Anisotropic emission of interface fluctuation quantum dots

We calculate energy levels, dipole moments and radiative broadening of interface fluctuation quantum dots. For optically allowed states, the dipole moment grows proportionally to the lateral quantum dot radius while the radiative broadening saturates towards the quantum well radiative broadening for large lateral quantum dot radii. This is accompanied by a change in the angular emission pattern, concentrating emission in forward and backward direction. Optically forbidden states do not couple to light propagating in the growth direction yet they may have a considerable radiative broadening due to spontaneous emission in other directions. Received 20 March 2002 Published online 25 June 2002     

Excitons and electron-hole plasma in quasi-two-dimensional systems

A theoretical study of many-body effects in quasi-two-dimensional electron-hole systems is presented. The renormalized single-particle energies and the exciton binding energy are calculated as functions of the carrier density and temperature. A simple model for the nonlinear excitonic absorption and refraction is proposed.     

Two-dimensional spectral shearing interferometry for few-cycle pulse characterization

We present a new method for measuring the spectral phase of ultrashort pulses that utilizes spectral shearing interferometry with zero delay. Unlike conventional spectral phase interferometry for direct electric-field reconstruction, which encodes phase as a sensitively calibrated fringe in the spectral domain, two-dimensional spectral shearing interferometry robustly encodes phase along a second dimension. This greatly reduces demands on the spectrometer and allows for complex phase spectra to be measured over extremely large bandwidths, potentially exceeding 1.5 octaves.     

High-efficiency stepwise contraction and adsorption nanolithography

A new miniaturization protocol is demonstrated using stretching and relaxation of an elastomer substrate. A designed microstructure is formed on the stretched substrate and subsequently becomes miniaturized when the substrate relaxes. More importantly, the miniaturized structures can be transferred onto a new substrate for further miniaturization or can be utilized as stamps for nanolithography of designated materials. As an example of this approach, an elastic mold was first cast from a Si mold containing periodic line arrays of 1.5-microm line width. Upon relaxation, line width is reduced to 240 nm. The new elastomer may be used as stamps for micro- and nanofabrication of materials such as proteins. The polymer surface roughness or wrinkling behavior at nanoscale is found to follow classic stability model in solid mechanics. This observation provides means to design and control the surface roughness to meet specific requirements.