Mapping local charge recombination heterogeneity by multidimensional nanospectroscopic imaging.

As materials functionality becomes more dependent on local physical and electronic properties, the importance of optically probing matter with true nanoscale spatial resolution has increased. In this work, we mapped the influence of local trap states within individual nanowires on carrier recombination with deeply subwavelength resolution. This is achieved using multidimensional nanospectroscopic imaging based on a nano-optical device. Placed at the end of a scan probe, the device delivers optimal near-field properties, including highly efficient far-field to near-field coupling, ultralarge field enhancement, nearly background-free imaging, independence from sample requirements, and broadband operation. We performed ~40-nanometer-resolution hyperspectral imaging of indium phosphide nanowires via excitation and collection through the probes, revealing optoelectronic structure along individual nanowires that is not accessible with other methods.

Electrical detection of the spin resonance of a single electron in a silicon field-effect transistor.

The ability to manipulate and monitor a single-electron spin using electron spin resonance is a long-sought goal. Such control would be invaluable for nanoscopic spin electronics, quantum information processing using individual electron spin qubits and magnetic resonance imaging of single molecules. There have been several examples of magnetic resonance detection of a single-electron spin in solids. Spin resonance of a nitrogen-vacancy defect centre in diamond has been detected optically, and spin precession of a localized electron spin on a surface was detected using scanning tunnelling microscopy. Spins in semiconductors are particularly attractive for study because of their very long decoherence times. Here we demonstrate electrical sensing of the magnetic resonance spin-flips of a single electron paramagnetic spin centre, formed by a defect in the gate oxide of a standard silicon transistor. The spin orientation is converted to electric charge, which we measure as a change in the source/drain channel current. Our set-up may facilitate the direct study of the physics of spin decoherence, and has the practical advantage of being composed of test transistors in a conventional, commercial, silicon integrated circuit. It is well known from the rich literature of magnetic resonance studies that there sometimes exist structural paramagnetic defects near the Si/SiO2 interface. For a small transistor, there might be only one isolated trap state that is within a tunnelling distance of the channel, and that has a charging energy close to the Fermi level.

APPLIED PHYSICS: How to Be Truly Photonic.

Photonic crystals behave toward light waves as semiconductors do toward electron waves. Yablonovitch discusses a report by Noda et al., who have made a photonic crystal with unprecedented performance, using GaAs, the best material for integration into optoelectronic devices. According to Yablonovitch, the work thus represents a significant step toward photonic integrated circuits.

Engineered omnidirectional external-reflectivity spectra from one-dimensional layered interference filters.

Owing to optical refraction, external rays that are incident upon a high-refractive-index medium fall within a small internal cone angle. A one-dimensionally periodic Bragg structure can reflect over an angular acceptance range that is greater than the small internal refraction cone, if the internal refractive-index contrast is sufficient. Thus Winn et al. [Opt. Lett. (to be published)] charted the range of refractive indices at which omnidirectional external reflection occurs. A wide spectral gap requires a high-index contrast. It is proposed that, by chirping or grading the periodicity of the structure, one can cover an arbitrarily wide spectral range with only a modest index contrast and, furthermore, that arbitrary spectral shapes can be produced. The graded-periodicity approach requires only a modest index contrast, provided that the average refractive index is >2 .

Single-shot two-photon exposure of commercial photoresist for the production of three-dimensional structures.

We report the use of an amplified femtosecond laser for single-shot two-photon exposure of the commercial photoresist SU-8. By scanning of the focal volume through the interior of the resist, three-dimensional (3-D) structures are fabricated on a shot-by-shot basis. The 800-nm two-photon exposure and damage thresholds are 3.2 and 8.1TW/cm(2), respectively. The nonlinear nature of the two-photon process allows the production of features that are smaller than the diffraction limit. Preliminary results suggest that Ti:sapphire oscillators can achieve single-shot two-photon exposure with thresholds as low as 1.6TW/cm(2) at 700 nm, allowing 3-D structures to be constructed at megahertz repetition rates.

Two-dimensional photonic-crystal vertical-cavity array for nonlinear optical image processing.

We investigate the electromagnetic properties of a two-dimensional (2-D) photonic-crystal array of vertical cavities for use in nonlinear optical image processing. We determine the 2-D photonic band structure of the array, and we discuss how it is influenced by the degree of interaction between cavities. We study the properties of defects in the 2-D lattice and show that neighboring cavities interact through their overlapping wave functions. This interaction can be used to produce nearest-neighbor nonlinear Boolean functions such asand, or, and xor, which are useful for optical image processing. We demonstrate the use of 2-D photonic bandgap structures for image processing by removing noise from a sample image with a nearest-neighbor and function.

The chemistry of solid-state electronics.

Since the original theoretical insights of Bardeen and Shockley about 40 years ago, the progress of solid-state electronics has been paced by the ability to control chemical bonding structures, particularly at surfaces and interfaces. The functioning of solid-state devices depends on being able to produce interfacial structures with a minimum number of defective chemical bonds. A series of chemical discoveries and insights, on germanium (Ge) and silicon (Si) surfaces and gallium arsenide-aluminum arsenide (GaAs-AlAs) interfaces, has brought the electronics revolution to its present state of development. In most cases, the technological consequences of these accidental discoveries could not be accurately foreseen. With that caution, the technological prognosis for some current research is also reviewed.

Thermodynamics of daylight-pumped lasers.

We cast the problem of very-low-threshold, daylight-pumped lasers in a general thermodynamic framework. We calculate that the requirements to reach threshold are that the Stokes shift of such a laser be greater than 13.3kT and that the absorption ratio of the pump/emission band and the geometrical aspect ratio both be greater than exp (13.3). We describe some luminescent material systems that might be able to satisfy these rigid requirements.