Is there a linewidth theory for semiconductor lasers?

Semiconductor laser generation begins at a critical injection when the gain and loss spectra touch each other at a singular frequency. In the framework of the standard (Schawlow-Townes-Lax-Henry) theory, the finite linewidth results from the account of fluctuations associated with the random spontaneous emission processes. This approach is based on the assumption that in the mean-field approximation the singular frequency generation persists for injection levels higher than critical. We show that this assumption in the framework of the Boltzmann kinetic equation for electrons and photons is invalid and therefore the standard description of semiconductor laser linewidth lacks theoretical foundation. Experimental support of the standard theory is also questionable

Waveguiding power of photonic crystal slabs

We consider the waveguiding by thin patterned slabs embedded in a homogeneous medium. In the longwave limit, the wave spectra of slabs are found to be well described by a single frequency-independent parameter, which we call the "guiding power". The guiding power can be evaluated in an effective medium approximation, similar to the Maxwell Garnett theory, but modified for the local field corrections specific to the two-dimensional geometry. The guiding power is different for the transverse magnetic (TM) and transverse electric (TE) polarizations. We show that the confinement factor of TM waves in a porous layer with high index ratio can exceed that for a homogeneous layer. Similarly enhanced confinement of TM waves is demonstrated for a layer of elongated cylinders or elliptic inclusion with a high axis length ratio. The effect originates from the suppression of local field effects and the increasing internal field in the inclusion. It may be useful in the design of far-infrared or THz quantum cascade lasers.Comment: 15 pages, 5 figure

Levy Flight of Photoexcited Minority Carriers in Moderately Doped Semiconductors: Theory and Observation

Spatial spread of minority carriers produced by optical excitation in semiconductors is usually well described by a diffusion equation. The classical diffusion process can be viewed as a result of a random walk of particles in which every step has the same probability distribution with a finite second moment. This allows applying the central limit theorem to the calculation of the particle distribution after many steps. However, in moderately doped direct-gap semiconductors the photon recycling process can radically modify the spatial spread. For this process, the steps in the random walk are defined by the reabsorption length of photons produced in radiative recombination. The step distribution has an asymptotic power-law decline. Moments of this distribution diverge and the displacement is governed by rare but large steps. Random walk of this kind is called the Levy flight. It corresponds to an anomalously large spread in space and a modified ("super-diffusive") temporal evolution. Here we discuss the first direct observation of the hole profile in n-doped InP samples over distances of the order of a centimeter and more than two orders of magnitude in hole concentration. Luminescence spectra and intensity were studied as a function of distance from the photo-excitation in a rather unusual geometry (homogeneous excitation of the wafer edge and observation of the luminescence spectra from the broadside). The intensity is proportional to the minority-carrier concentration and exhibits a slow power-law drop-off with no changes in the spectral shape. This power law gives a direct evidence of Levy-flight transport. It has enabled us to evaluate the index of the distribution, the characteristic distance of the minority-carrier spread and the photon recycling factor. The results are in good agreement with the theoretical analysis.Comment: 14 pages, 3 figure

Modal control in semiconductor optical waveguides with uniaxially patterned layers

Uniaxially patterned dielectric layers have an optical anisotropy that can be externally controlled. We study the effects of patterning the cladding or the core layer of a 3-layer optical waveguide on the polarization properties of propagating radiation. Particular attention is paid to the case when the core material is a semiconductor with optical gain. We discuss a number of devices based on incorporating an uniaxially patterned layer in the structure design, such as a polarization-insensitive amplifier, a polarizer, an optically-controlled polarization switch, and an optically controlled modal coupler.Comment: 9 pages 7 figure

Positive Margins And Other Factors Associated With Survival In Early Stage Oral Cavity Squamous Cell Cancer: Prognostic Impact And Quality Measure

Objective: The aim of this work was to determine the prognostic impact of positive margins in early oral cavity squamous cell cancer and evaluate the utility of positive margin incidence as a surgical quality measure. Study design and setting: Retrospective analysis of the National Cancer Data Base Subjects and methods: Patients with oral cavity squamous cell cancer diagnosed between 1998 and 2011 who were treated with surgical resection were sampled. Univariate and multivariate analyses of overall survival and incidence of positive margins were performed. Results: A total of 6,830 patients were included in the survival analysis. Overall survival at 5-years was 69.7%. On multivariate analysis, neck dissection (HR 0.79, 95% CI 0.76-0.94) and treatment at academic/research institutions (HR 0.88, 95% CI 1.01-0.99) were associated with improved survival, while positive margins (HR 1.27, 95% CI 1.08-1.49), insurance through Medicare (HR 1.45, 95% CI 1.25-1.69) or Medicaid (HR 1.96, 95% CI 1.60-2.39), and adjuvant radiotherapy (HR 1.31, 95% CI 1.16-1.49), or adjuvant chemotherapy (HR 1.34, 95% CI 1.03-1.75) were associated with compromised survival. A total of 20,602 early oral cancer patients were identified for analysis of factors associated with positive margins. Margin status was reported in 94.8% of cases, and positive margins occurred in 7.5% of those cases. Incidence of positive margins by institution varied from 0% to 43.8%, with median incidence of 7.1%. Positive margins were associated with clinical factors including stage II disease (OR 1.75; 95% CI 1.55-1.98), intermediate grade (OR 1.20; 95% CI 1.04-1.37), high grade (OR 1.68; 95% CI 1.39-2.03), and floor of mouth (OR 1.78; 95% CI 1.52-2.08), buccal mucosa (OR 2.06 95% CI 1.59-2.68), and retromolar locations (OR 2.40, 95% CI 1.85-3.11). Positive margins were also associated with treatment at non-academic cancer centers (OR 1.23; 95% CI 1.04-1.44) and institutions with low oral cancer case volume (OR 1.45; 95% CI 1.23-1.69). Conclusion: Positive margins portend a poor prognosis in early oral squamous cell cancer. The incidence of positive margins is associated with clinicopathologic factors as well as treatment and institution factors and can serve as an effective surgical quality measure for early oral cavity squamous cell cancer

Legal Problems of Vocational and Professional Training during the Soviet Period of Stagnation

This article investigates the legal methods used to regulate professional training under Soviet labor law. It will examine relevant norms of labor legislation, the views of Soviet labor law specialists, and existing practice