Optics & Photonics: Twenty-First Century Technology
In the same way that electronics became pervasive in the twentieth century, optics and photonics will play a critical role in enabling the development and refinement of manufacturing, medical, sensing, telecommunications, and defense technologies in the twenty-first century.
Optics is the study and the manipulation of electromagnetic radiation. Optical scientists and engineers work primarily with the part of the spectrum that spans from x-rays to the far infrared. Optical engineers use the discoveries of scientists to design and fabricate optical equipment that advances industry and improves life by making products that are safer, faster, and easier to use.
Photonics is the technology that deals with the generation, manipulation, transport, detection, and use of light energy, whose quantum unit is the photon. Photons carry information through fiber optic cable, retrieve information in laser radar and other sensor devices, and can transport light energy (laser machining and laser surgery).
The field of optics and photonics is all about light—pervasive, primordial, and life-giving. It is what enables us to see, and a source of energy and life.
From mirrors, first used thousands of years ago, to early seventeenth century lenses ground for the first microscopes and telescopes, to the first laser in 1960, to today’s high-resolution microscopes and the revolutionary electron-beam nanolithography tool, the field of optics and photonics has served as a crucial enabler in the unfolding story of technological progress and innovation.
The field of optics and photonics, by its very pervasive nature, touches, enables, and accelerates the work of many divergent fields—electrical engineering, physics, chemistry, and material science. The National Research Council report on “Optical Science and Engineering for the Twenty-first Century” highlights some exciting trends:
Progress during the past decade has been extraordinary. Optical fiber for communication is being installed worldwide at a rate of 1,000 meters every second. Just 10 years ago, only 10 percent of all transcontinental calls in the United States were carried over fiber optic cables: today 90 percent are.
Meeting the computing and communications needs of the next ten to twenty years will require advances across a broad front, and many capabilities will have to advance a hundredfold to keep pace with this rapidly growing, high-speed global telecommunications network.
Healthcare and Life Sciences
Optics is enabling a wide variety of new therapies from laser heart surgery to the minimally invasive knee repairs made possible by arthroscopes containing optical imaging systems. Optics is providing new biological research tools for visualization, measurement, analysis, and manipulation.
Over the next decade, optical techniques are expected to enable such breakthroughs as early detection of breast cancer and “needless” glucose monitoring for people with diabetes.
Energy and the Environment
Optics is enabling advances in lighting sources and light distribution systems, real-time measurements of industrial emissions, and global environmental monitoring. This field is also enabling dramatic advances in photovoltaic solar cells, a crucial alternative to conventional energy sources.
We are poised to dramatically reduce the electricity consumption devoted to lighting, presently claiming 20 percent of U.S. electricity consumption. Advances in photovoltaic cells may enable solar energy to provide up to half of the world’s energy needs by the middle of the next century.
Optical technology has become a ubiquitous part of national defense. It enables sophisticated satellite surveillance systems for intelligence gathering, night vision imaging and missile guidance units, and lasers used for everything from targeting and range fielding to navigation.
Optical technology will continue to be crucial to national defense. The Department of Defense has a significant stake in optics, because of existing technologies, and future developments such as high power-directed energy weapons that this field affords.
Looking forward to the twenty-first century, the National Research Council report states, “As we move into the next century, light will play an even more significant role, enabling a revolution in world fiber-optic communications, new modalities in the practice of medicine, a more effective national defense, exploration of the frontiers of science, and much more.”
The Future of Optics and Photonics
Optical solutions to engineering problems are being increasingly found in many diverse applications from medicine, communications, entertainment, sensing, and homeland security. Promising areas of research include:
- pharmacogenomics (tailoring the choice of drug or other treatments based on the specific genetic makeup of an individual)
- chemical sensing for identifying biological agents and explosives
- security imaging through fog, sand, areosols, and other scattering media
- passive optical networks for ultra-broadband access
- optical spectroscopy of nanoscale systems
- solid-state lighting for general illumination
- photonic integration technology
Biophotonics describes the use of light to image, detect, and manipulate biological systems. It has applications in biology to study molecular processes, and in medicine to study tissue and blood, and to diagnose and treat diseases in a non-invasive way.
Gold nanoparticles stick to cancer cells and make them shine.
(Courtesy of M. El-Sayed)
Today optics is playing a major role in medicine through laser surgery and various optical diagnostic techniques. For instance, lasers are being used routinely in laser-refractive surgery of the cornea to correct for visual defects and in laser angioplasty to remove blockages in arteries. Advances in understanding the interaction of light with tissue has led to photodynamic therapy as a possible treatment of some cancers. In photodynamic therapy, drugs injected into a patient can be selectively activated by exposing to light the area of interest, leading to the photochemical destruction of tumors. Other widely used applications of biophotonics include advanced imaging techniques such as scanning confocal microscopes that can provide three-dimensional images of biological tissues in vivo, and DNA sequencing for genomics.
Colloidal crystals assembled from hydrogel nanoparticles.
(Courtesy of L. A. Lyon)
Researchers at Georgia Tech are working actively to further advance the understanding of the interaction of light with biomolecules and various nanomaterials, including gold nanoparticles and nonlinear organic molecules such as two-photon-absorbing molecules. These advances will enable better imaging devices and lead to new diagnosis, treatment, and drug delivery systems.
Diffractive and Holographic Optics
Diffractive and holographic optics makes use of the wave nature of light and the principles of diffraction and interference effects. Pioneered by scientists like Thomas Young during the eighteenth century, optical interference is at the basis of many areas including spectroscopy. More recent applications of optical interference includes holography, discovered by Dennis Gabor in 1947, which allows the recording of three-dimensional images and is considered for future optical storage and imaging technologies.
Contour map of the interferogram of an electro-optic diffractive lens.
(Courtesy of B. Kippelen)
Today, diffractive optical elements are playing a central role in spectrometers and in modern imaging systems. They are employed in the latest lens technologies in which chromatic aberrations (color defects) of conventional refractive lenses can be minimized. Diffractive optics, found on almost every credit card in the form of a sparkling hologram, is increasingly used on bank notes and other security applications. It plays a central role in dense wavelength demultiplexing (DWDM), one of the key technologies in optical communications.
Image of the interferogram of an electro-optic diffractive lens.
(Courtesy of B. Kippelen)
By studying the interaction of light with new nano structured media, researchers at Georgia Tech are developing next-generation diffractive and holographic elements that will find use in tomorrow’s reconfigurable optical networks, in the next generation of ultrafast processors that will be based on optical interconnects, and in high-speed, high-capacity holographic storage systems.
The field of nonlinear optics was initiated shortly after the discovery of the laser with the experiment of second-harmonic generation by Franken and colleagues in 1961. Since then, the field has grown considerably and covers numerous aspects ranging from fundamental studies of light-matter interactions to applications in lasers, optical communications, and biology.
Photograph of a three-dimensional photonic crystal structure fabricated by two-photon-induced photopolymerization.
(Courtesy of J. Perry)
Today, nonlinear optics plays an increasing role in almost every area of optical technology. Examples of applications of optical nonlinear phenomena include frequency conversion of lasers (e.g. green laser pointers); the routing, switching, and amplification of optical signals; confocal laser scanning microscopy; two-photon microscopy to generate depth-resolved images of biological tissues.
Photograph of the third-harmonic generation at an infrared (1,500 nm) laser beam in an organic thin film. Infrared light is converted into visible green light.
(Courtesy of B. Kippelen)
Nonlinear optics will play a major role in the development of future optical data networks with increased transmission capacity by enabling the controlled switching and routing of optical signals. These devices will be based on the ability to modulate the refractive index of nonlinear materials with an electric field or with a light beam. Nonlinear optics will continue to provide new imaging tools with capabilities that go far beyond that of conventional microscopes. Other applications in biology include novel drug delivery techniques and nonlinear photodynamic therapy for cancer treatment.
SHG FROG traces (and retrieved intensities and phases) of 1.55-micron fiber-laser pulses compressed in dispersion-decreasing fiber and then dispersed in SMF-28 with varying intensity. Traces taken by Fredrik Fatemi, Naval Research Laboratory, Washington, D.C., and published in Opt. Lett. 2002.
Research in nonlinear optics at Tech is focusing on several key aspects, including the development of new materials with unprecedented nonlinear optical properties, the very precise characterization of ultrashort laser pulses using frequency conversion, the understanding of light propagation in optical fibers, and the scanning of tightly focused laser beams to fabricate high-resolution three-dimensional objects by photoinduced polymerization
Optical Communication Systems
Optical communications includes all methods of using light to communicate. An early demonstration was performed by Alexander Graham Bell [1847-1922], who showed that it was possible to modulate light by use of a membrane that vibrated in response to sound, thus demonstrating a free-space optical link. These free-space optical transmission links have applications today using lasers as rapidly deployable optical links with large bandwidth capacity. However, most modern optical systems rely on guiding the light in glass fibers that exhibit exceptionally low loss. The advent of the laser was followed by extensive efforts to understand and reduce the loss of glass. In the1960s to early 1970s the attenuation of glass fiber was reduced from more than 1,000 dB/km to less than 20 dB/km. The chief advantage of using light is the enormous bandwidth or signal carrying capacity that arises from the nearly 200THz carrier frequencies commonly used in fiber systems. Using only a 10GHz modulation rate, a single channel optical link can simultaneously transmit 129,000 telephone calls or can transmit more than 100 standard CDs in one minute.
Optoelectronic signal processing: A multi-segmented photodetector separately detects power from the different modes of a multimode fiber, enabling mitigation of differential modal delay and dramatically improving fiber bandwidth.
(Courtesy of S. Ralph)
An optical communications system includes methods of modulating, transporting, and detecting light. Therefore, optical communications relies heavily on the advances in nearly all of the other optical and photonic research areas. Optical communications systems today routinely carry 10Gbps on each wavelength, and systems using wavelength multiplexing methods have been used to demonstrate aggregate data rates in excess of ten Terabits per second on a single fiber. In addition to the increasing data rates, the reach or distance of optical links has also increased, and undersea links are common. Advances in fiber optic optical components have allowed optics to extend closer to the end user. All of these advances in optical communications have enabled the Internet to flourish.
In optics as in many other areas of science and engineering, advances in materials are driving technological innovations. For instance, the development of low-loss glass in the 1960s and 1970s opened the door to fiber-optic communications. During the same time, advances in semiconductors based on the elements gallium, arsenic, and phosphorus paved the way to the development of light-emitting diodes and solid-state laser diodes.
Photograph of a blue light-emitting diode based on new compounds semiconductors developed at Tech.
(Courtesy of R. Dupuis)
Today, the use of these light sources has become ubiquitous and continues to create new applications such as in high-efficiency lighting. Traditional traffic lights based on incandescent light bulbs are being replaced with brighter, more power-efficient, and longer-lasting solid-state light-emitting diodes (LEDs).
Photograph of the optical texture of a liquid crystal film fabricated from organic semiconducting molecules imaged with polarized microscope.
(Courtesy of S. Marder and B. Kippelen)
Optical materials are generally divided into several classes: inorganic, organic, and hybrid. Organic materials such as synthetic polymers are mainly comprised of the elements carbon, hydrogen, and oxygen. The rich diversity of carbon chemistry enables the synthesis of complex molecules with tailored optical, electronic, and mechanical properties. Fluorescent molecules, for instance, are being used in biological applications. Today’s contact lenses are mainly based on flexible, highly transparent plastic materials. Sometimes, optical materials with unique properties can be generated by mixing inorganic materials with organic ones. For instance, hybrid materials based on semiconductor nanoparticles embedded in a semiconducting polymer matrix are being developed for renewable power generation.
In recent years, increased focus has been given to tailoring the optical properties of materials by reducing their dimensions to the nanometer scale. These advances in nanomaterials are key enablers for nanotechnologies.
Quantum chemical modeling
Optical Systems and Technology
Optics began with visible light, because the only detector was the human eye. The sources of light were the sun, stars, and candles. The main optical material was glass, and systems, such as telescopes, were simple.
An infrared laser is used to probe artificial smoke clouds.
(Courtesy of G. Gimmestad)
Today, optical systems and optical technology span the spectrum from the ultraviolet to the far infrared. Research areas include sources (such as lasers and LEDs), detectors, materials, and electro-optical systems that are developed to meet specific needs. Advances in electro-optics have enabled much of the ubiquitous technology that defines a modern society, including devices for entertainment, communication, and medicine.
Undergraduate students use an eye-safe LIDAR system to study the atmosphere.
(Courtesy of G. Gimmestad)
Much of Georgia Tech’s optical systems work is related to sensors and sensing systems, with many applications: atmospheric characterization for visibility and air quality, hazard detection for transportation safety, detection of chemical and biological hazards, threat detection for military and security purposes, environmental monitoring, process analysis, biomedical diagnostics, and other applications involving lasers and imaging systems. Advanced imaging sensors incorporate multi-spectral, hyperspectral, and/or polarimetric technologies.
Extensive research programs and capabilities are in place in infrared technology and in atmospheric laser radar (LIDAR), along with modeling and simulation software to predict the performance of electro-optical sensing systems in various scenarios. Optical sensor work has expanded in recent years, and that expansion is expected to continue due to an increased emphasis on situational awareness. Advanced techniques for image interpretation and processing are also being developed to make full use of new sensor capabilities.
Handheld quantum cascade laser-based mid-infrared sensor.
(Courtesy of B. Mizaikoff)
Optical Systems and Technology offers many new research opportunities in sensor technology and image interpretation, along with opportunities to simulate, model, and test new technologies. Furthermore, optical chemical sensor and biosensor concepts along with novel molecular recognition chemistries are being developed.
Optoelectronics and Nanophotonics
Optoelectronics relates to devices that function as electrical-to-optical or optical-to-electrical transducers. Examples of such devices include photodiodes, laser diodes, and integrated optical circuits and are commonly used in numerous applications in our daily lives. Optoelectronic devices can deal with the mutual conversion of electrical and optical energy and can be used in many sensing applications.
Inspection of a silicon solar cell developed at the University Center of Excellence for Photovoltaics at Georgia Tech.
(Courtesy of A. Rohatgi)
Examples of electrical-to-optical transducer effects include the ability to change the refractive index of a material with an applied field. These processes discovered by Friedrich Pockels and John Kerr during the second half of the nineteenth century are used today to modulate laser beams at high speed for optical communication. The birth of modern-day optoelectronics was marked by the invention of the laser in 1960. Since then, semiconductor lasers have transformed the way we listen to music and watch movies. Today, every household in the United States owns several lasers. Photodetectors are used in digital photography, and solar cells initially discovered at Bells Laboratories in 1954 form the basis of solar panels and are expected to play a major role in clean power generation in the future.
Photograph of a two-dimensional photonic crystal device fabricated in the Microelectronics Research Center (MiRC). Periodic structures with dimensions in the 100s of nanometers are fabricated utilizing the JEOL JBX-9300FS electron beam lithography system followed by inductively coupled plasma etching on a silicon on insulator (SOI) and amorphous silicon on insulator (ASOI) substrates.
(Courtesy of C. Summers)
Nanophotonics is an emerging field that builds on recent advances in nanoscale science and engineering. By controlling the growth of materials at a nanometer scale, scientists are developing new optical materials with unprecedented performance. Likewise, new patterning techniques such as the e-beam lithography capabilities at Georgia Tech allow the patterning and reproduction of optoelectronic devices with nanoscale resolution. That increased resolution leads to increased integration, but more importantly allows the design of new artificial devices with intricate optical and electrical properties.
Photograph of a 3-x-3 array of organic light-emitting diodes developed at Georgia Tech’s Center for Organic Photonics and Electronics.
(Courtesy of B. Kippelen)
The field of quantum optics emphasizes the investigation and control of the basic interaction processes between light and matter. The initial motivation for research in this field, in the 1960s, was the idea that some properties of detected light fields could not be explained by classical models of fluctuating light sources, but required a quantum mechanical description in terms of photons. Powered by advances in laser physics in the 1970s and 1980s, the field has grown to have a strong impact on areas such as laser spectroscopy and interferometry, investigations of photon statistics, laser cooling and trapping of atoms and molecules, the physics of ultra-cold atomic gases, and quantum information. Ideas from quantum optics have also found application in experiments that involve the wave nature of atoms, the field of matter-wave optics.
Ultrafast optics is the study and application of optical pulses and optical phenomena with duration shorter than a few picoseconds. Ultrafast optics became possible with the invention of short pulse lasers which can now routinely produce optical pulses of a few femtoseconds (10-15). These short pulses provide the “clock” or ruler by which researchers can create and measure events which occur on the femtosecond time scale. To appreciate the precision allowed by such fast events, consider that the ratio of one second to one femtosecond is the same as the ratio of seconds elapsed since the big bang to one second.
Continuum generation in micro-structured fiber; strong nonlinearities produce a white light continuum from a narrow band laser. The white light source is useful as a spectroscopic and frequency standard source.
(Courtesy of S. Ralph)
Ultrafast techniques have also been applied to imaging, time resolved femosecond spectroscopy and the generation and measurement of optical pulses. Ultrafast optical techniques allow the investigation of highly non-equilibrium carrier dynamics in semiconductors and ultrafast optical interactions with biological systems. Ultrafast optical methods have also allowed the generation of precision light sources for optical frequency metrology. Indeed, the recent Nobel Prize in Physics, awarded to Drs. R. Glauber, J. Hall, and T. Henesch, was, for their work using repetitive trains of ultrashort pulses which have a spectrum that consists of an evenly spaced comb consisting of thousands of sharp spectral lines.
Detail of a micro-structured optical fiber used to generate a white light continuum from a narrow band laser.
(Courtesy of S. Ralph)
Short pulse technologies allow the generation of high peak powers, PetaWatts, for the study of the effects of very high electric fields. When combined with optoelectronic devices, ultrashort optical pulses can be used to generate and measure electrical pulses of the shortest possible durations. These signals, which have bandwidths in excess of one terahertz (1012Hz), can be guided and used to investigate the operation of new electrical interconnects or integrated circuits, or they can be formed into radiating “terahertz beams” and used for spectroscopic applications in the far infrared.
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