Two-Photon Photovoltaic Effect in Silicon

Optical amplification, wavelength conversion, and a myriad of other functions that were once considered to be beyond silicon's reach have been made possible by the material's nonlinear optical properties. The common feature of such devices is the high optical intensity that is required to induce the nonlinear optical interactions. Concurrent with the useful nonlinearities (Raman and Kerr) are two-photon absorption and free carrier scattering, which are two related and harmful phenomena that render silicon lossy at high intensities. This paper explores the use of the two-photon photovoltaic effect as a means to counter these phenomena in an energy-efficient manner. The effect reduces losses due to free carrier scattering and serendipitously scavenges the optical energy lost to two-photon absorption. Analytical and numerical modeling of the two-photon photovoltaic effect in silicon devices is presented. The model is validated through comparison with experimental results and is used to establish the limits of this approach for creating energy-efficient silicon photonic devices.     

Pulsed laser annealing of self-organized InAs/GaAs quantum dots

The effect of pulsed laser annealing (PLA), using an excimer laser, on the luminescence efficiency of self-organized InAs/GaAs and In_0.4Ga_0.6As/GaAs quantum dots has been investigated. It is found that such annealing can enhance both the peak and integrated photoluminescence (PL) efficiency of the dots, up to a factor of 5–10 compared to as-grown samples, without any spectral shift of the luminescence spectrum. The improved luminescence is attributed to the annealing of nonradiative point and extended defects in and around the dots.     

Gain Enhancement in Cladding-Pumped Silicon Raman Amplifiers

Core-pumped silicon Raman amplifiers reported so far have exhibited gains of 4 dB or less in continuous-wave operation, limited by free-carrier absorption. Here we propose a device design that mitigates the problem of free-carrier absorption and leads to significantly higher gain values. In the new design, we surround the silicon waveguide with an additional rib-shaped passive cladding with a refractive index between that of silicon and the silica buffer. By injecting the pump power into the cladding, instead of directly into the silicon core, the overlap of the pump power with the silicon core is reduced and thus the effect of free-carrier absorption on the pump is weakened. The amplifier may therefore be made much longer, and higher total gains be achieved than in conventional core-pumped amplifiers. We present a detailed analysis of a design where a total gain as high as 18 and 34 dB may be achieved with pump powers of 300 mW and 1 W, respectively. We describe the model we have developed to design and optimize cladding-pumped silicon Raman amplifiers. Explicit expressions are formulated for the nonlinear effective areas for stimulated Raman scattering and two-photon absorption in silicon waveguides, taking into account the differing tensorial structures of these nonlinearities.     

Linewidth enhancement factor and near-field pattern in tunnel injection In/sub 0.4/Ga/sub 0.6/As self-assembled quantum dot lasers

Measurements of ultra-low linewidth enhancement factor in ridge-waveguide tunnel injection In/sub 0.4/Ga/sub 0.6/As/GaAs self-assembled quantum dot lasers have been made and the results compared with similar quantum well lasers. Values of /spl alpha//spl sim/3.8 were measured in the quantum well lasers, and /spl alpha/ was /spl les/0.7 in the quantum dot lasers. The consequent suppression of filamentation in the quantum dot devices has also been observed.     

Silicon Photonics

After dominating the electronics industry for decades, silicon is on the verge of becoming the material of choice for the photonics industry: the traditional stronghold of III-V semiconductors. Stimulated by a series of recent breakthroughs and propelled by increasing investments by governments and the private sector, silicon photonics is now the most active discipline within the field of integrated optics. This paper provides an overview of the state of the art in silicon photonics and outlines challenges that must be overcome before large-scale commercialization can occur. In particular, for realization of integration with CMOS very large scale integration (VLSI), silicon photonics must be compatible with the economics of silicon manufacturing and must operate within thermal constraints of VLSI chips. The impact of silicon photonics will reach beyond optical communication-its traditionally anticipated application. Silicon has excellent linear and nonlinear optical properties in the midwave infrared (IR) spectrum. These properties, along with silicon's excellent thermal conductivity and optical damage threshold, open up the possibility for a new class of mid-IR photonic devices     

Small-signal modulation characteristics of p-doped 1.1- and 1.3-/spl mu/m quantum-dot lasers

We have investigated the small-signal modulation characteristics of 1.1and 1.3-/spl mu/m p-doped quantum-dot lasers in order to evaluate the potential of acceptor doping. The maximum measured 3-dB bandwidth of the 1.1- and 1.3-/spl mu/m lasers are 11 and 8 GHz, respectively, which are only marginally higher than those in the corresponding undoped devices.     

Measurement of modal gain in 1.1 /spl mu/m p-doped tunnel injection InGaAs/GaAs quantum dot laser heterostructures

Single-pass optical gain of self-assembled InGaAs/GaAs p-doped tunnel injection quantum dot laser heterostructures emitting at 1.1 /spl mu/m is measured by the multisection device technique. The heterostructures, consisting of three layers of quantum dots in the active region, demonstrate net modal gain as high as 57 cm/sup -1/.     

A Brouwer domain invariance approach to boundary behavior of Nyquist maps for uncertain systems

The boundary preserving properties of a fixed frequency Nyquist mapping from a compact, convex region of uncertainty inR ^" onto the Horowitz template are investigated via a topological approach. In the casen=2, the boundary preserving properties of the Nyquist mapping and its inverse are shown to be corollaries of the Brouwer domain invariance and the Carathéodory conformal invariance of the boundary. The casen>2 is derived from the casen=2 by taking many two-dimensional slices through the domain. Finally, the edge test results are examined in the light of the Brouwer domain invariance. The second author was supported by NSF Grant ECS-95-10656.