Waveguide-enhanced Raman platform promises rapid diagnosis of disease

Sensor chips: chemically adaptable to detect small molecules

http://optics.org/news/9/4/39

Portable and easy-to-use photonics devices can potentially offer substantial advantages in the diagnosis and prevention of disease, and a new project at the University of Southampton Optoelectronics Research Centre (ORC) could see a significant new step made towards this goal.

Supported with £1 million from the EPSRC Healthcare Technologies Programme, ORC and its collaborators at Southampton's Department of Chemistry and the UK Defence Science and Technology Laboratory (Dstl) intend to develop a low-cost instrument suitable for use in GP surgeries, hospital wards, or in remote communities, using cheap, disposable plug-in sensor chips.

These chips will be chemically adaptable to detect small molecules, proteins or DNA, with fluid samples simply being dropped onto the sensor surface, to rapidly detect infection and diagnose disease.

According to ORC, the new project will build upon research by Southampton's Zilong Wang on waveguide-enhanced Raman spectroscopy (WERS), along with work on biochemical detection currently underway between the University and Dstl.

In principle WERS offers a solution to the inherent issue of low sensitivity to which Raman spectroscopy is prone. Rather than employing free-space optical components, WERS incorporates Raman spectroscopy into a waveguide structure, allowing the waveguide to transport incident light, excite Raman scattering, and collect the emission.

Wang's research, published as part of his PhD studies at Southampton, demonstrated how guided light in a suitable dielectric thin-film waveguide can be squeezed to dimensions similar to its wavelength, which results in a large optical intensity over a long propagation range. Analyte molecules close to the waveguide's surface then interact with the evanescent field of the waveguide, becoming Raman excited and allowing their emission to be collected.

Environmental monitoring and security
This effect could be particularly valuable for analytical applications in real-world biological sensors, if it allows detection of particular species to be made through repeatable activity over relatively large enhanced surfaces, rather than the highly localized enhancements often employed to produce single-molecule detection.

"Highly specific, sensitive sensors interfaced with portable, easy-to use, low-cost instruments are needed for rapid point-of-care infection diagnostics, leading to better targeted therapy, shorter time to treatment and reduced morbidity," notes OFC in its WERS research literature.

"We aim to realize a generic, flexible, compact sensing platform with high sensitivity and selectivity, building upon our recent work on WERS to realise a sensor chip which shows surface enhancements comparable to those of surface-enhanced Raman spectroscopy, with improved application flexibility and manufacturability. These will have wide application in the diagnosis of disease."

As well as point-of-care applications, the platform could also prove valuable in broader settings such as environmental monitoring and border security, and should be readily configurable for new locations and analytical challenges.

In its project announcement, ORC indicated that the sensor will first be demonstrated using clinical samples from volunteers who have been exposed to controlled experimental infections in the NIHR Southampton Clinical Research Facility, and will also be assessed as a potential method of analysis for international priority pathogens such as Ebola and plague.

Researchers use asymmetry to generate tunable terahertz light

Graham Pitcher
http://www.newelectronics.co.uk/

University of Southampton researchers have found that two dimensional nanostructures with an asymmetric design can trigger the emission of tunable light at terahertz frequencies and say the system has unprecedented efficiency. 

The team, which also included researchers from Imperial College London, found that quantum wells can enhance light emission in a spectral range that is technically challenging.

Nathan Shammah, from Southampton University's Quantum Light and Matter group, said: "As the 2D nanostructures can be manufactured with an asymmetric design, this allows light to interact with trapped electrons in a way that is not otherwise allowed. This interaction process, leading to the emission of light at lower frequencies, has not been observed in atoms because those are very symmetrical systems and symmetry rules prevent the transitions that trigger this light emission from happening."

In their paper, published in Physical Review B, the researchers predict that, by targeting a 2D asymmetric nanostructure with laser light tuned at resonance with the electronic transitions that can occur in the nanostructure, the 2D device would emit light at frequencies which can be tuned simply by changing the laser power.

Shammah added: "This mechanism is perfectly suited for the terahertz frequency range, which spans from above the current Wi-Fi bandwidth to below the visible light spectrum, where the lack of practical light emitters constitutes a serious technological gap."

It is hoped the findings will have an impact on photonic and optoelectronic devices across a broad range of applications, including medical imaging and security scanning.

Polariton Lasers Light Up at Low Power

http://spectrum.ieee.org/semiconductors/optoelectronics/polariton-lasers-light-up-at-low-power

Two independent groups of researchers come up with a new kind of laser

By Neil Savage

Posted 15 May 2013 | 21:18 GMT

Image: Nature

Polariton Pillars: The 2-micrometer-wide pillars are key to confining polaritons in a new kind of electrically powered laser.

A new type of laser has the potential to be much more energy efficient than conventional lasers, according to two groups of scientists who separately came up with very similar designs for it.

Known as a polariton laser, the device isn’t, strictly speaking, a laser at all. Conventional lasers work through stimulated emission of radiation: Electrons in a laser cavity are raised to a high-energy state, and when they drop to a lower state, they emit the excess energy as photons, producing a coherent beam of light.

This new device, however, is based on the stimulated scattering of polaritons. A polariton is a “quasiparticle,” a mixture of an electron-hole pair (also known as an exciton) and a photon, which can exist only within a crystal. When energy is pumped into the system, the exciton-polaritons absorb it and then quickly release it as photons—the stimulated scattering that creates the laser beam. In a conventional laser, the majority of electrons must be in a high-energy state before lasing can begin. Such a “population inversion” isn’t required with polariton lasers, so it takes less energy to run them.

 Since 1996, when the concept was first described, researchers have built polariton lasers that used light from other lasers to pump energy into the system. Now two groups, working independently, have built devices that run on electricity, a key step in turning any laser from a laboratory curiosity into something useful.

“This is a big deal,” says Pallab Bhattacharya, a professor of engineering at the University of Michigan, whose paper describing his team’s work appears online in Physical Review Letters on Wednesday. “A real device is one that is electrically injected. This makes it a practical device.”

“It is not a laser in the common sense, but it shares a lot of characteristics with the conventional laser,” says Sven Höfling, a researcher at the University of Würzberg, in Germany. He and colleagues from Iceland, Japan, Russia, Singapore, and the United States published a paper on their work in Thursday’s issue of Nature.

Illustration: Nature

Reflected Glory: The polariton laser cavity contains for distributed Bragg reflectors (DBRs) surrounding a quantum well (QW) made of indium gallium arsenide.

One major difference between conventional and polariton lasers is that the lasing threshold—the amount of energy it takes to stimulate the light emission—is orders of magnitude lower in polariton lasers. Bhattacharya says these early prototypes have a threshold current of 12 amperes per square centimeter and will presumably improve. By comparison, he says, it took years of research to make advanced experimental lasers based on quantum dots—tiny clumps of semiconductor material—with a similar threshold current.

Polariton lasers can also be switched on and off much faster than conventional lasers can, Battacharya says, which allows signals to be encoded onto the laser beam very quickly. That means the laser might be useful for low-power (and therefore less-expensive) optical telecommunications and light amplification. Polariton lasers might also be used to trigger lasing at terahertz wavelengths. They could be used to build cheaper, more compact terahertz lasers, which could be a safer alternative to X-ray scanners in spectroscopy and security applications.

Alexey Kavokin, a physicist at the University of Southampton, in England, who is familiar with Höfling’s results, says polariton lasers might also be used in optical logic circuits. Because they can switch from on to off or the reverse in mere picoseconds, and because the polarization of their light might be controlled, polariton lasers would make excellent AND and NOT gates, he says. But these lasers have limits. For example, Kavokin says, they aren’t candidates for high-power applications, such as cutting and welding, because pumping more energy into them destroys the polaritons and thus ends the lasing effect.

Both groups’ lasers were built using gallium arsenide, and both use a magnetic field to make the scattering more efficient. Both also operate only at extremely low temperatures, on the order of 30 kelvins. Höfling says the next step in the research will be to try to build an electrically pumped polariton laser that operates at room temperature; optically pumped room-temperature versions already exist. The lasers are still at a fairly basic stage of research, he says, and it could be a long time before anyone builds a commercial polariton laser.

Both Höfling and Bhattacharya were surprised to learn of each other’s papers, which were released in the same week. “That’s a tremendous coincidence, and a big deal. We validate each other,” Bhattacharya says. “The two taken together, it’s a big boost for the field.”

About the Author

Neil Savage, based in Lowell, Mass., writes about strange semiconductors, unusual optoelectronics, and other things. In the April 2013 issue he reported on a breakthrough that could lead to a way to combined CT scanners and MRI machines.http://m.spectrum.ieee.org/semiconductors/optoelectronics/polariton-lasers-light-up-at-low-power

Why do we need polariton lasers?

http://spie.org/x90791.xml?highlight=x2404&ArticleID=x90791

Alexey Kavokin Polariton lasers' unusual physical principles make them promising for optical communications and generating terahertz radiation. 23 October 2012, SPIE Newsroom. DOI: 10.1117/2.1201210.004477 The word ‘laser’ originally stood for ‘light amplification by stimulated emission of radiation.’ In polariton lasers, the radiation is emitted spontaneously, but it possesses all the characteristic properties of laser light: it is coherent in the first and second order and monochromatic. The concept of polariton lasers was formulated by Imamoğlu and coauthors in 1996.1 They are based on a type of quasiparticle called the exciton-polariton, which is made of light and matter and occurs in suitably designed semiconductor crystal structures. Exciton-polaritons arise from interactions between excitons (neutral quasiparticles formed from bound electron-hole pairs) and photons (for instance, modes of visible light trapped in a semiconductor structure). Being bosons, exciton-polaritons can form condensates, similar to the Bose-Einstein condensates observed in gases of cold atoms. These condensates, in which large quantities of exciton-polaritons accumulate in a single quantum state, form the basis of the polariton laser. The exciton-polariton lifetime is much shorter than a nanosecond, and they decay by passing their energy to photons, which escape from the crystal. Being generated by identical exciton-polaritons, these emitted photons form monochromatic, coherent light. Polariton lasers have been realized in semiconductor microcavities: multilayer crystal structures in which light confined between two parallel mirrors strongly interacts with the elementary excitons in the crystal. In 1998, Le Si Dang and coauthors observed polariton lasing at liquid-helium temperature.2 The first room-temperature polariton laser with optical pumping was realized in 2007 by a collaboration of the Southampton and Lausanne groups.3 At present, optimized schemes of electrically pumped polariton lasers based on gallium nitride (GaN) microcavities are discussed in the literature4 (see Figure 1).   Figure 1. Schematic of an electrically pumped polariton laser based on a gallium nitride (GaN) microcavity with embedded indium gallium nitride (InGaN)/GaN quantum wells (QWs). (Adapted with permission.4) TCO: Transparent conducting oxide. Al: Aluminum. EBL: Electron blocking layer. SiO2: Silica. DBR: Distributed Bragg reflector. MQWs: Multiple quantum wells. nid: Nonintentionally doped. FS: Freestanding. Polariton lasers are expected to have lower thresholds than conventional semiconductor lasers.4 Their output power is quite limited, however, because exciton-polaritons dissociate at strong pumping. Consequently, the area of application for polariton lasers still needs to be defined. Two promising directions stand out: high-speed optical polarization switches and compact sources of terahertz radiation. Optical polarization switches, or spin switches, enable light of a chosen circular polarization to be switched on and off in an optoelectronic device. Conventional approaches to such switching are typically based on nonlinear optical effects, which require high power and external optical elements. Spin switches based on polariton lasers instead exploit the spin properties of exciton-polaritons and the strong interactions induced between exciton-polaritons by their matter (exciton) component. Like photons, exciton-polaritons have two values of spin polarization, corresponding to left and right circularly polarized light, respectively. The polarization of light emitted by a polariton laser is governed by the spin of its exciton-polariton condensate, which can be controlled by external fields. The first optical spin switches based on semiconductor microcavities were realized by Amo and our coworkers in 2010, with gigahertz switching speeds.5 A low-power continuous-wave laser primes the system in a ready state so that a little additional ‘probe’ laser light will switch on the polariton laser. Such spin switches are important building blocks for polariton integrated circuits, which would carry information as domains of spin-polarized exciton-polaritons. Polariton integrated circuits would have an advantage of lower energy losses and faster information transfer compared to conventional electronic circuits. For the terahertz radiation application, we recently proposed vertical-cavity surface-emitting terahertz lasers based on polariton lasers.6 Emission of terahertz radiation would be stimulated by a condensate of exciton-polaritons in these devices. Unlike many other types of terahertz laser, this design does not require a waveguide or a laser cavity for terahertz photons, allowing the whole structure to be microscopic. Tuning of the emission frequency may be achieved by shifting the optical excitation beam on the surface of a graded microcavity. Tunable terahertz lasers have wide areas of application in medicine, communication technologies, and security. Polariton lasers will bring fundamental effects of many-body quantum physics to our everyday life. Their unique physical properties make them suitable for novel types of spin switches and terahertz lasers, which have many applications. The most promising materials for polariton lasing are the wide-band-gap semiconductors GaN and zinc oxide (ZnO), which allow for room-temperature action. We are working toward commercialization of polariton lasers. The next milestones along that path are the experimental demonstration of an electrically pumped polariton laser, the realization of polarization modulators and amplifiers based on polariton lasers, and demonstration of vertical-cavity terahertz lasing. This work has been supported by the Royal Society and the European Union IRSES (International Research Staff Exchange Scheme) project POLAPHEN (Polarization Phenomena in Quantum Microcavities). Alexey Kavokin School of Physics and Astronomy University of Southampton Southampton, UK and Charles Coulomb Laboratory University of Montpellier 2 Montpellier, France Alexey Kavokin received his PhD at the Ioffe Institute of the Russian Academy of Sciences in 1993. In 1998 he became a professor at Blaise Pascal University in France, and in 2005 he moved to the UK, where he is Chair of Nanophysics and Photonics in the Department of Physics and Astronomy at the University of Southampton. References: 1. A. Imamoğlu, R. J. Ram, S. Pau, Y. Yamamoto, Nonequilibrium condensates and lasers without inversion: exciton-polariton lasers, Phys. Rev. A 53, p. 4250-4253, 1996. 2. L. S. Dang, D. Heger, R. André, F. Boeuf, R. Romestain, Stimulation of polariton photoluminescence in semiconductor microcavity, Phys. Rev. Lett.81, p. 3920-3923, 1998. 3. S. Christopoulos, G. Baldassarri Höger von Högersthal, A. J. Grundy, P. G. Lagoudakis, A. V. Kavokin, J. J. Baumberg, G. Christmann, Room-temperature polariton lasing in semiconductor microcavities, Phys. Rev. Lett. 98, p. 126405, 2007. 4. I. Iorsh, M. Glauser, G. Rossbach, J. Levrat, M. Cobet, R. Butté, N. Grandjean, M. A. Kaliteevski, R. A. Abram, A. V. Kavokin, Generic picture of the emission properties of III-nitride polariton laser diodes: steady state and current modulation response, Phys. Rev. B 86, p. 125308, 2012. 5. A. Amo, T. C. H. Liew, C. Adrados, R. Houdré, E. Giacobino, A. V. Kavokin, A. Bramati, Exciton-polariton spin switches, Nat. Photon. 4, p. 361-366, 2010. 6. A. V. Kavokin, I. A. Shelykh, T. Taylor, M. M. Glazov, Vertical cavity surface emitting terahertz laser, Phys. Rev. Lett. 108, p. 197401, 2012. Copyright © 2012 SPIE About SPIE  |  Author Information  |  Privacy Policy  |  Sitemap  |  SPIEDigitalLibrary.org