Singapore - Photonics: Graphene’s flexible future

Plots showing that surface plasmons are more confined when propagating along on a monolayer of graphene (G) than they are along a thin film of gold (Au).

Theoretical calculations show graphene’s potential for controlling nanoscale light propagation on a chip

Semiconductors have revolutionized computing because of their efficient control over the flow of electrical currents on a single chip, which has led to devices such as the transistor. Working towards a similar tunable functionality for light, researchers from the A*STAR Institute of High Performance Computing (IHPC), Singapore, have shown how graphene could be used to control light at the nanometer scale, advancing the concept of photonic circuits on chips1.

Graphene, which is made from a single layer of carbon atoms, has excellent electronic properties; some of these are also useful in photonic applications. Usually, only metals are able to confine light to the order of a few nanometers, which is much smaller than the wavelength of the light. At the surface of metals, collective oscillations of electrons, so-called ‘surface plasmons’, act as powerful antennae that confine light to very small spaces. Graphene, with its high electrical conductivity, shows similar behavior to metals so can also be used for plasmon-based applications, explains Choon How Gan of IHPC, who led the research.

Gan and co-workers studied theoretically and computationally how surface plasmons travel along sheets of graphene. Even though graphene is a poorer conductor than a metal, so plasmon propagation losses are higher, it has several key advantages, says team member Hong Son Chu. “The key advantage that makes graphene an excellent platform for plasmonic devices is its large tunability that cannot be seen in the usual noble metals,” he explains. “This tunability can be achieved in different ways, using electric or magnetic fields, optical triggers and temperature.”

The team’s calculations indicated that surface plasmons propagating along a sheet of graphene would be much more confined to a small space than they would traveling along a gold surface (see image). However, the team also showed that surface plasmons would travel far better between two sheets of graphene brought into close contact. Furthermore, by adjusting design parameters such as the separation between the sheets, as well as their electrical conductivity, much better control over surface plasmon properties is possible.

In the future, Gan and his co-workers plan to investigate these properties for applications. “We will explore the potential of graphene plasmonic devices also for the terahertz and mid-infrared regime,” he explains. “In this spectral range, graphene plasmonic structures could be promising for applications such as molecular sensing, as photodetectors, or for optical devices that can switch and modulate light.”

The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing

References

1. Gan, C. H., Chu, H. S. & Li, E. P. Synthesis of highly confined surface plasmon modes with doped graphene sheets in the midinfrared and terahertz frequencies. Physical Review B 85, 125431 (2012). |  article

Singapore – Photonics - Think thin, think vibrant

Schematic of the tunable color filter. The combination of a gold film with ring-shaped holes and the use of liquid crystals (red and green) enables pixels of a defined color that can be turned on and off.

A thin liquid crystal film on gold sheets makes an ultra-compact color filter

Flat panel displays, mobile phones and many digital devices require thin, efficient and low-cost light-emitters for applications. The pixels that make up the different colors on the display are typically wired to complex electronic circuits that control their operation. Jing Hua Teng at the A*STAR Institute of Materials Research and Engineering and co-workers have now developed a display technology that requires a much simpler architecture for operation. They demonstrated that combining a thin perforated gold film with a liquid crystal layer is all that it takes to make an efficient color filter1.

“Our color filters are a lot thinner and more compact than conventional thin-film-based color filters,” says Teng. “The colors of these filters can be tuned with ease so they are very versatile in applications.”

The color selection of the devices comes from the patterned gold film. The collective motions of the electrons on the film surface — the so-called surface plasmons — absorb light at wavelengths that depend on the details of these patterns. In the present case, the patterns are narrow, nanometer-sized rings cut out of the films (see image). As the diameter of the rings changes, so does the color of the metal film. Pixels of a different color can be realized simply by patterning rings of different sizes across the same gold film.

To realize a full display, however, each of these pixels needs to be turned on and off individually. This is where liquid crystals come in.

Liquid crystals are molecules that can be switched between two different states by external stimuli, such as ultraviolet light. In their normal state the crystals let visible light pass through so that the pixel is turned on. But when ultraviolet is also present, the structure of the liquid crystal molecules will change so that it absorbs visible light (i.e. the pixel is turned off). This process can be repeated over many cycles without degrading the device itself.

Although the device works in principle, it remains a concept on the drawing board for now. This is because there are still many issues that need to be overcome, for example, the optimization of the switching speed and the contrast between ‘on’ and ‘off’ states. In future work, the researchers will need to extend their ideas so that their device can serve a larger area and produce the fundamental colors red, green and blue.

Teng and his team are quite optimistic that they will achieve this soon.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering

References

1. Liu, Y. J., Si, G. Y., Leong, E. S. P., Xiang, N., Danner, A. J. & Teng, J. H. Light-driven plasmonic color filters by overlaying photoresponsive liquid crystals on gold annular aperture arrays. Advanced Materials 24, OP131–OP135 (2012). | article

Singapore - Photonics: Sensing with holes

SPP sensing. Nanohole films can be used in two different configurations to sense molecules in a water solution. In the reflection mode (top), light is directed at the sample from the water side. In the transmission mode (bottom), light is directed at the sample from the back, leading to different SPP properties. The SPP field intensity is represented by the color plot. The optical fields on the top and bottom are calculated for different resonance frequencies.

Molecular sensors based on nanoholes in metallic films are shown to be ideal for medical diagnosis

The detection of small quantities of molecules is important for a myriad of applications, ranging from gas sensing to biomedical diagnostics. The majority of these applications require the sensors to be cheap and disposable, yet sensitive enough to detect molecules down to the single-molecule level. Ping Bai and co-workers at the A*STAR Institute of High Performance Computing and the Institute of Materials Research and Engineering have now studied the properties of thin metallic films with holes in them that are particularly promising for molecular sensing1.

Metallic thin films with nanometer-sized holes in them are known to transmit light of particular wavelengths very efficiently. The efficiency arises from surface plasmon polaritons (SPPs) — the collective movements of electrons on the metal surface — which are able to focus light into tiny spots much smaller than the wavelength of light used (see image).

These SPPs can be used to detect the molecules through the fluorescence of tracer molecules attached to them. This fluorescence is also strongly enhanced by the SPP and can easily be detected by a microscope even for small quantities of molecules. “The whole setup is ultra-compact to support a point-of-care sensing system,” explains Bai.

Bai and his colleagues studied two sensing arrangements. In the first arrangement, light is directed at a film with nanoholes at an oblique angle from the same side as the sample. In the second arrangement, the film is illuminated from the back so that light is travelling through the holes first. The researchers found that each scheme has its own advantages.

In the ‘reflection’ scheme, the SPP effect is stronger as the light is directly aimed at the sample and does not have to cross the metal film. However, a thicker film is needed so that the light does not pass through. In the ‘transmission’ scheme, the intensity of the light emitted by the molecules is weaker, but the advantage there is that filters and other sensors can possibly be included with the metal film, and the film thickness can be much thinner.

“There is therefore no clear advantage for either sensing modes of such films,” says Bai. “One thing that is clear from the study, however, is the clear benefits of using metal films with nanoholes as a molecular sensing platform,” says Bai.

“This is merely a snapshot of our whole project. Ultimately, our sensing technology will be utilized in hospitals and test centers, for example, in prostate cancer screening, or even used at home just like glucose test kits,” adds Bai.

The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing and the Institute of Materials Research and Engineering

References

1. Wu, L., Bai, P., Zhou, X. & Li, E. P. Reflection and transmission modes in nanohole-array-based plasmonic sensors. IEEE Photonics Journal 4, 26–33 (2012). | article

Singapore - Photonics: The smaller the better

A variety of nanoplasmonic waveguides of complex shapes

Waveguides that combine metallic and semiconductor structures can be made more compact

Increasing the areal density at which electronic components can be integrated onto a computer chip has always been key to the revolution of technological applications. However, achieving the same feat in the world of optics has been proven difficult as light waves cannot be compressed to sizes below their wavelength by conventional semiconductor-based opticalwaveguides.

Metallic structures, in theory, are able to provide such functionality through so-called plasmonic effects. In practice, however, the large optical losses have hampered the implementation of such schemes. Combining the benefits of conventional optics with plasmonics, Shiyang Zhu and co-workers at the A*STAR Institute of Microelectronics have now demonstrated how structures made of semiconductor and metals represent a more viable approach to effectively miniaturize optical circuits1.

Plasmonic effects are based on motions of electrons at the surface of metals that act like an antenna on incoming light. They can be very effective to squeeze light into small volumes, although transport losses when guiding light along such small volumes are much higher than for conventional semiconductor waveguides.

Zhu and colleagues observed waveguides based on semiconductor silicon. First, ridges are etched out of silicon chip to form the basis for the waveguide architecture. The surface of the silicon is then oxidized to provide electrical insulation of the silicon before it is covered in a thin copper layer (see image).

This architecture has the benefit of very efficiently squeezing light into the waveguide via the surrounding copper layer, but travels mostly along the core made of silicon and not the metal. Silicon is transparent for light at telecommunications frequencies and thus shows low losses. ”These waveguide structures are not only compatible with the fabrication processes of silicon computer chips,” says Zhu. “More importantly, the use of silicon and silicon oxide and related semiconductors enables further possibilities to potentially achieve other effects, such as light amplification, and control over the plasmon properties.”

Having previously shown that such waveguides are able to guide light efficiently, the researchers have now demonstrated a number of complex photonic structures, including the splitting of light beams at multiple junctions, the propagation of light across multiple kinks and steps, resonator structures that show light interference effects and many more.

“This is only a first step towards the varied and complex effects possible with these structures,” says Zhu. “The next step is to demonstrate some of the active functionality, especially to combine waveguides with ultracompact plasmonic light modulators based on related designs for complete functional nanoplasmonic circuits.”

The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics

References

1. Zhu, S., Lo, G. Q. & Kwong, D. L. Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO₂-Si-SiO₂-Cu nanoplasmonic waveguides. Optics Express 20, 5867–5881 (2012). | article 

Singapore - Photonics: Beam me up

‘Tractor beams’ of light that pull objects towards them are no longer science fiction

Tractor beams are a well-known concept in science fiction. These rays of light are often shown pulling objects towards an observer, seemingly violating the laws of physics, and of course, such beams have yet to be realised in the real world. Haifeng Wang at the A*STAR Data Storage Institute and co-workers have now demonstrated how a tractor beam can in fact be realized on a small scale1. “Our work demonstrates a tractor beam based only on a single laser to pull or push an object of interest toward the light source,” says Wang.

Based on pioneering work by Albert Einstein and Max Planck more than a hundred years ago, it is known that light carries momentum that pushes objects away. In addition, the intensity that varies across a laser beam can be used to push objects sideways, and for example can be used to move cells in biotechnology applications. Pulling an object towards an observer, however, has so far proven to be elusive. In 2011, researchers theoretically demonstrated a mechanism where light movement can be controlled using two opposing light beams — though technically, this differs from the idea behind a tractor beam.

Wang and co-workers have now studied the properties of lasers with a particular type of distribution of light intensity across the beam, or so-called Bessel beams. Usually, if a laser beam hits a small particle in its path, the light is scattered backwards, which in turn pushes the particle forward. What Wang and co-workers have now shown theoretically for Bessel beams is that for particles that are sufficiently small, the light scatters off the particle in a forward direction, meaning that the particle itself is pulled backwards towards the observer. In other words, the behaviour of the particle is the direct opposite of the usual scenario. The size of the tractor beam force depends on parameters such as the electrical and magnetic properties of the particles.

Although the forces are not very large, such tractor beams do have real applications, says Wang. “These beams are not very likely to pull a human or a car, as this would require a huge laser intensity that may damage the object,” says Wang. “However, they could manipulate biological cells because the force needed for these doesn’t have to be large.”

Such applications are the driving force for future experimental demonstrations of such pulling effects. The technology could, for example, be used to gauge the tensile strength of cells, which would be useful to investigate whether cells have been infected. “For instance, the malaria-infected blood cell is more rigid, and this technology would be an easy-to-use tool to measure this,” adds Wang.

The A*STAR-affiliated researchers contributing to this research are from the Data Storage Institute

References

1. Novitsky, A., Qiu, C.-W. & Wang, H. Single gradientless light beam drags particles as tractor beams. Physics Review Letters 107, 203601 (2011). | article

Singapore - Photonics: strong vibrations

Terahertz (THz) generation. A strong THz emission from the center of the device is observed in the tip-to-tip design (top). The electrodes are the black lines in the center of the device. The colours show the electric field from low (blue) to high (red) values. Much weaker electric fields and THz emission are seen in the interdigitated electrode design (bottom).

From Ref. 1 © 2012 H. Tanoto

A new approach to generating terahertz radiation will lead to new imaging and sensing applications

Terahertz (THz) electromagnetic radiation has promising properties for a wide range of applications. The low energy of the radiation means that it can pass through materials that are otherwise opaque, opening up uses in imaging and sensing — for example, in new security scanners. In practice, however, applications have been difficult to implement. Terahertz radiation is a difficult portion of the electromagnetic spectrum to utilize. The frequencies of the region are higher than the mega and gigahertz frequencies achievable with conventional electronic circuits, but are too low-frequency to be compatible with optical instruments.

“The key challenges for THz technology are the development of a compact high power source and high sensitivity detector operating at room temperature,” explains Jinghua Teng of the A*STAR Institute of Materials Research and Engineering. A recent discovery made by Teng’s team of a new, efficient protocol for THz wave generation that utilizes the enhancement of light between nanometer-scale electrical contacts may provide a solution.1

One method for creating continuous THz radiation involves directing two optical laser beams of almost similar frequencies at a suitable nonlinear material, such as certain semiconductors causing light emission exactly at the frequency difference of the two laser beams. If this difference is sufficiently small, the radiation produced falls within the THz spectrum.

However, this process is rather inefficient and requires strong light fields. Fortunately, strong amplification of light can occur near small metallic objects that act as mini antennas. This antenna effect occurs with the small metal contacts that are needed to link the non-linear material that creates the THz emission — in the current case a variant of the common semiconductor gallium arsenide.

Normally, these electrical contacts are arranged such that they resemble the fingers of interlocked hands reaching into each other. However, the A*STAR researchers developed a revised design in which the electrodes are arranged tip to tip (see top of the above image). This means that the gap between the electrodes is much narrower and also results in the alignment of the electrical field with the THz light waves, which leads to a considerably stronger antenna enhancement.

Using the new arrangement the A*STAR team were able to generate THz radiation of about 100 times the strength of that produced by conventional systems. The work suggests that these devices can be miniaturized significantly for compact yet powerful THz sources. “This approach will greatly facilitate the applications of THz technology in areas such as gas sensing, non-destructive inspection and testing, high resolution spectroscopy, product quality monitoring and bio-imaging,” says Teng.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering

References

1. Tanoto, H. et al. Greatly enhanced continuous-wave terahertz emission by nano-electrodes in a photoconductive photomixer. Nature Photonics 6, 121–126 (2012). | article

Singapore - Photonics: Getting a fair compensation

Compensation doping can improve the efficiency of silicon optical modulators

Silicon is widely used in electronics devices, such as computer chips and solar cells. It is also becoming the material of choice for making photonic devices that lie at the heart of communications, including light-emitting diodes, photodetectors and optical modulators.

One of the drawbacks of silicon photonic devices is 'insertion loss' — the loss of optical signals when these devices are integrated into the optical network. Silicon optical modulators, for example, may have comparable switching speed and modulation efficiency as optical modulators made of other materials such as lithium niobate, but their insertion loss can be on the double. Improving the optical performance of silicon modulators is highly desirable as these devices are compatible with complementary metal–oxide–semiconductor (CMOS) technology that is widely used in today’s electronic devices.

Xiaoguang Tu and co-workers at the A*STAR Institute of Microelectronics have now demonstrated how to improve silicon modulators by using an appropriate way of doping the silicon with electrons and holes1.

Doping can provide active modulators with extra electrons and holes. The process normally involves implanting acceptor and donor impurities in the main component of the modulator — the silicon waveguide. Unfortunately, the extra carriers produce a reduction in efficiency due to their light absorption. The loss efficiency of silicon modulators is typically 20% worse than that of lithium niobate modulators. There are several possible routes to minimize the loss efficiency, but they all tend to degrade the devices in one way or another. 

Tu and his team overcame the problem using an approach called compensation doping (see image). In this approach, the central area of the silicon waveguide is highly doped as usual, so that the electrons and holes remain on opposite sides of the central plane. Moving away from the centre, however, the doping is reduced so that the total number of carriers and the light they absorb is compensated.

The researchers monitored several characteristics of the devices while varying the profile of the non-compensated region on the cross-section of the modulator. They found that in the best-case scenario, the loss efficiency of silicon modulators was comparable to that of lithium niobate modulators without affecting the modulation efficiency or the shifting speed.

“With these improvements, silicon modulators may become a main competitor of lithium niobate modulators currently on the market,” says Tu. “These modulators may also be the perfect candidate for future integrated photonics and electronics circuits.” Tu and his team are now working towards improving the performance of silicon modulators further by exploring new structure designs and doping profiles.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics

References

1.     Tu, X., Liow, T.-Y., Song, J., Yu, M. & Lo, G. Q. Fabrication of low loss and high speed silicon optical modulator using doping compensation method. Optics Letters 19, 18029–18035 (2012). | article