Laser Pulses Control Single Electrons in Complex Molecules

Garching and Munich researchers (L to R) Regina de Vivie-Riedle, Philipp von den Hoff, Matthias Kling, and Irina Znakovskaya. [Photograph compliments of Thorsten Naeser of Max Planck Institute of Quantum Optics]



[This is an invited article based on recent works of the authors and their team members -- 2Physics.com]



Authors: Matthias Kling and Regina de Vivie-Riedle

Affiliations: Max Planck Institute of Quantum Optics, Garching and
Ludwig-Maximilians University, Munich, Germany

Link to 'Attosecond Imaging' Group >>

Predatory fish are well aware of the problem: In a swarm of small fish it is hard to isolate the prey. A similar situation can be found in the microcosm of atoms and molecules, whose behavior is influenced by “swarms” of electrons. In order to achieve control over single electrons in a bunch, ultrashort light pulses of a few femtoseconds duration are needed.

Electrons are extremely fast. In atoms and molecules they move on attosecond timescales [1]. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the hydrogen molecular ion and its isotopes, their control with such light pulses is relatively easy and was demonstrated by Kling et al. [2, 3]. These first studies on controlling electrons in molecules with waveform controlled light pulses sparked, however, the question, whether the steering of electrons in more complex systems is feasible.

In our recent work [4] we have now managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes more difficult as their states lie energetically very close to each other. Control of the electric field waveform E(t) = E₀(t) cos(ωt + φ), with envelope E₀(t), and frequency ω, is achieved by stabilization and control of the carrier envelope phase (CEP) φ and constitutes a new paradigm of coherent control that can be applied to steer electrons in atomic and molecular systems [1].

We used visible (740 nm) laser pulses with only 4 femtoseconds duration. The control of electron dynamics in the system was experimentally determined via an asymmetric distribution of C⁺ and of O⁺ fragments after the breaking of the molecular bond. The measurement of C⁺ and O⁺ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the CEP of the laser pulse.

Theoretical methods based on ab initio quantum mechanics were developed for multi electron systems and applied to analyze the ionization and subsequent electron localization process. The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process takes only ca. 1.7 femtoseconds. The collision produces an electronic wave packet which induces a directional motion of electrons along the molecular axis. The excitation and subsequent interaction with the remaining part of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry.

From theoretical analysis it became clear that in the performed experiments, it was also possible to image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed to explore this process in the outermost HOMO (highest occupied molecular orbitals) and HOMO-1 orbitals. The ionization of the molecules is found to take place with a distinct angular dependence with respect to the laser polarization direction. This dependence is seen in fragments from the dissociation following ionization (see Fig. 1 for recorded C+ fragments). Our observation is in good agreement with theoretical calculations (shown as black line in Fig. 1) [4]. The ionization process itself also gives a contribution to the observed asymmetry, which strongly depends on the duration of the laser pulses [4].

Fig. 1: The detachment of electrons from carbon monoxide molecules by femtosecond laser pulses leads to a characteristic angular distribution of the molecular ions and their fragments. The angular distribution resembles the structure of orbitals from which electrons have been ionized. The figure shows the angular distribution of C⁺ ions measured in the laser-induced dissociative ionization of CO via velocity-map imaging (VMI) for CEP = Π. The laser polarization is vertical and the number of observed ions is displayed in color.

Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors – technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics [5] aiming at computing speeds on attosecond timescales.

References
[1] M. F. Kling, and M. J. J. Vrakking, "Attosecond Electron Dynamics", Annu. Rev. Phys. Chem. 59, 463 (2008). Abstract.
[2] M. F. Kling, Ch. Siedschlag, A. J. Verhoef, J. I. Khan, M. Schultze, Th. Uphues, Y. Ni, M. Uiberacker, M. Drescher, F. Krausz, M. J. J. Vrakking, "Control of Electron Localization in Molecular Dissociation", Science 312, 246 (2006). Abstract.
[3] M. F. Kling, Ch. Siedschlag, I. Znakovskaya, A. J. Verhoef, S. Zherebtsov, F. Krausz, M. Lezius, M. J. J. Vrakking, "Strong-field control of electron localization during molecular dissociation", Mol. Phys. 106, 455 (2008). Abstract.
[4] I. Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M. J. J. Vrakking, R. de Vivie-Riedle, M. F. Kling, "Attosecond Control of Electron Dynamics in Carbon Monoxide", Phys. Rev. Lett. 103, 103002 (2009). Abstract.
[5] E. Goulielmakis, V. S. Yakovlev, A. L. Cavalieri, M. Uiberacker, V. Pervak, A. Apolonski, R. Kienberger, U. Kleineberg, F. Krausz, "Attosecond Control and Measurement: Lightwave Electronics", Science 317, 769 (2007). Abstract.

Shor's Quantum Factoring Algorithm Demonstrated on a Photonic Chip

From L to R: Jeremy L. O'Brien, Alberto Politi, Jonathan C. F. Matthews (photo by: Carmel King)

A primitive quantum computer that uses single particles of light — photons — whizzing through a silicon chip to perform a mathematical calculation has been reported by a team of physicists and engineers in 'Science'. This is a major step forward in the quest to realise a super-powerful quantum computer and the first time such a calculation has been performed on a photonic chip.

The chip takes four photons that carry the input for the calculation, it then implements a quantum programme (Shor’s algorithm) to find the prime factors of 15, and outputs the answer – 3 and 5.

“This task could be done much faster by any school kid,” said PhD student, Alberto Politi, from the University of Bristol who, together with fellow PhD student Jonathan Matthews performed the experiment, “but this is a really important proof-of-principle demonstration.”

Image: The waveguide chip used to perform the algorithm

Finding prime factors may seem like a mathematical abstraction, but it lies at the heart of modern encryption schemes, including those used for secure internet communication. The ability of quantum computers to simulate quantum systems may also prove to be a powerful tool in the development of new materials or pharmaceuticals.

The team from the University of Bristol’s newly established Centre for Nanoscience and Quantum Information have spent several years developing devices where photons propagate in silica waveguides — much like in optical fibres — micro-fabricated on a silicon chip.

“This approach results in miniature, high-performance, and scalable devices,” said Professor Jeremy O’Brien, Director of the Centre for Quantum Photonics, who led the research. “The realisation of a quantum algorithm on a chip is an extremely important step towards an all-optical quantum computer”

“Despite recent advances, the ability to perform even small-scale quantum algorithms has largely been missing,” said Matthews. “For the last few years, researchers at the Centre for Quantum Photonics have been working towards building fully functional quantum circuits on a chip to solve this issue,” added O’Brien.

Past 2Physics article by Jeremy O’Brien and Alberto Politi:
"Silicon Photonics for Optical Quantum Technologies"

The team coupled four photons into and out of the chip using optical fibres. On the chip the photons traveled through silica waveguides that were brought together to form a sequence of quantum logic gates. The output was determined by which waveguides the photons exited the chip in. By detecting the photons at the output of the device they confirmed high-performance operation of the quantum algorithm.

“As well as quantum computing and quantum metrology, ‘on-chip’ photonic quantum circuits could have important applications in quantum communication, since they can be easily integrated with optical fibres to send photons between remote locations,” said Politi.

O’Brien concurred and added: “The really exciting thing about this result is that it will enable the development of large scale quantum circuits for photons. This opens up all kinds of possibilities”.

Reference
"Shor’s Quantum Factoring Algorithm on a Photonic Chip",
Alberto Politi, Jonathan C. F. Matthews, Jeremy L. O'Brien,
Science, Vol. 325. no. 5945, p. 1221 (2009). Abstract.

Operation of an Electrical Amplifier Close to the Quantum Limit

Dartmouth researchers (L to R) Joel Stettenheim, Alex Rimberg, and Weiwei Xue

[This is an invited article based on recent works of the author and his team members -- 2Physics.com]

Author: Alex Rimberg

Affiliation: Dept of Physics and Astronomy, Dartmouth College, USA

Link to Rimberg Group >>

Classically, it is possible to imagine purely passive measurements in which an instrument collects information from some measured system without disturbing it in any way. Measurements of quantum mechanical systems, in contrast, must always be active. A measuring device, no matter how sophisticated, must influence what it is being used to measure; such influence is commonly referred to as backaction. Since the backaction associated with the measurement randomly changes the behavior of the system, the act of measurement must always introduce additional noise. The result is a strict lower bound on the minimal noise an amplifier can introduce for a given sensitivity [1]. This bound on amplifier performance is essentially a manifestation of the uncertainty principle, and implies that there is a well-defined limit on how "good' an amplifier can be.

In a paper recently published in Nature Physics, researchers at Dartmouth College have operated an electrical amplifier that very nearly approaches this quantum limit [2]. The amplifier in question is a superconducting single electron transistor (S-SET), which is well-known to be one of the world's most sensitive detectors of electrical charge. It has been suggested for sometime that the SET can be closely approach the quantum limit [1,3]. However, technical limitations have prevented researchers from approaching the limit by closer than a factor of roughly 20.

To understand the difficulties researchers have faced, it is necessary to have some understanding of how the SET is usually operated as a charge detector. The most sensitive approaches are currently based on the radio-frequency SET technique (RF-SET) [4], in which a radio-frequency wave is reflected off the SET,and the reflected wave is amplified by a secondary classical amplifier. When a charge moves near the SET, its conductance changes -- causing changes in the amplitude of the reflected wave.

The SET is a high-impedance device (about 25 kOhm) while coaxial cable is relatively low impedance (usually 50 Ohm). To make energy transfer from the SET to later classical amplifiers more efficient, an LC matching network is used to impedance match the SET to the coaxial line. In principle, it is possible to make the impedance matching and power transfer nearly perfect. In practice, however, unless great care is taken, the matching network will be imperfect, and some power coming from the SET will be lost. The loss occurs either by having the outgoing power reflected back toward the SET, or lost in the matching network, or both.

Why is this a problem? A quantum-limited amplifier disturbs the system it is measuring, collects all possible information based on the disturbance, and transmits the information, via a chain of classical amplifiers, to the laboratory (the macroscopic world). If any information is lost, either by dissipation or through being buried in the inevitable classical noise of the amplifier chain, the result is to move the measurement away from the quantum limit: the same disturbance occurs but the measurement uncertainty is higher. In the case of the RF-SET, if the matching network is lossy, or impedance matching is imperfect, the result will necessarily be less than quantum limited performance. Worse, in most cases the impedance matching is imperfect enough that noise from the classical amplifier chain dominates the measurement. Note however, that even if the classical amplifiers introduced no noise of their own, imperfect matching necessarily implies a departure from the quantum limit.

In order to optimize the matching network, the Dartmouth researchers developed fully superconducting on-chip matching networks consisting of a superconducting spiral and a parasitic capacitance [5]. The resulting networks are nearly lossless, and due to their very small parasitic capacitance, provide excellent impedance matching at their resonant frequency of 1 GHz. As a result, the power transfer from the S-SET to the subsequent amplifiers is vastly improved, allowing the Dartmouth team to measure the quantum noise of the S-SET near a particularly useful operating point for the first time.

The particular operating point chosen was a feature known as the Double-Josephson-quasiparticle (DJQP) resonance that occurs at bias voltages too small to break Cooper pairs at both junctions. Instead, charge is transferred through the S-SET by means of a complex cycle of Cooper pair and quasiparticle tunneling. A special characteristic of the DJQP cycle is that when operated here, the S-SET has been predicted to have a combination of charge sensitivity and backaction that will allow it to closely approach the quantum limit [3].

By measuring the quantum noise of the S-SET near this feature, it was possible to demonstrate that the S-SET can either emit or absorb energy from the resonator, depending on its precise bias conditions. Classical amplifiers are characterized by a singlenoise parameter because they are equally likely to emit or absorb energy. Quantum mechanically, however, an amplifier may be much more likely to emit than absorb, or vice versa, depending on its precise operating conditions. As a result, two parameters are required to describe the noise. Here, the noise was described by a damping rate that described the S-SET's net tendency to emit or absorb energy from the LC tank circuit, and an effective temperature that describes the degree of asymmetry between emission and absorption. The resulting values of the effective temperature and damping, shown in Fig. 1, constitute the first complete and quantitative characterization of the quantum noise of the S-SET near the DJQP resonance.

Fig. 1: (a) S-SET damping rate and (b) S-SET effective temperature. Together, these give a complete and quantitative description of the S-SET quantum noise.

In addition, the charge sensitivity of the S-SET near the DJQP resonance was shown to be excellent, approaching the world record for RF-SET operation. By estimating the charge fluctuations on the S-SET island, it was possible to determine the backaction the S-SET would likely have on a system such as a quantum dot. Ignoring the noise of the classical amplifiers, the S-SET operated within a factor of 3.6 of the quantum limit, a factor of five improvement over the nearest previous results.

Near quantum limited amplifiers such as this one could have a host of applications in the fields of quantum computation and quantum measurement. They would allow fast, efficient measurement of qubits, might lead the way to direct observation of quantum charge oscillations, and could potentially be used in the preparation of exotic squeezed quantum states.

References
[1] "Amplifying Quantum Signals with the Single-Electron Transistor,"
M. H. Devoret and R. J. Schoelkopf, Nature 406, 1039(2000). Abstract.
[2] "Measurement of Quantum Noise in a Single-Electron Transistor near the Quantum Limit," W. W. Xue, Z. Ji, Feng Pan, Joel Stettenheim, M. P. Blencowe, A. J. Rimberg, Nature Phys. 5, 660(2009). Abstract.
[3] "Resonant Cooper Pair Tunneling: Quantum Noise and Measurement Characteristics,"
A. A. Clerk, S. M. Girvin, A. K. Nguyen and A. D. Stone, Phys. Rev. Lett. 89, 176804 (2002). Abstract.
[4] "The Radio-Frequency Single-Electron Transistor (RF-SET): A Fast and Ultrasensitive Electrometer," R. J. Schoelkopf, P. Wahlgren, A. A. Kozhevnikov, P. Delsing and D. E. Prober, Science, 280, 1238 (1998). Abstract.
[5] "On-Chip Matching Networks for Radio-Frequency Single-Electron Transistors," W. W. Xue, B. Davis, F. Pan, J. Stettenheim, T. J. Gilheart, A. J. Rimberg and Z. Ji, Appl. Phys.Lett. 91, 093511 (2007). Abstract.

Phonon Laser Demonstrated Using Trapped Ions

For decades there has been interest in phonon lasers, a device that operates in analogy to optical lasers, except replacing light with vibrational energy. Phonon lasers would amplify motion to the point of coherence through the process of stimulated emission, whereby the presence of energy quanta increases the chances that additional quanta are emitted. Now, researchers at the Max Planck Institute of Quantum Optics in Garching, Germany have realized such a device using a cold, harmonically bound, magnesium ion.

Reporting in the journal Nature Physics [1], the ion was first laser cooled to a temperature of around 1 milliKelvin in an electromagnetic trap, and then a second laser was applied, whose wavelength was precisely adjusted so as to create mechanical (not optical) amplification through the process of stimulated emission of phonons. The phonons are associated with the center-of-mass motion of the ion in the electromagnetic trap. As such, phonon stimulated emission causes the ion to oscillate, once a threshold condition is achieved (figure).

Sequence of time-averaged images showing coherent motion of the phonon laser. At the far left, the magnesium ion is cooled to approximately 1 milli-Kelvin using a red-detuned laser. In each subsequent image, an additional, blue-detuned pump laser is stepped in intensity. Beginning around the fifth trace, laser threshold is achieved at which point mechanical amplification compensates mechanical damping. Beyond this threshold pumping level, the motion is sustained by stimulated emission of phonons and stabilized by saturation of the amplification. The ions oscillate vertically in the figure, and the brighter end points in each image result from the ion slowing and eventually stopping to reverse motion.

“The possibility of phonon laser action has been considered dating back to the earliest days of the optical laser, nearly 50 years ago,” notes Kerry Vahala, who is currently a guest scientist at the Max Planck Institute of Quantum Optics in Garching, on leave from Caltech. He adds that the possible ways to realize these devices are as numerous as those used for conventional optical lasers, and ultimately, many other types of phonon lasers including those in the solid-state are possible.

“Laser-cooled ions are, however, a very good starting point to explore the physics of these intriguing devices,” notes Thomas Udem. Udem observes that they can be precisely controlled and manipulated, because of a multi-decade effort directed towards exploration of quantum phenomena using chains of cold ions. That history is also interesting in another respect.

“The physical mechanism that is responsible for amplification and stimulated emission of phonons has, over the last decades, been misunderstood as a different phenomenon referred to as heating,” says Maximilian Herrmann. He explains that the regime of heating was always associated with laser pumping using a wavelength that is blue-detuned, relative to an optical transition of the trapped ion. In stark contrast, the Garching team was able to show that the regime is, in fact, one of phonon laser action.

In the experiment, the ion was simultaneously cooled using a red-detuned laser and also pumped to create stimulated phonon amplification using the blue-detuned beam. The cooling laser also allowed the researchers to introduce a controlled amount of damping into the system. To understand the origin of amplification in the ion system, consider the so-called small-signal regime with the blue detuned pumping beam. This interaction involves a second-order quantum process in which a pump photon is absorbed, followed by emission of center-of-mass phonon.

Herein, the role of atomic transition damping is crucial. As is also true in a conventional optical laser oscillator, the transition damping is fast enough so as to quench the underlying Rabi dynamic. As a result, the rate of emission of phonons can be shown to contain a stimulated component that is proportional to the number of phonons, and also a component that is purely spontaneous (and responsible for starting the motion in the first place). The stimulated term, as in a conventional laser, produces the amplification that ultimately creates the coherent motion.

The transition from purely thermal motion to coherent motion is marked by a threshold pumping power, where stimulated amplification balances ion motional damping. As pumping is further increased through this threshold, coherence emerges and is visible in images of the ion motion (figure). Ultimately, the large signal motion of the ion saturates the stimulated amplification. The saturation process in the ion is, itself, interesting as it involves a competing phonon absorption process. In effect, as the coherent motion grows in amplitude, this phonon absorption competes more effectively with the stimulated emission and saturates the net amplification. Generally speaking, saturation of amplification is an essential feature of all regenerative oscillators, and makes possible stable coherent motion.

The mechanism of amplification through scattering of a pump photon and generation of a stimulated phonon is in some ways analogous to the process of Raman optical amplification in which a pump photon scatters to produce a stimulated (and amplified) optical Stokes wave [2]. In that process, a phonon is also produced, but is so strongly damped that it effectively serves to only assist the overall amplification of light as an intermediate process. In the ion phonon laser system, on the other hand, this situation changes dramatically.

The researchers note that the pump photon creates both a polarization excitation and a phonon; but in a peculiar twist, the phonon switches out of its conventional “supporting” role, and, because of the very low damping rate of phonons in the ion system, takes-on the lead role analogous to the optical Stokes mode in the conventional optical Raman process. This feature of the stimulated phonon process in the ionic system can be made mathematically precise; and also helps to explain, as is true in optical Raman lasers, operation without an obvious inversion.

Current research is directed towards controlling the phonon laser using tools and techniques that can be adapted from the laser world. One example is a process called injection locking whereby a weak, external control field is used to phase-synchronize the phonon laser with an external reference. The team has recently used this method to image the coherent motion of the ion as it oscillates. Also interesting is the possibility of studying excitations in chains of ions and even two-dimensional arrays of ions.

Concerning applications, Professor Theodor Hänsch notes, “the fact that the forces involved here are so weak suggests that this phenomenon might prove useful as a weak force probe.” He adds, “it is always important when a phenomenon or idea can be reduced to practice.” Along these lines, the Garching work has already inspired realization in a very different system of phonon laser action [3].

Reference
[1] K. Vahala, M. Herrmann, S. Knünz, V. Batteiger, G. Saathoff, T. W. Hänsch & Th. Udem, “A phonon laser,” Nature Physics, 5, 682 – 686 (2009). Abstract.
[2] Shen, Y. R. & Bloembergen, N. “Theory of stimulated Brillouin and Raman
scattering,” Phys. Rev. 137, 1787–1805 (1965). Abstract.
[3] Ivan S. Grudinin, O. Painter, Kerry J. Vahala, “Phonon laser action in a tunable, two-level photonic molecule,” arXiv:0907.5212v1 (2009).

Tiniest Semiconductor Laser

Xiang Zhang [Photo courtesy: Roy Kaltschmidt/ Lawrence Berkeley National Laboratory]

In an advanced online publication of the journal Nature dated Aug. 30, a team of researchers from USA and China -- led by Xiang Zhang of University of California, Berkeley -- has reported the creation of the world's smallest semiconductor laser, capable of generating visible light in a space of only 5 nanometer -- smaller than a single protein molecule. The team not only successfully squeezed light into such a tight space, but found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser action. The research was performed at the NSF Nanoscale Science and Engineering Centre of University of California -- Berkeley, the Materials Sciences Division of Lawrence Berkeley National Laboratory, and the State Key Lab for Mesoscopic Physics and School of Physics of the Peking University -- China.

The achievement helps enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.

[Image courtesy of Xiang Zhang Lab/UC Berkeley] Left: Light being compressed and sustained in the 5 nanometer gap — smaller than a protein molecule — between a nanowire and underlying silver surface. Right: Electron microscope image of the hybrid design shown in the schematic.

While it is traditionally accepted that an electromagnetic wave - including laser light - cannot be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons 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 excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.

Recently, another team of researchers from Norfolk State University, Purdue University and Cornell University reported the creation of "spaser-based nanolasers" which were spheres 44 nanometers in diameter - more than 1 million could fit inside a red blood cell [Read 2Physics article dated August 22, 2009]. Those nanolasers are based on 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 UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed 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 state that is a key characteristic of a laser.

The Berkeley team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire - 1,000 times thinner than a human hair - with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single 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, loss is significantly diminished. With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.

[Image courtesy of Xiang Zhang Lab/UC Berkeley] Left: Light being compressed and sustained in the 5 nanometer gap — smaller than a protein molecule — between a nanowire and underlying silver surface. Right: Electron microscope image of the hybrid design shown in the schematic.

"When you are working at such small scales, you do not have much space to play around with," said Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."

Trapping and sustaining light in radically 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; in this study, the researchers measured a six-fold increase in the spontaneous emission rate of light in a gap size of 5 nanometers.

"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."

"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 a co-lead author of the paper.

Scientists hope to eventually shrink light down to the size of an electron's wavelength, which is about a nanometer, or one-billionth of a meter, 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."

Reference
"Plasmon lasers at deep subwavelength scale"
Rupert F. Oulton, Volker J. Sorger, Thomas Zentgraf, Ren-Min Ma, Christopher Gladden, Lun Dai, Guy Bartal & Xiang Zhang,
Nature advance online publication 30 August 2009 doi:10.1038/nature08364; Abstract

[Our presentation of this work is based on a write-up by Sarah Yang of University of California, Berkeley]