New Generation of Minute Lasers Steps Into the Light

Microchip-based diode lasers have had a good run. They’re at the heart of CD and DVD players, computer disc optical drives, and a host of medical devices. Together, these and other applications add up to a sweet $3.5 billion market. But diode lasers can’t do it all. Researchers have struggled to get them to produce the long-wavelength light — ranging from the mid-infrared to Terahertz frequencies — that is highly sought after for applications from explosives detection to biomedical imaging. Researchers have also had a tough time making the lasers out of silicon, the workhorse of computer technology, an advance that could vastly improve computer processing speeds by enabling chips within computers and local networks to send signals through high-speed glass fibers instead of metal wires. Now a spate of advances could finally help chip-based lasers leap those hurdles.

In recent months groups at the University of California, Los Angeles (UCLA), and Intel Corp. have reported major strides in making “Raman” lasers out of silicon. Like other lasers, the new silicon-based devices trap light waves, force their peaks and troughs into orderly alignment, and then release them in energetic beams. The one downside is that in order to work, these lasers must be primed by light from another laser. But 2 weeks ago, a group at Harvard University in Cambridge, Massachusetts, reported creating a chip-based Raman laser that works when fed electricity. “Over the past 5 months, this field has exploded,” says Philippe Fauchet, an optics expert at the University of Rochester in New York.

The lasers take their name from the Indian physicist Chandrasekhara Venkata Raman, who discovered the principle behind them in 1928. When monochromatic light passes through a transparent material, he found, most of the photons emerge with their wavelength unchanged. Others, though, collide with atoms in the material and lose or gain energy, causing them to emerge at a shorter or longer wavelength.

The effect lies at the heart of fiber optic-based commercial devices called Raman amplifiers, which boost longer-wavelength optical signals streaming through glass fibers for long-distance data transmission and telecommunications. The devices work by using an initial high-energy “pump” pulse to prime the fiber so that when photons in a data pulse pass through, they stimulate the release of additional photons at the same energy, amplifying the pulse. By reflecting the growing light pulse back and forth through a transparent fiber, engineers can create a Raman-based fiber-optic laser. But because the Raman effect is so slight in glass fibers, these devices typically require kilometers of fiber to work.

The good news is that the Raman effect is 10,000 times stronger in pure silicon than in glass. “We can do in centimeter-sized devices in silicon what is done in kilometers in glass,” says Mario Paniccia, who directs Intel’s photonics technology laboratory in Santa Clara, California. At least, that’s the theory. Unfortunately, silicon has an appetite for eating laser photons. When an incoming laser pulse — known as the pump pulse — is trained on silicon, silicon atoms can absorb two photons simultaneously. The energy excites one of the atom’s electrons, freeing it to roam through the crystal. Such mobile electrons are strong photon absorbers and quickly quench any amplification of laser photons in the material.

Last fall, UCLA optoelectronics researchers Ozdal Boyraz and Bahram Jalali were the first to overcome this problem and create a silicon-based Raman laser. In the 18 October 2004 issue of Optics Express, the pair reported that to prevent the buildup of excited electrons, they zapped their silicon chip with a staccato of pulses, each lasting just 30 trillionths of a second, or picoseconds. Between pulses they gave the excited electrons time to relax back to their ground state, so they wouldn’t reach a level that kept photons from building up in the material. The UCLA device, however, wasn’t pure silicon: It also used 8 meters of optical fiber to carry the emerging laser light back to the silicon crystal for additional passes in order to boost the output of the Raman-shifted pulse.

Three months later, researchers at Intel did away with the optical fiber. In the 20 January issue of Nature, a team led by Paniccia reported creating the first all-silicon-based Raman laser. Like the UCLA device, it relied on pulsing an incoming beam, but mirrors in the silicon bounced the light back and forth without the need for the fiber. The Intel team also added another trick: They routed the light down a path within the chip lined with positive and negative electrodes. When the researchers applied a voltage, charged particles swarmed to the electrodes, sweeping the mobile electrons out of the path of the incoming photons. As a result, the team could blast the silicon chip with a stronger pump pulse to increase the output of the Raman-shifted laser light. Last month in Nature, the Intel team reported another improvement, the first silicon Raman laser that emits a continuous beam of photons. Boyraz and Jalali jumped back into the fray as well, reporting in the 7 February issue of Optics Express that they had incorporated an electric modulator into their optically pumped device to switch their new lasers on and off.

The string of advances, Fauchet says, sets the stage for a host of innovations, such as silicon-based optoelectronic devices to replace copper wires in speeding short-distance communication between computers, as well as other military, medical, and chemical detection applications. By leveraging the semiconductor industry’s decades of experience in fabricating silicon components, the new work could help slash costs for optical components. “It’s a potential sea change that allows you to do new things because they are cheap,” Fauchet says.

The new lasers have their drawbacks. “The major limitation of Raman lasers is that to get a laser you need another [pump] laser,” Fauchet says. “Ideally, you would like to have an electrically pumped laser. That would be the Holy Grail.”

As if on cue, in the 24 February issue of Nature, researchers led by Federico Capasso of Harvard reported just such a device. Unlike the previous lasers, however, the new one is made from alloys of aluminum, gallium, indium, and arsenic rather than silicon and works in a different manner. Known as a “quantum cascade” (QC) laser, it consists of hundreds of precisely grown semiconductor layers. As electrons pass through the layers, they lose energy at each step, giving up photons, which combine to create the laser beam.

Capasso and his colleagues at Harvard and Lucent Technology’s Bell Laboratories in Murray Hill, New Jersey, had spent a decade building QC lasers that emit light in the mid-infrared range. In hopes of extending their reach to longer, Terahertz frequencies, Capasso teamed up with theorist Alexey Belyanin of Texas A&M University in College Station, who had suggested modifying the device by adding new sections that use the Raman effect to shift the initial laser light to a longer wavelength. In essence, the group created a pair of Raman lasers on a single chip: one that converts electricity into an initial pump laser, and a second that shifts the light to longer wavelengths. The new QC Raman lasers turn out beams of infrared light with a wavelength of 9 micrometers. Capasso says his team is working to create similar devices that turn out beams at Terahertz frequencies, which are widely sought after for use in detecting explosives and other chemicals. Fauchet notes that the advance doesn’t produce the shorter wavelength photons ideal for telecommunications, but “it demonstrates you don’t need an external laser to get a Raman laser,” he says.

No matter which of the new Raman lasers proves most successful, the devices look likely to extend diode lasers’ run for a long time to come.

[Complete article viewable at www.sciencemag.org]