the 1.5 \im range, is compatiblewith standard silicon processing meth-ods, and is easy to process," says research-er Sjoerd Hoogland. "PbS colloidal nano-crystals do have these properties, with anadditional advantage that they potentiallycan be pumped electrically. Although op-tical gain had been demonstrated yearsago, no lasing operation was obtained.From our optical analysis, we learnedthat high-quality resonators were neededto provide a sufficient amount of feed-back. Therefore we opted for a ring-lasergeometry in the form of a microcapillarytube, which did not require additionalprocessing." Researcher Ted Sargent adds,

"We need first to demonstrate room-tem-perature behavior, and then to show elec-trically injected lasing, and then we willpursue commercialization."

Gail Overton

REFERENCE1. S. Hoogland et a l , Optics Express 74(8) 3273

{April 17, 2006).

300 NewProducts!

Jusl some of ihe7600 products inour new catalog.

Request yourFREE copy today!

800.363.1992www.edmundoptics.com/LF(•••!i'.-)!i L.fvV06@f-'d[riundoptics.com

Visit us at SEMICON WEST.Booth #3045