Are Laser Manufacturers Blue with Envy?
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SPECIAL REPORT 141 lattice - the spacing between its atoms - must be an almost perfect match for the Are Laser Manufacturers semiconductor’s lattice. Gallium nitride grows at temperatures near 1000 °C, a fac tor that further limits possible substrates. Blue with Envy? In fact, only two materials match both lat tice and temperature requirements: silicon Charles T. Whipple carbide and sapphire. The former is pro hibitively expensive, and the latter’s lattice Blue (or purple) diode lasers should hit the market this year and change doesn’t match ideally. Early on, sapphire all the rules In consumer electronics, data storage, printing and displays substrates caused so many defects in the gallium nitride semiconductor layer that devices wouldn’t lase. Lyricists have been infatuated with blue for They thought they’d found the key in Then, in 1986, Isamu Akasaki and his centuries. Songs speak of blue eyes, blue 1969 when RCA Laboratories in Princeton Nagoya University research group laid velvet, blue angels and singing the blues. in the US, developed crystalline thin films down a sacrificial layer of aluminum Scientists’ and engineers’ love affair with of gallium nitride. But the next step was 2: nitride on the sapphire, and topped it with blue light, however, hearkens back only a years in coming. a smooth layer of gallium nitride. The team few decades, to the 1960s and the invention Thin-film semiconductors must be also discovered how to make p-doped galli of semiconductor lasers. grown on a substrate, and the substrate’s um nitride by adding continued over page potassium titanyl phosphate (KTP) crystal. excited into higher energy levels, thus An alternative approach for the realization enhancing the up-conversion process. Laser Sources of visible laser sources is up-conversion Visible up-conversion lasing at room pumping by energy transfer processes of temperature has already been demonstrat Giinter Huber, Hamburg University two adjacent excited ions or two step pump ed in Tm-doped crystals and in various Jean-Pierre Huignard, Thomson CSF, France processes as ground state plus excited state rare earth doped fluorozirconate fibres. absorption. Er3+ is a very interesting ion for continu Compact and efficient diode pumped solid- ous wave (cw) up-conversion to the green state laser sources in the near infrared and Up-conversion Lasers spectral region. visible spectral region are required for Lasers which emit at higher frequencies Recently, research teams at Hamburg many applications such as measurement than the pump light usually are called up- and Hannover demonstrated a cw Pr,Yb- techniques, communications, and surgery. conversion lasers. In these lasers the active fibre up-conversion laser with 1.1 W output Besides Nd3+, various efficient diode- ion is excited by internal up-conversion of power at 635 nm (see photo). This device pumped near infrared rare earth lasers near infrared or red light via multistep was pumped with 5.5 W Ti-sapphire radia have been operated with Er3+, Tm3+, Ho3+ photon excitation or cooperative energy tion near 850 nm. The pump mechanism in and Yb3. Compact solid-state lasers in the transfer and emits anti-Stokes visible the ZBLAN-fibre is mainly due to avalanche visible spectral region are of potential light. The advent and rapid improvement up-conversion. High power diode pumping interest, especially for display, medical, and of high power laser diodes in the red and of this system seems also feasible. high-density optical data storage appli near infrared spectral ranges have caused The ions Tm3+, Er3+, Pr3+ provide up- cations. Recently, optical efficiencies of new interest in the development of up- conversion schemes which lead to visible more than 20 per cent with respect to the conversion lasers. The output wavelength cw laser operation with near infrared pump power were obtained in Nd:Y3Al5012, of laser diodes can be tuned to match the pumping. The simple scheme of up-con Nd:YV04, and Nd:LaSc3(B03)4 lasers by absorption lines of the active laser ion, version lasers make them attractive candi internal frequency doubling with a resulting in a substantial fraction of ions dates for further research. The beam quality of laser sources can Photograph of the be improved by efficient phase conjugate 1.1 Watt continuous wave up-conversion laser architectures based on 4 wave-mix fibre laser showing ing interactions using two different non the pump input linear mechanisms: the gain gratings in coupling optics, the saturable amplifiers, and the photorefrac- fibre, and the red tive gratings in doped BaTi03 crystals. output (dark grey Both mechanisms have shown excellent fibre tip, top left of picture) capabilities for the correction of severe spatial aberrations. A nearly diffraction limited beam was obtained even when the average power and the repetition rate of the source was varied. Both configurations are attractive for compact and efficient diode pumped solid-state lasers with a dif fraction limited beam quality. 1998 July/August News Europhysics 142 SPECIAL REPORT magnesium and annealing it with electron Matsushita is ready to match that price. Fasol on data storage and laser printing beams. Down the road, however, as production and adds surgery, contaminant detection A young Japanese researcher named volume increases, it will be easier for and covert communications as possibili Shuji Nakamura watched Akasaki’s work Nichia to reduce prices on its simple semi ties. In addition, the US Navy has said it is with interest. Employed by a small compa conductor laser than for Matsushita to cut interested in blue lasers for optical com ny, Nichia Chemical Industries in the cost of its more complicated device. munication through water; seawater Tokushima, Japan, Nakamura had spent Some say Nichia is two years ahead of absorbs less blue light than longer wave more than a decade playing catch-up with the pack, but that may not be true. Manijeh lengths. Other possibilities include flat Japan’s big electronics companies. He Razeghi and colleagues at Northwestern screen displays and projection TVs. developed red light-emitting diodes (LEDs University, Illinois in the US, have demon Blue LEDs offer nearly as many possibil for Nichia, but his heart was all blue. ‘I strated continuous-wave (CW) room-tem ities, some of which have already been real decided to do research on blue LEDs,’ perature operation of InGaN/GaN multi ized. There’s a full-colour display made of Nakamura explained,‘because they had quantum-well lasers, observing no consid 300,000 red, green and blue LEDs at long been a dream of mine, going back to erable degradation in laser characteristics Hachiko Square near bustling Shibuya the days when I was doing LED materials during lifetime testing of 140-plus hours. Station on Tokyo’s Yamanote commuter research on GaP in 1979.’ Cree Research of Durham, North Carolina train line. At the moment, LED colour dis After a year at the University of Florida in the US, also has achieved a CW blue plays are commercially competitive with in the US, learning how to grow crystals laser beam, but so far its duration is only a projected displays when the diagonal mea with metallorganic chemical vapour depo few seconds. surement exceeds 2.5 meters. But they may sition, Nakamura returned to Nichia to Several university and corporate labs soon be competitive at 1.25 meters. take up his search for a blue laser in have achieved pulsed blue lasers: Boston Most colour copiers use fluorescent light earnest. ‘At that time, I tried not to read the University, the University of California at when scanning, but true blue light sources [scientific] papers on III-V nitrides,’ he Santa Barbara, Hewlett-Packard Co., the could lead to significant improvements in said.‘Instead, I followed up on the lessons 1 Xerox Palo Alto Research Center and SDL colour scanners and even colour facsimile learned from my experiments.’ in the United States; and Meijo University, machines. It took Nakamura two years to achieve a Toshiba, Fujitsu and Pioneer in Japan. In The Blue Laser Diode, a book by breakthrough: his two-flow method of Some use GaN, some use silicon carbide Fasol and Nakamura, Fasol writes that growing GaN films. Once that barrier was substrates and some offer double het LEDs are likely to replace incandescent broken, his advances came in virtual leaps erostructure for greater efficiency. light bulbs in traffic signals. This year, a and bounds: p-doping of GaN, InGaN In one interesting development, Leo number of LED traffic lights have been put growth, blue LEDs introduced in 1995 and Schowalter and his group of researchers at into test operation in Japan to see how they now blue lasers. Today, Nakamura’s blue Rensselaer Polytechnic Institute in Troy, work in various weather conditions. The GaN lasers have operated in continuous- New York State in the US, found a way to Japanese government has an ongoing pro wave mode for more than 4000 hours at grow aluminum nitride crystals large gram to replace incandescent and fluores elevated temperatures, which extrapolates enough to be sliced into semiconductor cent lighting with LEDs. Nichia already to approximately 10,000 hours at room substrates. Schowalter pointed out that markets white LEDs that could replace temperature. ‘because aluminum nitride endures conventional room lighting. extreme heat, it can be used for microelec ‘Biological applications of LEDs are very Quest for the Blue tronic devices on jet engines.’ interesting to me,’ said Nakamura.‘You see, Nakamura’s breakthroughs, first with LEDs Hearing of the development, Nichia’s plants only need red and blue light for and then with blue lasers, triggered a rash Nakamura said,‘If they’ve found some photosynthesis. Tests show that plants grow of blue laser programs around the world.