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QC lasers changing spectroscopy
The situation is much different in QC lasers because of their layered structure. As researchers at both Bell Labs In spectroscopy, quantum cascade (QC) lasers are relative- and IBM discovered years ago, sandwiching a layer of low ly new, but it’s fair to say they are bursting on the scene. band-gap material between two high band-gap layers Just five years after their introduction, these tiny, “tailorkeeps electrons confined to particular energy levels. This able” semiconductor devices are used for a variety of IR arrangement is known as the “quantum well”, and it sensing techniques, both gas and liquid. They are slated to allows researchers to specify the electron transitions. Furanalyze the atmospheres of other planets, and at the same thermore, if the high band-gap layers are thin enough— time, they have entered perhaps tens of atomic laynew territory on Earth— ers wide—electrons can the commercial market. In tunnel from one quantum addition, the recent well to the next. announcement of a bidirecIf quantum wells are tional QC laser, which designed carefully, an elecemits at two wavelengths tron can tunnel through a (Science 1999, 286, series of them, dropping to 749–752), is opening up lower and lower energy even more possibilities. levels in the process. This “Why use QC lasers?” is what happens in QC asks Steven W. Sharpe of lasers. After entering the the Pacific Northwest first well at the highest National Laboratory level, E2, the electron tun(PNNL). “That’s easy— nels to the next well, where they work.” it drops to E1 and emits a Invented in 1994 at Bell photon. Because lasing Labs, now the research and General schematic energy diagram for the bidirectional QC requires more electrons in development arm of Lucent laser. (a) Under a positive applied voltage, electrons travel from E2 than E1—known as a Technologies by Jérôme left to right. (b) Under a negative applied voltage, electrons “population inversion”—a Faist, Federico Capasso, travel from right to left. (Adapted with permission. Copyright third well, E0, is included. Alfred Cho, and colleagues, 1999 American Association for the Advancement of Science.) The drop to E0 ensures the QC laser is a step that electrons spend very beyond its semiconductor little time in E1 and makes diode cousins. Actually, it’s many steps beyond them, it easy to achieve a population inversion. (Every quantum thanks to its “staircase” design, in which electrons cascade well has three energy levels, not one, but electrons tunnel down layer after layer of material, emitting photons as because of the tiny dimensions of the layers.) A trio of these wells is called an “active region”, and it they go. is the basis of the QC laser (Science 1994, 264, 553–556). Materials by design. Materials-minded people—such as Changing the thickness of the layers changes the E2-to-E1 Lucent’s Claire Gmachl, who helped create the bidirectransition and, thus, the wavelength of the emitted photional laser last year—like QC lasers for their versatility. tons. Therefore, a single material can be used for devices Compared with traditional semiconductor lasers, QC over a range of wavelengths. Indeed, the newest QC lasers lasers are very easy to modify. For example, changing the can operate at 3.5–17 µm, according to Gmachl. wavelength is simply a matter of changing the thickness of In fact, in a QC laser, multiple active regions are lined the layers. “You turn your ideas into a design for a real up—each at a lower energy than the previous one—with system and then determine how to grow [the necessary] injector regions between them to help relay the electrons, crystals and make them work,” says Gmachl. says Faist. As an electron cascades down each of these Modifying traditional semiconductor lasers is much steps, it emits a photon. Thus, in theory, the most recent more involved, according to Faist, “because you have to devices, which have 36 active regions, generate 36 phochange the crystal to change the color.” That is because tons for each electron, says Gmachl. This arrangement the laser’s wavelength depends on the material’s inherent gives QC lasers their power. band gap. Because the associated conduction and valence Spectroscopic applications. Unlike the lead salt lasers bands are fixed, the wavelength can be adjusted only by often used for fine mid-IR spectroscopy, QC lasers operate changing the material’s composition. This means developat room temperature and are stable when exposed to ing a new fabrication technique for each kind of laser.
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news GOVERNMENT AND SOCIETY humidity. They have a narrow bandwidth, and their emission wavelength is very reproducible. They also generate much higher power than their predecessors (~8 mW vs 10–100 µW). “Now you can dream of doing cheap spectroscopy based on optical sensing because you have the source,” says Faist, who is now at the University of Neuchâtel (Switzerland) and is a co-founder of the company Alpes Lasers, which develops and commercializes QC laser technology. Most applications of QC lasers are for gas sensing. Sharpe and his colleagues at PNNL and the Stevens Institute of Technology are working with Lucent researchers to develop high-resolution chemical vapor methods for noninvasive medical diagnostics and for law enforcement applications. Richard Zare’s group at Stanford University has used QC lasers for cavity ring-down spectroscopy, and the spin-off company Information Diagnostics won a NIST grant to develop the method further. In addition, the Jet Propulsion Laboratory is developing QC laser spectrometers to sample the atmospheres of Mars, Venus, and several planetary satellites. The Geneva, Switzerland-based company Orbisphere has taken an even bigger step and incorporated a QC laser into its commercial photoacoustic spectroscopy instrument for gas analysis. Work in liquid sensing also is beginning. Last year, Faist, Antoine Müller, and colleagues at the Univesity of Neuchâtel and the Swiss Federal Institute of Technology reported large electrical tuning ranges— 40 cm–1 and 20 cm–1—for a QC laser at –10 ˚C and room temperature, respectively (Appl. Phys. Lett. 1999, 75, 1509–1511). This feature is useful for spectroscopy of liquids or solids, which may have wide absorption features. In addition, the Neuchâtel researchers and co-workers at the Vienna University of Technology
(Austria) have used QC lasers for flow injection and observed a 50-fold improvement over results obtained with an FT-IR (Anal. Chem. 2000, 72, in press). Bidirectional lasers. The bidirectional semiconductor laser is expected to follow in the footsteps of its unipolar predecessor and break new ground in differential spectroscopy, which requires two wavelengths. “Before the bidirectional laser, we had worked on two-color lasers,” says Gmachl. “But they emitted two wavelengths at the same time, and then you had the problem of peeling apart those two wavelengths in some way.” This problem disappears with the bidirectional laser because the wavelengths alternate in time, depending on the applied polarity. Bidirectional lasers use exactly the same materials as earlier QC lasers, Gmachl says, but the layers are designed for “double use”. At one polarity, everything is normal—the active region generates photons, and the injector region transports electrons. “[But] if we change the polarity, these two types of regions change their tasks,” she explains. The structures could emit at the same wavelength in both directions, but decoupling them produces two independent lasers in a single device. And because the polarities can be switched very rapidly (in hundreds of picoseconds), the device should be useful for many applications. “In hindsight, it looks very intuitive. You look back and say, ‘What a waste of layer structure!’,” Gmachl says. But the bidirectional laser required a special design that became obvious only after the researchers decided to alternate the wavelengths in time, she explains. To Gmachl, that is precisely the advantage of working with QC lasers. “Once you have the goal in mind,” she says, “quantum cascade lasers, by being designer materials, allow you to go for it.” Elizabeth Zubritsky
National Nanotechnology Initiative In his fiscal year 2001 budget, President Clinton is requesting $227 million to create a new National Nanotechnology Initiative (NNI). The initiative is designed to boost basic research in nanoscale science and engineering, with roughly 70% of the proposed funding earmarked for universities. One goal of NNI is to create centers and networks of excellence that would encourage the shared use of academic facilities. Funding also would be set aside for developing a nanotechnology infrastructure, including instrumentation, modeling, simulation, and user facilities. In addition, studies of the ethical, legal, social, and economic impact of nanotechnology would be supported. If approved, the National Science Foundation would receive an added $217 million; the Department of Defense and the Department of Energy each would receive ~$100 million more; and the National Institutes of Health, NASA, and the Department of Commerce would receive increases of $18–36 million.
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