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QUANTUM-RING LASERS:Whispering-Cave-Mode Lasers Emit in Blue-Violet

Lord Rayleigh wrote about the two-dimensional whispering gallery mode (WGM) in 1910 after a visit to the dome of St. Paul’s cathedral in London. The whispering cave mode (WCM) is a threedimensional (3D) effect?a toroid with circular helix symmetry?which recent studies have shown can be used to create photonic-quantum-ring (PQR) lasers that emit in the blue-violet part of the spectrum. A research team at Pohang University of Science and Technology (POSTECH, Pohang, Korea) first created 3D WCM lasers that emit in the infrared and red part of the spectrum. To achieve this, professor O’Dae Kwon and his group stacked mesas of vertically reflecting distributed-Braggreflector (DBR) structures above and below a few active 80 A gallium-arsenide and gallium-indiumphosphide quantum wells. The resulting 3D WCM laser of photonic quantum rings avoided the problem of in-plane light spreading found in 2D WGM lasers, and generated a donut-like band of 3D helical modes. One such photonic-quantum-ring device of 15 μm in diameter featured an ultra-lowthreshold current of 11.5 μA, about a thousandth of that needed for vertical-cavity surface-emitting lasers (VCSELs) of the same diameter. These multimode devices emitted around a central wavelength of 848 nm, exhibiting increasing threshold current and decreasing linewidth with larger device diameter. The team observed the narrowest linewidth with an optical spectrum analyzer to date from a 10 μm PQR of 0.55 A at an injection current of 800 μA. The next iterations of the infrared PQR device involved single-mode electrically pumped lasers made of a hyperboloid drum shape only 0.9 μm across. These devices exhibited a linewidth of 0.46 A at 838.5 μm, and a tiny threshold current of 300 nA, the smallest ever observed among quantum well, wire, or dot-type lasers. Although the external quantum efficiency suffered from soft lasing turn-on behavior, the emission efficiency of the PQR laser was very high?more favorable than that of lightemitting diodes (LEDs). Such lasers could be used to replace LEDs in high-end displays in the near future. The researchers then used various vertical galliumnitride (GaN) structures to extend their PQR work to blue wavelengths from 420 to 470 nm (see Fig.). In one version, a “reverse-mesa” approach with microholes etched in the vertical-quantum-well structure enabled unexpected “convex whisperinggallery” lasing via gain-guiding effects. This “weird” laser also exhibited very low quantum-ring-like thresholds (6 μA per pixel for 256x256 arrays, and 0.3 μA per pixel for mega-pixel arrays at room temperature) and surface-normal dominant multimode emissions. The hole lasers are easily fabricated, readily scalable, and, says Kwon, may become sought-after for next-generation interconnects or nano-bioengineering for its potential to anchor submicron fibers. “In general,” said Kwon, “the blue laser has been like a holy grail?it has been very difficult to achieve surface-normal lasing. Existing blue photonic-crystal LDs still require relatively high current. The new blue PQR achieves surface-normal lasing easily, even with the modest design of less than 95% to 70% vertical-pair reflection, thanks to the unique 3D helix WCM phenomena. And its ultralow threshold implies it can outperform LEDs while overcoming the thermal and material problems of the LED.” Future challenges associated with 3D WCM PQR lasers include studies of 3D device theory and simulations, angular moment studies, understanding of carrier-photon interactions, and chaotic dynamics research on modified structures. [Laser Focus World, Volume: 44 (March, 2008)] Professor O’Dae Kwon Department of Electronic and Electrical Engineering Tel: +82-54-279-2212 Fax: +82-54-279-8119 E-mail: odkwon@postech.ac.kr

Graphene Nanoribbon Spin-Valve Device

Spin-valve devices are a key component of a magnetoresistive random access memory. Mr. Woo Youn Kim and Professor Kwang Soo Kim of Department of Chemistry of POSTECH predicted supermagnetoresistance in a graphene nanoribbon device, the article of which has appeared in Nature Nanotech (3, 408-412, 2008). The reported graphene nanoribbon spin-valve device shows extremely large magnetoresistance (ten thousand times larger than that of conventional devices), which promises high speed access, and good sensitivity. The striking enhancement originates from the peculiar symmetry of band structures of graphene nanoribbon in addition to the spin symmetry. The characteristic symmetry of the graphene band structure plays the role of a spin filter in perfectly transmitting the spin current in the case of symmetric band alignments between both ends of the nanoribbon and in completely forbidding the spin current in the case of orthogonal symmetric alignments. This phenomenon is highly contrasted to conventional spin-valves of giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR) which utilizes only the spin symmetry and antisymmetry. The discovery of the GMR phenomenon was awarded the Nobel Prize in Physics last year. The present supermagnetoresistance ideally approaches to the infinite. Thus, the predicted new physics would open a new pathway much beyond the current limit of spin-valve devices. Professor Kwang Soo Kim Department of Chemistry Tel: +82-54-279-2110 Fax: +82-54-279-8137 E-mail: kim@postech.ac.kr

A New View on Quantum Complementarity

According to laws of quantum mechanics a physical entity may possess either particle-like or wave-like properties. A particle exhibits wave properties when one cannot tell the path the particle may take among different paths available. Once the path information is obtained, however, the wave-like properties of a particle are supposed to disappear. In any case, it is not possible to observe both the wave and particle properties simultaneously, which is known as complementarity of a physical entity. In early days around the advent of quantum mechanics the concept of complementarity was considered only in hypothetical thought experiments. More recently, however, the progress in the nano-scale artificial fabrication technique enables one to examine the validity of complementarity through the direct experimental realization. To that purpose, one can resort to the double-slit-type interference involving photons, atoms, or electrons in solids. Traditionally the reduction of the wave properties to the particle ones was thought to result from a momentum transfer to a physical entity while getting its path information. If one attempts to find the path that a physical entity (i.e., an electron) takes in a double-path interferometer, for instance, a momentum transfer causes uncertainty in the phase of the affected wave packet in one path, which results in the suppression of the interference between the partial waves along the two paths. Although this point of view was physically easy to accept, it began to face a challenge since the late twentieth century. It was debated that, in a certain circumstance, only the quantum correlation or the entanglement may lead to the path information even without a momentum transfer, which in turn suppresses the quantum interference. In this study, we adopt a "closed-loop-type" Aharonov-Bohm electron interferometer [Fig. (a)] to find a clue to the fundamental cause of complementarity. The electron interferometer was fabricated on a two-dimensional electron gas existing at the interface of a GaAs-AlGaAs heterojunction semiconducting wafer. Electrons in the twodimensional gas are laterally confined to be transferred along the two arms of the Aharonov-Bohm interferometer by the electron-confining gates patterned on the surface of a heterojunction wafer and negative voltages applied on them. In the interferometer, a quantum dot is embedded in one arm of the interferometer. The detection of the electrons through this quantum dot is made by monitoring the conductance of the quantum point contact (QPC), which is placed in proximity to the quantum dot and thus electrostatically coupled to the quantum dot. Once the electron path information is obtained by the QPC detector in this double-path interferometer the quantum interference is supposed to be suppressed in proportion to the electron detectability. Complementarity of electrons in this kind of solid-state double-path interferometer was already observed in 1998 by Moty Heiblum's group in Weizmann Institute of Science, Israel. Different from the multi-terminal open-loop-type double-path interferometer used by the Weizmann group, our closed-loop-type electron interferometer has only two terminals (the source and the drain). Thus, in our interferometer, multiple turns of electron passage around the interferometer loop is possible in principle. In the case of the double-path interferometer with only a single turn of electron passage, the charge detection always provides the path information. In this case, one cannot tell whether the suppression of the interference due to the charge detection is caused by a momentum transfer or simply by the quantum entanglement. In our closed-loop interferometer, however, the charge detection does not necessarily provide the path information. In Figure (b) the detector responds to the electrons passing the red path only; thus, the path detection is equivalent to the path information. But for the multiple-turn path shown in Fig. (c) both the red and blue paths are through the quantum dot once so that the charge detection does not provide the path information. If only the path information, irrespective of the momentum transfer, suppresses the wave nature of electrons the charge detection will suppress the firstharmonic interference for the path in Fig. (b) but will not affect the second-harmonic interference for the path in Fig. (c). In case the momentum transfer is the cause of the suppression of the interference both the first and second harmonics of the interference will be affected by the charge detection. In our measurements the amplitudes of the first and second harmonic interferences were monitored as a function of the voltage bias of the charge detector. Measurements show that the first harmonics decrease linearly with the voltage bias of the detector while the second harmonics remain unaffected. The results of this study decisively demonstrate that the path information itself rather than the momentum transfer is the essential element of determining the particle-wave nature of an electron or quantum complementarity. Professor Hu-Jong Lee Department of Physics Tel: +82-54-279-2072 Fax: +82-54-279-5564 E-mail: hjlee@postech.ac.kr

Stepping Toward Printed Electronics:Inkjet Printing of Organic Semiconductors

Direct printing of functional electronic materials may provide a promising route to low-cost fabrication of integrated circuits. From this point of view, ink-jet printing has received special attention as a direct patterning technique for the cost-effective fabrication of organic electronic devices such as organic thin-film transistors (OTFTs) (Figure 1) and organic photovoltaic cells. Professor Kilwon Cho and Ms. Jung Ah Lim of the Department of Chemical Engineering have succeeded in fabricating high-performance OTFTs by inkjet printing of organic semiconductor. This result entitled “Self-Organization of Inkjet-Printed Triisopropylsilylethynyl Pentacene via Evaporation- Induced Flows in a Drying Droplet” was published and introduced in the January issue of the Advanced Functional Materials as the cover story. To produce organic electronic devices with high performance via inkjet printing, the uniform deposition of organic semiconductor thin-film with desired molecular ordering by inkjet printing has become an essential challenge because charge carrier transport in organic electronic devices is strongly influenced by the crystalline microstructure and morphology of the organic semiconductor film. However, the uneven distribution and random orientation of organic semiconductor molecules were commonly observed in most of organic semiconductor films printed from homo-solvent, which was disadvantageous to electrical property of the devices. Prof. Cho’s research group reported for the first time that inkjet printing of organic semiconductor films with uniform morphology and a desired molecular orientation can be achieved by varying the composition of the solvent mixture. They found that self-aligned crystals of organic semiconductor with highly ordered crystalline structures can be inkjet printed in presence of the minor solvent, which has a higher boiling point and a lower surface tension than the major solvent (Figure 2). These self-aligned organic semiconductor crystals can be used successfully to produce highperformance organic transistors. They illustrated that this approach makes use of the evaporation-induced flows, in particular the convective and Marangoni flows that occur in an inkjet printed droplet during drying process. The convective flow that transports the molecules in a droplet to the perimeter of droplet can be counterbalanced, depending on the solvent composition, by the Marangoni flow that is induced by the surface tension gradient between the periphery and the interior of the droplet from regions with low to regions with high surface tension. They have confirmed that high performance inkjetprinted transistors can be obtained by optimizing the deposit morphology and crystalline structure of inkjet-printed organic semiconductor films by controlling the evaporation-induced flow in the printed droplets. This finding may offer an excellent way to control the molecular ordering of organic semiconductors for the direct-write fabrication of high-performance organic electronics. This development of printed electronics will realize new electronic products such as wearable electronics in garments, electronic toys in giveaways, or even electronic bar code on a yogurt cup in the near future. Prof. Kilwon Cho (Dept. of Chemical Engineering) received his Ph.D in polymer science from the University of Akron in 1986, and worked as a researcher at IBM Research Center. He has taught at POSTECH since 1988. Professor Kilwon Cho Department of Chemical Engineering Polymer Research Institute Tel: +82-54-279-2270 Fax: +82-54-279-8298 E-mail: kwcho@postech.ac.kr

Lesson from Lotus Leaf:Photoreversibly Switchable Smart Surface

Professor Kilwon Cho and Doctor Ho Sun Lim of the Department of Chemical Engineering discovered the smart surface that can switch reversibly from extreme water-hating (superhydrophobic) to dramatic water-loving (superhydrophilic) via exposure to ultraviolet or visible light (J. Am. Chem. Soc., 2006, 128, 14458). Recently, this smart surface was introduced with the subject, “Self-Cleaning Materials: Lotus Leaf-Inspired Nanotechnology” in a hot issue of materials science field in Scientific American. The lotus grows in muddy water, but its leaves, when they emerge, are seemingly never dirty. The effect is caused by the combination of two features of the leaf surface: its waxiness and the microscopic bumps (a few microns in size) that cover it. The innumerable bumps on a lotus leaf transform its waxy surface into an extremely water repellent or superhydrophobic one - the contact angle exceeds 150 degrees, material and water on it forms nearly spherical droplets with very little surface contact. Raindrops roll easily across such a surface, removing all dirt. Since the lotus has this exceptional purity, many researchers have been interested in synthetic self-cleaning materials, some of which are based on this ‘lotus effect’. And this research has also broadened into an entirely new science of wettability, self-cleaning and disinfection. Especially, smart surfaces that respond to external-stimuli are one of the newly attractive research fields due to their great advantages in wide applications. Their tunable wettability might be achieved by many means: ultraviolet light, electricity, temperature, solvent and acidity. In 2006 Professor Cho’s research group achieved complete switchability by adding a compound based on the azobenzene molecule to the top of a silicapolyelectrolyte multilayer film. A hydrophobic group on the end of the azobenzene molecules, along with the roughness of the layers, makes the surface superhydrophobic, but under ultraviolet light the azobenzene compound changes chain configuration from straight-chain (trans) form to bent (cis) one and converts it to superhydrophilic. Visible light promptly restores the original condition. Selective exposure to light carves the pattern on the surface, and the surface has been designed to attract or push away water in waffle-shaped pattern. Light irradiation made the exposed surface hydrophilic. In these regions, water spreads out over as much surface as possible. In the remaining hydrophobic parts, water gathers itself into a sphere to avoid contacting the surface. Scientific American has suggested that this kind of control could have major applications in the field of microfluidics, such as the microarrays now used for drug screening and other biochemical tests. For instance, hydrophilic pathways could be closed or opened by switching parts of them to be hydrophobic of hydrophilic. Professor Kilwon Cho (Department of Chemical Engineering) received his Ph.D in polymer science from the University of Akron in 1986, and worked as a researcher at IBM Research Center. He has been with POSTECH since 1988. Department of Chemical Engineering Tel: +82-54-279-2270 Fax: +82-54-279-8298 E-mail: kwcho@postech.ac.kr

Ultrahigh Density Arrays of Conducting Polymer Nanorods Fabricated

Ultrahigh density arrays of conducting polymer nanorods have been fabricated through a joint research project by Professors Jin Kon Kim and Su-Moon Park of Pohang University of Science and Technology (POSTECH), Dr. Jae-Woong Yu of Korea Institute of Science and Technology (KIST), and Professor Thomas P. Russell of the University of Massachusetts at Amherst, U.S.A., as reported in Nano Letters, vol. 8, 2315-2320 (2008). Nanoporous templates, for instance, track-etched polymer membrane and anodized aluminum oxide (AAO) membrane, have been widely used to prepare conducting polymer nanotubes, nanowires or nanorods. But, the former contains randomly distributed nanochannels with an areal density of only ~109 pores/cm2 which precludes the fabrication of ultrahigh density arrays of conducting polymer nanorods. With the AAO membranes, well-defined arrays of conducting polymer nanotubes and nanowires have been synthesized by electrochemical polymerization. However, upon removal of the AAO matrix, the array of nanostructure collapses onto the substrate, losing its orientation. To overcome these disadvantages, we employed nanoporous block copolymer template with a diameter of ~25nm and a density of ~1011 pores/cm2, which was prepared by the mixture films of polystyrene-blockpolymethylmethacrylate (PS-PMMA) and PMMA homopolymer. Then, ultrahigh density arrays of conducting polypyrrole (PPy) nanorods are fabricated directly on the indium-tin oxide (ITO) coated glass by an electropolymerization within the nanoporous template, as schematically depicted in Figure 1. As shown in field-effect scanning electron microscopy (FESEM) images given in Figures 2a and 2b, nanorods did not collapse, and the retained self-supporting arrays of conducting polymers oriented normal to the substrate surface, even after the template was removed. Since PPy chains are well-confined in the nanosized holes, they are highly aligned to the template length direction, which results in much higher conductivity compared with continuous PPy film. This high degree of chain orientation was confirmed by high resolution transmission electron microscopy (HR-TEM) image, given in Figures 2c and 2d, even though the separation distance for two neighboring PPy main chains is as small as 0.37nm. We also determined the high orientation of PPy chain by using grazing incidence Xray diffraction (GIXD) experiment with a synchrotron source in beamline 5A of Pohang Light Source. We also fabricated PPy, poly (3,4-ethylenedioxythiopene) (PEDOT) and poly (3-hexyl thiopene) (P3HT) nanowire (or nanorods) arrays on flexible substrates, like ITOcoated polycarbonate and poly (ethylene terephthalate) thin films, which greatly expand the utility and versatility of this process. These ultrahigh density arrays of conducting polymer nanorods have applications as sensor materials, nanoactuators, and organic photovoltaic devices. Professor Jin Kon Kim Department of Chemical Engineering Tel: +82-54-279-2276 Fax: +82-54-279-8298 E-mail: jkkim@postech.ac.kr Professor Su-Moon Park Department of Chemistry Tel: +82-54-279-2102 Fax: +82-54-279-3399 E-mail: smpark@postech.ac.kr

New Year’s Message from the President

As the eventful year of 2008 comes to a close and the New Year of 2009 begins, I wish all of you a New Year full of happiness and love. 2009 is the Year of the Ox. There is an old saying that goes, “Ox pace takes you a thousand miles,” which means slow and steady steps take you much farther. This year’s start is cloudier and gloomier than ever. The recent global economic crisis has been even more brutal than the foreign exchange crisis we experienced a decade ago. At a time of such hardship, we should learn from the wisdom of the ox, not panicking or getting nervous, but being persistent to stay on with slow but steady paces to reach the goal. A crisis and an opportunity are two sides of the same coin. We shall take this unparalleled economic crisis as a chance to take up a new challenge. Especially in times like this, we need to uphold the attitude we had in the very beginning and once again arm ourselves with spirits of a challenger and pioneer to commit to building a globally competitive university. This year will bring us the results of some big and important initiatives that will greatly influence the future advancement of the University. The outcome of our effort, embarked upon last year, to establish a Max Planck Institute in Korea, will be of great significance not only to our University but to the Korean basic science community at large. Final decision by the Max Planck Society is expected by October this year. The government has finally approved the proposal to upgrade our Synchrotron Light Source worth over US$70 million. The Pohang Light Source (PLS) will be upgraded from 2.5GeV to 3.0GeV with 10 more undulator/wiggler beamlines within 3 years. It will provide global scientific community with drastically improved beams with higher stability and brightness. At the same time, we will continue our effort to secure public funds to install a free electron laser, the 4th generation radiation source, to generate and supply coherent X-ray beams with another million times higher brightness and femtosecond shutter speed. Last year, a number of proposals to establish new interdisciplinary departments and programs to attract internationally renowned scholars were selected and funded as a part of the ambitious “World Class University (WCU)” initiative by the Ministry of Education, Science and Technology. I expect that it will provide a valuable opportunity for restructuring and improving our academic activities and drastically enhancing global visibility which is critical for achieving our Vision 2020. Successful launching of the WCU programs in 2009 will certainly become of the corner stone for our journey to the global top rank university by the year 2020. Reinforcement of undergraduate education is also a crucial issue throughout this year. The Residential College System and the English Certification System, both introduced last year, have been implemented and operated quite successfully. Starting this year, the Mathematics Certification System will be launched, and the curriculum reform is under way, aimed for enforcement in 2010. We shall offer educational programs of world top level and do our best to nurture global talents equipped with courage, creativity and communication skills to excel and succeed. Also starting from this year, we plan to select new students based on our own admissions calendar and criteria. New admission officers will proactively excavate and invite talents, who, gifted particularly in science and mathematics, match the images and qualifications of the founding educational principles of POSTECH. Admission will be given based on the criteria including high school record, essay, recommendation, extra curricular activities, followed by a rigorous oral examination and interview on scientific and leadership aptitude, thus deemphasizing the standardized test scores. In closing, I would like to ask all of you to prepare yourselves with positive attitudes toward change and innovation. The circumstances we face this year, internally and externally, are very unpredictable. Not only income from endowment, but also governmental and industrial support will be affected by the current economic situation. Careful selection and prioritization must be made in budget operation, and every one of University members should participate proactively in change and innovation required for our journey to the world top university. I hope that we will not falter in complacency, but strive and be one step closer toward achievement of our dream and vision in the New Year. May health and blessing be with you and your family. Happy New Year! Sunggi Baik President, POSTECH