News Article | November 8, 2016
Direct doping of nanoparticles in glass shows potential for smart applications A new and versatile method for integrating light-emitting nanoparticles, without loss of their unique properties, into glass is demonstrated. Light-emitting upconversion nanocrystals (UPNCs)—tiny particles studded with active lanthanide ions (Ln3+)—can convert IR excitation radiation into higher-energy emissions. Improved understanding and manipulation of upconversion properties at the nanoscale has recently fueled the development of new-generation UPNCs.1 These new-generation nanocrystals emit higher brightness upconversion through the use of high-irradiance excitation (which can unlock the previously inactive emitters).2 Alternatively, clustering ytterbium ion (Yb3+) sensitizers, in arrays at the sublattice level, are used to promote localized excited states.3 Furthermore, the UPNCs are promising for various applications, including biological sensing, anti-counterfeiting, photon energy management, and volumetric displays. The realization of many of those applications would be particularly helped if the new-generation UPNCs could be incorporated into glass. It remains a challenge, however, to infuse glass with UPNCs that have tailored nanophotonic properties. The glass ceramic technique is the conventional integration approach for in situ growth of nanocrystals inside a glass.4, 5 In this technique, a glass, which contains precursor ions for the nanocrystals—see Figure 1(a)—is heated above the glass transition temperature to yield the crystal seeds. These seeds then undergo further growth, and thus form nanocrystals across the glass volume: see Figure 1(b). Although this in situ method is promising for certain nanocrystals, it runs into significant chemical and physical disadvantages when dealing with UPNCs. These disadvantages—related to production conditions, solubility restrictions, and post-annealing events—mean that the desired optical properties in hybrid glass are hard to achieve (and they can cause increased light scattering).6 Figure 1. Schematic illustration of active lanthanide ions (Ln3+) distributions in various glasses, i.e., glass containing (a) Ln3+ ions (Ln3+-doped glass), (b) nanocrystals (NCs) grown in situ (glass ceramics), and (c) directly doped NCs (NC-doped glass). The small gray dots in (c) represent the Ln3+ ions in the NC-doped glass that were dissolved from NCs. To pursue high levels of compositional and structural control over UPNCs in glass, we have thus devised a versatile ‘direct doping’ approach6—see Figure 1(c)—as an alternative to the conventional glass ceramic technique. In our direct doping approach, as-synthesized nanoparticles are injected into the molten glass and are then integrated to create a highly controllable hybrid material.7, 8 With this technique, we advantageously combine the flexible selection of glass and sophisticated syntheses of unique nanocrystals. In this way, we can achieve far more control over the composition, concentration, and nanostructure of UPNCs in glass. In particular, the success of our approach lies in the manipulation of the correct doping temperature and dwell time of the nanocrystals in the glass melt. To thus ensure the survival and even dispersal of UPNCs across the glass, we first determined a suitable glass melting temperature for doping and dispersing ytterbium- and erbium-doped lithium yttrium fluoride (LiYF :Yb, Er) nanocrystals. The maximum doping temperature of LiYF is given by its decomposition threshold. In addition, we determined the lower limit of the doping temperature from the glass melt viscosity of TZN tellurite (where TZN denotes tellurium dioxide–zinc oxide–sodium oxide, or 75TeO –15ZnO–10Na O). We thus identified the doping temperature window of LiYF :Yb, Er as 550–625°C, as marked by the blue region in Figure 2(a). At higher temperatures within this window, both the desired dispersion and the detrimental dissolution of nanocrystals in the glass melt are accelerated. By examining three different doping temperatures within the window, we confirmed that 577°C is the optimal temperature for achieving the balanced survival and dispersion of UPNCs in glass. Similarly, a prolonged dwell time aids the dispersion of the nanocrystals, but enhances their dissolution in the glass melt. From three different dwell times (3, 5, and 10min), we demonstrated that the 5min dwell time, at 577°C, was the most promising. Figure 2. Characterizations of NC-doped tellurium dioxide–zinc oxide–sodium oxide (TZN) glasses. (a) Differential thermal analysis of lithium yttrium fluoride (LiYF ) NCs, used to determine their decomposition temperature. The blue band represents the doping temperature (T ) window that is suitable for directly doping the LiYF NCs in a TZN glass melt. Δ: Energy change. T : Melting temperature. #: Decomposition threshold. (b) Transmittance spectra of bulk TZN glasses doped with different amounts of LiYF NCs. Er: Erbium. (c) Normalized upconversion spectra of Er3+-doped TZN glass, TZN glass doped with 170ppm of LiYF NCs (at three different locations: P1, P2, and P3), and a suspension of LiYF NCs. a.u.: Arbitrary units. (d) A 3D reconstruction of TZN glass doped with 67ppm of LiYF NCs. This is produced by stacking 100 x–y planes (10 ×10μm) of upconversion images, with a depth increment (i.e., between the frames) of 1.5μm. (e) Optical attenuation curves (between 500 and 1300nm) of blank, Er3+-doped TZN, and NC-doped TZN glass fibers. Solid curves represent the data, and the range of the standard error is shown by the shaded regions. We thus used this optimum doping temperature and dwell time as the pre-set conditions to prepare a series of UPNC-doped TZN glasses. We find—see Figure 2(b)—that all of these samples exhibit high optical transmittance (very close to the maximum transmission of blank TZN glass). According to Rayleigh–Gans–Mie theory,9 we ascribe the negligible amount of light scattering in our glasses to their low-doping concentration (i.e., ≤170ppm w/w), the partial dissolution, and the absence of serious agglomerations of nanocrystals. Furthermore, we obtained almost identical x-ray diffraction patterns and Raman spectra from the blank TZN glass and our UPNC-doped TZN glasses, which suggests that our hybrid glasses retain the original glass network. We have also used optical methods to thoroughly inspect our doped glasses. For example, we used the hypersensitivity of Er3+ emissions—Figure 2(c)—to validate the survival of the UPNCs in the glass and to quantify the dissolution fraction of the doped UPNCs as 30–60%. In addition, we used upconversion scanning confocal microscopy—see Figure 2(d)—to produce the first 3D in situ visualization of UPNC dispersion in glass. To obtain this volumetric 3D imagery and thus visualize the spatial distribution of UPNCs in TZN glass, we reconstructed 100 scanned x–y planes. We also acquired the light attenuation spectrum (between 500 and 1300nm) of the samples. We observe a loss of 0.28±0.06dB/m for a UPNC-doped TZN glass fiber. This value is intermediate to the loss from blank TZN (0.35±0.02dB/m) and from an Er3+-doped TZN (0.08±0.06dB/m) glass fiber: see Figure 2(e). These loss results indicate that dissolution of the nanocrystals has occurred and the absence of serious nanocrystal agglomerations in the TZN glass fibers. In summary, we have used our direct doping approach to successfully integrate UPNCs (which have unique properties) in TZN glass fibers. We have thus demonstrated that this new methodology can be used to overcome key obstacles in the conventional glass ceramics technique. We now plan to use core-shell nanoparticles (which are surrounded by an additional robust layer of material) to ensure that the nanoparticles remain intact and are better dispersed within the glass. We will also generalize our direct doping approach so that it can be used to embed other nanoparticles (with interesting photonic, electronic, and magnetic properties) in glass and to thus advance smart glass technology for a wealth of applications, e.g., in biomedical engineering, remote radiation sensing, and 3D volumetric displays. We thank colleagues and collaborators at Macquarie University and the University of Melbourne (Australia) for their contribution to this work. We also gratefully acknowledge financial support from the Australian Research Council (grants DP130102494 and CE140100003), as well as from the Commonwealth and South Australia State Government (funding for the OptoFab node of the Australian National Fabrication Facility). University of Adelaide Jiangbo (Tim) Zhao is an associate investigator at the Australian Research Council (ARC) Centre of Excellence in Nanoscale Biophotonics and a research associate at the University of Adelaide. He holds a PhD from Macquarie University, Australia. His research interests lie in the interdisciplinary area of photonics, materials, and biomedical science, and are focused on light–matter interactions, luminescence in solid-state and nanoscale structures, and photonics-based devices for practical applications. Heike Ebendorff-Heidepriem received her PhD in chemistry from the University of Jena, Germany, in 1994. She subsequently held two prestigious fellowships and received the Weyl International Glass Science Award. Between 2001 and 2004 she worked at the Optoelectronics Research Centre at the University of Southampton, UK, and she has been at the University of Adelaide since 2005. She is currently one of the leaders of the Optical Materials and Structures Theme and is the deputy director of the Institute for Photonics and Advanced Sensing. She is also a senior investigator of the ARC Centre of Excellence in Nanoscale Biophotonics. Her research is focused on the development of novel optical glasses, fibers, surface functionalization, and sensing approaches. 1. A. Nadort, J. Zhao, E. M. Goldys, et al., Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties, Nanoscale 8, p. 13099-13130, 2016. 3. J. Wang, R. Deng, M. A. MacDonald, B. Chen, J. Yuan, F. Wang, D. Chi, et al., Enhancing multiphoton upconversion through energy clustering at sublattice level, Nat. Mater. 13, p. 157-162, 2014. 5. A. Herrmann, M. Tylkowski, C. Bocker, C. Rüssel, Cubic and hexagonal NaGdF4 crystals precipitated from an aluminosilicate glass: preparation and luminescence properties, Chem. Mater. 25, p. 2878-2884, 2013. 7. H. Ebendorff-Heidepriem, Y. Ruan, H. Ji, A. D. Greentree, B. C. Gibson, T. M. Monro, Nanodiamond in tellurite glass part I: origin of loss in nanodiamond-doped glass, Opt. Mater. Express 4, p. 2608-2620, 2014. 8. M. R. Henderson, B. C. Gibson, H. Ebendorff-Heidepriem, K. Kuan, S. Afshar V., J. O. Orwa, I. Aharonovich, et al., Diamond in tellurite glass: a new medium for quantum information, Adv. Mater. 23, p. 2806-2810, 2011.
News Article | February 15, 2017
Novel nanoscale transfer printing technique for precise positioning of nanowire lasers Semiconductor nanowires, with lasing emission at room temperature, can be transferred in a controlled way to specific locations on diverse substrates and organized into bespoke spatial patterns. Semiconductor nanowire (NW) lasers provide coherent light sources with highly localized emission and extremely small footprints. Such lasers may thus have the potential to revolutionize the field of photonics.1–3 Indeed, NW lasers are expected to play a key role in future optoelectronic systems, i.e., with applications in nanophotonic integrated circuits for on-chip communications and computing, in ultrasmall laser sensors for healthcare, and in light-cell interfaces for biological sciences.1–3 The extremely small dimensions of NW lasers, however, mean that it is technologically challenging to accurately manipulate and integrate them into functional systems. Their transition from laboratory environments to real-life, industrially relevant products is thus limited. In the past, several approaches have been proposed for the manipulation of NWs. These include optical tweezers,4 Langmuir-Blodgett assembly processes,5 the use of microscope probes,6 or contact printing techniques.7 All these techniques, however, have associated problems. For example, the NWs must be in solution, complex equipment is required, they provide reduced positioning accuracy, they do not allow individual NWs to be manipulated, or heterogeneous NWs cannot be integrated within the same system. The precise, simple, and efficient manipulation of single-NW lasers is thus still to be realized. At the Institute of Photonics (IOP) of the University of Strathclyde, UK, we have thus developed a new technique—known as nanoscale transfer printing (nano-TP)—to tackle the challenge of single-NW laser manipulation.8, 9 Transfer printing technology (originally introduced by John Rogers at the University of Illinois10) involves the use of polymer stamps to capture semiconductor structures in a controlled manner and to subsequently release them onto diverse substrates. This approach provided a revolutionary platform for hybrid fabrication of a wide range of novel optoelectronic systems (e.g., semiconductor lasers printed on silicon substrates11 and LEDs printed on flexible and diamond substrates12). In turn, these systems had a large impact in many different technologies, such as visible light communications, flexible optoelectronics, and photonic integrated circuits. In our nano-TP technique we use bespoke polymer microstamps (μ-stamps), which have reduced dimensions and controlled shapes, to capture/release indium phosphide (InP) NW lasers.13 To fabricate (at the Australian National University) the NW lasers we used in our work, we grew vertically aligned InP NWs on an InP substrate, as shown in Figure 1(a). Before performing our nano-TP study, we removed the NWs from the growth substrate and used mechanical means to randomly scatter them onto a silicon (Si) substrate: see Figure 1(b). The final lasers have a lasing emission—see Figure 1(c)—at room temperature in the ∼840–890nm wavelength range.13 Figure 1. Scanning electron images of indium phosphide (InP) nanowire (NW) lasers that are (a) vertically aligned (as-grown) on an InP substrate and (b) randomly scattered on a silicon (Si) surface. (c) Image of lasing emission (in the 840–890nm range) from the NW lasers. Scanning electron images of indium phosphide (InP) nanowire (NW) lasers that are (a) vertically aligned (as-grown) on an InP substrate and (b) randomly scattered on a silicon (Si) surface. (c) Image of lasing emission (in the 840–890nm range) from the NW lasers. 8 To fabricate our μ-stamps, we used polydimethylsiloxane (PDMS), i.e., an elastomeric and adhesive polymer that conforms to the shape of the NW when pressed against it. The NW therefore adheres to the μ-stamp and enables its capture. After lifting off the NW in our approach, we align the μ-stamp (with the NW attached) at a targeted location on a secondary substrate (where the NW is then released). The different mechanisms and stages of our nano-TP process are illustrated in Figure 2.8 Figure 2. (a) Schematic diagrams illustrating the six stages of the nanoscale transfer printing (nano-TP) technique, i.e., showing (1) alignment of the NW and microstamp (μ-stamp), (2) surface (growth substrate) contact, (3) NW lift-off, (4) alignment of the μ-stamp with the receiving substrate, (5) surface (receiving substrate) contact, and (6) NW release. (b) Sequence of microscope images illustrating the transfer of an InP NW laser (5μm in length, 660nm in diameter) from a Si substrate (left) to a silica surface (right), through pressing with a v-shaped μ-stamp. (a) Schematic diagrams illustrating the six stages of the nanoscale transfer printing (nano-TP) technique, i.e., showing (1) alignment of the NW and microstamp (μ-stamp), (2) surface (growth substrate) contact, (3) NW lift-off, (4) alignment of the μ-stamp with the receiving substrate, (5) surface (receiving substrate) contact, and (6) NW release. (b) Sequence of microscope images illustrating the transfer of an InP NW laser (5μm in length, 660nm in diameter) from a Si substrate (left) to a silica surface (right), through pressing with a v-shaped μ-stamp. 8 We have also used our nano-TP approach to demonstrate the precise transfer of InP NWs that have different dimensions (i.e., diameters of 435, 660, and 920nm) from a primary Si substrate to targeted locations on heterogeneous surfaces (e.g., PDMS, silica, or gold).8 We find that the NWs retain their lasing emission characteristics after the nano-TP process is completed. Furthermore, our technique permits the formation of micrometric spatial patterns from NW lasers on diverse substrates. For example, we show bright and dark micrographs of an ‘IOP’ pattern in Figure 3. We fabricated this pattern—in which all elements (i.e., NWs) kept their lasing emission—with the use of 435nm-diameter InP NWs.8 Figure 3. (a) Bright and (b) dark micrographs of an ‘IOP’ pattern formed from InP NWs on polydimethylsiloxane, through the use of nano-TP. Scale bar marks 20μm. (a) Bright and (b) dark micrographs of an ‘IOP’ pattern formed from InP NWs on polydimethylsiloxane, through the use of nano-TP. Scale bar marks 20μm. 8 In summary, we have developed a novel transfer printing technique—known as nano-TP—that enables precise positioning of NW lasers onto heterogeneous substrates. We can also use our approach to organize the NW lasers into bespoke micrometric patterns. Notably, the NWs retain their lasing emission even after the TP protocols have been completed. Our enabling technology thus opens up potential new routes for accurate integration of NW lasers onto functional nanophotonic systems. Our future plans involve the use of nano-TP to incorporate NW lasers into waveguiding technological platforms, and to thus develop reduced footprint nanophotonic integrated circuits for applications in information and communication technologies (e.g., integrated nanoscale light sources in on-chip communications/computing systems) and healthcare (e.g., nanolaser integrated sensing modules). Financial support for this work was provided by the University of Strathclyde (through the Strathclyde's Chancellor Fellowship Program), the Australian Research Council, and the UK's Engineering and Physical Sciences Research Council (grant EP/I029141/1). We thank the Australian National Fabrication Facility, ACT Node, for access to the growth facilities used in this work. University of Strathclyde Antonio Hurtado has more than 10 years of research experience in photonics in the UK, US, and Spain. He was awarded two prestigious Marie Curie Fellowships by the European Commission and a Strathclyde Chancellor's Fellowship, after which he was appointed as lecturer in the University of Strathclyde's Institute of Photonics. 4. P. J. Pauzauskie, A. Radenovic, E. Trepagnier, H. Shroff, P. Yang, J. Liphardt, Optical trapping and integration of semiconductor nanowire assemblies in water, Nat. Mater. 5, p. 97-101, 2006. 5. S. Jin, D. Whang, M. C. McAlpine, R. S. Friedman, Y. Wu, C. M. Lieber, Scalable interconnection and integration of nanowire devices without registration, Nano Lett. 4, p. 915-919, 2004. 6. H. Xu, A. Hurtado, J. B. Wright, C. Li, S. Liu, J. J. Figiel, T.-S. Luk, et al., Polarization control in GaN nanowire lasers, Opt. Express 22, p. 19198-19203, 2014. 7. Z. Fan, J. C. Ho, Z. A. Jacobson, R. Yerushalmi, R. L. Alley, H. Razavi, A. Javey, Wafer-scale assembly of highly ordered semiconductor nanowire arrays by contact printing, Nano Lett. 8, p. 20-25, 2008. 8. B. Guilhabert, A. Hurtado, D. Jevtics, Q. Gao, H. H. Tan, C. Jagadish, M. D. Dawson, Transfer printing of semiconductor nanowires with lasing emission for controllable nanophotonic device fabrication, ACS Nano 10, p. 3951-3958, 2016. 9. A. Hurtado, B. J. E. Guilhabert, M. J. Strain, N. Laurand, C. Jagadish, M. D. Dawson, Nanoscale transfer printing for heterogeneous device integration. Presented at SPIE Photonics West 2017. 10. M. A. Meitl, Z.-T. Zhu, V. Kumar, K. J. Lee, X. Feng, Y. Y. Huang, I. Adesida, R. G. Nuzzo, J. A. Rogers, Transfer printing by kinetic control of adhesion to an elastomeric stamp, Nat. Mater. 5, p. 33-38, 2006. 11. X. Sheng, C. Robert, S. Wang, G. Pakeltis, B. Corbett, J. A. Rogers, Transfer printing of fully formed thin-film microscale GaAs lasers on silicon with a thermally conductive interface material, Laser Photon. Rev. 9, p. L17-L22, 2015. 12. A. J. Trindade, B. Guilhabert, E. Y. Xie, R. Ferreira, J. J. D. McKendry, D. Zhu, N. Laurand, et al., Heterogeneous integration of gallium nitride light-emitting diodes on diamond and silica by transfer printing, Opt. Express 23, p. 9329-9338, 2015. 13. Q. Gao, D. Saxena, F. Wang, L. Fu, S. Mokkapati, Y. Guo, L. Li, et al., Selective-area epitaxy of pure wurtzite InP nanowires: high quantum efficiency and room-temperature lasing, Nano Lett. 14, p. 5206-5211, 2014.
News Article | December 7, 2016
Scientists at The Australian National University (ANU) have designed a nano crystal around 500 times smaller than a human hair that turns darkness into visible light and can be used to create light-weight night-vision glasses. Professor Dragomir Neshev from ANU said the new night-vision glasses could replace the cumbersome and bulky night-vision binoculars currently in use. "The nano crystals are so small they could be fitted as an ultra-thin film to normal eye glasses to enable night vision," said Professor Neshev from the Nonlinear Physics Centre within the ANU Research School of Physics and Engineering. "This tiny device could have other exciting uses including in anti-counterfeit devices in bank notes, imaging cells for medical applications and holograms." Co-researcher Dr Mohsen Rahmani said the ANU team's achievement was a big milestone in the field of nanophotonics, which involves the study of behaviour of light and interaction of objects with light at the nano-scale. "These semi-conductor nano-crystals can transfer the highest intensity of light and engineer complex light beams that could be used with a laser to project a holographic image in modern displays," said Dr Rahmani, a recipient of the Australian Research Council (ARC) Discovery Early Career Researcher Award based at the ANU Research School of Physics and Engineering. PhD student Maria del Rocio Camacho-Morales said the team built the device on glass so that light can pass through, which was critical for optical displays. "This is the first time anyone has been able to achieve this feat, because growing a nano semi-conductor on a transparent material is very difficult," said Ms Camacho-Morales from the Nonlinear Physics Centre at ANU. The innovation builds on more than 15 years of research supported by the ARC through CUDOS, a Centre of Excellence, and the Australian National Fabrication Facility. The research is published in Nano Letters and is being presented by Dr Rahmani at the Australian Institute of Physics Congress in Brisbane this week: http://pubs. The paper and related images are available via this Dropbox link: https:/ Watch a video interview with the researchers on the ANU YouTube channel: https:/
News Article | December 7, 2016
Abstract: Scientists at The Australian National University (ANU) have designed a nano crystal around 500 times smaller than a human hair that turns darkness into visible light and can be used to create light-weight night-vision glasses. Professor Dragomir Neshev from ANU said the new night-vision glasses could replace the cumbersome and bulky night-vision binoculars currently in use. "The nano crystals are so small they could be fitted as an ultra-thin film to normal eye glasses to enable night vision," said Professor Neshev from the Nonlinear Physics Centre within the ANU Research School of Physics and Engineering. "This tiny device could have other exciting uses including in anti-counterfeit devices in bank notes, imaging cells for medical applications and holograms." Co-researcher Dr Mohsen Rahmani said the ANU team's achievement was a big milestone in the field of nanophotonics, which involves the study of behaviour of light and interaction of objects with light at the nano-scale. "These semi-conductor nano-crystals can transfer the highest intensity of light and engineer complex light beams that could be used with a laser to project a holographic image in modern displays," said Dr Rahmani, a recipient of the Australian Research Council (ARC) Discovery Early Career Researcher Award based at the ANU Research School of Physics and Engineering. PhD student Maria del Rocio Camacho-Morales said the team built the device on glass so that light can pass through, which was critical for optical displays. "This is the first time anyone has been able to achieve this feat, because growing a nano semi-conductor on a transparent material is very difficult," said Ms Camacho-Morales from the Nonlinear Physics Centre at ANU. ### The innovation builds on more than 15 years of research supported by the ARC through CUDOS, a Centre of Excellence, and the Australian National Fabrication Facility. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.
Karouta F.,Australian National Fabrication Facility
Journal of Physics D: Applied Physics | Year: 2014
In this paper, general aspects of the reactive ion etching (RIE) technique will be described, such as anisotropy, loading effect, lag effect, RIE chemistries and micro-masking, followed by a brief overview of etching dielectrics (SiOx, SiNx) and crystalline Si. The second section of the paper is dedicated to etching III-V compound semiconductors where, based on RIE results of GaN material, a simple and practical thermodynamic approach is exposed, explaining the criteria for selecting the best chemistry for etching a specific material and explaining the GaN etching results. Finally, a comprehensive study of etching InP-based materials using various chemistries will be discussed, as well as their various photonic applications. © 2014 IOP Publishing Ltd.
Malinauskas M.,Vilnius University |
Farsari M.,IESL FORTH |
Piskarskas A.,Vilnius University |
Juodkazis S.,Australian National Fabrication Facility |
Juodkazis S.,Vilnius University
Physics Reports | Year: 2013
Research into the three-dimensional nanostructuring of photopolymers by ultrashort laser pulses has seen immense growth over the last decade. In this paper, we review the basic principles and the most important developments and applications of this technology. We discuss the mechanisms the linear and nonlinear light absorption at tight focusing conditions, and we present some typical laser writing conditions with numerical examples. The photochemistry of traditional and novel photopolymers together with strategies for their photosensitization for laser structuring by ultra-short pulses are discussed. We also discuss current and potential future applications in diverse fields such as metamaterials, plasmonics, micro-optics, and biomedical devices and implants. © 2013 Elsevier B.V.
News Article | August 25, 2016
Photodetectors are used to convert light into an electrical signal. They are at the heart of night-vision goggles,1 are the eyes of autonomous farming robots,2 and are used as sensors in applications ranging from medical diagnostic tools to consumer electronics. Although photodetectors are available for almost any desirable visible or IR wavelength, the sensitivity of these devices is limited, which makes it difficult to detect faint signals. One limitation of the signal-to-noise ratio comes from thermal excitation of electrons.3 This noise source can be reduced by decreasing the thickness of the photodetector, but this also reduces the absorption (i.e., the signal) and prevents net gains in the signal-to-noise ratio. Research in nanophotonics has shown that light–matter interactions can be dramatically altered by carefully sculpting the nanoscale structure of a medium. For example, it has been shown that light can be absorbed very strongly in metamaterials composed of ultrathin, finely structured layers of metals and semiconductors.4, 5 The metallic constituents, however, lead to parasitic losses. These losses reduce the generated electric current (and the signal) and are thus incompatible with standard fabrication techniques, particularly at visible wavelengths. In our work, as part of a collaboration of Australian researchers, we have been examining how strong light interactions—similar to those observed in metamaterials—can be engineered with relatively simple and easy to fabricate structures, and without the use of metals. In our structures, we etched narrow grooves into the absorbing (top) layer. This means that the vertically incoming light is directed sideways, so that the light travels through the layer over a long path length and has a greater chance of being absorbed (Figure 1). In this way, we recently demonstrated near-perfect absorption (meaning 100% of the incident light is absorbed, with zero reflection or transmission) in a semiconductor layer that is only 0.041μm thick (less than 1/15th the wavelength of the light).6 The quintessential property of diffraction gratings is that they give rise to diffracted orders of light. In other words, if light of wavelength λ is incident on a grating with period d at angle ϑ, then light is reflected at angles of −ϑ+mλ/d (where m is an integer). In our perfect absorber structures, we use a diffraction grating as both the absorbing medium and the coupling element. The grating diffracts light at approximately 90°, causing it to travel parallel to the surface in either a waveguide mode (if the real part of the material's dielectric constant, ∊, is more than zero) or as a surface plasmon polariton (if the real part of ∊ is less than 0), so that the absorption can be significant. To achieve maximal absorption, the thickness, period, and volume fraction of the grating must be carefully designed so that the incident light is ‘critically coupled’ to the leaky (almost bound) mode of the layer (i.e., the rate at which light is absorbed must exactly match the rate at which it is incident). We fabricated a 41nm-thick antimony sulfide grating structure that we placed a precise distance (about a quarter of a wavelength) above a metallic reflector. The minimum feature size is close to 100nm even when it is used to target visible wavelengths around 600nm. It is thus possible to fabricate this grating using the standard technique of electron beam lithography. Using a confocal microscope, we measured a peak absorptance of 99.3% at a wavelength of 591nm, which is in excellent agreement with theory (Figure 2). A planar reference sample absorbs 7.7% at this wavelength. We also infer that the absorption within the grating is 98.7%, with only 0.6% absorbed by the silver mirror. Although we used only one specific material in our experimental demonstration, our analytic and numerical results (obtained by numerically solving the Maxwell equations with a combination of finite element and modal methods)7 show that perfect absorption can be achieved with a very wide range of commonplace semiconductors. There is no minimum intrinsic loss of the material, therefore perfect absorption can always be achieved (although the bandwidth of strong absorption becomes smaller as the material's intrinsic loss decreases). Another advantage of our approach is that it is straightforward to vary the absorption peak wavelength by adjusting the thickness of the top layer and the spacing of the rulings. In addition, the underlying physical principle is simple, robust, and widely applicable. In summary, we have shown that carving grooves into a weakly absorbing semiconductor can increase its absorption to nearly 100% if the grooves are correctly proportioned. This suggests that the sensitivity of photodetectors of any target wavelength can be increased. This is particularly important in the IR part of the spectrum (e.g., as used for night vision), where detectors currently require cooling to reduce noise. Our work also opens up the possibility of using new, affordable materials. Our next steps are to focus on the semiconductors used ubiquitously in the optoelectronics industry, which will allow us to rapidly realize applications of our structures. This work was conducted as part of a collaboration of Australian researchers at the University of Sydney, the Australian National University, National Computational Infrastructure, and the University of Technology Sydney. The authors gratefully acknowledge funding from the Australian Renewable Energy Agency and the Australian Research Council. Computational resources were provided by the National Computational Infrastructure and the NeCTAR Research Cloud, Australia, while experimental facilities were provided by the Australian National Fabrication Facility and Centre for Advanced Microscopy at the Australian National University.
News Article | April 5, 2016
Researchers from The University of Queensland have, for the first time, used laser light to cool a special form of quantum liquid, called a superfluid. Lasers are widely used to cool gases and solid objects, but have never before been applied to cool a quantum liquid. The findings were released April 5, 2016, in the journal Nature Physics. Superfluids are quantum liquids with a strange property—much like electrical currents in superconductors, the flow of a superfluid never stops. Lead author Glen Harris, who is now building on these experiments with a team at Yale University, said this unique property was a key feature for many of the proposed applications of superfluids. “The applications of this research range from improved sensors for navigational systems to the development of quantum devices and fundamental exploration of the quantum physics of turbulence, or the turbulent motion of quantum fluids when cooled to temperatures close to absolute zero,” Harris said. In the experiments, the team created a superfluid helium film on a silicon chip. They then used a bright laser beam to draw energy out of waves on the surface of the superfluid, cooling them. In addition to laser cooling, the research team showed that combining superfluid with microphotonics allows extremely precise measurements of superfluid waves. The project’s chief investigator Professor Warwick Bowen said that this research provides a pathway towards replacing state-of-the-art inertial systems used in navigation systems. “Previous experiments have shown that ultra-precise inertial sensing is possible using superfluid helium. However, these experiments relied upon bulky architectures somewhat akin to a plumbing system for water,” Bowen said. "The ability to cool, measure, and control superfluid waves on a silicon chip brings a new level of scalability and integrability to such sensors," Bowen said. The experiments were performed by an international team of researchers from Australia, New Zealand, France, Belarus and Ireland. They would not have been possible without micro- and nanofabrication infrastructure within Australian National Fabrication Facility (ANFF) supported by the National Collaborative Research Infrastructure Strategy (NCRIS). The project was funded by the ARC Centre of Excellence for Engineered Quantum Systems.
News Article | April 5, 2016
Lasers are widely used to cool gases and solid objects, but have never before been applied to cool a quantum liquid. The findings were released today in the journal Nature Physics. Superfluids are quantum liquids with a strange property - much like electrical currents in superconductors, the flow of a superfluid never stops. Lead author Dr Glen Harris, who is now building on these experiments with a team at Yale University, said this unique property was a key feature for many of the proposed applications of superfluids. "The applications of this research range from improved sensors for navigational systems to the development of quantum devices and fundamental exploration of the quantum physics of turbulence, or the turbulent motion of quantum fluids when cooled to temperatures close to absolute zero," Dr Harris said. In the experiments, the team created a superfluid helium film on a silicon chip. They then used a bright laser beam to draw energy out of waves on the surface of the superfluid, cooling them. In addition to laser cooling, the research team showed that combining superfluid with microphotonics allows extremely precise measurements of superfluid waves. The project's chief investigator Professor Warwick Bowen said that this research provides a pathway towards replacing state-of-the-art inertial systems used in navigation systems. "Previous experiments have shown that ultra-precise inertial sensing is possible using superfluid helium. However, these experiments relied upon bulky architectures somewhat akin to a plumbing system for water," Professor Bowen said. "The ability to cool, measure, and control superfluid waves on a silicon chip brings a new level of scalability and integrability to such sensors," Professor Bowen said. The experiments were performed by an international team of researchers from Australia, New Zealand, France, Belarus and Ireland. They would not have been possible without micro- and nanofabrication infrastructure within Australian National Fabrication Facility (ANFF) supported by the National Collaborative Research Infrastructure Strategy (NCRIS). Explore further: Superfluids: Observation of 'second sound' in a quantum gas More information: G. I. Harris et al. Laser cooling and control of excitations in superfluid helium, Nature Physics (2016). DOI: 10.1038/nphys3714
News Article | April 25, 2016
The Australian Institute for Nanoscale Science and Technology (AINST) has been officially opened in Sydney. The new AUS$150 million Sydney Nanoscience Hub will reportedly be most advanced facility for nanoscience in the region, where design, fabrication and testing of devices can occur under one roof. The award-winning Sydney Nanoscience Hub was co-funded with AUS$40 million from the federal government, includes teaching spaces alongside publicly available core research facilities that will support fundamental research as well as the work of start-ups and established industry. The Institute hosts some of the capabilities of the Australian National Fabrication Facility and of the Australian Microscopy and Microanalysis Research Facility – both co-funded by the National Collaborative Research Infrastructure Strategy (NCRIS). Researchers at the Institute contribute to two Australian Council Centres of Excellence: CUDOS, the Centre for Ultrahigh bandwidth Devices for Optical Systems; and EQuS, the Centre for Engineered Quantum Systems. ‘The Australian Institute for Nanoscale Science and Technology continues the University of Sydney’s tradition in addressing multidisciplinary issues in a unique way to ensure that we are ready to solve the great challenges of science, engineering and beyond,’ said vice-chancellor Dr Michael Spence. This story is adapted from material from AINST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier.