Search

An ultralow- $\text{V}_{\pi }\text{L}$ photonic switch device is demonstrated utilizing the high optical and electrical field confinement in silicon slot waveguides coupled with the strong electro-optic response of nematic liquid crystals. A silicon photonic directional coupler switch with a modulation efficiency of 0.0195 $\text{V}\cdot $ mm and a loss-efficiency product of 0.0624 $\text{V}\cdot $ dB is achieved. The 1.5 mm long device is based on two-mode interference within a single slot waveguide resulting in a $\text{V}_{\mathrm {\pi }}$ of 0.013 V and an extinction ratio of $\sim ~9$ dB at 1550 nm wavelength. The power consumption of the photonic switch is estimated to be below 0.6 nW and it possesses a response time of < 1.5 ms. A comparative performance study between the directional coupler switch and a Mach-Zehnder Interferometer (MZI) switch is performed. The directional coupler switch is projected to have a smaller footprint and lower optical loss compared to a similar design MZI switch making it a strong candidate for switch matrix designs and applications.

The design and performance of optical switches are crucial for switch-based beam steering devices to be competitive among existing beam steering technologies for light detection and ranging (LIDAR) applications. An efficient liquid crystal (LC) filled directional coupler (DC) optical switch is proposed. Device performance as a function of geometric design is analyzed through numerical simulations. The analysis reveals that the DC is a promising candidate to replace more popular Mach-Zehnder Interferometer (MZI) configurations for optical switches when ultra-low $\mathrm{V}_{\pi}\mathrm{L}$ performance is important.

We demonstrate low loss ultra-compact bends in PECVD silicon rich silicon nitride and utilize them towards realizing efficient thermo-optic MMI switches with Pπ = 23 mW and a footprint of 68 x 68 µm2.

The design, fabrication, and characterization of low loss ultra-compact bends in high index (n = 3.1 at {\lambda} = 1550 nm) PECVD silicon rich silicon nitride (SRN) is demonstrated and utilized to realize efficient, small footprint thermo-optic phase shifter. Compact bends are structured into a folded waveguide geometry to form a rectangular spiral within an area of 65 x 65 \mum2 possessing a total active waveguide length of 1.2 mm. The device features a phase shifting efficiency of 8 mW/{\pi} and a 3 dB switching bandwidth of 15 KHz. We propose SRN as a promising thermo-optic platform combining the properties of silicon and silicon stoichiometric nitride, Comment: 4 page, 3 figures, 1 table

A highly sensitive silicon photonic temperature sensor based on silicon-on-insulator (SOI) platform has been proposed and demonstrated. A two-mode nano-slot waveguide device structure cladded with a nematic liquid crystal (LC), E7, was adopted to facilitate strong light-matter interaction and achieve high sensitivity. The fabricated sensor was characterized by measuring the optical transmission spectra at different ambient temperatures. The extracted temperature sensitivities of the E7-filled device are 0.810 nm/°C around room temperature and 1.619 nm/°C near 50°C, which match well with simulation results based on a theoretical analysis. The results obtained represent the highest experimentally demonstrated temperature sensitivity for a silicon-waveguide temperature sensor on SOI platform. The slot waveguide directional coupler device configuration provides submicron one-dimensional spatial resolution and flexible selection in LC materials for designing temperature sensitivity and operational temperature range required by specific applications.

Ruyi-Jinhuang Gao (RJG) is an ointment prepared from Ruyi-Jinhuang San (RJS), a traditional formula of powder-type Chinese medicine, usually used in anti-inflammatory analgesic care in China. The present study is aimed to detect the anti-inflammatory signaling of RJG exposed by an experimental model of lipopolysaccharides (LPS)-induced pro-inflammatory expression of inducible nitric oxide synthase (iNOS) and prostaglandin endoperoxide synthase (PGH synthase-2, COX-2). LPS-induced activation of iNOS and COX-2 has been recognized to increase cytokines and nitric oxide (NO), which play predominant roles in inflammation. In the culture of RAW264.7 cells, RJG concentration-dependently inhibited LPS-induced iNOS and COX-2 expression. However, the herbal components of RJG displayed different results, including increase and decrease of both protein expressions. Among them, inhibition by Curcuma Zedorarica Rhizoma (Cu), Atractylodis Lancea Rhizoma (At), and Glycyrrhizae Radix (Gl) are more potent than that by others. Inhibition by Cu and At are associated with their cytotoxicity. Glycyrrhizae Radix (Gl), with lower cytotoxicity in comparison with Cu and At, is the most potent component of RJG on inhibiting LPS-induced iNOS and COX-2 expression. In comparison with LPS-induced tumor necrosis factor-α (TNF-α), croton oil-induced formation of TNF-α and interleukin-4 (IL4), RJG did not increase the pro-inflammatory cytokines. These facts indicated that RJG could not significantly sensitize an immunologic response during the treatment of non-wounded inflammation on body surface. Ruyi-Jinhuang Patch (RJP), a pharmaceutical preparation of RJG, has proven to have significant relief of cutaneous inflammation-associated somatic pain in clinical use.

We demonstrate the thermo-optic properties of silicon-rich silicon nitride (SRN) films deposited using plasma-enhanced chemical vapor deposition (PECVD). Shifts in the spectral response of Mach-Zehnder Interferometers (MZIs) as a function of temperature were used to characterize the thermo-optic coefficients of silicon nitride films with varying silicon contents. A clear relation is demonstrated between the silicon content and the exhibited thermo-optic coefficient in silicon nitride films, with the highest achievable coefficient being as high as (1.65+/-0.08) x 10-4 K-1. Furthermore, we realize an SRN Multi-Mode Interferometer (MMI) based thermo-optic switch with over 20 dB extinction ratio and total power consumption for two-port switching of 50 mW., 10 pages, 7 figures, submitted to a journal

To evaluate the changes in marginal bone levels (MBL) and soft tissue dimension around platform-switched implants with the implant-abutment junction (IAJ) placed at the crest or 1.5-2 mm subcrestally.In all, 96 platform-switched implants were placed in either the posterior maxilla or mandible in 48 partially edentulous patients in a split-mouth study. All implants were provisionally restored after 4-5 months and definitively after 6 months (T6). Radiographic assessment of MBL was assessed at implant placement (T0), T6, 12 months (T12), and 18 months (T18) after placement. Mid-buccal soft tissue and papilla measurements were performed at T6, T12, and T18.In all, 43 patients with 86 implants completed the study. The T18 examination showed an implant survival rate of 100% in both groups. Analysis showed that MBL varied as a function of IAJ location, which indicated more coronal bone levels with subcrestal (2.39 ± 0.08 mm) than with epicrestal placements (0.88 ± 0.08 mm) (p .05). Greater average marginal bone loss was found in the subcrestal group (0.40 ± 0.07 mm) compared to the epicrestal group (0.13 ± 0.08 mm) although no statistically significant difference was found at T18 (p .05). Levels of mid-buccal soft tissue had no significant changes over time, regardless of group (p .05). There was a significant difference in increase in papilla between T6 and T12 and T18 (p = .005 and .001), but not between T12 and T18 (p = .61). These papilla levels and changes were similar between groups (p .05).The MBL changes around platform-switched implants with same geometry were not affected by the epicrestal or subcrestal location of the IAJ. Furthermore, the location of the IAJ did not affect the implant survival and soft tissue dimensions. However, no bone loss was located apical to the IAJ when the implants were placed subcrestally.

Setting: Mzuzu Central Hospital (MZCH), Mzuzu, and Chitipa District Hospital (CDH), Chitipa, Malawi. Objective: To compare management and outcomes of human immunodeficiency virus (HIV) exposed infants in early infant diagnosis (EID) programmes at MZCH, where DNA polymerase chain reaction (PCR) testing is performed on site, and CDH, where samples are sent to MZCH, between 2013 and 2014. Design: Retrospective cohort study. Results: Of infants enrolled at MZCH (n = 409) and CDH (n = 176), DNA PCR results were communicated to the children's guardians in respectively 56% and 51% of cases. The median time from sample collection to guardians receiving results was 34 days for MZCH and 56 days for CDH. In both hospitals, only half of the dried blood spot (DBS) samples were collected between 6 and 8 weeks. More guardians from MZCH than CDH received test results within 1 month of sample collection (25% vs. 10%). Among the HIV-positive infants, a higher proportion at MZCH (92%) started antiretroviral therapy than at CDH (46%). The relative risk (RR) of death was higher among infants with late DBS collection (RR 1.3, 95%CI 1.0-1.7) or no collection (RR 5.8, 95%CI 4.6-7.2), and when guardians did not receive test results (RR 8.3, 95%CI 5.7-11.9). Conclusion: EID programmes performed equally poorly at both hospitals, and might be helped by point-of-care DNA PCR testing. Better programme implementation and active follow-up might improve infant outcome and retention in care.

The aim of this report is to present a case with a 10-year follow-up of a single anterior tooth dental implant replacement with a xenogenous bone graft in a patient with a large bone defect associated with a cleft lip and palate. At 10 years postloading, the implant was healthy and well-integrated. Although the implant restoration appeared slightly intruded due to growth and development at that time, the patient was happy with the esthetic result and refused corrections of the uneven occlusal plane.

A major challenge in gallium nitride (GaN) vertical power devices and other large bandgap materials is the high defect density that compromises the performance, reliability, and yield. Defects are typically nucleated at the heterointerface where there are both lattice and thermal mismatches. Here, we report the selective area growth (SAG) of thick GaN on Si and on the newly available Qromis Substrate Technology™ (QST) substrates that lead to a significant reduction of the defect densities to a level that is nearly comparable to that on native substrates by defect annihilation. We performed a parametric study of the electrical properties of the SAG GaN layers by fabricating and characterizing Schottky barrier diodes for SAG GaN layer thicknesses of 5, 10, 15, and 20 μm for GaN-on-Si, GaN-on-QST, and GaN-on-GaN diodes. While thicker layers led to a significant reduction in defect densities and improvement in the diode forward current characteristics, the GaN-on-QST diodes exhibited nearly similar characteristics to the GaN-on-GaN diodes. Further improvement in the device structure and/or SAG growth for GaN-on-Si is needed to achieve a comparable performance as the defect densities in the GaN-on-Si are comparable to that of GaN-on-QST substrates.

We describe a new class of electric motor/generator technology with specific power more than 10kW/kg for a variety of applications including terrestrial, airborne hybrid and all electric propulsion. The technology utilizes a new type of turn-less electro-mechanics configuration intimately integrated with dedicated inverters which utilize high current, low voltage GaN MOSFET switches.

In this paper, we review the key performance of optical modulators operating in the wavelength range of 1.3–1.5 μm for fiber-optic link applications and compare the design of two nanoscale modulators which are based upon (a) second-harmonic generation (SHG) from GaN nanopillars and (b) electrooptic polymer in nanoslot waveguides, respectively.

Second-harmonic generation (SHG) from single GaN nanpoillar is reported. A model for the SHG processes in the GaN nanopillar as a function of diameter is presented; the analysis showed quantitatively that the SHG is dominated by its surface area. The effective second order nonlinear optical susceptibility increases as the diameter of the GaN nanopillar decreases, reaching a value of 136 pm/V at 150nm diameter, making them attractive for modulator applications.

Thermal resistances of AlGaN/GaN high electron mobility transistors (HEMTs) on polycrystalline diamond (with thermal conductivity >1500 W/m K) and on Si substrates are respectively extracted and compared. GaN HEMTs on diamond and on Si with the same device dimensions were fabricated and their direct-current (DC) characteristics were compared. Thermally-induced negative differential output resistance in the saturation region at DC was observed for GaN-on-Si HEMT but was negligible for the GaN-on-diamond device at comparable power densities, which is consistent with reduced thermal resistance. Thermal resistances of the HEMTs on Si and diamond substrates determined experimentally from analysis of I–V characteristics agree with estimates based on simulation.

A modal analysis is conducted for analyzing the absorption profile of high power waveguide photodiodes designed for analog optical link. The excitation of guided modes with large filling factor in the absorber is identified as a limiting factor for the performance of waveguide photodiodes at high optical power, including power handling capability, and bandwidth-efficiency product. A waveguide photodiode design, which spatially separates the input waveguide from the absorber in the lateral direction, is analyzed and experimentally demonstrated to suppress the excitation of mode with large filling factor. Photocurrent>60 mA under − 4 V bias is measured, with 0.80 A/W responsivity. This design illustrates that high power handling capability can be achieved without compromising the bandwidth-efficiency product.

Present research in widebandgap semiconductors is concentrated predominantly in continuous-wave applications. While their applicability for pulsed power is recognized, little work has been done in taking advantage of the individual characteristics, such as the large drift velocities, of the candidate materials. Analysis of the relative merits of the key semiconductor technologies—Si, GaAs, SiC, and GaN—in the pulsed-power regime is presented, including limitations associated with majority carrier operation and thermal transients. We will also discuss a GaN photoconductor with a vertical topology, which is particularly well suited to higher current applications and also is geometrically more applicable for integration to transmission lines without degrading the high blocking field (2 MV/cm) now available. The relation between the blocking voltage and conduction current typical of majority carrier operation is analyzed and we will show how optical carrier generation provides the much higher switching power associated with bipolar operation.

We describe the technology and experimental work done at Polarix Corporation on photoconductive switch-based high-power microwaves systems for airborne counter-improvised explosive device (IED) applications. We address the fluence requirement for nanosecond pulse range for coupling to IED harnesses both theoretically and experimentally. We describe some of the key elements of the technology and the system being developed. They include silicon backed biased junction switch technology, low inductance–high current switch integration with thin film Blumlein type circuits, multiswitch multicycle generation, and radiation concepts using asymmetric Wheeler–Chu apertures for side firing small diameter but long unmanned aerial vehicles platforms. We will also present the experimental results showing switch source and radiator performance.

High-power and high-linearity photodiodes (PDs) are of critical importance to achieve high-efficiency analogue fibre links. A small-signal analysis is utilised on the nonlinear responsivity of the modified uni-travelling-carrier PDs to understand the theoretical limitation on the third-order output intercept point (OIP3) of the link. It is theoretically shown that a nonlinear responsivity depending on both the voltage and the current can give rise to an enhanced OIP3 at certain bias and current values.

We studied the impact of optical pulse coding techniques on Brillouin optical time-domain analysis (BOTDA) systems experimentally. The results show that complementary-coding-based schemes can provide significant enhanced signal-to-noise ratio (SNR) to the BOTDA system. Using complementary coding, the average time can be reduced significantly and so does the measurement time. The complementary-coded pulses with individual pulse duration of 0.1 ns allowed us to realize temperature sensing over 50 m of single-mode fiber with 1-cm spatial resolution.

A study of the reverse-leakage-current mechanisms in metal-organic-chemical-vapor-deposition (MOCVD)-grown GaN Schottky-barrier diodes is presented. An analysis is carried out of the characteristics of GaN Schottky diodes as well as of diodes with an InGaN surface layer to suppress the reverse leakage current and increase the breakdown voltage. The experimental results of the diodes with InGaN surface layers showed a ~ 40-V breakdown voltage increase and a significant leakage-current reduction under high reverse bias, in comparison with the design with GaN only. Such improvements are attributed to the reduced surface electric field and the increased electron tunneling distance induced by the polarization charges at the InGaN/GaN interface. We also report the effect of a high-pressure (near atmospheric pressure) MOCVD growth technique of the GaN buffer layer to further improve the leakage current and breakdown voltage.

Semiconductor quantum well (QW) light-emitting diodes (LEDs) have limited temporal modulation bandwidth of a few hundred MHz due to the long carrier recombination lifetime. Material doping and structure engineering typically leads to incremental change in the carrier recombination rate, whereas the plasmonic-based Purcell effect enables dramatic improvement for modulation frequency beyond the GHz limit. By stacking Ag-Si multilayers, the resulting hyperbolic metamaterials (HMMs) have shown tunability in the plasmonic density of states for enhancing light emission at various wavelengths. Here, nanopatterned Ag-Si multilayer HMMs are utilized for enhancing spontaneous carrier recombination rates in InGaN/GaN QWs. An enhancement of close to 160-fold is achieved in the spontaneous recombination rate across a broadband of working wavelengths accompanied by over tenfold enhancement in the QW peak emission intensity, thanks to the outcoupling of dominating HMM modes. The integration of nanopatterned HMMs with InGaN QWs will lead to ultrafast and bright QW LEDs with a 3 dB modulation bandwidth beyond 100 GHz for applications in high-speed optoelectronic devices, optical wireless communications, and light-fidelity networks.

A novel fabrication procedure is developed that allows for the direct measurement of the linear electrooptic coefficient of semiconducting nanowires to determine their viability for use in electrooptic devices. Vertically aligned InP nanowires are transferred from their growth substrate to a glass substrate using a host polymer, while still retaining the alignment of the nanowires. The linear electrooptic coefficient of the InP nanowires exhibited a 1-2 orders of magnitude enhancement over bulk InP and ranged from 31 to 147 pm/V. The figure of merit, n3r, exhibited a factor of 20 enhancement over lithium niobate and ranged from 1010 to 4817 pm/V.

We demonstrate the use of an area selective zinc in-diffusion process as a simple and efficient technique for the fabrication of integrated photonic devices. By controlling the profile of the diffusion front and the zinc depth the insertion losses of the devices can be minimized. Using this technique an integrated 1x2 Mach-Zehnder analog modulator was fabricated. Our experimental results demonstrate that the modulator exhibits excellent linearity for both TE and TM polarizations over a wavelength range of 40nm. The measured on-chip losses in the order of 3dB are obtained, which is significantly lower compared to the use of isolation trenches.

Single-crystal InAs nanowires (NWs) are synthesized using metal-organic chemical vapor deposition (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO(2)/n(+)-Si substrate with a global n(+)-Si back-gate and sputtered SiO(x)/Au underlap top-gate. For top-gate NWFETs, we have developed a model that allows accurate estimation of characteristic NW parameters, including carrier field-effect mobility and carrier concentration by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temperature field-effect mobility as high as 6580 cm(2) V(-1) s(-1), which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW.

The construction of a prototype Schwarzschild-Couder telescope (pSCT) started in early June 2015 at the Fred Lawrence Whipple Observatory in Southern Arizona, as a candidate medium-sized telescope for the Cherenkov Telescope Array (CTA). Compared to current Davies-Cotton telescopes, this novel instrument with an aplanatic two-mirror optical system will offer a wider field-of-view and improved angular resolution. In addition, the reduced plate scale of the camera allows the use of highly-integrated photon detectors such as silicon photo multipliers. As part of CTA, this design has the potential to greatly improve the performance of the next generation ground-based observatory for very high-energy (E>60 GeV) gamma-ray astronomy. In this contribution we present the design and performance of both optical and alignment systems of the pSCT.

We study nanopatterned multilayer hyperbolic metamaterials with tunable plasmonic properties for enhancing fluorescent molecules and LEDs at different working wavelengths. About two order of magnitude of spontaneous emission rate enhancement was demonstrated.

High power photodiodes were proposed in prior works for conversion of broadband modulated optical signals to electrical signals for transmission at high frequencies. At moderate to high optical powers, the linearity of the photodiode becomes a main concern for achieving high linearity transmission in fiber link. In this invited talk the mode analysis for understanding the absorption profile of high power coupled waveguide photodiodes is presented. which also illuminated the design parameters pertinent for high saturation power and linear detection.

This paper presents results of normally-off AlGaN/GaN MOS-HEMTs fabricated with a two-step gate recess technique which includes a chloride based Inductively Coupled Plasma (ICP) etch followed by HCI and NH 4 OH surface treatment. The latter can effectively smoothen evenly the ICP etched surface. The two-step gate recessed device with atomic layer deposited (ALD) Al 2 O 3 as gate dielectric delivers a threshold voltage (V th ) of +1V and a maximum current density per gate width (I max ) of up to 0.583 A/mm which is 90% that of an un-recessed gate depletion-mode AlGaN/GaN HEMT device (V th of −3.5V) fabricated from the same epitaxial wafer.

This paper presents a proposal for a novel integrated tunable coupler device called programmable coupler ladder, based on Titanium diffused lithium niobate waveguide and Y-junction reflector. Unlike the traditional serial to parallel converter, the coupler ladder sorts the output bits in the time axis using a built-in delay waveguide. With a proper control signal it can perform signal processing at the bit level. It also can generate coherent multi-channel outputs with theoretically arbitrary amplitude and phase from continuous input light source. Its application in optical microwave beam forming is briefly described. The key component, built-in delay line based on Y-junction reflector, has been experimentally verified via a loop resonator structure. I dB loss is found for each Y-junction reflector, which enables a practical coupler ladder. The loop itself is also an important device for optical signal processing.

A simple and comprehensive modeling approach is developed for analyzing the frequency response of segmented traveling-wave optical modulators. The approach is based on the microwave transmission (ABCD) matrix theory. The case study for a GaAs traveling-wave Mach-Zehnder modulator (MZM) verifies this analysis approach with excellent agreement to the reported experimental results; the analyses for the quantum-well-based MZMs and electroabsorption modulators indicate that the segmented traveling-wave design can provide much better bandwidth than the lumped-element or the continuous-traveling-wave counterparts, with a few decibels penalty in the electrical-to-optical (E/O) conversion gain if low-loss optical waveguides are available. Meandered transmission line design, which provides more design freedom, is also analyzed using this modeling approach.

This tutorial describes the basic principles and performance analysis of optical intensity modulators using electrooptic and electroabsorption effects, for use in analog and digital communication systems. These include lithium niobate modulators, semiconductor electroabsorption modulators, semiconductor Mach-Zehnder modulators, and polymer modulators.

We describe how the material properties of III–V semiconductors, including bandgap, band structure, band offset, refractive index, absorption, and ionization coefficient, are exploited for lasers and photodetectors for fiber-optic communications. The material systems discussed for 1.3 μm and 1.55 μm light emission include the more traditional GaInAsP and AlGaInAs on InP, the more recently investigated GaInAs quantum dots and low-bandgap GaInNAs on GaAs as well as GaAsSb/GaAs Type II structures, and the potentially viable GaN/AlGaN from intersubband transitions (i.e., between quantized conduction-band energy levels). As an example of photodetector applications, GaInAsP/InP and wafer-fused GaInAs/Si are discussed in terms of gain and noise factor for use in avalanche photodiodes with separate absorption and multiplication regions.

This paper proposes the integration of an antenna with a photodetector for high capacity wireless communications. A waveguide photodetector (WGPD), illuminated on the side, is used to convert the RF-modulated optical power into a microwave signal. That microwave signal is used to feed an antenna built on the same plane as the photodetector. The WGPD is a standard p-i-n device grown on a semi-insulating InP. The performance of this RF/photonic antenna in the frequency range 17–20 GHz is studied theoretically and experimentally. An equivalent circuit model for the WGPD is developed to predict the photodetector impedance as a function of frequency. The photodetector is matched to the input impedance of the antenna. The agreement between measurement and the circuit model results for the WGPD impedance is very good. It is envisioned that a large number of such RF/Photonic antenna elements could be networked together into a star configuration, feeding in and out of a radio hub. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 179–184, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10550

Integrated metrology in the lithography cluster is a promising solution to tighten process control. It is shown that optical CD metrology using YieldStar, an angular resolved scatterometer, meets all requirements in terms of precision, process robustness, throughput and matching to CD-SEM, the current tool-of-reference. The same metrology tool supports also diffraction-based overlay metrology. Using an appropriate sampling plan and the full scanner correction capabilities, overlay control can be improved. The throughput of the integrated tool is sufficient to support high volume sampling plans for combined CD and overlay monitoring and control, with 100% lot coverage.

Bandwidth requirements continue to drive the need for low-power, high speed interconnects. Harnessing the mature CMOS technology for high volume manufacturing, Silicon Photonics is a top candidate for providing a viable solution for high bandwidth, low cost, low power, and high packing density, optical interconnects. The major drawback of silicon, however, is that it is an indirect bandgap material, and thus cannot produce coherent light. Consequently, different integration schemes of III/V materials on silicon are being explored. An integrated CMOS tunable laser is demonstrated as part of a composite-CMOS integration platform that enables high bandwidth optical interconnects. The integration platform embeds III-V into silicon chips using a metal bonding technique that provides low thermal resistance and avoids lattice mismatch problems. The performance of the laser including side mode suppression ratio, relative intensity noise, and linewidth is summarized.

An integrated tunable CMOS laser for silicon photonics, operating at the C-band, and fabricated in a commercial CMOS foundry is presented. The III-V gain medium section is embedded in the silicon chip, and is hermetically sealed. The gain section is metal bonded to the silicon substrate creating low thermal resistance into the substrate and avoiding lattice mismatch problems. Optical characterization shows high performance in terms of side mode suppression ratio, relative intensity noise, and linewidth that is narrow enough for coherent communications.

In this letter, we demonstrate a design of GaN-based Schottky diodes for microwave varactor applications. Using suitable InGaN surface layer structures, the Schottky diodes exhibit an enhanced high-voltage handling capability. The breakdown voltage is increased by ~40 V, and the reverse leakage current is significantly suppressed under high reverse voltages. The breakdown-voltage enhancement and leakage-current suppression are attributed to a reduction in the surface electric field and an increase in the electron tunneling distance due to polarization-induced charges at the InGaN/GaN interface.

Polarized neutron and X-ray reflectivity methods have been used to study the permalloy epitaxial thin films on sapphire with Cr as a buffer layer. The reflectivity result indicates that the magnetic interface is smoother than the chemical interface. It is possible that the magnetic moment of the intrusive Ni or Fe atoms across the interface can be assimilated by the near-by Cr atoms or the other way around.

Theoretical analysis and numerical calculations are presented for ultrahigh-speed (>50 GHz) traveling-wave electroabsorption modulators (TW-EAM's), including effects of velocity mismatch, impedance mismatch, and microwave attenuation. A quasi-static equivalent circuit model is used to examine the TW-EAM microwave properties, including the effect of photocurrent. Due to the optical propagation loss of the waveguide, the TW-EAM waveguide length for maximum RF link gain is currently limited to 200-300 /spl mu/m. The discussion indicates that the carrier transit time in the intrinsic layer may not severely limit the TW-EAM bandwidth. Three TW-EAM design approaches are discussed: low-impedance matching; reducing the waveguide capacitance; and distributing the modulation region.

Using the method of projection operators based on plane wave expansion, we theoretically investigate photonic band structures in two-dimensional metallodielectric photonic crystals, especially, square arrays of dielectric and perfectly conducting cylinders. Existence of photonic band gaps in metallodielectric photonic crystals and validity of the proposed numerical method are demonstrated.

A novel approach to the design of electroabsorption waveguide modulators for use in broadband, analog radio frequency (RF) links is presented. The approach utilizes a linear combination of two electroabsorption effects (the Franz-Keldysh effect and the quantum-confined Stark effect) to improve the spurious-free dynamic range of the waveguide modulator. Measured transfer curves verify simultaneous improvement in linearity and modulation efficiency for an exemplary combination structure. Residual absorption is modeled to determine the impact on modulation efficiency.

Planar separate-confinement, double-heterostructure, single-quantum-well photoelastic GaAs/AlGaAs lasers have been fabricated using a novel yet practical processing technique involving thin-film surface WNi stressors for waveguiding and ion implantation for isolation. A p++-GaAs contact layer regrown by chemical beam epitaxy has been used to improve the WNi ohmic contacts to the lasers. Even without bonding on heat sinks, these planar photoelastic lasers operate at continuous wave at room temperature. The lowest threshold is 29 mA for a cavity length of 178 μm and a stressor width of 5 μm. The internal quantum efficiency above threshold is 75%. The characteristic temperature is 114 K. The main waveguiding mechanism of the photoelastic lasers is determined to be weak index guiding with the beam waist in the junction plane measured 10 μm behind the end facet.

We introduce the method of projection operators based on plane-wave expansion to compute photonic band structures in periodic media containing perfectly conducting elements. Eigenfunctions of a unit configuration potential generate suitable projection operators in the form of a set of eigenvectors in the Fourier domain. By simply applying a projection operator onto a proper subspace, a quadratic or cubic eigensystem for finitely conducting media can be transformed into an ordinary symmetric eigensystem in the limit of perfect conductors. The procedure is equivalent to finding solutions of wave equations under the condition that the electromagnetic fields are entirely zero inside periodic perfect conductors. The methodology developed here, in fact, can be viewed as a generalization of the conventional metal waveguide or cavity theory. The method is numerically handy, fast, and readily extendible to general metallodielectric photonic crystals. As examples, we present photonic band structures in two-dimensional metal and metallodielectric cylinder structures.

Analog optical links using electroabsorption modulators have a gain limit caused by the photocurrent in the modulator. The gain limit results in a minimum link noise figure as well. The gain limit is due to the voltage-dependent absorption causing a voltage-dependent component to the photocurrent; the gain limit applies to a modulator that has this relationship

We present a unique Si photonics platform with butt-coupled III-V and SOI waveguides fabricated in a wafer-scale CMOS process. This enables high-efficiency lasers, modulators, photodetectors, and tuning-free Mux/Demux, which is ideal for TB/s optical interconnects.

Fabrication tolerances for control of the Bragg wavelength of gratings in waveguides are studied. Techniques are demonstrated to hold the period of the grating constant to /spl plusmn/0.04 nm over the majority of the exposed wafer area, and ridge waveguides fabricated with standard thin film process equipment are found to have Bragg wavelengths constant to within /spl plusmn/0.2 nm. Additionally, adjacent ridges with differing widths are written in a single photomask/etch process step and found to controllably shift the Bragg wavelength over a 10-20-nm band suitable for a distributed Bragg reflector (DBR) laser array or a monolithic comb filter for wavelength division multiplexing (WDM) filtering/routing arrays.

We have used cross-sectional scanning tunneling microscopy (STM) to study interface structure in arsenide/phosphide heterostructures grown by gas-source molecular beam epitaxy (GSMBE) and by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). High-resolution images of GSMBE samples consisting of GaAs interrupted at 200A intervals with a 40 s P2 flux reveal substantial, growth-temperature-dependent incorporation of phosphorus with nanometer-scale lateral variations in interface structure. STM images of InGaAs/InP multiple quantum well structures grown by LP-MOVPE show evidence of interface asymmetry and extensive atomic cross-incorporation at the interfaces. Data obtained by STM have been corroborated by high-resolution x-ray diffraction and reflection high-energy electron diffraction. Together, these studies provide direct information about nanometer-scale grading and lateral nonuniformity of arsenide/phosphide interfaces that can occur under these growth conditions.