Pei-Cheng Ku

Slow-light optical buffers: capabilities and fundamental limitations

This paper presents an analysis of optical buffers based on slow-light optical delay lines. The focus of this paper is on slow-light delay lines in which the group velocity is reduced using linear processes, including electromagnetically induced transparency (EIT), population oscillations (POs), and microresonator-based photonic-crystal (PC) filters. We also consider slow-light delay lines in which the group velocity is reduced by an adiabatic process of bandwidth compression. A framework is developed for comparing these techniques and identifying fundamental physical limitations of linear slow-light technologies. It is shown that slow-light delay lines have limited capacity and delay-bandwidth product. In principle, the group velocity in slow-light delay lines can be made to approach zero. But very slow group velocity always comes at the cost of very low bandwidth or throughput. In many applications, miniaturization of the delay line is an important consideration. For all delay-line buffers, the minimum physical size of the buffer for a given number of buffered data bits is ultimately limited by the physical size of each stored bit. We show that in slow-light optical buffers, the minimum achievable size of 1 b is approximately equal to the wavelength of light in the buffer. We also compare the capabilities and limitations of a range of delay-line buffers, investigate the impact of waveguide losses on the buffer capacity, and look at the applicability of slow-light delay lines in a number of applications.

Slow light in semiconductor quantum wells.

We experimentally demonstrate slow-light via population pulsation in semiconductor quantum well structures. A group velocity as small as 9600 m/s is inferred from the measured dispersive characteristics. The transparency window exhibits a bandwidth of 2 GHz

Slow light in semiconductor heterostructures

This paper presents an overview of slow light in semiconductor heterostructures. The focus of this paper is to provide a unified framework to summarize and compare various physical mechanisms of slow light proposed and demonstrated in the past few years. We expand and generalize the discussions on fundamental limitation of slow light and the delay–bandwidth product trade-off to include gain systems and other mechanisms such as injection locking. We derive the maximum fractional delay and compare the differences between material dispersion and waveguide dispersion based devices. The delay–bandwidth product is proportional to the square root of the device length for a material dispersion based device but has a linear relationship for a waveguide dispersion based device. Possible scenarios to overcome the delay–bandwidth product limitation are discussed. The prospects of slow light in various applications are also investigated. (Some figures in this article are in colour only in the electronic version)

Novel Epitaxial Nanostructures for the Improvement of InGaN LEDs Efficiency

We demonstrated that the efficiency of an InGaN LED can be improved by using a novel epitaxial nanostructure, namely, the nanostructured semipolar (NSSP) gallium nitride (GaN). The NSSP GaN template was fabricated on a c-plane GaN surface using a standard GaN metal-organic chemical vapor deposition tool on c-plane sapphire substrates. We showed that the surface of NSSP GaN consisted of two semipolar orientations: (10-11) and (11-22). InGaN/GaN multiple quantum wells (MQWs) fabricated on NSSP GaN exhibited negligible quantum-confined Stark effect (QCSE) and a 30% improvement in internal quantum efficiency as compared to planar c-plane InGaN/GaN MQWs. Using time-resolved photoluminescence (PL), a considerable improvement in radiative recombination lifetime was also observed. We fabricated and characterized semipolar InGaN LEDs on NSSP GaN that emitted at 543 nm and showed negligible QCSE. The NSSP GaN structure can also be applied to improve the photon extraction efficiency of InGaN-based LEDs. The surface texturing was performed in situ together with the LED epitaxy without additional ex situ etching processes. The in situ surface texturing improved the PL intensity by a factor of two. An electrical injection LED structure employing in situ surface texturing was also demonstrated.

Inducing electron spin coherence in GaAs quantum well waveguides: spin coherence without spin precession

Electron spin coherence is induced via light-hole transitions in a quantum well waveguide without a DC magnetic field. The spin coherence is detected through effects of quantum interference in spectral domain nonlinear optical response.

Slow-light in semiconductor quantum wells

We experimentally demonstrate slow-light via population pulsation in semiconductor quantum well structures. A group velocity as small as 9600 m/s is inferred from the measured dispersive characteristics. The transparency window exhibits a bandwidth of 2 GHz

Semiconductor all-optical buffers using quantum dots in resonator structures

A variable semiconductor all-optical buffer is proposed. The buffer uses electromagnetically-induced transparency in quantum dots. With resonator structures, the performance can be further improved. A device storing 168 bits in a 10 Gb/s system can be obtained at room temperature.

Fundamental limitations of slow-light optical buffers

We show that slow-light optical delay-line buffers are constrained by fundamental physical limitations. We compare the capabilities of a variety of buffer technologies including electromagnetically induced transparency, population pulsations, photonic crystal filters, and fibre-based delay lines.

Thermal oxidation of AlGaAs: modeling and process control

A simple physical model is developed for the thermal oxidation process of AlGaAs using the continuity equation. The model is based on the principle of oxidant mass conservation. Theoretical calculations are compared with experimental data to a good agreement. The model is then applied to the study of VCSEL batch fabrication. Several control parameters are discussed including AlGaAs layer thickness, aluminum composition, initial mesa size, spacing between two adjacent devices, oxidation time, and oxidation temperature.

Slow Light Using P-Doped Semiconductor Heterostructures for High-Bandwidth Nonlinear Signal Processing

We propose a novel scheme for slow light based on a resonant three-level lambda system (RTLS) in a p-doped semiconductor heterostructure. Numerical simulations show that a slow-down factor of 145 and a slow-down-bandwidth product exceeding 200 THz can be achieved in semiconductor quantum wells at room temperature. These figures of merit make the RTLS slow light especially useful for the enhancement of the optical nonlinearity in high bandwidth all-optical signal processing applications.