Active optics, adaptive optics, and laser guide stars

Optical astronomy is crucial to our understanding of the universe, but the capabilities of ground-based telescopes are severely limited by the effects of telescope errors and of the atmosphere on the passage of light. Recently, it has become possible to construct inbuilt corrective devices that can compensate for both types of degradations as observations are conducted. For full use of the newly emerged class of 8-meter telescopes, such active corrective capabilities, known as active and adaptive optics, are essential. Some physical limitations in the adaptive optics field can be overcome by artificially created reference stars, called laser guide stars. These new technologies have lately been applied with success to some medium and very large telescopes.     

Gravitational lens optics

Several instances of multiple imaging of cosmologically distant sources by intervening galaxies and galaxy clusters have been discovered over the past decade. These "gravitational lenses" have distinctive optical properties. Pointlike sources such as quasars generally produce two or four images when lensed, whereas extended sources such as galaxies produce spectacular arcs and rings. The salient features of most of the observations can be reproduced with the use of simple elliptical lens models that approximate the lenses made by ellipsoidal mass distributions such as are common in the universe. In addition to illustrating simple optics in operation on a cosmological scale, multiple images and arcs provide useful probes of the lensing galaxies and clusters. Also, gravitational lenses can make magnified images of cosmologically distant sources and may eventually furnish important cosmographic data such as the Hubble constant.     

Single-cycle nonlinear optics

Nonlinear optics plays a central role in the advancement of optical science and laser-based technologies. We report on the confinement of the nonlinear interaction of light with matter to a single wave cycle and demonstrate its utility for time-resolved and strong-field science. The electric field of 3.3-femtosecond, 0.72-micron laser pulses with a controlled and measured waveform ionizes atoms near the crests of the central wave cycle, with ionization being virtually switched off outside this interval. Isolated sub-100-attosecond pulses of extreme ultraviolet light (photon energy approximately 80 electron volts), containing approximately 0.5 nanojoule of energy, emerge from the interaction with a conversion efficiency of approximately 10(-6). These tools enable the study of the precision control of electron motion with light fields and electron-electron interactions with a resolution approaching the atomic unit of time ( approximately 24 attoseconds).     

Adaptive optics revisited

From the earliest days and nights of telescopic astronomy, atmospheric turbulence has been a serious detriment to optical performance. The new technology of adaptive optics can overcome this problem by compensating for the wavefront distortion that results from turbulence. The result will be large gains in resolving power and limiting magnitude, closely approaching the theoretical limit. In other words, telescopic images will be very significantly sharpened. Rapid and accelerating progress is being made today by several groups. Adaptive optics, together with the closely related technology of active optics, seems certain to be utilized in large astronomical telescopes of the future. This may entail significant changes in telescope design.     

Transformation optics using graphene

Metamaterials and transformation optics play substantial roles in various branches of optical science and engineering by providing schemes to tailor electromagnetic fields into desired spatial patterns. We report a theoretical study showing that by designing and manipulating spatially inhomogeneous, nonuniform conductivity patterns across a flake of graphene, one can have this material as a one-atom-thick platform for infrared metamaterials and transformation optical devices. Varying the graphene chemical potential by using static electric field yields a way to tune the graphene conductivity in the terahertz and infrared frequencies. Such degree of freedom provides the prospect of having different "patches" with different conductivities on a single flake of graphene. Numerous photonic functions and metamaterial concepts can be expected to follow from such a platform.     

Phonon optics and phonon propagation in semiconductors

Recent experiments involving the propagation of extremely high frequency, short-wavelength acoustic phonons in semiconductors are described. Such phonons, which play an important role in thermal energy transpor and nonradiative recombination processes, can be used as sensitive microscopic probes of electronic, defect, and interface states. Experiments on phonon transmission thrugh epitaxially grown bulk material as well as thin-film superlattice structures in the semiconductor gallium arsenide are described. Such thin-film periodic structures can be used to build frequency-selective phonon filters and reflectors, which can in turn be used to manipulate phonon diagnostic beams in technologically important materials.     

Giant birefringent optics in multilayer polymer mirrors

Multilayer mirrors that maintain or increase their reflectivity with increasing incidence angle can be constructed using polymers that exhibit large birefringence in their indices of refraction. The most important feature of these multilayer interference stacks is the index difference in the thickness direction (z axis) relative to the in-plane directions of the film. This z-axis refractive index difference provides a variable that determines the existence and value of the Brewster's angle at layer interfaces, and it controls both the interfacial Fresnel reflection coefficient and the phase relations that determine the optics of multilayer stacks. These films can yield optical results that are difficult or impossible to achieve with conventional multilayer optical designs. The materials and processes necessary to fabricate such films are amenable to large-scale manufacturing.     

Looking to the future of quantum optics

Light has provided both fundamental phenomenology and enabling technology for scientific discovery for many years, and today it continues to play a central role in fundamental explorations and innovative applications. The ability to manipulate light beams and pulses with the quantum degrees of freedom of optical radiation will add to those advances. The future of quantum optics, which encompasses both the generation and manipulation of nonclassical radiation, as well as its interaction with matter, lies in the rich variety of quantum states that is now becoming feasible to prepare, together with the numerous applications in sensing, imaging, metrology, communications, and information processing that such states enable.