Advances in sensing, imaging, and metrology over the last decade have been critically dependent on optics and photonics, and precision sensing has moved progressively to optically based measurements. Optical techniques are already at the core of some of the most precise measurements. For example, the NIST-F1 cesium time standard in use in the United States since 1998, around the time of the publication of the National Research Council’s (NRC’s) report Harnessing Light: Optical Science and Engineering for the 21st Century,1 exploits laser cooling of cesium atoms, optical monitoring of fluorescence, and various other optical techniques to lock in the microwave frequency of the atomic clock, and a second generation of such a system is under construction.2 This chapter describes the advances made in these technologies since 1997.
Precision metrology is important for advances in the following: fundamental research that relies on precision measurements, communication that relies on precision timing for high data rates and long ranges, and the Global Positioning System (GPS), which relies on precision timing. GPS devices were just becoming commercially available in 1998, and now they are in nearly every cell phone. The advent of octave-spanning optical frequency combs allows a small table-top apparatus
1 National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, D.C.: National Academy Press.
2 Jefferts, S.R., T.P. Heavner, T.E. Parker, and J.H. Shirley. 2007. NIST cesium fountains—Current status and future prospects. Acta Physica Polonica A 112(5):759-767.
to provide a direct link between radio frequency (RF) and optical standards, which took several rooms to perform at only a few laboratories around the world a decade ago. Now this capability is commercially available. Since the NRC’s 1998 study, miniature atomic clocks on a chip have been developed to provide precise local measurements. Quantum cascade lasers on the market extend the range of chip-scale laser sources for near and remote sensing applications into the middle-infrared wavelength range of the electromagnetic spectrum (3-30 µm). The field of terahertz imaging has matured to the point of deployable systems in airports and other points of entry into our nation for the secure and efficient passage of trade goods. New construction—such as bridges, tunnels, dams, skyscrapers, pipelines, railroad tracks, and power plants—and renovation of civilian and military infrastructure around the world routinely have many kinds of active and passive optical sensors (for example, of vibration, temperature, strain, displacement, and cracks) embedded for the real-time monitoring of operation and for the forecasting of hazardous conditions before disaster strikes. Optical sensors are also common in cars, trucks, airplanes, and ships.
Optics and photonics advances have enabled advances in precision manufacturing, which have enabled further improved sensors. Low-cost, high-resolution cameras in cell phones now make advanced digital imaging available to a substantial fraction of the world’s population with capabilities comparable with the best high-end cameras of a decade ago. Those components will enable a new wave of secondary niche markets that have the potential to have a significant impact on the U.S. economy and job pool. This broad growth of optical sensing and metrology—from the most precise scientific applications to universal consumer devices—makes the next decade an exciting time for optics and photonics in sensing and measurement, in research, and in consumer and industrial applications and offers significant opportunities for U.S. leadership.
Advanced photonic measurements and applications have had a profound impact on our daily lives. For example, GPS has had a significant impact on navigation. In the late 1990s, consumer GPS devices were only beginning to enter the market. Now this capability is a commonplace consumer item found in cell phones, car navigation equipment, and even pet identification tags. GPS relies on precision timing to enable high-resolution positioning, which also enables high data rates and long-range communications. That timing is enabled by several advances in photonics, such as compact atomic clocks on a chip (see Figure 8.1). Sensing and metrology have enabled a new level of integrated-circuit (IC) manufacturing, which has driven the entire consumer electronics industry. Those advances have
FIGURE 8.1 Schematic (left) and photograph (right) of a microfabricated atomic clock. The total volume of the device is less than 1 cm3, making it practical for use in handheld, battery-powered electronics. (See source for detailed image labels.) SOURCE: Reprinted, with permission, from Knappe, S., L. Liew, V. Shah, P. Schwindt, J. Moreland, L. Hollberg, and J. Kitching. 2004. A microfabricated atomic clock. Applied Physics Letters 85:1460.
also enabled the incorporation of low-cost, high-resolution imaging sensors in a broad range of consumer devices (such as cell phones and tablets). The proliferation of low-cost sensors connected by a high-bandwidth data transfer capability will enable the rapid growth of applications that would not have been economically viable without this large technology base. One example will be low-cost medical sensing devices that leverage consumer electronics components.
Since the NRC’s 1998 study, advances in octave-spanning optical combs have enabled a small table-top apparatus that provides a direct link between RF and optical frequency and time standards—apparatus that used to take several rooms full of specialized equipment. Such advances have narrowed the gap in measurement capabilities between premium laboratories with specialized equipment and those with modest funding, and this will be a game changer in advancing both basic and applied research.
Photonic measurement and application advances have enabled improvements
in manufacturing (for example, in lithography, machining, cutting, and welding), which have provided improved devices that are used to make improved sensors. That spiral threading of improvements feeds itself. Although the United States tends not to compete well in high-volume manufacturing, there is now a market opportunity for leveraging the application of these improved capabilities, as in the examples above, from consumer devices to address lower-volume niche sensor markets.
There has been a steady progression from RF to optically based sensing, which has advanced significantly since the Harnessing Light appeared in 1998. One example is in synthetic aperture imaging. Synthetic aperture radar (SAR) has been used since the 1950s; however, only in the last decade have advances in photonics enabled simultaneously agile and stable optical sources that have made SAR viable at optical wavelengths. The move to optically based sensing is partially due to the potential for improved resolution made possible by the much shorter wavelength. However, in many systems the resolution requirements are modest. In those cases, the primary motivations are to achieve easily interpreted imaging and improve illumination efficiency. The shorter wavelength enables a smaller illumination area because of diffraction, and the reflectivity at optical wavelengths closely matches what we are accustomed to viewing with our eyes. In contrast, typical SAR images require significant training for interpreting the resulting data.
Since the NRC’s 1998 study, there have been significant advances in emitter and detector materials for practical sources and sensors at new wavelengths. One example is the substantially improved capability at wavelengths near 2 µm, which is important for atmospheric research and military sensing. Significant advances in devices have also enabled photon-counting detectors to be extended to Geiger-mode detector arrays and to photon-number-resolving Geiger-mode detectors. Such advanced photon-counting techniques need to be expanded not only to higher count rates but to exploitation of novel quantum states of light in advanced optical sensors that are likely to come onto the horizon in the next decade or so.3,4,5 Moreover, current research will potentially provide a true linear-mode single-photon detector that will open new doors for sensing, imaging, and metrology.
3 An example is the planned incorporation of squeezed quantum states of light in the advanced Laser Interferometer Gravitational Wave Observatory (LIGO). Johnston, Hamish. 2008. Prototype gravitational-wave detector uses squeezed light. Physics World. Available at http://physicsworld.com/cws/article/news/33755. Accessed August 1, 2012.
Since the issuance of Harnessing Light: Optical Science and Engineering for the 21st Century 14 years ago,6 the role of optics in advanced photonic measurements and applications has undergone a revolution. New fields have blossomed, such as the advent of carrier-envelope mode locking (which earned the 2005 Nobel Prize in physics for John L. Hall and Theodor W. Hänsch),7 which enables highly coherent pulse trains and precisely spaced lines of optical frequency (about 1 cycle per second, 1 Hz) that span more than an octave in spectrum from middle-infrared to deep blue. That enables a direct link between RF and optical standards in a small table-top apparatus; such precision makes possible extremely precise spectroscopy for metrological applications, which would have been impossible when Harnessing Light was published. Moreover, the availability of mass-market optical imagers (such as fairly high-resolution cell phone cameras) is making possible personalized sensing and imaging applications. Such applications are likely to be not only affordable but highly precise in sensitivity and resolution because of the tight linking that exists between the optics and the sophisticated onboard electronic signal processing tools. We have also seen significant new technological opportunities emerge as nanotechnology has increasingly enabled new kinds of optical and optoelectronic structures, some without precedent in the classical optical world. Nanophotonic structures that are patterned or fabricated on sub-wavelength scales open new or enhanced functions for almost any application in which tailoring the properties of light is important.
While the new scientific developments are breathtaking and will continue to spawn new directions in advanced photonic measurements and applications in laboratories worldwide, it is the category of transitioning to mass-market devices that might have a much greater impact on the economy and people’s daily quality of life. Imagine an optics-enabled attachment to one’s cell phone that allows monitoring of blood glucose by simply inserting a finger into an orifice in the attachment, thereby avoiding pricking one’s finger several times a day. What if the same attachment had sensing elements that recorded other vital signs at the same time, to keep track of the user’s general well-being and issue an early warning when a trend in some vital measure was spotted?
Much has happened in science in the years since Harnessing Light appeared. Many scientific breakthroughs that were in their infancy in 1998 have matured,
6 National Research Council. 1998. Harnessing Light.
7 More information on the Nobel Prize is available at http://www.nobelprize.org/nobel_prizes/physics/laureates/2005/. Accessed August 18, 2011.
have penetrated the marketplace, and are having an impact on our lives.8 Of course, many other breakthroughs are just beginning to be understood. The following are some the exciting areas of science and technology that are being pursued aggressively today:
• Development of coherent sensing and imaging techniques;
• Emergence of highly coherent optical pulse trains (carrier-envelope mode locking made possible by highly nonlinear and novel microstructure optical fibers);
• Development of attosecond pulse trains by means of high-harmonic generation;
• Table-top availability of extreme intensities by means of chirped pulse amplification;
• Terahertz and middle-infrared sources of radiation (for example, quantum-cascade lasers);
• High-power fiber lasers;
• Advances in non-linear optics, quasi-phase matching, photonic bandgap fibers, and magneto-optics;
• Nano optics and plasmonics, negative index materials, and transformation optics;
• Advances in controlled generation of quantum light states and their manipulation and detection;
• Advances in detector technologies, wider wavelength coverage, pixel count, quantum limited operation, and single-photon and photon-number resolved counting;
• Advances in adaptive optical techniques, guide stars, deformable mirrors, and turbulence control; and
• Computational imaging and sensing.
Some of these areas are expected to mature technologically and lead to new applications that will penetrate the marketplace or make existing applications work better in the coming years. The next section presents a few of the major advances with an eye toward the technologies that might have the most impact on society in the future.
It should be noted that the list of scientific advances above only briefly touches on subjects pertaining to quantum information science and technology. Light plays
8 Optical coherence tomography is one example. More information is available at Optical Coherence Tomography News, http://www.octnews.org/. Accessed October 26, 2011. The ubiquitous social networking enabled by massive wavelength division multiplexing (WDM) optical communications is another example.
an important role in almost all implementations of quantum processing, not just quantum communications and the so-called linear-optics paradigm of quantum computing. Those subjects are aptly covered in the National Research Council report Controlling the Quantum World: The Science of Atoms, Molecules, and Photons.9 In a similar vein, scientific advances in astronomy are only briefly touched on where adaptive optics and photon-counting arrays are playing a transformative role in Earth-based telescopes and photon-counting arrays are likely to play a similar role in space-based telescopes, such as the James Webb Space Telescope.10 The focus of this chapter is the advances that may have direct applications in sensing, imaging, and metrology systems.
There have been significant changes in advanced photonic measurements and applications since the publication of Harnessing Light.11 The changes have created new capabilities, improved the resolution and precision of measurements, and provided capabilities to modest facilities that were previously available in only a few locations around the world. Some of the significant changes are highlighted here.
Around the time that Harnessing Light was published, the Système International (SI, or International System of Units) definition of the second was changed from the 1967 definition—the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the hyperfine levels of a cesium-133 atom—to include the stipulation of ground state at a temperature of 0 K. The change is made practical by the extremely low temperature that is available from the use of optical cooling of collections of cesium atoms to temperatures as low as 1.3 µK. Combined with the 1983 change in the definition of the meter (defined as the path traveled by light in vacuum in 1/299,792,458 second), the change in the definition of the second reflects the continued importance of optics and photonics in precision measurements. The definition of the kilogram is also undergoing a fundamental change: the current definition defines the kilogram as the mass of the
9 National Research Council. 2007. Controlling the Quantum World: The Science of Atoms, Molecules, and Photons. Washington, D.C.: The National Academies Press.
11 National Research Council. 1998. Harnessing Light.
international prototype kilogram, and the new definition relates it to the equivalent energy of a photon by means of Planck’s constant.12
When light passes through a medium, such as glass, its wavelength usually is not affected; such transmission of light through matter is termed linear optics. However, when the strength of the light is high, nonlinear optical phenomena occur, one of which is harmonic generation. Consider what happens when we turn the volume up too high in a loudspeaker. Instead of clean, pure tones, we get distortion, which consists of higher harmonics of the pure tones and other frequencies produced by mixing the tones. In similar fashion, light is a wave—just like a sound wave, but made of electromagnetic (EM) energy. When the light passing through a material gets too intense, harmonics of the light wave can be created. Blue light, for example, is the second harmonic (one-half the wavelength and twice the frequency) of near-infrared light and can be created by a non-destructive change in the response of the medium to the intense lightwave. Such phenomena are captured by the field of nonlinear optics.
By using the techniques of laser mode-locking and chirped-pulse amplification, scientists in the United States, Europe, and Japan have learned to create compact, cost-effective table-top sources of highly intense pulses of light. When such pulses of light are focused on inert gases, extreme nonlinear optical phenomena occur.13 Generation of these high-harmonics leads to extremely short pulses of light at a very short wavelength (the second harmonic is one-half the wavelength, the third harmonic is one-third the wavelength, and so on). Scientists at JILA (University of Colorado, Boulder) have created table-top sources of coherent x rays14 by such methods of extreme nonlinear optics.15 Such x-ray light sources are likely to have a revolutionary impact on such applications as imaging and lithography on the nanoscale. (See Box 8.1.)
12 Mohr, P. 2010. “Recent Progress in Fundamental Constants and the International System of Units.” White paper. Third Workshop on Precision Physics and Fundamental Physical Constants. Available at http://physics.vniim.ru/SI50/files/mohr.pdf. Accessed January 17, 2012.
13 As the second and third harmonics are generated, which themselves can become very intense, this can cause generation of harmonics of the harmonics, which generate further harmonics, and so on.
14 Popmintchev, T., M.-C. Chen, P. Arpin, M.M. Murnane, and H.C. Kapteyn. 2010. The attosecond nonlinear optics of bright coherent x ray generation. Nature Photonics 4:822-832.
15 Kapteyn, H.C., M.M. Murnane, and I.P. Christov. 2005. Extreme nonlinear optics: Coherent x rays from lasers. Physics Today 58:39-44.
Table-Top, Lensless, Soft-X ray Microscope
The optical microscope has contributed greatly to our understanding of the world around us. Unfortunately, the smallest object that can be imaged is determined—and limited—by the wavelength of the light used. To visualize much smaller objects on the nanoscale, x-ray microscopes are needed. A team led by the Kapteyn—Murnane research group at JILA (University of Colorado, Boulder) has recently demonstrated a table-top, lensless, soft-x-ray microscope with a resolution that is very close to the wavelength of the extreme ultraviolet light used. A lensless microscope uses a computer algorithm to analyze the scatter patterns produced from the illuminated sample. Figure 8.1.1 shows imaging of a test sample with 13-nm coherent light. A resolution of 92 nm is obtained. Higher-repetition-rate ultrafast lasers currently under development will significantly reduce image capture time and thus improve resolution toward the wavelength-limited value. This table-top soft-x-ray diffraction microscope should find applications in biology, medicine, nanoscience, and materials science.
FIGURE 8.1.1 Lensless diffractive imaging combined with multiple-reference fast Fourier transform holography. The spatial autocorrelation of the object can be retrieved. Further refinement of the image to a resolution of 50 nm is possible with phase-retrieval algorithms to recover the spatial frequency information scattered at high angles. SOURCE: Reprinted with permission from McKinnie, I., and H. Kapteyn. 2010. High-harmonic generation: Ultrafast lasers yield x rays. Nature Photonics 4(3):149-151.
SOURCE: McKinnie, I., and H. Kapteyn. 2010. High-harmonic generation: Ultrafast lasers yield x rays. Nature Photonics 4(3):149-151.
Among the many attributes of laser light (monochromaticity, directionality, polarization purity, and brightness), the brightness or intensity (power density) is the most used property. Applications include cutting, welding, printing, data storage,
and many more. Much has been accomplished in the technology of boosting laser beams to high, and sometimes lethal, power.16 However, amplifying laser light without affecting its other attributes presents several challenges.
Modern nanofabrication techniques allow us to control the size, shape, and structure of the material used with features on deeply sub-optical-wavelength scales, thereby opening up a broad range of technical opportunities. Such controlled fabrication means that optical properties can be tailored by the size, shape, or structure rather than by just the natural properties of materials themselves. Structures with controlled dimensions from tens to hundreds of nanometers fabricated in dielectrics, semiconductors, and metals17 allow a broad range of new optical possibilities, such as photonic crystal structures, metamaterials,18 compact high-quality-factor micro-ring resonators, and other nanometallic and plasmonic structures.19 Those approaches offer new ways of concentrating or manipulating light20 for enhancing or controlling sensing of various kinds,21 such as chemical sensors, or such techniques as Raman scattering, and allow us to tailor optical response, such as spectral sensitivity, in ways beyond conventional optics. The science and basic technology of many such opportunities have been increasingly explored in research over the last decade as various nanofabrication tools have become more available.
Sensing with surface plasmon phenomena,22 in which light is concentrated very near the surface of a metal, has been exploited in commercial biochemical sensing devices since the 1990s. Small changes in refractive index resulting from specific biochemical activity can be detected in very small detection volumes. The
16 More information on chirped-pulse amplification is available at http://www.rp-photonics.com/chirped_pulse_amplification.xhtml. Accessed January 17, 2012.
17 von Freymann, G., A. Ledermann, M. Thiel, I. Staude, S. Essig, K. Busch, and M. Wegener. 2010. Three-dimensional nanostructures for photonics. Advanced Functional Materials 20:1038-1052.
18 Chen, H., C.T. Chan, and P. Sheng. 2010. Transformation optics and metamaterials. Nature Materials 9:387-396.
19 Brongersma, M.L., and V.M. Shalaev. 2010. The case for plasmonics. Science 328:440-441.
20 Schuller, J.A., E.S. Barnard, W. Cai, Y.C. Jun, J.S. White, and M.L. Brongersma. 2010. Plasmonics for extreme light concentration and manipulation. Nature Materials 9:193-204.
21 Richens, J.L., P. Weightman, W.L. Barnes, and P. O’Shea. 2010. “In Vivo Spectroscopic Imaging of Biological Membranes and Surface Imaging for High-Throughput Screening.” Chapter 17 in Nanoscopy and Multidimensional Optical Fluorescence Microscopy, A. Diaspro, ed. Boca Raton, Fla.: CRC Press.
22 Homola, J. 2008. Surface plasmon resonance sensors for detection of chemical and biological species. Chemical Reviews 108:462-493.
use of nanometallic particles is expected to improve such sensitivities further.23 Another example of a nanometallic approach, recently commercialized, uses sub-wavelength holes in metals to allow optical detection of individual nucleotides in DNA sequencing.24
Nanophotonic techniques with dielectrics or metallic nanostructures show promise for making extremely compact spectrometers. Quantum mechanical properties can also be tailored once dimensions can be controlled on about a 10-nm or smaller scale. For example, quantum-dot (QD) fluorescent tags for biological experiments allow the fluorescent color to be controlled by choice of the size of the quantum dots.25
An ideal laser—and many practical lasers approach this ideal limit—emits light in the form of what is called a coherent state, so termed by Roy J. Glauber26 in the early 1960s. In this quantum state, the light quanta (photons) exit the laser at random times, forming a Poisson distributed stream of photons even though the emitted light beam has constant power in the case of a continuous-wave laser. At the macroscopic level, the EM field associated with the emitted light wave approaches a sinusoid much like that seen on a string when it is repetitively shaken. Microscopically, however, the same randomness causes the wave to possess an uncertainty in its amplitude (height of the crests and troughs) and phase (zero-crossing points of the wave amplitude), but in this wave picture the uncertainty can be tied to the fluctuations in the vacuum EM field that permeates all space. The fundamental uncertainty caused by the vacuum field cannot be removed, but its effect can be manipulated in judicious ways to bypass its degrading effect on precise measurements in some situations. For example, the uncertainty in the amplitude can be traded at the expense of the uncertainty in the phase and vice versa, whereas the uncertainty product remains unchanged, as dictated by the Heisenberg uncertainty
23 Offermans, P., M.C. Schaafsma, S.R.K. Rodriguez, Y. Zhang, M. Crego-Calama, S.H. Brongersma, and J. Gómez Rivas. 2011. Universal scaling of the figure of merit of plasmonic sensors. ACS Nano 5:5151-5157.
25 Alivisatos, P. 2004. The use of nanocrystals in biological detection. Nature Biotechnology 22:47-52.
26 Roy J. Glauber shared the 2005 Nobel Prize in physics “for his contribution to the quantum theory of optical coherence.” More information is available at http://www.nobelprize.org/nobel_prizes/physics/laureates/2005/. Accessed November 14, 2011.
principle,27 a fundamental law of quantum physics. Such novel quantum light states have been called squeezed states, and tremendous progress has been made in the development of sources of squeezed light in the last couple of decades.28
One current grand challenge in the scientific world of sensing and precision measurement is the quest for the detection of gravity waves predicted by Einstein’s theory of general relativity. Even though these waves in the fabric of space-time continuum were predicted almost a century ago, their direct observation has eluded scientists. National-scale efforts are underway in different parts of the world to detect gravity waves, and the most advanced sensor is in the Laser Interferometer Gravitational-Wave Observatory (LIGO).29,30,31,32 It turns out that the strain sensitivity achieved in the current generation of LIGO does not reach a level that is high enough to ferret out the faint signatures of the gravity waves. The ultimate barrier to improving the strain sensitivity of the LIGO further is the above-discussed fundamental noise on the waves of light that bounce between the arms of LIGO’s giant interferometer. The use of squeezed light can lead to enhanced performance, and a prototype demonstration of the expected enhancement has been made (see Figure 8.2).33,34 It is expected that the use of this novel quantum state of light will play a pivotal role in the ultimate detection of gravity waves and in the opening of a new window on the universe. Continued development of highly efficient sources of squeezed light motivated by the grand challenge of detecting gravity waves, particularly those in the telecommunications wavelength bands prevalent in today’s
28 Vahlbruch, H., M. Mehmet, S. Chelkowski, B. Hage, A. Franzen, N. Lastzka, S. Goßler, K. Danzmann, and R. Schnabel. 2008. Observation of squeezed light with 10 dB quantum noise reduction. Physical Review Letters 100:033602-033606.
33 Goda, K., O. Miyakawa, E.E. Mikhailov, S. Saraf, R. Adhikari, K. McKenzie, R. Ward, S. Vass, A.J. Weinstein, and N. Mavalvala. 2008. A quantum-enhanced prototype gravitational-wave detector. Nature Physics 4(6):472-476.
34 The LIGO Scientific Collaboration. 2011. A gravitational wave observatory operating beyond the quantum shot-noise limit. Nature Physics 7(12):962-965.
Figure 8.2 View into the GEO600 central building in Schäferberg, Germany. In the front, the squeezing bench containing the squeezed-light source and the squeezing injection path is shown. The optical table is surrounded by several vacuum chambers containing suspended interferometer optics. SOURCE: Reprinted with permission from The LIGO Scientific Collaboration. 2011. A gravitational wave observatory operating beyond the quantum shot-noise limit. Nature Physics 7(12):962-965.
35 Mehmet, M., S. Ast, T. Eberle, S. Steinlechner, H. Vahlbruch, and R. Schnabel. 2011. Squeezed light at 1550 nm with a quantum noise reduction of 12.3 dB. Optics Express 19:25763-25772.
36 Dutton, Z., J.H. Shapiro, and S. Guha. 2010. LADAR resolution improvement using receivers enhanced with squeezed vacuum injection and phase-sensitive amplification. Journal of the Optical Society of America B 27:A63-A72.
Although high-resolution remote sensing with optical synthetic aperture radar is discussed in Chapter 4 on defense, the ability to do high-resolution imaging at long ranges can have applications in areas other than for the military. Planet Earth videos37 used defense-developed equipment for what at the time was long-range imaging, so animals could be remotely observed in their natural habitat without the observation changing the animals’ behavior. In disaster scenarios, long-range imaging can help to plan relief activities.
The performance of astronomical telescopes and free-space laser communication systems is severely limited by the effects of atmospheric distortion. Similarly, in microscopy and retinal imaging, optical aberrations can prevent one from achieving diffraction-limited resolution. “Adaptive optics (AO) is a technology that is used to improve the performance of optical systems by reducing the effect of wavefront distortions. It works by measuring the distortions in a wavefront and compensating for them with a device that corrects the errors, such as a deformable mirror or a liquid-crystal array.”38 Tremendous advances continue to occur in the technology and applications of adaptive optics.39 For example, in the not-too-distant future, a patient may, after having cataract surgery, be able to have a personalized aberration-corrected lens implanted that would give the person better vision than she or he had been born with.40
The ultimate technical challenge in sensing is to be able to detect something even at very low levels or with very high specificity, such as trace concentrations of toxic pollutants in the atmosphere, a specific biochemical structure, vibrations on the fuselage or wings of an airplane in order to gain early indications of crack formation, or variations in Earth’s gravity to facilitate a search for oil or other hidden objects. Imaging is sensing as a function of location to obtain a spatial rendering of whatever is being sensed. The goal of metrology is to ensure that the output
38BBC News. 2011. “‘Adaptive Optics’ Come into Focus.” Available at http://www.bbc.co.uk/news/science-environment-12500626. Accessed May 29, 2012.
39BBC News. 2011. “‘Adaptive optics’ Come into Focus.”
40 Chris Dainty, Professor of Applied Optics, National University of Ireland, Galway. Communication to the committee. May 15, 2011.
of a sensing device can be accurately tied to the sensed quantity, such as “This many units of the sensor reading correspond to this many grams of the pollutant per liter” in the first example above. Therefore, harnessing light for ever-more-advanced and reliable applications in advanced photonic measurements and applications is intimately tied to our basic understanding of how light interacts with matter and how we can manipulate and detect light at the very fundamental level. The technological advances since the publication of the NRC’s 1998 Harnessing Light report41 have already enabled new measurement capabilities and narrowed the gap between “high-end” laboratories and more modest facilities in terms of measurement capabilities. Those advances will be a significant catalyst for the next wave of advances in both fundamental and applied research. The proliferation of high-resolution sensors in consumer devices has enabled a market opportunity to leverage these new measurement capabilities for applications that would otherwise not be economically viable. Below are some examples of technological opportunities enabled by recent advances in sensing, imaging, and metrology.
Cost-Effective Biomedical Sensing Devices
The general field of nanophotonics is likely to remain promising and active in research in coming years for biochemical and biomedical sensing. Because many nanopatterning and nanofabrication tools (such as optical lithography developed for IC fabrication and other novel techniques, such as nanoimprint lithography42) are capable of mass manufacture of precisely controlled nanostructures, there is significant potential for implementing novel practical applications. Research focused on those application possibilities will be increasingly important. Highly chemical-specific and low-cost biochemical sensing will be a particularly important application.
Such devices as cell phone cameras already offer a ubiquitous optical sensing platform that is networked. Mobile phone subscriptions worldwide have passed 5 billion.43 Extensions of such technology—possibly with the addition of light sources to excite fluorescence, novel microscopy approaches, or more sophisticated spectral detection capabilities—may allow widely available remote medical or
41 National Research Council. 1998. Harnessing Light.
42 Osborne, M. 2005. “Enhanced Nanoimprint Process for Advanced Lithography Applications.” White paper. Available at http://www.fabtech.org/white_papers/_a/enhanced_nanoimprint_process_for_advanced_lithography_applications/. Accessed January 17, 2012.
43 Associated Press. 2010. “Number of Cell Phones Worldwide Hits 4.6B.” Available at http://www.cbsnews.com/stories/2010/02/15/business/main6209772.shtml. Accessed December 5, 2011.
physiological monitoring or diagnostic techniques44 with major impact on global health.
Exploiting the Quantum Detection and Manipulation of Light
At the macroscopic level, such as experienced when one is sitting in a lighted room, one perceives light to vary in a continuous, classical manner. For instance, a dimmer switch can control the brightness of light in a room and can be continuously varied from daylight conditions to the extreme darkness of nighttime. At the microscopic level, however, light consists of quantized packets of energy. A beam of light can be thought of as a flux of photons. When faint light is detected, instead of a detector output changing continuously, the detector observes random clicks corresponding to the absorption of specific photons by the detector. A familiar analogy is watching sand flow through an hourglass. When viewed from a distance, the falling of sand appears to be a smooth continuous flow. However, when viewed close up, it can be seen as the granular dropping of the sand particles. If one were to count the number of sand particles crossing the neck of the hourglass per second, one would obtain a randomly varying number from one second of counting to the next, and the flow rate would only seem to be constant. The same applies to the measurement of light by a detector. The light that one would want to detect after it interacts with the transducer in the sensor would have random variations (usually called noise) in the measured photon count, yielding uncertainty or error in the value of the sensed quantity. That kind of noise is called the shot noise, and the resulting error is a fundamental property of the process because it is related to the elementary nature of light. Thus, it would appear that the error due to shot noise would set ultimate limits on the sensitivity of sensing, imaging, and metrology systems. That is, the very basic granular nature of light would in general prevent us from sensing extremely weak signals.45
The quantum manipulation of the generation and detection of light, however, offers new opportunities. Research in the last couple of decades has shown that the arrangement of quanta in a beam of light can be manipulated. For example, instead
44 Zhu, H., S. Mavandadi, A.F. Coskun, O. Yaglidere, and A. Ozcan. 2011. Optofluidic fluorescent imaging cytometry on a cell phone. Analytical Chemistry 8(17):6641-6647.
45 For example, it is possible to reduce shot noise by means of squeezed light injection in the LIGO; this is leading to enhanced sensitivity in the quest for the detection of the gravity waves.
of being a random flow,46 the photons in a light beam can be regularized (photon antibunching)47 so that on detection the uncertainty in measurement would be reduced. Similarly, many other types of manipulations of photons in light beams can be made, such as creating paired photons that maintain their intimate quantum mechanical phase-coherent correlation (entanglement)48 no matter how far apart they are.49,50 Such novel photonic quantum states of light are already proving to be extremely potent. For example, there is the possibility of using entangled photons for creating shared secrets between remote users for the purpose of communicating securely.51 Such techniques of quantum cryptography have been demonstrated and are being commercialized,52,53 and there is much potential for ensuring the privacy of communications in ways that are tamperproof.54 However, much more research and technology development need to happen before the promise of global-scale, highly secure communications protected by the fundamental laws of quantum physics can be realized. For example, the current systems have limited reach owing to the lack of a suitable quantum repeater technology—unlike the ubiquitous optical amplifiers in the case of conventional optical communications—and are slow owing to poor quantum efficiency and low speed of single-photon detectors. Many promising paths of research and technology development are being pursued worldwide, but the United States is consistently losing ground in this field for lack
46 It turns out that ordinary lasers at their best emit light beams in the form of random flow of photons characterized by the so-called Poisson distribution. When such light is detected, the shot-toshot variation in the photon count (standard deviation) in a unit time interval equals the square root of the average photon count in that time interval. Detection of light is thus very uncertain when the irradiance is weak enough (low-light-level illumination) for the detector to see only a few photons over its response time.
47 Teich, M.C., and B.E.A. Saleh. 1990. Antibunched light. Physics Today (43)6:26-34.
48 Zeilinger, A. 2010. Dance of the Photons: From Einstein to Quantum Teleportation. New York, N.Y.: Farrar Straus Giroux.
49 Ursin, R., F. Tiefenbacher, T. Schmitt-Manderbach, H. Weier, T. Scheidl, M. Lindenthal, B. Blauensteiner, T. Jennewein, J. Perdigues, P. Trojek, B. Ömer, M. Fürst, M. Meyenburg, J. Rarity, Z. Sodnik, C. Barbieri, H. Weinfurter, and A. Zeilinger. 2007. Entanglement-based quantum communication over 144 km. Nature Physics 3:481-486.
50 Dynes, J.F., H. Takesue, Z.L. Yuan, A.W. Sharpe, K. Harada, T. Honjo, H. Kamada, O. Tadanaga, Y. Nishida, M. Asobe, and A.J. Shields. 2009. Efficient entanglement distribution over 200 kilometers. Optics Express 17:11440-11449.
51 Gisin, N., G. Ribordy, W. Tittel, and H. Zbinden. 2002. Quantum cryptography. Reviews of Modern Physics 74:145-195.
54 Scarani, V., H. Bechmann-Pasquinucci, N.J. Cerf, M. Dušek, N. Lütkenhaus, and M. Peev. 2009. The security of practical quantum key distribution. Reviews of Modern Physics 81:1301-1350.
of adequate support for basic science and technology development. For instance, Europe, Japan, and China have roadmaps for breaching the distance limit by way of low-Earth-orbit satellite terminals, but the U.S. funding agencies, once a leader in free-space quantum cryptography and communications, so far have announced no plans.55
The fundamental quantum nature of light is such that our ability to produce light beams with prearranged photonic structure (light of a specified quantum state) is intimately tied to our ability to measure the arrangement of photons in a light beam. Although tremendous progress has been made in the last couple of decades in “seeing” photons,56 it remains a technical challenge to detect light at single-photon resolution with a high degree of confidence and precision and certainly in a cost-effective manner. This is despite the widely accepted belief that the human eye is capable of resolving single or very small numbers of photons57 and that photomultiplier tubes capable of detecting light at the single-photon level have been around for over a half-century. Instead of measuring light with single-photon resolution, the current generation of instruments puts out either no click with high probability when no photons arrive or one click no matter how many photons arrive in the detector’s response time. In addition, when the photons do arrive, the probability of detection is very limited (about 70 percent for visible to near-infrared light and about 20 percent in the telecommunications waveband).58,59 Nonetheless, progress is being made; devices and instruments with arrays of single-photon detectors for imaging applications are beginning to appear on the market, and technologies based on superconducting devices have been demonstrated in research laboratories.
In addition to diagnosing the photonic structure of light beams, the technology of detecting light efficiently and reliably at the single-photon level will open a host of other opportunities because such technology will revolutionize how we quantify light. Measuring light level (brightness) is typically an analog measurement that is notoriously hard to make precise and accurate. Counting photons will turn such measurements into an inherently digital form by basing the measurements on fundamental
55 Hughes, R., and J. Nordholt. 2011. Refining quantum cryptography. Science 333:1584-1586.
56 National Research Council. 2010. Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays. Washington, D.C.: The National Academies Press.
57 Wolpert, H.D. 2002. “Life lessons: Photonic Systems in Nature Can Offer Technical Insights to Designers of Optical Systems and Detectors.” SPIE Newsroom. Available at http://spie.org/x25379.xml?ArticleID=x25379. Accessed August 1, 2012.
unit of energy.60 Because the precision resulting from counting increases with the count rate, the ability to count photons at a high rate would spawn new metrological applications of light.
Although many advances that originated in the United States address optical manufacturing capabilities, there is almost no high-volume manufacturing of sensors and imagers within the United States. However, the proliferation of devices developed for consumer products presents a significant marketing opportunity. Many niche sensor markets could not be addressed without the capabilities enabled by these devices. One example is in biomedical sensing. There are capabilities in microscope systems costing more than $400,000 that could be partially addressed in a small device costing less than $10,000 that leverages capabilities provided by high-volume consumer device components. Because the resulting sales could be about 1,000 per year, these markets would not be efficiently addressed by a large microscope manufacturer. However, a small company could profitably address such a market. These niche markets rely on moving research advances into the market efficiently while exploiting the capabilities of components developed and priced for high-volume markets. A small company could keep most of the created jobs within the United States by leveraging the manufacture of low-cost devices that have steadily moved overseas. To address this market opportunity efficiently, an efficient coupling between basic and applied research in optics- and photonics-related technologies with industrial application partners is critical. An efficient partnership in this field could significantly add to U.S.-based jobs at all levels.
For many years, the United States has benefited from a leadership position in research in optics and photonics. However, the research capabilities of many countries have been steadily improving, and the gap is rapidly narrowing. As discussed earlier, several advances over the last decade have hastened that narrowing, and cutting-edge measurement capabilities are now available to a much broader set of researchers. While continued research in fundamental optical sciences will be critical in maintaining a leadership position, it will also be critical for the U.S. economy to move those advances into the market efficiently to capture the financial benefit of generated intellectual property. Although high-volume manufacturing is not typically done within the United States, there is a significant market opportunity
60 Migdall, A. 1999. Correlated photon metrology without absolute standards. Physics Today 52:41-46.
for leveraging high-volume consumer components with research advances to address low-volume markets. Capitalizing these niche markets efficiently could have a significant impact on U.S.-based jobs.
Key Finding: Optics and photonics have been critically important to advances in precision metrology, which has had a significant impact since publication of the NRC’s 1998 study Harnessing Light: Optical Science and Engineering for the 21st Century (for example, GPS, communications, and manufacturing). The importance of optics and photonics is now reflected in the adoption of optics-based SI definitions of the second and the meter.
Key Finding: There is a significant opportunity for the U.S. economy to exploit niche sensor markets that leverage consumer components and cutting-edge research applications. One example is in biomedical sensing in which low-volume manufacturing of devices could efficiently be maintained within the United States by leveraging high-volume consumer components, such as the high-resolution networked imagers now almost universally available in the form of cell phone cameras. Exploiting this advanced technology could enable portable and/or remote health monitoring and diagnosis.
Key Finding: Techniques of extreme nonlinear optics that promise table-top, coherent sources of extreme ultraviolet (EUV) and x-ray light have been developed. If this promise becomes real, it will profoundly affect such applications as sub-nanometer-scale lithography and determination of the structure of complex matter (biological proteins, for instance) on the atomic scale, further enabling advances in fields such as optical machining that rely on progressively shorter illumination wavelengths to improve manufacturing tolerances. This increased precision will be important for maintaining advances consistent with Moore’s law of ICs.
Key Finding: The ultimate sensitivity of any advanced photonic measurement and application system is fundamentally tied to the intrinsic photonic granularity of light. Measuring light with single-photon resolution and accuracy at high speeds will therefore improve the performance of such systems tremendously in analogy to how counting cycles of light waves for shorter and shorter wavelengths is paving the way for more accurate and precise measurements of time (first key finding above).
Finding: Precision metrology has improved and become more widely available because of the significant technological advances since the NRC’s Harnessing Light study was published in 1998. One example is octave-spanning optical combs, which
provide a direct link between RF and optical standards within a small table-top apparatus that is now commercially available. At the time of the 1998 study, linking between RF and optical standards took instrumentation that filled several rooms and was performed at only a few locations around the world.
Finding: Several countries around the world have made significant advancements in photonics research capabilities in the measurement area, and the research leadership gap between these countries and the United States has significantly narrowed in many disciplines.
Finding: Progress in nanophotonics, plasmonics, metamaterials, and other related fields of science and technology is opening a broad range of possibilities for the enhanced sensitivity, greater specificity, lower size, and lower cost of sensors. These possibilities will have significant impacts in various fields, including biochemical sensing.
Entangled photons and squeezed states are new subjects of research and development in the optics and photonics field and allow sensing options never previously considered.
Key Recommendation: The United States should develop the technology for generating light beams whose photonic structure has been prearranged to yield better performance in applications than is possible with ordinary laser light.
Prearranged photonic structures in this context include generation of light with specified quantum states in a given spatiotemporal region, such as squeezed states with greater than 20-dB measured squeezing in one field quadrature, Fock states of more than 10 photons, and states of one and only one photon or two and only two entangled photons with greater than 99 percent probability. These capabilities should be developed with the capacity to detect light with over 99 percent efficiency and with photon-number resolution in various bands of the optical spectrum. The developed devices should operate at room temperature and be compatible with speeds prevalent in state-of-the-art sensing, imaging, and metrology systems. U.S. funding agencies should give high priority to funding research and development—at universities and in national laboratories where such research is carried out—in this fundamental field to position the U.S. science and technology base at the forefront of applications development in sensing, imaging, and metrology. It is believed that this field, if successfully developed, can transfer significant technology to products for decades to come.
Key Recommendation: Small U.S. companies should be encouraged and supported by the government to address market opportunities for applying research advances to niche markets while exploiting high-volume consumer components. These markets can lead to significant expansion of U.S.-based jobs while capitalizing on U.S.-based research.
Recommendation: U.S. funding agencies should continue to support fundamental research in optics and photonics. Important subjects for future research include nanophotonics, extreme nonlinear optics, and number-resolving photon counters for a truly linear-mode single-photon detector. Support should be provided for applying advances to devices for market application.
The fifth grand challenge question is partially supported by the discussion in this chapter and is thus repeated here with some supporting information.
How can the U.S. optics and photonics community develop optical sources and imaging tools to support an order of magnitude or more of increased resolution in manufacturing?
Meeting this grand challenge could facilitate a decrease in design rules for lithography, as well as providing the ability to do closed-loop, automated manufacturing of optical elements in three dimensions. Extreme ultraviolet is a challenging technology to develop, but it is needed in order to meet future lithography needs. The next step beyond EUV is to move to soft x rays. Also, the limitations in three-dimensional resolution on laser sintering for three-dimensional manufacturing are based on the wavelength of the lasers used. Shorter wavelengths will move the state of the art to allow more precise additive manufacturing that could eventually lead to three-dimensional printing of optical elements.