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:: Accurate optical surface profilometry ::
Accurate optical surface profilometry
Quang Duc Pham and Yoshio Hayasaki
A new method uses an ultra-stable mode-locked femtosecond laser, a single-pixel camera, and the compressive sensing technique to measure objects with depths of several centimeters.
AWT IMAGEMeasuring the surface profile of objects with large depth is becoming important in several industrial applications (e.g., car manufacturing). Surface profilometers can be used to determine the profile of an object by simply measuring the distance between a reference and sample points on the object. The accuracy of such measurements, however, is limited by the stability of the mechanical parts in the instrument.
Recently, absolute distance measurements have been made over a wide dynamic range and with high accuracy using a new system involving an optical frequency comb, which includes an ultra-stable mode-locked femtosecond (i.e., 10−15s) laser.1–4 This technology can now be developed to realize optical profilometry with wide axial dynamic range, i.e., beyond that of conventional methods.

We have created an optical profilometer that consists of a carrier-envelope phase-stabilized femtosecond laser (with a repetition rate of 76MHz), a single-pixel camera, electronic circuits, and a computer (see Figure 1).5–7 Our profilometer also uses the compressive sensing (CS) signal processing technique, which allows an object's profile to be reconstructed from fewer measurements than the number of sampling points.

Figure 1. Frequency comb profilometer with a single-pixel camera. SLM: Spatial light modulator. FSM: Frequency selection module. PhD: Photodetector. PD: Phase detector. Amp: Amplifier.
In our profilometer setup, the object wave travels through a liquid-crystal spatial light modulator (SLM) before it is detected by a photoreceiver. The reference wave is also detected by a photoreceiver. We designed the frequency selectable module (FSM) to choose the 13th-order harmonic of the laser repetition rate, and we obtain the phase difference between the spatially modulated object wave and the reference wave from the phase detector module (PDM). By integrating a two-step phase-shifting module into the PDM, we are able to eliminate the dependence of the phase measurement on the change in the intensity of the reflected light.

Although we used the CS technique to reduce the number of measurements that are required, it can only be applied to sparse signals.5, 6,8 We have designed a simple method to solve this problem and allow our profilometer to work in more useful situations. We display pseudorandom patterns on the SLM, and for each of these we measure 5000 samples of the phase information with intervals of 100μs. We then calculate the average phase that corresponds to the displayed pseudorandom pattern. We repeat this process until the number of measurements is sufficient for the CS reconstruction condition, and then extract the relative depth information at each point on the surface.5, 6 The SLM we use has a pixel size of 26μm, and we fixed the measurement range of our experiments at 200×200 pixels (5.2×5.2mm2).

We evaluated the accuracy and stability of our method by measuring the profile of a mirror with different pseudorandom patterns. We reconstructed the object's profile from the root mean square (RMS) error that we calculated from all the measured points (see Figure 2). We also measured a more complex object that was composed of three flat mirrors located in different axial positions: see Figure 3(a). The distance between the first and second mirrors was about 30mm, and about 50mm between the first and third mirrors. The resultant profile, which we obtained via 80 measurements of 10×10 pseudorandom patterns, is shown in Figure 3(b).

Figure 2. Accuracy, expressed as the root mean square (RMS) error of the measurements, of the new profilometry technique, using different numbers of pseudorandom patterns.


Figure 3. (a) An object composed of three plane mirrors located at different positions. (b) The measured surface profile of the object shown in (a), obtained from 10×10sample points.
We have developed a new method to measure an object's surface profile. By using an optical frequency comb we are able to measure large-depth objects with high axial dynamic range. Our measurements are made with submicrometer to micrometer accuracy, over centimeter to meter depth variations. Imaging can be conducted without mechanical scanning due to the inclusion of a single-pixel camera, and the CS technique means that profile reconstructions can be made with few measurements. The number of sampling points in our technique can be changed easily, up to the number of pixels in the SLM. In the future, we aim to improve the SLM contrast and the axial dynamic range of the system. We are also developing a more efficient data acquisition process so that the number of measurements required can be further reduced.

Quang Duc Pham, Yoshio Hayasaki
Center for Optical Research & Education
Utsunomiya University
Utsunomiya, Japan

1. W. Sibbett, A. A. Lagatsky, C. T. A. Brown, The development and application of femtosecond laser systems, Opt. Express 20, p. 6989-7001, 2012.
2. K. Minoshima, H. Matsumoto, High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser, Appl. Opt. 39, p. 5512-5517, 2000.
3. S. Yokoyama, T. Yokoyama, Y. Hagihara, T. Araki, T. Yasui, A distance meter using a terahertz intermode beat in an optical frequency comb, Opt. Express 17, p. 17324-17337, 2009.
4. J. Lee, Y. J. Kim, K. Lee, S. Lee, S. W. Kim, Time-of-flight measurement with femtosecond light pulses, Nat. Photon. 4, p. 716-720, 2010.
5. D. L. Donoho, Compressed sensing, IEEE Trans. Inf. Theory 52, p. 1289-1306, 2006.
6. E. J. Candès, T. Tao, Near optimal signal recovery from random projections: universal encoding strategies, IEEE Trans. Inf. Theory 52, p. 5406-5425, 2006.
7. W. Chan, K. Charan, D. Takhar, K. Kelly, R. Baraniuk, D. Mittleman, A single-pixel terahertz imaging system based on compressed sensing, Appl. Phys. Lett. 93, p. 121105, 2008.
8. Q. D. Pham, Y. Hayasaki, Optical frequency comb interference profilometry using compressive sensing, Opt. Express 21, p. 19003-19011, 2013.

source :http://spie.org/x105716.xml
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:: Photon up-converting devices for solar fuels ::
Photon up-converting devices for solar fuels
Karl Börjesson, Damir Dzebo, Bo Albinsson and Kasper Moth-Poulsen
Employing a molecular solution capable of triplet-triplet annihilation in a layered microfluidic device enables the solar spectrum to be locally modified toward the UV, increasing solar energy system efficiency.
AWT IMAGESolar power production and solar energy storage are important technologies on the road toward obtaining a sustainable society, independent of access to fossil fuels. To achieve this goal, challenges must be overcome regarding intermittency (the inherent changeability in the supply of solar energy) and load leveling (the variability in consumer demand for electricity). As a consequence, technologies for solar energy storage in the form of latent electrical power (voltaics) and energy-rich chemical bonds (solar fuels) are becoming increasingly important.
Solar energy reaches the earth in quanta known as photons, which can then be converted into useful energy. Although photons from solar irradiation vary in intensity, from the UV to the near-IR, the desired chemical transformations in solar fuels typically require high-energy photons. Since the requisite UV photons account for less than 10% of the total solar spectrum, this presents a problem for the overall efficiency of practical devices.
A process known as triplet-triplet annihilation (TTA) photon up-conversion provides a possible strategy toward circumventing this issue. A material capable of performing TTA can combine two low-energy photons to form one with high energy via annihilation.1–4 In a typical system, a triplet sensitizer molecule is excited by the absorption of solar light. This excited sensitizer then transfers its triplet energy to an annihilator molecule, also in a triplet state. At a sufficient intensity of irradiation, it becomes likely that two excited annihilator molecules will combine, emitting excess energy in the form of a high-energy photon. Lower-energy photons are thereby converted to higher-energy photons that can be used to drive solar fuels. TTA has traditionally required laser light and strict oxygen-free conditions to work efficiently, but the process was recently demonstrated even under lighting conditions analogous to natural sunlight.1, 2 Furthermore, new systems have been developed that operate in the presence of oxygen.5

We have fabricated a device that employs TTA photon up-conversion to increase the energy absorbed by a molecular solar thermal fluid. This fluid undergoes a molecular transition (isomerization) when photons are absorbed, enabling chemical energy storage. An example of an energy storage system based on this process can be seen in Figure 1.6–8 In a practical device, either thermal energy or a catalyst is then used to initiate the energy-releasing reaction when required.7
Figure 1. A schematic showing how a molecular solar thermal system could work. The arrows denote the path of the molecular solar thermal fluid flowing through the system.6 (Reproduced with permission from the American Chemical Society.)

Without photon up-conversion, the process has a maximal efficiency of about 10%. One of the limiting factors is the small fraction of UV photons in the solar spectrum,6 since the photochemical reaction typically requires photons with UV wavelengths (less than 450nm) to occur.8 The molecular system absorbs photons at these wavelengths, but is transparent to lower-frequency photons. Our device, shown in Figure 2(a), consists of a top layer that filters out UV light from the solar spectrum, allowing truncated white light through. The subsequent layers are fabricated using quartz glass, into which channels have been sintered to create microfluidic systems. The molecular solar thermal fluid—bis-(1,1-dimethyl-tridecanyl)-fulvalene-dirutheniumtetracarbonyl—is pumped through the second layer of the device. Long-wavelength photons are transmitted through this layer and into a third, containing a solution made up of sensitizer and annihilator molecules—2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine palladium(II) and 9,10-diphenylanthracene, respectively—dissolved in toluene,1, 2 which facilitate TTA. The photons are up-converted and transmitted back to the second layer of the device as high-energy photons, which in turn drive the photochemical energy storage reaction. This technique leads to a considerable increase in the conversion percentage: see Figure 2(b).
Figure 2. (a) Device employing photon up-conversion for solar energy storage. The upper (first) layer filters out UV light. The second layer consists of a microfluidic system sintered into quartz glass, through which a molecular solar thermal fluid is pumped. The microfluidic system of the third layer contains a molecular solution capable of performing triplet-triplet annihilation (TTA). Light that is not energetic enough to be absorbed by the second layer passes through to the third, where it can be up-converted and transferred back. (b) Solar energy conversion as a function of residence time with (blue) and without (red) photon up-conversion.4 (Reproduced with permission from the Royal Society of Chemistry.)
In summary, we have fabricated a device capable of converting lower-energy to higher-energy photons by employing a solution that enables TTA interactions. The molecular photon up-conversion system has enabled us to improve the efficiency of our device by 130% for truncated white light. This success highlights the potential use of photon up-conversion technologies in the context of solar energy fuel generation in the future.4 We are currently working to improve the energy efficiency of both the molecular solar thermal fluid and the photon up-conversion process.

Karl Börjesson
University of Strasbourg
Strasbourg, France
Karl Börjesson is currently working as a postdoctoral researcher. His research interests span the design, synthesis, and photophysical evaluation of both photochromic molecules and energy-transfer complexes, as well as the incorporation of such systems into practical devices.
Damir Dzebo, Bo Albinsson, Kasper Moth-Poulsen
Chalmers University of Technology
Gothenburg, Sweden
Damir Dzebo is a PhD candidate under Bo Albinsson. His work focuses on the characterization and optimization of TTA up-conversion systems with the use of a wide range of optical spectroscopic methods.
Bo Albinsson is a professor. He has a long-standing interest in mechanisms of energy and electron transfer reactions, with relevance for solar energy research. He has also recently developed DNA nanostructures functionalized with photoactive molecules and has a broad interest in advanced optical spectroscopic methods.
Kasper Moth-Poulsen is an assistant professor. He currently works in the design of new self-assembled materials, with the goal of fabricating materials from nanoparticles, nanorods, and tailor-made small molecules for a number of applications, ranging from single-molecule electronics to sensors and renewable energy.

1. S. Baluschev, T. Miteva, V. Yakutkin, G. Nelles, A. Yasuda, G. Wegner, Up-conversion fluorescence: noncoherent excitation by sunlight, Phys. Rev. Lett. 97, p. 143903, 2006. doi:10.1103/PhysRevLett.97.143903
2. S. Baluschev, V. Yakutkin, T. Miteva, G. Wegner, T. Roberts, G. Nelles, A. Yasuda, S. Chernov, S. Aleshchenkov, A. Cheprakov, A general approach for non-coherently excited annihilation up-conversion: transforming the solar-spectrum, New J. Phys. 10, p. 013007, 2008. doi:10.1088/1367-2630/10/1/013007
3. J.-H. Kim, F. Deng, F. N. Castellano, J.-H. Kim, High efficiency low-power upconverting soft materials, Chem. Mater. 24(12), p. 2250-2252, 2012. doi:10.1021/cm3012414
4. K. Börjesson, D. Dzebo, B. Albinsson, K. Moth-Poulsen, Photon upconversion facilitated molecular solar energy storage, J. Mater. Chem. A 1, p. 8521-8524, 2013. doi:10.1039/C3TA12002C
5. P. Duan, N. Yanai, N. Kimizuka, Photon upconverting liquids: matrix-free molecular upconversion systems functioning in air, J. Am. Chem. Soc. 135(51), p. 19056-19059, 2013. doi:10.1021/ja411316s
6. K. Börjesson, A. Lennartson, K. Moth-Poulsen, Efficiency limit of molecular solar thermal energy collecting devices, ACS Sustain. Chem. Eng. 1(6), p. 585-590, 2013. doi:10.1021/sc300107z
7. K. Moth-Poulsen, D. Ćoso, K. Börjesson, N. Vinokurov, S. K. Meier, A. Majumdar, K. P. C. Vollhardt, R. A. Segalman, Molecular solar thermal (MOST) energy storage and release system, Energy Environ. Sci. 5, p. 8534-8537, 2012. doi:10.1039/C2EE22426G
8. V. Gray, A. Lennartson, P. Ratanalert, K. Börjesson, K. Moth-Poulsen, Diaryl-substituted norbornadienes with red-shifted absorption for molecular solar thermal energy storage, Chem. Commun., in press, 2014. First published online: 1 November 2013. doi:10.1039/C3CC47517D

suorce: http://spie.org/x107549.xml
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:: Efficient nanoparticle synthesis ::
Efficient nanoparticle synthesis
Dorota Koziej
Integrating a microwave reactor with a microfluidic platform reduces the synthesis time of nanoparticles to 64ms.
AWT IMAGENanoparticles are essential building blocks for many energy-related applications, ranging from lithium ion batteries, catalysis, and photocatalysis to electrochromic windows. The ability to synthesize nanoparticles with well-defined size, shape, and structure is critically important. In recent years, there have been two main trends to improve the efficiency of nanoparticle synthesis by use of microwave or microfluidic reactors.1, 2 Microwave reactors provide a higher nanoparticle yield in shorter reaction times than conventional batch reactors,1 and microfluidic reactors have the separate advantage that reaction conditions, such as heat and mass transfer rates, can be independently controlled for small volumes of reaction solution.2 Integrating such miniaturized reactors with microwave heaters would combine the advantages of both and so improve nanoparticle synthesis.
Several such devices have been developed for aqueous solutions, which are suitable for biochemical applications such as heating a polymerase chain reaction.3 However, water is not a suitable solvent for low-temperature, low-pressure synthesis of crystalline nanoparticles. Recently, colleagues and I have developed a microfluidic-microwave device, operating at 700–900MHz, which allows precise tuning of the temperature of non-aqueous solvents such as benzyl alcohol, n-butanol, and ethylene glycol.4

We used two independent non-contact methods to determine the temperature of benzyl alcohol droplets flowing in fluorocarbon-based oil. Infrared temperature imaging provided quantitative information about the microwave heating of the benzyl alcohol droplets and heat transfer from the droplets: see Figure 1. Additionally, we measured the microwave heating of the benzyl alcohol droplets by fluorescence imaging with high temporal resolution. We can heat the benzyl alcohol droplets to 50°C in 15ms.
Figure 1. Top: A 2D temperature map measured at the surface of the microwave-microfluidic device with an IR camera. Bottom: A schematic of the microfluidic-microwave device. (Copyright the Royal Society of Chemistry.4)
We used our microfluidic-microwave device to synthesize tungsten oxide nanoparticles within benzyl alcohol droplets using the synthesis protocol for a conventional reaction in oil bath. The residence time of the droplets in the area exposed to microwave heating of 50°C was 64ms. After drying, the droplets formed a honeycomb-like microstructure: see Figures 2 and 3. There was more solid phase at the boundaries between the droplets than in the middle of the single droplet. Additionally, the dried droplets revealed a subtle nanostructure: inside the droplets we observed individual nanoparticles, whereas at the edge of the droplets they formed assemblies. Thus, our technique offers a route to simultaneously synthesizing and assembling nanoparticles.

Figure 2. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images. The green arrows show the area magnified in the subsequent images. (a) Droplets after heating in microwave oven on the chip, which self-assemble and adopt a honeycomb-like microstructure. (b) A closer look at the dried droplets shows the wrinkled microstructure inside the droplets. (c) Even closer examination shows there is more solid phase collected at the boundaries between the droplets than in the middle of the single droplet. (d) The darker regions inside the droplets consist of assemblies of primary nanoparticles. (e–f) HR-TEM images deliver the final proof of the nanoparticles' crystallinity. (Copyright the Royal Society of Chemistry.4)

Figure 3. Schematic showing the complexity of the observed structure at different magnifications. (Copyright the Royal Society of Chemistry.4)
In summary, we have shown that microwave heating of picoliter-sized droplets produces appropriate conditions to crystallize inorganic metal oxide nanoparticles and assemble them into complex structures. Although microfluidic reactors are often applied to synthesize crystalline nanoparticles, microwave heating has not previously been used. Our approach meets the requirement to translate the unique properties of nanoparticles to microscale objects.5 We are now working to develop a next-generation device that will demonstrate microwave-assisted synthesis on a chip as an attractive alternative to batch microwave reactors. By varying the frequency and power of the electromagnetic field, we will tune the nanoparticles' properties. We hope to show that the method is universally applicable and not restricted to synthesis of particular materials.
This work was supported by the Swiss National Science Foundation (2-77354-12) and the Swiss Federal Institute of Technology, Zurich.

Dorota Koziej
Swiss Federal Institute of Technology Zurich (ETH Zurich)
Zurich, Switzerland
Dorota Koziej is a team leader in the Laboratory for Multifunctional Materials, Department of Materials, at ETH Zurich. She is developing platforms for on-chip monitoring of nanoparticle crystallization, addressing fundamental questions with in situ spectroscopic synchrotron methods, and developing materials for energy-related applications.

1. M. Niederberger, Microwave-assisted nonaqueous routes to metal oxide nanoparticles and nanostructures, Microwaves in Nanoparticle Synthesis: Fundamentals and Applications, ch.8, p. 185-206, Wiley, 2013.
2. K. S. Elvira, X. Solvas, R. C. R. Wootton, A. J. deMello, The past, present and potential for microfluidic reactor technology in chemical synthesis, Nat. Chem. 5, p. 905-915, 2013. doi:10.1038/nchem.1753
3. D. Issadore, K. J. Humphry, K. A. Brown, L. Sandberg, D. A. Weitz, R. M. Westervelt, Microwave dielectric heating of drops in microfluidic devices, Lab Chip 9, p. 1701-1706, 2009. doi:10.1039/B822357B
4. D. Koziej, C. Floryan, R. A. Sperling, A. J. Ehrlicher, D. Issadore, R. Westervelt, D. A. Weitz, Microwave dielectric heating of non-aqueous droplets in a microfluidic device for nanoparticle synthesis, Nanoscale 5, p. 5468-5475, 2013. doi:10.1039/c3nr00500c
5. D. Koziej, A. Lauria, M. Niederberger, 25th anniversary article: metal oxide particles in materials science: addressing all length scales, Adv. Mater. 26(2), p. 235-257, 2014. doi:10.1002/adma.201303161

source: http://spie.org/x106171.xml

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:: Generating light with control of polarization direction ::
 | Post date: 2014/04/19 | 
Chih-Wei Hsu and Per Olof Holtz
AWT IMAGEGroup III-nitride semiconductor quantum dots on elongated pyramids enable direct generation of light with controllable polarization for imaging and display applications.
Linearly polarized light (LPL) is essential for liquid-crystal displays (LCDs), 3D visualization, and high-contrast imaging. Conventionally, LPL is generated by filtering the desired polarization from the unpolarized light of conventional light sources. However, this process results in the loss of at least 50% of the light. Therefore, a source that directly generates LPL, preferably with controllable polarization directions, is in high demand.
Current methods of generating LPL are complex, and do not enable control of polarization direction, being limited instead to only one direction. One method of overcoming this problem is to use quantum dots (QDs)—nanoscopic inclusions of one semiconductor material in another—that exhibit quantum mechanical effects. The emission properties of QDs are determined by their size and shape. However, conventional fabrication methods typically result in a high QD density (>1×106cm2), resulting in limited possibilities for controlling QD positions and emission properties. While there has been recent progress in developing methods to control these properties, the corresponding control of polarization has not been demonstrated.

In our novel approach, we explore a new type of nitride-based QD formed on top of elongated hexagonal micropyramids, which offer an effective path to generate light with controllable polarization directions.1 In addition, the positions and emission wavelengths of the QDs can be controlled within certain limits. The process is based on standard photolithography to create elongated patterns on a silicon nitride (SiN)-masked gallium nitride (GaN) template. We grew micropyramids and QDs using a metal-organic chemical vapor deposition system, with trimethyl gallium, trimethyl indium, and ammonia as precursors to provide Ga, indium (In), and nitrogen (N) atoms for the desired reactions.2 The interiors of the pyramids are composed of GaN, topped with InGaN QDs, and finally capped with a thin GaN layer. We used a continuous-wave UV laser operating at 266nm to excite the QDs, and collected and analyzed data on the QD emissions to reveal their properties and polarization directions.

The elongated patterns enabled us to define a long axis that determined the micropyramids' direction of growth, as shown in Figure 1(a) and (b). Such pre-defined elongation of the micropyramid determines the polarization direction of the emitted photons. The QDs exhibit a high probability of being well aligned with the elongation axis (up to about 90% for 1μm elongation in our investigation). The best polarization alignment is obtained for elongations parallel with the principal axes of the hexagonal crystal, that is, arranged in multiples of 60° angles with respect to each other, but polarization control in 30° steps is also possible: see Figure 1(c).

Figure 1. Elongated gallium nitride (GaN) pyramids topped with indium gallium nitride (InGaN) quantum dots. (a) Scanning electron micrograph of elongated GaN pyramid arrays. (b) Magnified images of elongated GaN pyramids that are determined by long axes oriented in different directions. An angular parameter, α, is defined as the angle between the long axis and the underlying GaN template. (c) Distribution histograms of measured polarization directions from elongated GaN pyramids with various values of α. The result from non-elongated GaN pyramids is also included for comparison. The blue line/arrow refers to the perfect polarization alignment between the elongation direction of the pyramid and the measured polarization direction of the light emitted from the dot.
With the InGaN alloy, it is possible to tune the emission wavelengths across the entire visible spectrum toward the IR by adding more indium. Moreover, an increased indium content in the dots further improved the polarization degree of the emitted photons. According to our theoretical computations, the ‘split-off’ energy—the energy difference between the lowest and the second of the three energy states—is the crucial parameter that determines the degree of linear polarization. Given a certain asymmetry, a decreasingly smaller split-off energy will result in a successively stronger degree of linear polarization of the emission. The split-off energies of III-nitride semiconductors are about one order of magnitude smaller than in other common semiconductors, such as arsenides and phosphides. InN has the smallest split-off energy among all III-nitrides. As a direct consequence, InGaN QDs can be considered as the best candidates for generating LPL at a desired wavelength. In our present work, the emission wavelengths of the investigated QDs are between 400 and 420nm with an average degree of linear polarization of 84% in a pre-determined direction. For comparison, typical GaN QDs grown on planar substrates emit between 310 and 350nm, with ana average degree of linear polarization of less than 65% without control of the polarization direction.3

Our approach could benefit the development of more energy-efficient polarized LEDs, for example, for back illumination of LCD displays. We can generate a universal light source for various applications by combining several pyramids, individually addressed and designed for different polarization directions. In addition, our demonstrated QD approach exhibits good single-photon statistics compared with conventional light sources, offering potential for polarized single-photon emitters and quantum cryptography applications. Our ultimate goal is a scalable, electrically driven single-photon emitter with controllable polarization directions. In future work, we will seek to ensure the formation of an individual QD on top of every pyramid.

Chih-Wei Hsu, Per Olof Holtz
Linköping University
Linköping, Sweden
Chih-Wei Hsu recently received his PhD and is continuing his work in semiconductor materials at the Department of Physics, Chemistry, and Biology. His research interests include fabrication and characterization of III-nitride composite structures for various applications

Per Olof Holtz heads the optical characterization group at the Department of Physics, Chemistry, and Biology, of which the main focus is semiconductor-based QD and wire structures.

1. A. Lundskog, C. W. Hsu, K. F. Karlsson, S. Amloy, D. Nilsson, U. Forsberg, P. O. Holtz, E. Janzén, Direction generation of linearly polarized photon emission with designated orientations from site-controlled InGaN quantum dots, Light: Sci. Appl. 3, p. e139, 2014.
2. C. W. Hsu, A. Lundskog, K. F. Karlsson, U. Forsberg, E. Janzén, P. O. Holtz, Single excitons in InGaN quantum dots on GaN pyramid arrays, Nano Lett. 11, p. 2415, 2011.
3. R. Bardoux, T. Guillet, B. Gil, P. Lefebvre, T. Bretagnon, T. Taliercio, S. Rousset, F. Semond, Polarized emission from GaN/AlN quantum dots: single-dot spectroscopy and symmetry-based theory, Phys. Rev. B 77, p. 235315, 2008.

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:: Dual-function graphene-based electronic devices ::
 | Post date: 2014/04/19 | 
Jong-Hyun Ahn
AWT IMAGEGraphene/poly(vinylidene fluoride-co-trifluoroethylene) multilayer films enable the fabrication of transparent, flexible nanogenerators and acoustic devices.

Transparent electrodes are an essential element in numerous electronic devices, including displays, solar cells, LEDs, and touch panels for smartphones and tablets. Indium tin oxide (ITO) is currently the most dominant material for this purpose due to its good electrical and optical performance, and moisture resistance. However, indium is becoming a scarce and expensive resource and requires a costly vacuum deposition process. In addition, its mechanical properties are weak, which severely restricts its application in flexible electronics. These issues have stimulated numerous developments with the aim of discovering new transparent electrode materials. Graphene has recently attracted considerable attention as an alternative to ITO due to its good electrical conductivity, optical transparency, and high mechanical strength.1, 2 However, for adoption in practical electronic devices, the electrical conductivity of graphene must be made comparable to that of ITO.3, 4 Although a chemical doping method has been developed, the initially high conductivity tends to degrade due to the adsorption of moisture and other chemical molecules under environmental conditions.
Purchase Nanotechnology: A Crash CourseAs an alternative approach to overcoming these limitations, we have developed a nonvolatile doping method using a piezoelectric material as substrate5 that could pave the way toward the application of graphene in high-performance flexible and transparent devices. Our electrostatic doping method employs the ferroelectric polarization of a piezoelectric material, which can effectively enhance the conductivity of graphene while maintaining good mechanical and optical properties, without degradation. After a poling process in which the material is subjected to large voltages (50–150V), the carbon-fluorine dipoles of the ferroelectric poly(vinylidene fluoride-co-trifluoroethylene)—P(VDF-TrFE)—polymer align along the poling direction. In other words, the net negative charge of the fluorine atoms is attracted to the positively biased graphene film. These nonvolatile ferroelectric dipoles are polarized in a direction perpendicular to the plane of the graphene film, which increases the number of positive holes in the material. This causes a downward shift of the Fermi level and thereby improves the electrical conductivity, resulting in p-doped graphene.

To demonstrate this technique, we have developed an acoustic actuator and a highly efficient nanogenerator using a multilayer film of graphene/P(VDF-TrFE)/graphene (GPG). We were able to fabricate a stand-alone P(VDF-TrFE)/graphene structure at a low processing temperature without an additional supporting substrate. The resulting GPG-multilayer-based actuators and nanogenerators exhibit good device functionality as well as great mechanical and optical properties.

Figure 1 shows the optical image of a GPG multilayer film, which is composed of a central P(VDF-TrFE) film and two graphene layers working as the top and bottom electrodes. The device is interconnected with a sound source to measure acoustic actuation, a red LED to measure its performance as a nanogenerator, and an amplifier. To characterize the performance of our GPG-based actuator, which produces mechanical vibrations in response to an amplified electrical signal, we drove the device with an amplified electrical signal within a broad frequency range using white noise. Figure 2(a) shows the frequency response of our fabricated acoustic device between 1 and 3000Hz. The GPG-based acoustic device is shown to operate well within the broad frequency range. The use of a graphene electrode in this device fulfills the requirement for mechanical endurance and optical transmittance, making it superior to other acoustic devices composed of conventional electrode materials such as ITO, conductive polymers, and carbon nanotubes.
Figure 1. (a) Image of a graphene/poly(vinylidene fluoride-co-trifluoroethylene)/graphene (GPG) device. The device is interconnected with a sound source to measure acoustic actuation, a red LED to measure its performance as a nanogenerator, and an amplifier. Inset shows the structure of GPG multilayer film. Two graphene layers work as top and bottom electrodes for the central poly(vinylidene fluoride-co-trifluoroethylene)—P(VDF-TrFE)—layer. (b) Schematic depiction of the dual functionality of the device as an acoustic actuator and a nanogenerator.
Figure 2. (a) Frequency response of GPG-based acoustic device: variation of sound pressure level (SPL) as a function of frequency within a broad region (1∼3000Hz). (b) Output voltage of the GPG-based nanogenerator as a function of time.
To characterize the performance of the GPG-based nanogenerator, we measured the output voltage and current density with respect to time. External strain was repeatedly applied to the GPG multilayer film for the generation of current-voltage. Figure 2(b) shows the variation of output voltage as a function of time. The GPG-based nanogenerator achieved an average output voltage of ∼2.5V, with a maximum value of ∼3V. The average current density was found to be over ∼0.2μAcm−2, while the maximum value was estimated to be ∼0.37μAcm−2. High electrical conductivity of the top and bottom electrodes is important for good nanogenerator performance. We believe that the nonvolatile doping of ferroelectric P(VDF-TrFE) polymer in graphene significantly increases the electrical conductivity and lowers the barrier for charge collection at the interface between P(VDF-TrFE) and the graphene electrodes, resulting in excellent performance. Another important advantage of our nanogenerator is its easy fabrication. Previously reported nanogenerators have necessitated several complicated fabrication processes, while ours requires only graphene transfer and solution-based P(VDF-TrFE), without any high-temperature processing or complex steps.6

In addition to its good electrical properties, the GPG device possesses excellent mechanical properties that can satisfy the increasing demand for rollable devices in next-generation flexible electronics. To confirm the rollability of our GPG structure, the device was rolled and released for up to 100 cycles. The relative change to the resistance of the film was found to be within a range of 0.2, indicating that GPG-based structures can be used as rollable, large-scale electronic devices.

In summary, we have demonstrated a dual-function device using a GPG multilayer film. The fluoropolymer P(VDF-TrFE) effectively increases the electrical conductivity of graphene films via dipole moment interactions. Our GPG-based device includes a nanogenerator, with performance characteristics comparable to those of conventional ZnO- and PZT-based nanogenerators, and an acoustic actuator with good frequency response and high sensitivity. In the future, we plan to focus on the development of defect-free transfer methods, high-quality graphene synthesis, and a doping approach with long-term stability to enable further progress toward flexible and wearable applications.

Jong-Hyun Ahn
Yonsei University
Seoul, Republic of Korea

1. A. K. Geim, Graphene: status and prospects, Science 324(5934), p. 1530-1534, 2009.
2. S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, Roll-to-roll production of 30-inch graphene films, Nat. Nanotechnol. 5, p. 574-578, 2010.
3. N. O. Weiss, H. Zhou, L. Liao, Y. Liu, S. Jiang, Y. Huang, X. Duan, Graphene: an emerging electronic material, Adv. Mater. 24(43), p. 5782-5825, 2012.
4. S.-K. Lee, K. Rana, J.-H. Ahn, Graphene films for flexible organic and energy storage devices, J. Phys. Chem. Lett. 4(5), p. 831-841, 2013.
5. S.-H. Bae, O. Kahya, B. K. Sharma, J. Kwon, H. J. Cho, B. Ozyilmaz, J.-H. Ahn, Graphene-P(VDF-TrFE) multilayer film for flexible applications, ACS Nano 7(4), p. 3130-3138, 2013.
6. Z. L. Wang, G. Zhu, Y. Yang, S. Wang, C. Pan, Progress in nanogenerators for portable electronics, Mater. Today 15(12), p. 532-543, 2012.

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:: Domesticating random lasers ::
 | Post date: 2014/04/19 | 
Matthias Liertzer and Stefan Rotter
When pumped with a spatially modulated light beam, random lasers can be tuned to emit into specific, predetermined directions.
AWT IMAGE Random lasers are exotic light sources that emit coherent light without any of the confining mirrors that are part of a conventional laser.1,2 Instead, the light inside a random laser is trapped by multiple scattering in a disordered medium and can thus be efficiently amplified. The downside is that random lasers emit their light at many different frequencies and in many different directions (see Figure 1).3,4 Scientists have therefore thought about how to ‘domesticate’ random lasers by making both their emission spectrum and directionality externally tunable.5–9 A tunable random laser holds great promise for a range of applications. It could be used as a multipurpose device with a customized functionality that is determined not by its design but rather by external control knobs.
 Overcoming the uncontrollable nature of random lasers, however, is difficult. The disordered medium that determines much of the laser's properties consists of a random and typically immobile distribution of scatterers (see Figure 1). Engineering this medium to produce a specific output is possible, in principle, but laborious.5, 6 Rather than trying to control the lasing medium, it seems more promising to modify the spatial profile of the pump beam that provides the external energy (see Figure 2).7–9 Digital spatial light modulators—devices that allow a user to control 2D light patterns—make imprinting any arbitrary pattern on a pump beam relatively straightforward. The difficulty, however, lies in determining which pump pattern leads to the desired lasing frequency or mode.

Figure 1. A random laser typically consists of a disordered medium (right) and is optically pumped using a beam of light (left). In the case of a homogeneous pump pattern, the laser will typically emit into arbitrary directions (bottom left).
Solving this problem is similar to finding the optimal pump grating in a distributed feedback laser, only we are dealing with a random laser that does not have well-behaved periodic laser modes. In a random laser, the mode patterns are highly irregular and unique to each medium.3, 4 Every laser, therefore, requires a specially designed ‘pump grating’ to achieve the desired output. However, once the pump grating is known, it always produces the same output for that specific laser.
Figure 2. By optimizing the spatial profile of the pump beam with a spatial light modulator, it is possible to enhance the emission of the random laser into a single direction while reducing the emission in all other directions. The disordered medium used here features the same distribution of scatterers as in Figure 1.
Fortunately, it is possible to find the right pump grating without knowing how the scatterers in a random laser are distributed. The trick is to set up an optimization procedure based on a feedback loop between the pump grating profile and the laser output. The profile can be iteratively improved to produce an output that is closer to the desired specifications. We have recently demonstrated how this approach can be used to trick a random laser into emitting in a single predetermined direction.9
In our theoretical study we look at two conceptually different cases. In the first case, the random medium is strongly scattering, such that the laser modes are strongly trapped inside the medium with low lasing thresholds as a result. Since all lasing modes are already well-defined by the structure of the medium itself, the pump profile can only select one mode at the expense of all the others. To achieve unidirectional emission, we can therefore choose a pump profile that simply selects the mode with the smallest deviation from the desired far-field emission pattern to be the first lasing mode.
In the second case, where the scattering in the medium is weak, the laser modes are only weakly trapped by the medium with, correspondingly, a higher lasing threshold. In this limit, the pump profile has a much stronger influence on the lasing modes and has the potential to shape, rather than just select, them. It is here that the full potential of our optimization technique can be brought to bear.

Starting with a randomly chosen initial configuration and using a gradient-based optimization technique, we iteratively adjust the pump profile to shape the first laser mode according to a predetermined narrow angular distribution of emission. We typically require fewer than 20 initial guesses in our optimization procedure to reach a result that matches the target function to better than 95% (see Figure 2). This level of agreement, which we find based on ‘pump-shaping’ the modes, is much better than what can be achieved by simply ‘pump-selecting’ laser modes in the limit of strongly scattering media. Based on this encouraging result, we also managed to push our optimization procedure even further by making a random laser emit in two predetermined directions simultaneously.9
In all the cases we studied, the pump profiles we obtained through the optimization procedure are highly irregular. It is not evident why a specific profile leads to a given emission pattern when applied to a random medium. Clarifying this open question certainly constitutes an interesting theoretical challenge for the future. From the experimental point of view, the obvious next step would be to implement our envisioned setup with a random laser that is pumped through a spatially modulated light beam (see Figure 2). Given the exciting recent progress on the experimental side,10 we believe that our ideas can be realized with presently available setups. Such an experimental realization would not only be interesting from the fundamental physics point of view, but would also enlarge the scope of random laser applications.
We would like to acknowledge funding from the Vienna Science and Technology Fund through project MA09-030 as well as from the Austrian Science Fund through projects F25-P14 (SFB IR-ON) and F49-P10 (SFB NextLite). We are also indebted to the administration of the Vienna Scientific Cluster for free access to their computational resources.

Matthias Liertzer, Stefan Rotter
Institute for Theoretical Physics
Vienna University of Technology
Vienna, Austria
Matthias Liertzer studied physics at the Vienna University of Technology as well as at the Swedish Royal Institute of Technology and at the Swiss Federal Institute of Technology, Zurich. He is currently doing his PhD in Stefan Rotter's group, focusing on the physics of micro- and random lasers.

Stefan Rotter is a professor for theoretical physics with a research focus on nonlinear dynamics and complex scattering. With a background in the theory of mesoscopic systems, he started working on lasers during a postdoctoral fellowship at Yale University. He now leads a multigroup research effort dedicated to this topic.

1. H. Cao, Lasing in random media, Waves Random Media 13, p. R1-R39, 2003.
2. D. S. Wiersma, The physics and applications of random lasers, Nat. Phys. 4, p. 359-367, 2008.
3. H. E. Türeci, L. Ge, S. Rotter, A. D. Stone, Strong interactions in multimode random lasers, Science 320, p. 643-646, 2008.
4. H. E. Türeci, A. D. Stone, L. Ge, S. Rotter, R. J. Tandy, Ab initio self-consistent laser theory and random lasers, Nonlinearity 22, p. C1-C18, 2009.
5. D. S. Wiersma, S. Cavalieri, Light emission: a temperature-tunable random laser, Nature 414, p. 708-709, 2001.
6. S. Gottardo, Resonance-driven random lasing, Nat. Photon. 2, p. 429-432, 2008.
7. X. Wu, Random lasing in weakly scattering systems, Phys. Rev. A 74, p. 053812, 2006.
8. N. Bachelard, J. Andreasen, S. Gigan, P. Sebbah, Taming random lasers through active spatial control of the pump, Phys. Rev. Lett. 109, p. 033903, 2012.
9. T. Hisch, M. Liertzer, D. Pogany, F. Mintert, S. Rotter, Pump-controlled directional light emission from random lasers, Phys. Rev. Lett. 111, p. 023902, 2013.
10. N. Bachelard, S. Gigan, X. Noblin, P. Sebbah, Turning a random laser into a tunable singlemode laser by active pump shaping, arXiv:1303.1398 [physics.optics] , 2013.

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انجمن اپتیک و فوتونیک ایران Optics and Photonics Society of Iran
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