Ali Najmaie

Nanoscale porous silicon waveguide for label-free DNA sensing

Porous silicon (PSi) is an excellent material for biosensing due to its large surface area and its capability for molecular size selectivity. In this work, we report the experimental demonstration of a label-free nanoscale PSi resonant waveguide biosensor. The PSi waveguide consists of pores with an average diameter of 20nm. DNA is attached inside the pores using standard amino-silane and glutaraldehyde chemistry. Molecular binding in the PSi is detected optically based on a shift of the waveguide resonance angle. The magnitude of the resonance shift is directly related to the quantity of biomolecules attached to the pore walls. The PSi waveguide sensor can selectively discriminate between complementary and non-complementary DNA. The advantages of the PSi waveguide biosensor include strong field confinement and a sharp resonance feature, which allow for high sensitivity measurements with a low detection limit. Simulations indicate that the sensor has a detection limit of 50nM DNA concentration or equivalently, 5pg/mm2.

Pure spin current from one-photon absorption of linearly polarized light in noncentrosymmetric semiconductors

We show that one-photon absorption of linearly polarized light should produce pure spin currents in noncentrosymmetric semiconductors, including even bulk GaAs. We present 14x14 k.p model calculations of the effect in GaAs, including strain, and pseudopotential calculations of the effect in wurtzite CdSe.

Spin current injection by intersubband transitions in quantum wells.

We show that a pure spin current can be injected in quantum wells by the absorption of linearly polarized infrared radiation, leading to transitions between subbands. The magnitude and the direction of the spin current depend on the Dresselhaus and Rashba spin–orbit coupling constants and light frequency and, therefore, can be manipulated by changing the light frequency and/or applying an external bias across the quantum well. The injected spin current should be observable either as a voltage generated via the anomalous spin-Hall effect, or by spatially resolved pump–probe optical spectroscopy.

Optical injection and coherent control of a ballistic charge current in GaAs/AlGaAs quantum wells

We report all-optical injection and coherent control of a ballistic charge current in GaAs/AlGaAs quantum wells. This current arises through quantum interference of one- and two-photon absorption of ∼100 fs pulses with parallel linear polarizations, and its magnitude can be controlled by adjusting the relative phase of the incident pulses. By monitoring differential transmission using a spatially resolved optical pump–probe technique, we observe evidence of carrier motion associated with this ballistic current. Results are consistent with a theoretical treatment specific to quantum wells, and are qualitatively similar to previous measurements in bulk GaAs.

Generation of spin currents via Raman scattering

We show theoretically that stimulated spin-flip Raman scattering can be used to inject spin currents in doped semiconductors with spin-split bands. A pure spin current, where oppositely oriented spins move in opposite directions, can be injected in zinc blende crystals and structures. The calculated spin current should be detectable by pump-probe optical spectroscopy and anomalous Hall effect measurement.

Ultrafast extrinsic spin-Hall currents

We consider the possibility of ultrafast extrinsic spin-Hall currents, generated by skew scattering following the optical injection of charge or pure spin currents. We propose a phenomenological model for this effect in quantum well structures. An injected charge current leads to a spin-Hall-induced pure spin current, and an injected pure spin current leads to a spin-Hall-induced charge current. The resulting spin or charge accumulation can be measured optically.

All-Optical Control of Charge and Spin in GaAs: Densities and Currents

Charge and spin are two of the fundamental properties and defining characteristics of electrons and holes in semiconductors. The manipulation of charge — the density and the current — has formed the basis of the ongoing solid-state electronic revolution of the last fifty years. Recently, it has been realized that a deeper understanding and more complete control of spin could lead to novel data processing and storage schemes. This interest has sparked the rapid growth of the field of semiconductor “spintronics”, where electron spin, as opposed to charge, is used to carry information [1–4].

Coherent control of electron propagation and capture in semiconductor heterostructures

We theoretically study the use of quantum interference to coherently control the transverse direction in which carriers are optically injected in a semiconductor heterostructure, and the subsequent transport and capture of these carriers. We consider a structure consisting of three quantum wells, where carriers can be ejected from the middle one in a given direction by coherently controlled optical pulse excitations. After traveling through the barrier, electrons are slowed down by space-charge effects, and can be captured in the side wells by emitting a phonon. If the side wells are different, the coherent control of the injection can be monitored optically. We propose a design of a AlGaAs heterostructure for a possible experimental realization of the effects considered in this paper.

Porous Silicon Waveguides for Biosensing Applications

Porous silicon waveguides are designed and fabricated to achieve well-defined resonances suitable for high sensitivity biosensing. Two coupling schemes are investigated based on fabrication, measurement complexity, coupling losses, resonance quality, and DNA detection sensitivity.

Femtosecond dynamics of ballistic charge current gratings in GaAs

We investigate the dynamics of ballistic charge currents, optically-injected into GaAs by quantum interference of 1550 and 775 nm, 150 fs pulses. Space-charge induced transport and ambipolar diffusion successively define the relaxation dynamics on a 500 fs-20 ps timescale.