This article assesses the impact of export policy on 21st century United States space power. The efficacy of current export controls in preventing the proliferation of space technologies and maintaining United States advantage is evaluated. The study finds that space launch technology has been and remains highly globalized; the United States has lost significant international communications satellite market share, but it maintains a tangible lead in position, navigation, and timing systems; and the recent growth in foreign imaging systems, both radar and optical, have eroded any advantage the United States once enjoyed. Furthermore, export controls have not appreciably slowed the internationalization of space and breaking dependency on the United States is a common underlying theme. The paper goes on to highlight the unintended consequences stemming from current policy, including limiting access to advanced technology of foreign origin, limiting access to foreign-born expertise, and adding "fog" and "friction" to the execution of programs that include foreign content. This paper finds current export control policy is incongruent with 21st century space power programs and initiatives, such as Operationally Responsive Space, Coalition Space, and "Soft" Space Power. Export control reform is suggested herein on the basis of developing a trusted community of trading partners.

We compare the results of different optical vertical-cavity surface-emitting laser models on the position-dependent effects of thin oxide apertures. Both scalar and vectorial models as well as hybrid models are considered. Physical quantities that are compared are resonance wavelength, threshold material gain, and modal stability. For large device diameters and low-order modes, the agreement between the different models is quite good. Larger differences occur when considering smaller devices and higher order modes. It is also observed that the spread in the resonance wavelengths is smaller than that for the threshold material gain.

We report a tunable red vertical cavity surface emitting laser (VCSEL) with a flexible micro-electro-mechanical (MEM) top distributed Bragg reflector. The peak emission wavelength is electrically tunable over a range of 15 nm from 644 to 659 nm.

We present a theoretical analysis of the optical physics of tapered oxide apertures in long- and short-cavity VCSELs. We apply our quasi-exact vector finite element model to a USC (long cavity) and U.Texas (short cavity) VCSEL to compute the electric field distribution, transverse confinement factor, diffraction rate, and threshold gain of the fundamental lasing mode. Making qualitative reference to the Hegblom, et al model, we analyze our results to deduce the fundamental physical effects of the tapered oxide aperture. We iind that tapered oxides reduce diffraction loss through two separate physical phenomena: (1) a reduction in transverse confinement yielding a flatter phase front, and (2) an effective lens which acts to refocus the naturally diffracting wave front. We further find that in most VCSELs an inherent trade-off exists between minimizing the diffraction loss and maximizing the optical mode-to-gain interaction. To achieve the ultimate goal of (near) thresholdless lasing, this trade-off must be overcome: diffraction loss must be eliminated while simultaneously minimizing the mode volume. We conclude with a suggestion for a novel cavity design, which in theory achieves this goal.

A tunable prototype red light-emitting n-p-n VCSEL with an n-doped micro-electromechanical (MEM) top distributed Bragg reflector is reported. The peak emission wavelength is tunable over a range of about 15 nm from 644 to 659 nm

We present a comparison between a semi-analytic parasitic mode weighted index method and a quasi-exact vector finite element method. We derive the parasitic mode weighted index method by augmenting the older weighted index method to capture diffraction. Diffraction is included by coupling the weighted index eigenmode to a set of resonant parasitic modes. We apply both techniques to an oxide-apertured, oxide-DBR VCSEL to compute the lasing mode wavelength, threshold gain, and transverse confinement factor.

We have performed quantum-mechanical analyses of strain- symmetrized Ge/Si QWIPs grown upon a realized buffer layer of Si0.4Ge0.6 on Si. The multi-quantum-well (MQW) QWIP has 50 angstrom-thick compressively strained p-doped Ge quantum wells and 200 angstrom-thick tensile-strained Si- rich SiGe barriers. This MQW allows shorter-wavelength IR sensing than prior-art unbuffered Si0.64Ge0.36/Si asymmetrically strained QWIPs because the valence band offsets are approximately 3x larger. We predict normal- incidence higher-temperature operation over the 1.7-to-3.8- micrometers wavelength band using the bound-to-bound and bound-to- continuum transitions HH1-SO1 and HH1-SO-C respectively. We expect that the p-i-p Ge/Si MQW pixels analyzed here can be fashioned into 2D imaging arrays, and that the arrays can be integrated monolithically with Si readout circuity. We also anticipate that multi-spectral IR imaging will be feasible by the technique of vertical epitaxial stacking of 'sub- QWIPs' within each pixel - where each sub-QWIP has a narrowband spectral response that differs from its neighbor's, and each sub-QWIP's electrical readout current is independent of its neighbor's.

This paper presents modeling and simulation results on Si-based quantum-well intersubband THz detectors and THz lasers (tasers) in the 3 to 10 THz range where the intersubband transition energy is 12 to 41 meV. The incoherent cryogenically cooled (4K to 20K) quantum well terahertz detector (QWTD) consists of p-type Si0.9Ge0.1 QWs with Si barriers on an Si substrate, or of p-Si0.85Ge0.15/Si on a relaxed Si0.97Ge0.03 buffer on Si. The QWTD senses THz radiation at normal incidence (the XY polarization on the HH1 to LH1 transition) or at edge-illumination (the Z polarization on the HH1 to HH2 transition). Resonant-cavity enhancement, coupling to Si THz waveguides, and integration with SiGe transistor preamplifiers appear feasible for QWTDs. The quantum staircase taser is a simplified far-infrared version of the quantum cascade laser in which each superlattice transfer region is replaced by a thin tunnel-barrier layer. We have adapted to group IV the m-V idea of Sun, Lu, and Khurgin; the "inverted mass taser". On a Si0.81Ge0.19 substrate, we find that an inverted effective mass exists in LH1 at k(x) = 0.013 Angstrom(-1) in 9-nm single-wells of Si0.7Ge0.3 with 5-nm Si barriers. Selective electrical injection of holes into LH1 at T = 77K is postulated. This offers local-in-k-space LH1-HH1 population inversion and tasing at 7.2 THz. Since the taser emission is XY-polarized, the active MQW staircase (a set of identical square QWs) is suitable for insertion into a vertical cavity surface-emitting taser. The VCSET would have resonator thickness of lambda/2n = 6 mu m, and Bragg mirrors constructed from SiO2/Si multilayers.

We present a new semi-analytic technique for estimating the diffraction loss and threshold gain of oxide apertured microcavity VCSELs. Apart from a few geometric simplifications, our calculation is based on a rigorous first-principles analysis of the modal fields. By combining the threshold gain with the electronic bandstructure and optical matrix elements, we calculate the threshold currents of microcavity VCSELs and obtain good agreement with experiments.

We experimentally investigate the modal properties of vertical cavity surface emitting lasers with vertically coupled quantum dot active regions. Etched air-post structures with aluminum-gallium-oxide apertures and aluminum-oxide distributed Bragg reflectors are electrically-pumped below the lasing threshold. The wavelengths of the resonant cavity modes are revealed by room temperature electroluminescence measurements. In concert with our earlier theoretical predictions, we find that the resonant cavity modes blueshift as the radius of the oxide aperture decreases.

A classical model of spontaneous emission in resonant cavity light emitting diodes is developed. The model is based on a plane wave expansion of a randomly distributed ensemble of noninteracting optical dipole emitter pairs placed within a planar microcavity. The model accounts for losses due to leaky guided modes and intracavity absorption. Given an arbitrary device structure, the model predicts the total power emitted, the radiation pattern, and the relative magnitude of photocurrent generated in an intra or extracavity photodetector. Selected modeling results for an example device emitting at 650 nm are included.

We present a full-vector, finite element analysis of oxide apertured VCSELs, focusing on the optical properties required for low threshold design. We examine several versions of an 870 nm oxide DBR, oxide aperture VCSEL design to gain insight into the physical processes determining diffractive loss. Our results suggest the diffraction may be modeled as a coupling loss to the parasitic (transverse) mode continuum. In this approach, the loss is determined from two competing factors: (1) the lasing mode penetration into the radial cladding region, and (2) the relative alignment of the eigenmode and parasitic mode wavevectors. We also find the characteristic blueshift resulting from the transverse optical confinement.

We report a new full vector finite element model for analyzing the optical properties of azimuthally symmetric oxide-apertured vertical-cavity surface-emitting lasers (VCSEL's), Our model allows for quasi-exact calculation of the lasing mode blueshift, threshold gain, and field profile. Through a detailed analysis of a sample VCSEL, we ascertain the physical effects which determine diffractive or parasitic mode loss. They are: 1) the background density of parasitic modes and 2) the coupling strength between the lasing mode and the parasitic mode continuum. The coupling strength is in turn determined by the relative alignment between the lasing and parasitic mode propagation vectors and the lasing mode penetration into the oxide region. This analysis improves our understanding of the optical physics of apertured VCSEL's and should enable the next leap down in lasing threshold.

Threshold currents in small-aperture VCSELs are likely to be dominated by diffraction losses. We;fie have developed a semianalytic technique to estimate the lasing mode energies, field profiles and cavity losses-including absorption, mirror, and diffraction losses-in oxide-apertured VCSELs. By coupling these modes to the full, nonparabolic electronic bandstructure, and by solving the resulting multimode related rate equations, we can model the light verses current charcteristics of microcavity VCSELs. We apply our model to a low-threshold VCSEL structure and calculate threshold currents of 30-40 mu A, in qualitative agreement with experiments. We estimate spontaneous emission factors beta as large as 1.7x10(-2) for a 1.5 mu m radius device.

As the aperture size of oxide-apertured vertical-cavity surface-emitting lasers (VCSELs) shrinks, the lasing wavelength blueshifts. We have calculated this effect using both a scalar effective index model and a full-vector weighted index model. Results were compared against experimental data for two different VCSEL designs emitting near 780 and 850 nm. We find that the full-vector weighted index calculation matches the data remarkably well, while the scalar effective index calculation underestimates the blueshift.

Presents a semi-analytic full-vector method for calculating the spatial profile, optical confinement factor resonant frequency, absorption loss, and mirror loss of lasing modes in cylindrically symmetric microcavity vertical-cavity surface-emitting lasers (VCSEL's). It can be shown that this method gives the best separable approximation for the electric and magnetic vector potentials. Our technique can model the entire VCSEL structure and can treat complex media. We apply the method to etched-post and oxide-apertured VCSEL's designed for 980-nm emission and find a blueshift in cavity resonance as the cavity radius shrinks. We also find a minimum optical cavity radius below which radially bound lasing modes cannot be supported. This radius depends on the device geometry and lies between 0.5 and 1 /spl mu/m for the devices studied. Once this model is augmented to include diffraction losses-the dominant loss mechanism for conventional small aperture lasers-it will provide a complete picture of lasing eigenmodes in microcavity VCSEL's.

We present an extension of the weighted index method (WIM) for calculation of VCSEL lasing mode threshold gain. The WIM is a modal technique which gives the best separable solution for the VCSEL modes in a variational sense. The approach for inclusion of threshold gain in the WIM is a generalization of the classic Fabry-Perot round-trip amplitude and phase conditions.

This new finite element method model is expected to be valuable for the design of microcavity devices. It can be used to optimize optical mode control by examining changes of size, shape, number, and location of native oxide layers. It may also be combined with semiconductor gain calculations to determine the higher-order mode suppression level for various microcavity surface emitting laser designs. Finally, it may be used to analyze VCSEL lasing and spontaneous emission near-field structure. This information is of considerable importance in the design of optical interconnect and communication systems.