LASER TECHNOLOGY

Less excitement for more gainTodd D. Krauss

In theory, semiconductor nanocrystals are highly suitable laser materials, not least because the colour of their light is tunable over a wide range. In practice, they are difficult — but not impossible — to deal with.

At the heart of any laser is a material that, when ‘pumped’ by an external energy source, amplifies a light beam. Colloidal semicon-ductor nanocrystals are particularly prom-ising materials for a new and improved generation of lasers, but attempts over the past decade to construct efficient lasers out of them have proved frustrating: optical amplification was observed only under the most extreme and impractical of pumping conditions1. On page 441 of this issue, Klimov et al.2 describe how they finely tuned the energy states of a nano crystal to make pumping significantly easier. Their breakthrough could lower optical pumping thresholds by orders of magnitude, and so open the door for the practical use of colloidal nanocrystals in applications from telecommunications to medical diagnostics.

Colloidal semiconductor nanocrystals are crystalline particles typically between 2 and 10 nanometres in size3. When an electron in such a nanocrystal is promoted to an excited energy state, it leaves behind a locally posi-ti vely charged region, a ‘hole’. The electron and hole attract each other electrostati-cally, rather as the proton and electron in a hydrogen atom do, and form a particle called an exciton. Once created, this exciton in the nanocrystal decays into a photon extremely efficiently. Semiconductor nanocrystals are thus typically strong emitters of light.

In addition to their bright, robust fluores-cence, as seen above, colloidal semiconductor nanocrystals have other unusual character-istics. Remarkably, the colour of the crystals’ light emission can be tuned over a wide range simply by changing their size — an inherently quantum-mechanical effect4. They can also potentially emit in wavelength regions in the infrared, which current laser technology can-not easily reach5. Furthermore, an organic-mol-ecule surface coating of the nanocrystals, such as trioctylphosphine oxide, can provide chemi-cal reactivity and easy processability in solu tion, allowing them to be integrated without fuss into existing photonic-device configurations.

Despite all these advantages, the develop-ment of colloidal nanocrystal lasers has been severely hampered by the problem of achiev-ing long-lived optical amplification. Optical amplification in a material occurs under a condition known as a ‘population inversion’, when — unusually — more members of a given population, say, of electrons in a semiconduc-tor, exist in an excited state than in lower energy states. When a material that has a population inversion interacts with a photon of just the right colour, it will, in a process first described6 by Albert Einstein, not absorb the photon, but instead will be ‘stimulated’ to emit two photons of identical colour to the incident photon.

For a successful laser, a photon in the

initial laser beam must stimulate emission of two photons from the optical-gain medium before the population inversion is lost. For a typical nanocrystal, the lowest excited state can accommodate two excitons, so a popula-tion inversion, and successful lasing, require on average more than one exciton to be present per nanocrystal (Fig. 1a, overleaf). But because many excitons are confined to a minuscule vol-ume in a nanocrystal, they interact strongly, causing one exciton to annihilate another in a process known as Auger recombination7. This process takes place incredibly fast, typically in much less than 100 picoseconds. Once it has occurred, any population inversion is ruined.

Achieving optical gain with nanocrystals therefore requires intense optical pumping with very short laser pulses, such that an ava-lanche of photons arrives at the nanocrystals before Auger processes have begun. Some limited progress has been made by replacing the nanocrystals with semiconductor nano-rods8, in which Auger lifetimes are slightly longer 9. Nevertheless, for the past decade it has been widely assumed that lasers based on emission from excitons in colloidal nano-particles would be made impractical by the excitons’ short Auger lifetimes.

Klimov et al.2 used cleverly engineered nanocrystals to split the exciton, funnelling the holes into a shell made of zinc selenide and

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trapping the electrons in a core of cadmium sul-phide. The relatively large physical separation of the electrons and holes means that a nano-crystal with one exciton has a different energy configuration from one with two excitons (Fig. 1b). In this case, a population inversion and optical amplification can occur for nanocrys-tals containing on average less than one exciton, thus completely avoiding the pitfalls of Auger recombination. Even more impressive is the fact

that, under typical pumping conditions for opti-cal gain, the lifetime of the excited state in these nanocrystals is almost 2 nanoseconds, 50 times longer than for typical colloidal nanocrystals2. The threshold for lasing depends inversely on optical-gain lifetime10, and so it should now be possible to reduce this threshold by several orders of magnitude.

A hugely significant advance would be the integration of nanocrystal optical-gain media

directly into a silicon optoelectronic device, such that the nanocrystals would be electrically (instead of optically) pumped. Such a scheme would find ubiquitous applications similar to the proliferation of electrically pumped semi-conductor-diode laser devices, now found in everything from CD players to laser pointers to barcode scanners. The lifetime of a typical nanocrystal excited state is still far too short for these electrical pumping schemes: a typi-cal semiconductor-diode laser has an excited-state lifetime of around ten nanoseconds11. But Klimov and colleagues’ advances2 bring this exciting possibility a lot closer. Stay tuned for further developments. ■

Todd D. Krauss is in the Department of Chemistry, University of Rochester, Box 270216, Rochester, New York 14627-0216, USA.e-mail: krauss@chem.rochester.edu

1. Klimov, V. I. et al. Science 290, 314–317 (2000).2. Klimov, V. I. et al. Nature 447, 441–446 (2007).3. Alivisatos, A. P. J. Phys. Chem. 100, 13226–13239 (1996).4. Brus, L. E. J. Chem. Phys. 79, 5566–5571 (1983).5. Wise, F. W. Acc. Chem. Res. 33, 773–780 (2000).6. Einstein, A. Phys. Z. 18, 121–128 (1917).7. Klimov, V. I., Mikhailovsky, A. A., McBranch, D. W.,

Leatherdale, C. A. & Bawendi, M. G. Science 287, 1011–1013 (2000).

8. Kazes, M., Lewis, D. Y., Evenstein, Y., Mokari, T. & Banin, U. Adv. Mater. 14, 317–321 (2002).

9. Htoon, H., Hollingsworth, J. A., Dickerson, R. & Klimov, V. I. Phys. Rev. Lett. 91, 227401 (2003).

10. Ebberly, J. H. & Milonni, P. W. Lasers (Wiley, New York, 1988).11. Tang, C. L. & Olsson, N. A. IEEE J. Quant. Electron. 18,

971–976 (1992).

Figure 1 | Splitting the exciton. a, An ensemble of conventional semiconductor nanocrystals (NCs) is pumped to have, on average, one electron–hole pair (‘exciton’) (filled circles, electrons; empty circles, holes; downward arrows, stimulated emission; upward arrows, absorption). When illuminated by laser photons, double excitations (NC 1) contribute to gain, a single excitation (NC 2) means effective transparency (absorption and emission cancel), and lack of any excitation at all (NC 3) contributes to loss. Overall, this ensemble would exhibit optical transparency; because the doubly excited nanocrystals also decay very quickly into singly excited nanocrystals through the process of Auger recombination, much higher pumping rates are needed to obtain optical gain. b, In Klimov and colleagues’ engineered core–shell nanocrystals2, the energy required to create a second exciton (blue) is larger than that needed for the first. All singly excited NCs can contribute to optical amplification when lasing at the singly excited energy, thus significantly lowering the energy threshold for optical gain. In this case, the laser beam does not have enough energy to create a second exciton, so avoiding Auger processes and, potentially, allowing much slower pumping rates.

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CIRCADIAN RHYTHMS

Metabolic clockworkBenedetto Grimaldi and Paolo Sassone-Corsi

The ‘body clock’ regulates the daily cycles of many physiological and metabolic processes, but just how is a mystery. New findings suggest that the cycling of energy metabolism is mediated by an activator of gene expression.

Intuitively, we all feel that the activities of our bodies follow cycles of repeated oscillations. Unmistakable examples are the sleep–wake cycle, the feeding rhythm and variations in body temperature and hormonal levels. Many of these cyclic oscillations are circadian (of around 24-hour periodicity), and are control-led by an interplay of numerous molecular factors. Such factors ensure the accuracy of the ‘body clock’, being organized in complex feedback loops that involve gene transcrip-tion and the events that follow it1. Liu and colleagues (page 477 of this issue)2 provide a tantalizing interpretation of the molecular pathways implicated in the circadian control of energy metabolism, placing the transcriptional regulator PGC-1α in a strategic position.

The anatomical centre of the mammalian circadian clock lies within about 15,000 neu-rons in a region of the anterior hypo thalamus in the brain called the suprachiasmatic

nucleus. One unanswered question is how the suprachiasmatic nucleus directs the oscillatory nature of so many physiological and metabolic functions. The unexpected finding 3 that most peripheral tissues of fruitflies, zebrafish and mammals contain intrinsically independent pacemakers indicates the presence of a ‘synchro-nization web’ that coordinates timing in all the tissues. This concept, coupled with the striking notion that the transcription of at least 10% of all cellular genes oscillates in a circadian man-ner4, underscores how profoundly the circa-dian transcriptional machinery influences a wide array of cellular functions.

PGC-1α is a transcriptional coactivator with an essential role in the maintenance of glucose, lipid and energy homeostasis5. It is highly responsive to a variety of environmental cues — from temperature to nutritional status and physical activity — and has been linked to several pathological conditions, including

obesity, diabetes and neurodegeneration5. This pivotal contribution of PGC-1α to the coor-dinated regulation of metabolic pathways has acquired yet another dimension with Liu and colleagues’ discovery2 of its intimate links with the circadian clock.

Indeed, Liu et al. found that mice lacking the PGC-1α gene show abnormal circadian rhythms, as well as disruptions in the oscilla-tory pattern of the expression of ‘clock’ genes such as Bmal1 and Rev-erbα. These PGC-1α-null mice also seem to have metabolic defects, such as aberrant body temperature and meta-bolic rate. The parallel with the outcome of targeted mutations of some circadian regula-tors in mice is striking: mutations of CLOCK and BMAL1 — two proteins that form a com-plex to stimulate the transcription of a vari-ety of clock-controlled genes — also cause both an alteration in circadian rhythms and metabolic abnormalities in lipid and glucose homeo stasis6,7. Thus, PGC-1α seems to occupy a privileged regulatory position, acting as an intimate link between metabolism and circadian control (Fig. 1).

What is the molecular mechanism through which PGC-1α influences circadian physiol-ogy? Liu and colleagues provide an interesting lead by showing that PGC-1α regulates the function of the ROR family of factors known as orphan nuclear receptors2. These factors oscil-late in a circadian manner8, and are involved in

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