Light to Photonics
It was only in the 17th century that Sir Isaac Newton showed that white light is made of different colours of light. At the beginning of the 20th century, Max Planck and later Albert Einstein proposed that light was a wave as well as a particle, which was a very controversial theory at the time. How can light be two completely different things at the same time? Experimentation later confirmed this duality in the nature of light. The word ‘Photonics’ appeared around 1960, when the laser was invented by Theodore Maiman. Photonics is the science and technology of generating, controlling, and detecting photons, which are particles of light. The characteristics of the waves and photons can be used to explore the universe, cure diseases, and even to solve crimes. Scientists have been studying light for hundreds of years. The colours of the rainbow are only a small part of the entire light wave range, called the electromagnetic spectrum. Photonics explores a wider variety of wavelengths, from gamma rays to radio, including X-rays, UV and infrared light. Photonics underpins technologies of daily life from smartphones to laptops to the Internet to medical instruments to lighting technology. The 21st century will depend as much on photonics as the 20th century depended on electronics. Even if we cannot see the entire electromagnetic spectrum, visible and invisible light waves are a part of our everyday life. Photonics is everywhere; in consumer electronics (barcode scanners, DVD players, remote TV control), telecommunications (internet), health (eye surgery, medical instruments), manufacturing industry (laser cutting and machining), defense and security (infrared camera, remote sensing), entertainment (holography, laser shows), etc. All around the world, scientists, engineers and technicians perform cutting edge research surrounding the field of Photonics. The science of light is also actively taught in classrooms and museums where teachers and educators share their passion for this field to young people and the general public. Photonics opens a world of unknown and far-reaching possibilities limited only by lack of imagination.
The photonics revolution is akin to the improvements seen in computers, where cell phones now have the same performance as the discrete-component supercomputers that took up entire warehouses decades ago. Silicon photonics is also likely to lead to many new applications, some of which can be imagined now. Circuits are already being developed for processing analog radio-frequency signals, particularly for the frequencies ranges that are difficult to control electrically. These are likely to yield ultra-stable oscillators, analog communication systems, or high sensitivity Terahertz imagers (like the ones currently used in airports but with improved sensitivity). It is also possible to steer light beams emitting from the chip by controlling the relative phase of the light (e.g., phased arrays), which will be particularly useful to robotics or self-driving cars. Photons can also be used to realise sensors that, when implemented with other biological or chemical technologies, can be used to detect minute changes in the environment, which will benefit fields from health care to security. And one of the ultimate goals of photonics has always been to realise an optical computer. While this still remains very far off due to limitations of photons (they do not interact strongly with each other), there are future computing technologies that photons may benefit, such as quantum computing. Light has the nice advantage that the signal speed is twice as fast as electrical signals. Imagine the benefit if we could use integrated optics to distribute clock signals in our processors. In best case scenario, we could make them twice as fast without making them smaller. The applications for integrated photonics are endless and will have direct impact on future supercomputers, improved health care, faster telecommunications, and longer lasting cell phones. As the integrated photonics efforts in the Rochester region ramp up, there will be tremendous opportunities for research, innovation, education, and commercialisation.
The U.S. National Academy of Sciences’ Harnessing Light Committee has released a landmark report discussing the current state of optical sciences and goals for the future. The NAS study, which is a follow up to the Harnessing Light report published in 1998, identifies the technological and economic opportunities the science has enabled, assesses trends in market needs, gives examples of where progress in photonics innovation has translated into economic benefits, and makes recommendations for future research and policies that are intended to advance the optics and photonics discipline. The NAS report examined the use of optics and photonics in seven content areas —communications, information processing and data storage; defense and national security; energy; health and medicine; advanced manufacturing; and strategic materials—focusing on the enabling nature of optics and photonics and their role in facilitating economic growth. The Harnessing Light committee members also made a number of specific recommendations on how to best capitalise on the opportunities optics and photonics provides.
Role in Quality of Life
Photonics is the technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. Photonics involves cutting-edge uses of lasers, optics, fiber-optics, and electro-optical devices in numerous and diverse fields of technology – alternate energy, manufacturing, health care, telecommunication, environmental monitoring, homeland security, aerospace, solid state lighting, and many others. Photonic technologies directly increase our quality of life. Photonics could bring out products in many fields related to our life.
For example, in telecom, fiber optical communication system has already advanced and upgraded the communication network; in healthcare, many low-cost and portable biomedical optical devices such as optical micro-endoscopy are being developed for application in setting-limited regions; in energy saving, novel solar cells with enhanced conversion efficiency using Photonic techniques are being researched. These are only several of the whole possibilities that Photonics could do. As we can see, photonics is really broad and has lots of applications. The applications of photonics as an “enabling” technology are extremely broad. Rapid growth in the number and complexity of photonics and photonics-enabled technologies has caused the demand for technicians to exceed supply.
Photonics is everywhere, from communication and healthcare, to materials processing for the factories of the future, to lighting and photovoltaics, and to consumer products like smart phones and other internet devices. It is sure that photonics in the 21st century will revolutionise healthcare and provide new ways of detecting, treating and preventing diseases. In manufacturing, laser processing will become a prerequisite for high-volume, environmentally-friendly and low-cost production. This is true in particular in the aerospace domain, where manufacturers are starting to make use of laser processing for machining composite materials, or for non-destructive treatment and control of these materials. Another growth area for photonics is in helping overcome the limitations of electronics in computers. Hopefully, the 21st century will offer the possibility to have the first generation of quantum computers. Lasers and other light beams are the ‘preferred carriers’ of energy and information for many applications. For example:
- Lasers are used for welding, drilling, and cutting of metals, fabrics, human tissue, and other materials.
• Coherent light beams (lasers) have a high bandwidth and can carry far more information than radio frequency and microwave signals.
• Fiber optics allow light to be ‘piped’ through cables.
• Spectral analyses of gases and solid substances provide positive identification and quantifiable concentrations.
Importance of Photonics for the Future
Our modern day electronic devices are based on transistors and other semiconductor devices. Moore’s law has proved to be nearly accurate as we develop better and smaller circuits having more number of transistors per square inch. However, there will be a time maybe after two decades (as predicted by Moore himself) where we may not be able to make any further development with the integrated circuit based devices. The future computing devices, be it quantum computers or for that matter even simple photonic integrated circuits, have already given us a new perspective to the power and versatility of computation in the future. With the ever increasing demand for faster and more efficient computing, photonics seems to be a promising candidate. Technology giants such as Intel, IBM and Google already have made huge investments in this direction. Wearable technology like the Google Glass and Microsoft’s Hololens have shown how we can use light to connect our digital world to our lives. So all we can expect is a better and a more interactive computing experience in the future.
Metamaterials are artificial, precision-engineered materials which can exhibit peculiar properties not found in natural materials, which make them interesting. Many scientists had predicted and modeled such artificial materials in electromagnetics, wave interactions and mechanics. However their true potential was realised only after they were fabricated at the end of the 20th century. One of the hot topics in metamaterial research and a simple applications to manufacture metamaterial cloaks (like the one Harry Potter had) to prevent from being sighted. As of now, there are no optical metamaterials that cover the entire visible range of the spectrum however there has been considerable progress with microwaves. Costly fabrication techniques have been an obstacle to metamaterial research. Metamaterials are predicted to be of great use particularly in defence applications to impart stealth and conceal units. Other applications of metamaterials include superlenses which are lenses that are almost free of aberrations and that can focus images below the diffraction limit. We can expect better antennas and other devices based on metamaterials in our mobile phones in the future.
Imaging allows us to see the various physical and chemical changes taking place in a system. With the first camera-like device referred to as ‘camera obscura’ to the modern day DSLRs, our cameras and imaging systems have changed drastically. Imaging plays a crucial role particularly in life science, medicine and security issues but is also important in many fields of physics and chemistry. Imaging with super-high frame rates has given us a way to study ultrafast phenomena like chemical and electron transfer reactions that occur in an infinitesimally small duration of time. This has been achieved with the help of pulsed femtosecond lasers which allow us to carry out pump-probe studies for repetitive events and also burst mode studies for non-repetitive events. Some famous ultrafast imaging systems for both repetitive and non-repetitive events are – the streak camera dubbed as Femto-photography, STEAM, STAMP, etc. Fingerprinting is the underlying concept for spectroscopy. Scanning devices at airports and other important locations for safety measures are based on such spectroscopic systems. We can expect cheaper, safer and faster imaging systems based on Terahertz radiation. Terahertz waves are electromagnetic waves which fall between the visible spectrum (1015 Hz) and high radio frequency waves (1010 Hz). Today we have high resolution cameras having as many as millions of pixels embedded in our smartphones to give us sharp images! Maybe in the future, we can have single pixel based efficient and smaller cameras which not only capture the visible spectrum but also infrared radiations.
The importance of light in material processing was understood after the development of photography. Since then laser-cutting of metallic blocks in the industry and various other processes were developed. Today, material processing using light has become highly sophisticated. 3D printing has been used so widely and in unimaginable ways. Nanofabrication and material processing at the nanoscale are very important not only for basic research but also for industrial applications. Structured hydrophobic surfaces which are based on the lotus leaf model have been demonstrated using femtosecond laser pulses on metals. Optical data storage has been revolutionised and new technologies have shown an almost 36 fold increase in data storage compared to the conventional Compact Discs of the same size. The conventional electron beam lithography used for making electronic devices at the nanoscale maybe someday replaced by an optical nanofabrication technique which is diffraction limited that is the minimum feature size depends on the wavelength of light used. Optical techniques are faster, portable and enable us to design 3D free standing structures like nanowires.
Energy & Communication
With the need for better renewable sources of energy, solar cells have demonstrated their mettle. Newer and newer designs based on bandgap engineering allow us to utilise the solar spectrum in the most efficient way and in the process avoiding damage to the cell. Moreover, detailed study of photosynthesis and light-matter interactions may someday allow us to fabricate the most efficient energy harvesting devices. This will help us solve our energy crisis and up to some extent thwart deterioration of our environment. Trapping solar energy in space and wirelessly transporting it to earth using lasers has also been proposed by NASA, JAXA etc. This approach is called Space-based Solar Power. We have already seen how wireless charging has made our lives hassle-free. Li-Fi (light based communication) is another idea which will give us an edge over the existing radio wave communication systems. Simple LED’s could be used as transmitters in this technique. With light we have more bandwidth and we can use division multiplexing techniques to send more information over the same channel. Use of entangled photon pairs for highly secure communication is another interesting future prospect for important online transactions.
Basic Sciences and Research
Last but not the least, I personally believe that there is a lot of scope for research and development in optics and photonics. Since the development of the first laser, we have seen so much of technological advancement in this field. The interdisciplinary nature of this science is bound to amaze us in the future! In the past few years, we have seen some great advancement like self-accelerating light beams which have interesting properties like self-healing and curved trajectories. Also, a lot of research on plasmonics has allowed us to think for newer optical devices. The ability to squeeze light, slow it down, and impart angular momentum to it and the ability to cool macroscopic objects with the help of light are some of the cool applications developed in the last few decades. Who knows what these photons have in store to amaze us.
Understanding Our Universe
With time, the average person’s understanding of the universe has grown, thanks to the efforts of untold numbers of scientists, researchers, engineers and others around the world. From America’s first image of the moon — taken by NASA’s Ranger 7 spacecraft in 1964 — to the remarkably clear pictures of Pluto’s cratered, mountainous and glacial terrains — acquired by the New Horizons spacecraft within the last year — optics and photonics technologies are bringing us ever closer to the planets in our own solar system and all that lies beyond. Today, light-based technologies are helping to improve our understanding of the universe. One such technology — optical spectrometry — is playing a role. It has allowed engineers to measure the spectral content of a beam of light — specifically, how many photons of a certain color it contains. Applying this technique to astronomy, we’re able to gain a formidable amount of information about the universe, for instance by measuring what elements stars and planetary atmospheres contain and at which temperature and pressure they are. Astronomy and photonics are now merging in the field of astrophotonics, which aims at using photonics to enhance astronomical instruments. This could mean that bulky free-space-coupled spectrometers may be replaced by miniaturised fiber-coupled, on-chip spectrometers. Photonics technologies also extend beyond studying planets. Advancements could ultimately determine if there is life beyond Earth but for now, there is an entire universe of information yet to be gathered and examined, and photonics is taking the front seat on the spacecraft.
Towards integrated photonics
Integrated photonics is the intersection of microelectronics and photonics. Microelectronics (design and fabrication of electronic devices, systems, and subsystems using extremely small components) has been the driver of technology and the world’s economy for several decades. Its success is a direct result of the integrated circuit where billions of electrical components (transistors, wires, resistors, capacitors, etc.) are seamlessly integrated together on silicon wafers using manufacturing processes that have followed the scaling trends of Moore’s law. Photonic technologies are now at a point similar to where microelectronics was in the early 1970s—where just a relatively small number of components were tediously integrated together. By leveraging the manufacturing equipment and techniques that made microelectronics a success, it is now beginning to be possible to realise the same economies of scale to make integrated photonic circuits. Since similar manufacturing technologies are being used, photonics and electronics can be directly integrated together to make both the electronic and photonic elements of the circuits function better—not only reducing size, weight, and power but enabling entirely new applications, many of which have not been envisioned.
In order to understand how integrated photonics works, it is important to first define the broader area of photonics which is the study of the generation, manipulation, and detection of light. Light is made up of photons, similar to how electric current is made up of individual electrons. However, photons have the distinct advantage that they travel at the speed of light and don’t consume any power during their propagation. For example, photons routinely travel across the entire universe (albeit after approximately 13 billion years) with just the energy required to initially produce them. Photons are also very efficient information carriers. They are electromagnetic waves (just like a radio wave) that oscillate at very high frequencies, and as a result can easily encode terabytes/second of information in their amplitude, phase, and/or polarisation. There have been many platforms for photonics over the decades, such as fiber optic networks, where discrete components (lasers, the actual fiber optic cable that transmits light and detectors) are separately manufactured and put together. In the early 2000s the promise of silicon as an integrated photonics platform emerged. It is ideal for manufacturing since silicon wafers are also used to make the vast majority of integrated electronic circuits. Early on, though, it was not clear how well silicon would work for photonics. But after multiple breakthroughs over the past decade it’s proven to excel at controlling light. Specifically, silicon is excellent at guiding light in “photonic wires,” known as waveguides, because it has a very high refractive index that tightly confines light and easily supports total internal reflection—even for a ~90-degree bend. Consequently, it is possible to realise very complex integrated photonic circuits that are now rapidly growing in density. Furthermore, silicon is transparent at the same wavelengths used for fiber optics, enabling direct interfacing of silicon photonic chips with optical fibers, which is key for many applications. However, for silicon to be the integrated photonics platform of the future, it also needed the ability to generate, control, and detect light.
Silicon itself is not ideal in these roles as it is an indirect bandgap semiconductor. In contrast, many III-V semiconductors (named from the groups on the periodic table), such as gallium arsenide and indium phosphide, are direct bandgap semiconductors and can easily be made into lasers. Fortunately, it is now possible to bond or even grow III-V lasers directly onto silicon through advances in manufacturing technology. III-Vs can also be used to detect light, but the most commonly used detector material is germanium, because it is straightforward to grow on silicon and is already used to make silicon transistors operate faster while using less power. It is now possible to also actively encode information on light by combining photonics and microelectronics. Light is Photonic Wafer: A working integrated photonic wafer made by RIT researchers. It contains thousands of integrated photonic devices including waveguides, filters, fiber-chip couplers, modulators, and more. These devices will make computers, Internet communications, and sensors operate at a much higher performance and at a much lower cost than what is available today. 8 Spring/Summer 2015 Research at RIT 9 Focus Area | Harnessing Light sensitive to the same electrons and holes that microelectronic devices excel at controlling. Specifically, free-carriers change the refractive index and absorption of silicon. As a result, by combining silicon photonic waveguides with PN diodes it is possible to change the transmission of the light electrically. These electro-optic modulator devices are now able to switch the light on/off at staggeringly high rates of greater than 40 GigaBits/second, while using incredibly low amounts of energy of less than 1 femtoJoule and have the potential to approach the same energy used by just a few state-of-the-art transistors. With all of the key components now in place the potential of silicon photonics is enormous. In just the last few years the number of devices that have been integrated together has rapidly grown to over 10,000. The natural application of these integrated photonic circuits is high bandwidth communications, particularly since data centers are expected to consume a few percent of the entire power generated in the United States and a vast majority of that power usage is used to simply move data around. Consequently, the integration of all of the previously used discrete components onto silicon photonic chips will yield dramatic reductions in power along with orders of magnitude improvements in bandwidth.
Challenges remain, however, with the biggest being the ability to cost effectively package photonic chips. Packaging currently accounts for most of the cost because optical fibers must be precisely positioned to the waveguides using time-consuming procedures. However, solutions based on microfabrication are now being realised and will dramatically improve packaging throughout and reliability.
Dr S S Verma
Department of Physics
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