Matter in new light and why light matters
Even after centuries of basic science, research into the nature of light and its potential use remains at the scientific frontier. The creation of intense laser pulses lasting for less than 10–17 seconds will make it possible to capture fast movements of electrons. In the coming decade, the creation of artificial materials with new optical properties, especially along lines inspired by biomaterials, will revolutionize optics.
As a fundamental mystery, light has inspired scientists for centuries. Across the electromagnetic spectrum, from long-wavelength infrared radiation through X-rays and high-energy γ-rays, light is an indispensable tool for probing matter in atomic and solid-state physics and astrophysics, as well as in chemistry, biology and medicine. Yet research into the fundamental nature of light and its potential uses remains among the most vibrant areas of science.
In the decade to come, optics research and applications will rapidly progress along several lines. As illustrated recently, promising avenues include the fundamental exploration of how light interacts with matter, the engineering of new materials with hitherto unknown optical properties, and efforts to put light to use in powerful new schemes for information processing and communication.
INTERACTION OF LIGHT WITH MATTER: RECENT ADVANCES
The movement of electrons in atoms or molecules, or inside bulk materials, underlies the properties of all matter, including the chemistry that sustains life. Electronic events take place on a short time scale of around 10–18 seconds (1 as)1, and it is only recently that physicists have devised laser light sources that come close to being able to take snapshots this quickly2. These laser pulses make ‘light-wave electronics’ possible, not only to study electron motion in precise detail, but also to steer individual electrons during chemical reactions so as to form desired chemical bonds. Short pulses of light with tailored properties have also been used to manipulate the collective quantum behaviour of many electrons in complex materials.
The interaction of intense X-ray laser beams with atomic nuclei has also opened up the new field of nuclear quantum optics3. An intense laser field might cause an electron to be released from an atom and then to collide again with that atom with high energy. This effect mimics the action of a high-energy particle accelerator and might soon be used to probe the physics of the atomic nucleus.
In the past decade, researchers have also made enormous progress in learning to design and fabricate artificial materials with optical properties unlike anything in nature, as well as tailoring these properties for specific applications. For example, photonic-crystal fibres, which are glass fibres with microscopic channels running parallel to the fibre axis (Fig. 1), have recently been exploited for optical sensing and to control fluid flows in microscopic geometries (an area known as microfluidics)4. Another promising area of advanced materials is nanophotonics, which explores the optical properties of single nano particles and molecules that are able to serve as unusually sensitive sources or detectors of light.
In the related and rapidly advancing area of biophotonics, researchers probe the interactions between biological materials and light, especially the emission, detection, absorption, reflection and modification of light from biomolecules in cells and tissues. This area presents many emerging opportunities, such as using light to manipulate DNA and in new devices for medical diagnostics, imaging, photochemistry or environmental sensing.
Old techniques too have been revitalized. In laser spectroscopy for example, the invention of the frequency-comb technique5 — a method to count individual oscillations of a light wave — allows precision in the measurement of the spectral lines of hydrogen of 1 in 1014. Building on this advance, researchers have been able to carry out precise tests of fundamental physical laws, especially those of the theory of quantum electrodynamics, which describes the interaction between light and matter. Spectroscopy is also being used in advanced applications in surprising areas such as energy storage. Light conversion in photovoltaic devices (‘artificial photosynthesis’) or photocatalytic water-splitting cells (which could produce hydrogen for a putative hydrogen-based economy) takes place one photon at a time in processes on the atomic or molecular scale. New spectroscopic techniques have provided information of immense value for such processes, which often lie beyond the reach of other kinds of experiment.
Research into light–matter interactions will advance in coming years, especially in a number of key areas including, for example, the continuing development of ‘free-electron lasers’, already creating flashes of light that are 109 times brighter and 104 times shorter than synchrotron radiation6. These pulses will allow scientists to track the making and braking of chemical bonds during reactions by following the positions of specific ions, or to image the structure of biomolecules directly from those present in a gas (without the current need for crystallization). Superfast imaging will also allow physicists to study the basic processes in solid material by which electronic excitations lose their energy to lattice vibrations7.
The frequency-comb technique has already improved the accuracy of measurements in time to one in 1017. This advance will further spur many new applications in telecommunications, global-positioning systems and elsewhere, including fundamental science (such as the study of gravity).
Efforts continue to improve the control of single quantum particles such as atoms or photons, which obey the laws of quantum physics and so exhibit fundamentally different properties from those of large-scale objects. A key challenge is to exploit quantum physics to gain full control of such individual quanta. In optics, this would mean controlling the properties of single photons, including their spatial and temporal shape, as well as their interactions with emitters and absorbers8. Achieving such control will stimulate important progress towards quantum computing and other applications of quantum physics in information processing9.
Work in modern optics is also overcoming barriers long thought to be insurmountable. For example, scientists previously believed that the wavelength of light limited the possible spatial resolution that can be achieved in far-field imaging. This limit has now been broken, however, and spatial resolution in the range of nanometers can be achieved10. Improved imaging techniques have a wide range of applications in cell biology for example, as well as in surface physics and chemistry.
Finally, on a related theme, progress in the ability of scientists to control the interactions of light with specially designed micrometer-sized and nanometer-sized structures has led to the discovery of surprising phenomena, such as materials with a negative refractive index, the possibility of making ‘cloaks of invisibility’, and slowing down and stopping the propagation of light. So far, all this is so far restricted to narrow frequency bands. Further exploration of these phenomena should trigger advances in fundamental science and has the potential to create new areas of technology.
The Max Planck Institute for Quantum Optics succeeded in real-time observation of electron-charge transport in solids. Scientists liberated electrons in tungsten crystal by attosecond ultraviolet pulses and used the electric field of an infrared laser pulse as an attosecond ‘stopwatch’ to measure the arrival time of the electrons at the surface. Tracking electrons as they traverse atomic layers in solids might help advance electronics to its ultimate speed limit at light frequencies (Cavalieri, A. L. et al. Nature 449, 1029, 2007).