"Long before it's in the papers"
January 28, 2015


Tiny “nanolaser” could change face of computing, telecom

Aug. 31, 2009
Courtesy UC Berkeley
and World Science staff

Re­search­ers say they have cre­at­ed the world’s small­est sem­i­con­duc­tor la­ser, a de­vice that can gen­er­ate vis­i­ble light in a space smaller than a pro­tein mol­e­cule.

“This work shat­ters tra­di­tion­al no­tions of la­ser lim­its, and makes a ma­jor ad­vance to­ward ap­plica­t­ions in the bi­o­med­i­cal, com­mu­nica­t­ions and com­put­ing fields,” said Xi­ang Zhang, head of the Un­ivers­ity of Cal­i­for­nia, Berke­ley re­search team be­hind the work.

Both pic­tures show a bright point of light from a sin­gle plas­mon la­ser em­a­nat­ing from the op­ti­cal set­up used by UC Berke­ley re­search­ers. These sem­i­con­duc­tor la­sers -- the world's small­est -- are ex­treme­ly ef­fi­cient, so the small amount of scat­tered light is clear­ly vis­i­ble, even in am­bi­ent room light­ing. Cam­era sat­u­ra­tion of the bright la­ser light gives the im­pres­sion of a larg­er spot. (Cred­it: Cour­te­sy of Xi­ang Zhang Lab, UC Berke­ley)

The sci­en­tists said their work could help lead to ap­plica­t­ions such as ti­ny la­sers that can probe, ma­ni­pu­late and meas­ure prop­er­ties of DNA mol­e­cules; op­tics-based tele­com­mu­nica­t­ions many times faster than cur­rent tech­nol­o­gy; and op­ti­cal com­put­ing in which light re­places elec­tron­ic cir­cuit­ry, with a re­sult­ing leap in speed and pro­cess­ing pow­er.

The find­ings were de­scribed in an ad­vance on­line pub­lica­t­ion of the jour­nal Na­ture on Aug. 30. 

Light is an elec­tro­mag­netic wave—an os­cilla­t­ion of the elec­tric and mag­net­ic fields that al­so drive the mo­tions of elec­tric cur­rents. 

It was long thought that an elec­tro­mag­netic wave, in­clud­ing la­ser light, can’t be fo­cused, or com­pressed, be­yond the size of half its wave­length­—lit­erally the length of a wave in a se­ries, from one wave peak to the next.

But re­search­ers have pre­vi­ously found a way to com­press light further, down to doz­ens of nanome­ters, or bil­lionths of a me­ter. This was done by link­ing light to the elec­trons, or charge-carrying sub­a­tom­ic par­t­i­cles, that os­cillate col­lec­tively at the sur­face of met­als. 

This in­ter­ac­tion be­tween light and os­cillating elec­trons is known as sur­face plas­mons.

Sci­en­tists have been rac­ing to con­struct sur­face plas­mon la­sers that can sus­tain and uti­lize these ti­ny op­ti­cal fluctua­t­ions. But the re­sist­ance in­her­ent in met­als makes these sur­face plas­mons dis­si­pate their en­er­gy very quick­ly, pre­vent­ing the build­up of elec­tro­mag­netic field nec­es­sary for la­sers to be cre­at­ed.

Zhang and his re­search team took a new ap­proach by pair­ing a wire of cad­mi­um sul­fide, 1,000 times thin­ner than a hu­man hair, with a sil­ver sur­face, the wire and sur­face be­ing sep­a­rat­ed by an in­su­lat­ing gap of 5 nanome­ters, the size of a pro­tein mol­e­cule. 

In this struc­ture, the gap re­gion stores light with­in an ar­ea 20 times smaller than its wave­length. Be­cause light en­er­gy is largely stored in this ti­ny non-metallic gap, en­er­gy loss is greatly re­duced, Zhang ex­plained. The re­search­ers could then work on am­pli­fy­ing the light, the key step to making a las­er.

For­tu­nate­ly, the ti­ny wire, or nanowire, “acts as both a con­fine­ment mech­an­ism and an am­pli­fier,” said Ru­pert Oul­ton, a re­search as­so­ci­ate in Zhang’s lab who first the­o­rized this ap­proach last year and is the stu­dy’s co-au­thor. 

Trap­ping and sus­taining light in such tight quar­ters cre­ates such ex­treme con­di­tions that the very in­ter­ac­tion of light and mat­ter is strongly al­tered, the study au­thors ex­plained. An in­crease in the spon­ta­ne­ous emis­sion rate of light is a tell­tale sign of this al­tered in­ter­ac­tion, they added; they meas­ured a six-fold in­crease in the spon­ta­ne­ous emis­sion rate of light in the gap.

Re­cent­ly, re­search­ers from Nor­folk State Un­ivers­ity in Vir­gin­ia re­ported las­ing ac­tion of gold sphe­res in a dye-filled, glass­like shell im­mersed in a so­lu­tion. The dye cou­pled to the gold sphe­res could gen­er­ate sur­face plas­mons when ex­posed to light.

The Berke­ley re­search­ers used sem­i­con­duc­tor ma­te­ri­als and fab­rica­t­ion tech­nolo­gies com­monly used in mod­ern elec­tron­ics ma­n­u­fac­tur­ing. Semi­con­ductors are mat­erials whose abi­lity to con­duct el­ectric charge is some­where be­tween that of metals and that of non-cond­uct­ors, or in­su­la­tors. 

By en­gi­neer­ing sur­face plas­mons in the ti­ny gap be­tween sem­i­con­ductors and met­als, they were able to sus­tain the strongly con­fined light long enough that its os­cilla­t­ions sta­bi­lized in­to the “co­her­ent,” or syn­chro­nized, state char­ac­ter­is­tic of a la­ser.

Sci­en­tists hope to even­tu­ally shrink light down to the size of an elec­tron’s wave­length, which is about a nanome­ter, so that the two can work to­geth­er on equal foot­ing. “The ad­van­tages of op­tics over elec­tron­ics are mul­ti­fold,” added Thom­as Zent­graf, a post-doctoral fel­low in Zhang’s lab and an­oth­er co-lead au­thor of the Na­ture pa­per. “For ex­am­ple, de­vices will be more pow­er ef­fi­cient at the same time they of­fer in­creased speed or band­width.”

* * *

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Researchers say they have created the world’s smallest semiconductor laser, a device that can generate visible light in a space smaller than a single protein molecule. “This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields,” said Xiang Zhang, head of the University of California, Berkeley research team behind the work. The scientists said their work could help lead to applications such as tiny lasers that can probe, manipulate and measure properties of DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry, with a resulting leap in speed and processing power. The findings are described in an advance online publication of the journal Nature on Aug. 30. Light is an electromagnetic wave—an oscillation of the electric and magnetic fields that also drive the motions of electric currents. It was long thought that an electromagnetic wave, including laser light, can’t be focused, or compressed, beyond the size of half its wavelength—literally the length of a wave in a series, from one wave peak to the next. But researchers have previously found a way to compress light down to dozens of nanometers, or billionths of a meter, by binding it to the electrons, or charge-carrying subatomic particles, that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons. Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical fluctuations. But the resistance inherent in metals makes these surface plasmons dissipate their energy very quickly, preventing the buildup of electromagnetic field necessary for lasers to be created. Zhang and his research team took a new approach by pairing a wire of cadmium sulfide, 1,000 times thinner than a human hair, with a silver surface, the wire and surface being separated by an insulating gap of 5 nanometers, the size of a protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, energy loss is greatly reduced, Zhang explained. The researchers could then work on amplifying the light. Fortunately, the tiny wire, or nanowire, “acts as both a confinement mechanism and an amplifier,” said Rupert Oulton, a research associate in Zhang’s lab who first theorized this approach last year and the study’s co-lead author. Trapping and sustaining light in such tight quarters creates such extreme conditions that the very interaction of light and matter is strongly altered, the study authors explained. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction, they added; they measured a six-fold increase in the spontaneous emission rate of light in the gap. Recently, researchers from Norfolk State University in Virginia reported lasing action of gold spheres in a dye-filled, glasslike shell immersed in a solution. The dye coupled to the gold spheres could generate surface plasmons when exposed to light. The Berkeley researchers used semiconductor materials and fabrication technologies commonly used in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the “coherent,” or synchronized, state characteristic of a laser. “What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale,” said Volker Sorger, a Ph.D. student in Zhang’s lab and study co-lead author. “Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement,” said Zhang. “This work can bridge the worlds of electronics and optics at truly molecular length scales.” Scientists hope to eventually shrink light down to the size of an electron’s wavelength, which is about a nanometer, so that the two can work together on equal footing. “The advantages of optics over electronics are multifold,” added Thomas Zentgraf, a post-doctoral fellow in Zhang’s lab and another co-lead author of the Nature paper. “For example, devices will be more power efficient at the same time they offer increased speed or bandwidth.”