Automatically assembling nanolenses

Dr Julian Lloyd has devised a scalable method to produce trimers capable of focusing light. Credit: Julian Lloyd

A team of researchers has created a scalable production method for a class of nanoscale lenses using the electrostatic forces between charged nanoparticles, enabling wider application of the technology.

These nanolenses use the gaps between triplets – or trimers – of different sized nanoparticles to focus light.

They are showing enormous promise in a range of optical and sensing applications due to their minuscule size and very high-yield enhancement. However, an expensive and laborious production process has limited their widespread uptake.

Dr Julian Lloyd, lead author of the research, and his colleagues’ method uses induced electrostatic forces to self-assemble the gold nanoparticle triplets. The process was developed at Monash University, Monash Centre for Electron Microscopy and the Melbourne Centre for Nanofabrication, ANFF VIC’s flagship facility and ANFF’s headquarters.

The method enables production of these trimers with a more than 60 percent yield, quantities that have never been achieved before, and removes the need for expensive “top-down” fabrication techniques traditionally used that inhibit the scalability of the fabrication process.

trimerAn illustration of the trimers focusing light. Credit: Soon Hock Ng

Julian’s trimers are short strings of nanoparticles 20nm, 30nm and 50nm in diameter. In his paper published in ACS Nano, Julian explains that by placing differing surface charges on the nanoparticles, the structures draw themselves into place.

Using a positively charged substrate as a base, the team add a number of negatively charged 30nm particles which neatly distribute themselves across the surface. Positively charged 20nm particles are added, each attracted to a 30nm particle, before a solution of positively charged 50nm particles is introduced. The particles align themselves because the 20 and 50nm particles repel each other, whilst both being drawn towards the centre 30nm particle.

“We can control the distance between individual trimers and also improve the trimer yield by tuning the surface charges,” Julian explained.

“As the whole assembly method relies on electrostatic interactions, it was vitally important to know the surface charges of the different components,” he continued. “The surface charge measurements were taken at the Melbourne Centre for Nanofabrication using the Centre’s Zeta sizer and a Zeta potential analyser.”

Ultrathin holograms open doors to 3D displays

Holographic image captured by illuminating the nanometric holograms using 445 nm continuous wavelaser beams. 532 and 632 nm light was also used. Credit: Zengji Yue.

A team led by Professor Min Gu from RMIT University and Beijing Institute of Technology has fabricated the world’s thinnest hologram – and it could revolutionise the way we interact with everyday technologies.

The team’s research, published in Nature Communications, takes a step closer to three-dimensional (3D) displays for smart devices by reducing physical dimensions of a hologram to the nanometer scale.

Holograms are the result of shining light on an interference pattern to recreate a seemingly 3D object within a film. Even the parallax effect is captured, meaning when the viewer moves, the image appears to move too.

An interference pattern is the result of light being split into two and travelling different paths of varying length. The additional time causes a phase difference in the light, resulting in peaks and troughs of intensity when the beams are recombined. Holograms were made conventionally made by splitting a laser beam – one direct, one detoured via an object – before hitting a film.

Since the ‘60s it’s been common practice to use computer-generated patterns – a process called computer-generated holography (CGH). CGH can in principle be applied to smart technologies but the physical size of holograms which currently ranges between micrometer to millimetres makes this currently impractical.

Holograms have to be thick enough to allow enough time for the phase difference to become noticeable.

The paper’s lead author, Dr Zengji Yue, explained that to reduce the depth the team began working with Antimony Telluride (Sb2Te3) that had been laser etched to feature the desired interference pattern.

 

(a) Original image of the dinosaur object. Note: this figure is not included under the article CC BY licence; Indominus Rex image is reproduced with permission from the publisher Comingsoon.net and copyright owner Universal Studios. (b,c) SEM images of the laser printed hologram patterns. The scale bar is 50 μm for b and 10 μm for c, respectively. (d–f) Holographic images captured by illuminating the nanometric holograms using 445, 532 and 632 nm continuous wavelaser beams. Scale bars for d–f are 1 mm. Credit: Zengji Yue.
(a) Nanofabrication process of a nanometric Sb2Te3 hologram using the direct laser write system. The upper inset displays the ablated pixels in the topological insulator thin film. (b) Holographic imaging procedure of a nanometric Sb2Te3 hologram using a continuous wavelaser beam. θ is the projection angle of the holographic image. Credit: Zengji Yue.
A diagram of internal light multiple reflections in the resonant cavity of the Sb2Te3 thin film. The semi-transparent arrows indicate interface reflections. Credit: Zengji Yue.

The new film acts as a resonance cavity – light is bounced between the surfaces, amplifying the phase difference. Sb2Te3 is rare in the respect that its refractive index at the surface is far lower than within the body of the material which helps to retain the light.

The team worked closely with one of the Melbourne Centre for Nanofabrication’s process engineers, Dr Lachlan Hyde, to use the Centre’s atomic layer deposition (ALD) capabilities as the basis to fabricate the Sb2Te3 hologram film. The Centre’s ellipsometry equipment was used to compare the sample’s refractive index to theoretical models.

The result is a 60nm thick film that produces the holographic images. Considering most of the processes are scalable, the new holograms could be produced on a large scale.  

The next step is to create smaller pixels to increase the resolution of the images, and to investigate dynamic displays.

Folding plasmene nanosheets

Self-folding plasmene nanosheets could be used for advanced identification applications Credit: Professor Wenlong Cheng

Professor Wenlong Cheng has been investigating how to make an unusual class of materials with exotic properties and unprecedented real-world applications.

Formally introducing plasmene nanosheets to the world, he and his team have created self-folding sheets of nanoparticles that can change the behaviour of light.

The nanosheets consist of a superlattice of nanoparticles, organised in a hexagonal pattern much like the arrangement of carbon atoms in the wonder-material, graphene.

Wenlong’s team creates the materials by sparsely distributing nanoparticles covered with protruding polystyrene hairs, or ligands, across the surface of a water droplet. By evaporating the water the surface area shrinks, bringing the particles closer together. Attractive van der Waals forces between the nanoparticles draw them together while repulsive forces between the ligands manoeuvre the nanoparticles into the desired hexagonal configuration.

The nanoparticle-laden water droplet is suspended over an open cavity, in this case a hole in a copper sheet, which gives the nanosheet a place to form. Once all the water has been removed, a continuous nanosheet forms neatly across this cavity like the membrane on a drum, allowing Wenlong to begin to play with it.

Using ANFF-Vic’s focused ion beam (FIB) instrument at the Melbourne Centre for Nanofabrication, the team “scores” specific patterns into the sheets, deliberately weakening them at key points. The weakened regions are akin to the paper folding lines introduced in origami, where the layout and severity of the folds define the final three-dimensional structures.

(Centre and Left) TEM image of a single free-standing soft plasmene nanosheet and zoomed-in view showing the ordered rhombic dodecahedral building blocks. (Right) SEM image of a complex 3D plasmene origami fabricated in conjunction with top down focused ion beam lithography.

This scoring process means the resultant shape’s properties can be tuned by manipulating the pattern cut into them and the order in which the folding is performed, enabling tailoring of the final product to meet specific end applications.

Researchers have investigated the properties of this self-folding before, but the designs of the well-defined origami structures of this kind have not been realised until now.

Light interacts and reflects off differing origami shapes in a way that is unique to each contour. By changing the structure, Wenlong can exploit his plasmene sheets to perform some useful tricks.
One useful application for these tiny sculptures is in nanophotonics as plasmonic waveguides for directing and switching light at the nanoscale by circumventing diffraction limits.

The plasmene nanosheets produced can also be used as dual-coded encryption security labels as it’s almost impossible to fraudulently create the shapes without the initial blueprint. This poses enormous potential benefits to the banknote industry, which is always searching for more secure methods of deterring counterfeiters.

The plasmene sheet’s semitransparency, mechanical softness and uniform response to light suggest that it could be an ideal material in the production of secure identifying labels for chemicals, currency and commodities, among others.

Twisting light for faster internet

Diagram of the polarisation rotation device. Inset: SEM image of silver nanorods on vanadium dioxide-coated silicon substrate. 

March 2015

With the advent of fibre optic networks the bandwidth of the internet increased dramatically. The bottlenecks to next-generation internet speeds are now the points at which the light carrying the signal is switched to an electronic signal.

By replacing current electronic switches with optical equivalents, these bottlenecks can be removed increasing throughput while reducing power consumption, waste, heat and maintenance costs.

A collaborative project between researchers at the University of Melbourne, MCN, CSIRO and Vanderbilt University has created a device which enables the user to rotate the reflected linear polarisation axis of light. Used in conjunction with a pair of linear polarisers this allows a passive routing of the reflected signal. The working is a phenomenon known as a localised surface plasmon (LSP), which is caused by resonant oscillations of the free electrons confined to a metallic structure smaller than the wavelength of light. An array of rods, each displaying two of these LSPs parallel to their two axes, permits altering the polarisation state of that light. By carefully setting the dimensions of the rods, it is then possible to use these LSPs in conjunction with a phase change material to rotate the reflected light’s polarisation axis. Dramatically increased optical switching is relevant for a range of optical devices from microprocessors and electronics to sensing and telecommunications.

Electron beam lithography and evaporation of ultra-thin silver films were used at MCN to create optical antennas on vanadium dioxide, a phase change material deposited by collaborators at Vanderbilt University. Atomic force microscopy and scanning electron microscopy was then used to characterise the vanadium dioxide film and image the antenna arrays. The facilities at the MCN enabled the refinement of the fabrication process and creation of the actual device.

This project has shown how a simple structure can twist the polarisation of reflected light. In principle this could be further developed such that its operation is ultra-fast. While the crystalline phase of vanadium dioxide was transitioned thermally, it has the potential to be switched optically on a sub-picosecond timescale, enabling switching orders of magnitude faster than current technology.

The device has been shown to work in the visible part of the spectrum, however moving the design to telecommunications wavelengths (near infrared, 1550 nm) will facilitate device longevity by enabling the use of gold rather than silver, as well as simple integration into current fibre optic networks. Long-term plans aim to implement a simple self-switching design using amplitude modulation of the incident signal.

Nanoscale optical circuits for light-speed information processing

An example of a nanoscale optical circuit that measures optical phase difference. a) A graphic of the circuit which consists of three gold nanorods supporting localised surface plasmon resonances. One of the nanorods outputs an optical signal proportional to the difference between the two inputs. b) An SEM image of the resist pattern on a gold film prior to etching.

c) A microscope image of a circuit when the optical phase difference across the inputs is zero. The circuit cannot be seen because there is no output for zero phase difference. d) A microscope image of a single circuit when the phase difference is 40o. The circuit is too small to resolve but the light output from it is easily observed.

Two examples of nano-plasmonic circuits based on evanescent coupling between localised surface plasmons that show the circuit diagrams and their representations as metal nanorods.

April 2014

Modern communications networks that support the internet rely on light waves to carry information. In these systems, digital signals are imposed on the light beams, which travel long distances on optical fibres.

Currently these systems rely on electronic devices to encode and decode the information. While electronics can be integrated into high-density circuits, the limited speed at which these circuits function limits the amount of data that can be encoded on the light beam. Conversely, optical circuits with much faster speeds can be created using optical waveguides integrated into silicon chips, however they cannot achieve the same density as electrical circuits.

Led by MCN Technology Fellow Dr. Tim Davis, researchers from CSIRO have been utilising the MCN facilities to develop nano-scale optical circuits which operate at exceptionally high speeds and which can be integrated into high-density circuits. These ultimately demonstrate optical information processing.

The circuits are based on configurations of gold nanorods that support localised surface plasmon resonances – electric charge oscillations that are excited by light on metal surfaces. The team fabricated an optical circuit that was barely 200nm wide which measured the phase difference of an incident light wave across its inputs.

The circuit has many applications including the demodulation of optical signals with information encoded by phase shift keying. These nanoscale circuits operate extremely quickly and are able to respond to signals thousands of times faster than conventional electronic circuits, at a rate of up to 50,000 GHz.

To achieve these extremely fast nanoscale circuits, the team utilised the Electron Beam Lithography (EBL) tool at MCN. A glass wafer was first covered in a thin layer of gold about 30nm thick, after which an electron beam sensitive resist was coated over the gold. This was then exposed by the EBL. After the resist was developed, the circuits were imaged in the scanning electron microscope to check the integrity of the fabrication. The final step was to etch the gold film in the ion-etching instrument, which removes the gold unprotected by the resist. The result is a set of metal nano-rods that form the optical circuit.

While the optical circuits are too small to be imaged in an optical microscope, it is still possible to see the light scattered from them. The team was able to show that a single circuit could output an optical signal related to the phase difference of the light wave incident at its inputs. Incredibly, the entire device is about one quarter of the wavelength of light and probes the optical phase difference within a small fraction of a wavelength. This can be likened to a nano-scale voltmeter probing the electric field of the light wave.

The measurement of optical phase can be viewed as a form of optical signal processing. It is possible to show that optical circuits based on localised surface plasmons can perform a variety of linear mathematical operations between optical signals, all at the nano-scale. This project demonstrates that optical circuits of this type are feasible and there are a large number of possible circuits that can be designed, providing a means for processing information encoded on light beams.

The next steps in the project are to develop a range of other optical circuits and combine them. The team has already shown that arrays of optical circuits can be used to create a metamateria which enables the modulation and switching of one light beam by another. They will also be creating artificial materials based on these circuits that will be used for image processing.

You can read more about this project in Measuring subwavelength phase differences with a plasmonic circuit—an example of nanoscale optical signal processing, published in Optics Info Base.

Metal nanoparticles lead the way towards solar water decontamination

Three lowest-order modes of an interacting triangle of particles. Mode a) is the highest in energy of the three and is a mode with radial symmetry. Modes b) and c) are degenerate modes which are characterised by having a net dipole moment and perpendicular polarisations. 

April 2014

It has long been known that solar energy can be used to create electric energy for heating and lighting. It can also be used however, to drive important chemical transformations such as the production of renewable chemical fuels by extracting hydrogen from water and hydrocarbons from carbon dioxide. Other uses include the sequestration of air pollutants such as greenhouse gases, low-cost water purification for developing countries, and the low carbon-footprint production of fine chemicals such as disinfectants and self–cleaning surfaces. There are countless other as yet unforeseen applications of solar to chemical energy transformations.

Existing approaches for harnessing solar energy for these applications are relatively inefficient and therefore not widespread. In order to improve the efficiency of solar-to-chemical energy conversion processes, it is important to maximise the light collection efficiency.

In this project, researchers from CSIRO led by MCN Technology Fellow Dr. Daniel Gomez, are creating novel metal nanoparticles capable of efficient light collection and focusing, in order to enhance the efficiency of light-driven chemical reactions. Utilising the Electron Beam Lithography (EBL) system, thin film deposition and characterisation tools available at MCN, the team have created networks of precisely controlled metal nanoparticles with precision down to 10nm.

Preliminary results indicate that certain nanoparticle arrangements can lead to an enhancement of light-driven decomposition of water pollutants by at least eight times the current rate of decomposition. They have achieved this by creating arrays of nanoparticles using aluminium, an abundant and inexpensive metal, which offers great potential for future deployment in large-scale applications.

The team are currently optimising the shapes and geometrical arrangements of the nanoparticles to achieve optimal rates of photochemical rates. Through this optimisation process they are addressing key fundamental questions, which will provide guidance in the development of processes for creating complex hydrocarbons from carbon dioxide.

You can read more about this project in The Dark Side of Plasmonics, published in Nano Letters.

Black silicon sensors for molecular contamination

Cages for cells on black Si. Mechanical stimulation/stress induced on cells can be tracked in time as cells are confined in the cages

July 2013

Black silicon provides a unique platform for a non-reflecting, all-direction-absorbing surface, which can be used for sensing and fingerprinting of molecular and microbial contamination. This is done by sensing the light scattered in air, water, food, body fluids by various compounds.

Detection of dyes from a family of carcenogenic compounds was made using black silicon. Nano-textured surfaces of black silicon are suitable as practical, cheap, single-use substrates for efficient measurements in water, food or medical fields. Their sensitivity of detection is superior to the commercial Klarite substrates currently used in industry.

A team of MCN engineers and Swinburne PhD students led by Professor Saulius Juodkazis perfected efficient, large-area fabrication of black silicon using plasma processing, after which they carried our different characterisation tests. These showed that black silicon substrates can be used as substrates for laser fabrication, sensors for light scattering and detection of tiny numbers of molecules. Black silicon is highly absorbant of light rays, with only 1% of light reflected from its surface, as opposed to the usual 35% of reflected light on other materials.

The most interesting outcome of this project is the demonstration that black silicon can be used as a sensing substrate. This was not initially obvious as the low reflectivity makes it a challenge to detect light scattered from black silicon. The group showed that this is not an issue for reaching high sensitivity performance. This is significant as a few millimetres of commercially available Klarite sensors cost more than a few inches of black silicon.

Single use sensors are must in the medical, food, water and air control industries. Black silicon can become a platform to develop such sensors using new, label-free and established surface functionalisation technologies.

The group plans to fabricate die-chips of black silicon for sensing and make them ready to use after coating with gold film. Black silicon substrates are promising for bio-physics research with the possibility for strongly influencing cell membrane functions with nano-needles of black silicon.

Ultra-thin optical sensors for the detection of toxic chemicals

Plasmonic nanosheets from horizontally aligned gold nanorods (H Sheet) and from vertically aligned gold nanorods (V Sheet) 

July 2013

Development of a new, ultra-thin 2D optical material enables the rapid, sensitive and inexpensive detection of toxic chemicals in air, water and soil.

Surface Enhanced Raman Scattering (SERS) is an extremely powerful technique with the potential to identify fingerprint vibrations of trace chemical species. However, the high cost, portability and reproducibility are factors which prevent SERS from widespread use - commercially available Klarite® SERS substrates cost around $40 dollars. In addition, SERS-active structures are supported on rigid glass surfaces, which limits where can Klarite® SERS substrate be used. This makes it difficult for trace chemical identification on a topographically complex surfaces such as door handles.

To combat these problems, Associate Professor Wenlong Cheng has used nanofabrication facilities at MCN to successfully fabricate ultra-thin, plasmonic nanosheets for rapid, inexpensive and sensitive detection of toxic species in air, water and solids.

Associate Professor Cheng’s group has developed simple, yet efficient wet chemical nanofabrication approaches to obtain free-standing, monolayered, highly-ordered plasmonic nanosheets. The sheets could be as thin as 2.5nm but could have macroscopic lateral dimensions, corresponding to an aspect ratio of above 1 million. Such nanosheets are high-performance SERS substrates which achieve at least 10 times more sensitivity than commercial Klarite® SERS substrate.

The unique technology developed for ultrathin 2D optical has potentially huge impacts in society. This may include smart diagnostics, better displays, and more efficient solar energy systems. The project is currently expanding the potential to translate the technology into real-world products.

You can read more about this project in Free-Standing Plasmonic-Nanorod Superlattice Sheets, published in ACS Nano.

Thinner, faster electronics and data transfer devices

A schematic illustraion of integrated graphene silicon hybrid photonic circuit

July 2013

Graphene is the new kid on the block for electronics as well as photonics. The atomically thin material is stronger than steel and more conducting than copper, making it an ideal candidate for thin, fast and robust electronics.

The unique properties of graphene allow multiple functions of signal emitting, transmitting, modulating and detection to be realised in one material. Graphene shows superior properties to silicon and III-V semiconductors in terms of its high thermal conductivity, high optical damage threshold, and high third-order optical nonlinearities. These macro-scale properties are important for photonic devices. Furthermore, the fabrication of graphene-based devices is compatible with existing semiconductor processes. The complementary role of graphene to silicon photonics inspires the design of new structures targeted to improve the figures of merit in optical modulation and photodetection, which include optical response, optical bandwidth, and area efficiency.

By taking advantage of graphene’s infinitesimal thinness and ultra-wideband electro-optical response, Dr. Qiaoliang Bao from Monash University is able to fabricate the smallest optical modulation device and the thinnest photodetector. His work will offer one of the most promising approaches to achieve the high-density electronic/photonic integration necessary for the optical interconnects required in high performance computing.

The present state of development in integrated photonics can be compared to the situation before the integrated circuit revolution more than 50 years ago. Breakthroughs similar to what silicon transistors had achieved are expected for integrated photonics. Graphene-based photonics devices will deliver much better performance as well as many new functions to fulfil the stringent demands for broadband data transferring and translating. Conventionally used silicon-based materials are not broadband optical materials, which is where graphene-based devices have so much potential.

The fabrication of graphene-based optical modulator is compatible with existing silicon-based nanofabrication processes, which is one of the core expertise at MCN. The state-of-the-art lithographical tools, variety of thin film deposition systems, advanced characterisation tools and optoelectronic measurement systems available at MCN are highly important for the success of this research.

You can read more about this work in Graphene Photonics, Plasmonics, and Broadband Optoelectronic Devices, published in ACS Nano.

Nanoscale antennas the future of telecommunications

Scanning Electron Microscopy image of a 30nm thick silver J-pole antenna fabricated on glass with a thin gold coating. Dimensions: width 30nm, long arm length 120nm, short arm length 60nm.

July 2013

Researchers from the University of Melbourne and CSIRO, including Dr Tim James, Dr. Tim Davis and Associate Professor Ann Robertts, together with MCN Senior Process Engineer, Matteo Altissimo have developed nanometer sized optical anteanna based on every-day radio frequency designs. The novel designs explored are focused enhancing radiation from a single photon emitter, critical components in secure optical telecommunications systems and novel biosensing systems.

The nano-antenna structures, specifically the J-pole optical antennas, were fabricated at the MCN using the Vistec EBPG 5000 electron beam lithography (EBL) system, with very high resolution EBL resist, which enabled the 30nm minimum feature sizes to be achieved.

The ability to fabricate optical nano-structures of such high fidelity enables Australian researchers to create nano-optical devices at world-class standards. This will enable novel technologies such as high efficiency single photon sources to be developed locally which is the aim of this project, but more broadly facilitates the research into leading edge nano optical devices.

The presented J-pole presents the first step in the project to develop a high-efficiency single photon source which is critical to enhancing biological research and the security of telecommunications systems.

The team will next look at integration of the nano-antenna structures with single-photon sources such as NV centres in nano-diamond and inorganic quantum dots, which again requires the high precision and accuracy provided by the EBL tool at the MCN.