Longwave IR lattice matched L-valley Ge/GeSiSn waveguide quantum cascade detector.

We propose a lattice-matched Ge/GeSiSn quantum cascade detector (QCD) capable of operating in the longwave infrared. The optical absorption and carrier transport based on intersubband transitions all occur within the L-valley of the conduction band of the group-IV material system using N-doped quantum wells (QWs). The waveguided lattice matched structure can be deposited strain free on top of a Ge buffer grown on Si substrate, and is end-coupled to low-loss on-chip Ge waveguides. We optimized the QCD structure through the analysis of the photoresponsivity and detectivity D*. The QCD operates in photovoltaic mode with narrow spectral response that is peaked anywhere in the 9 to 16 µm range, tunable by design. This work aims to push the optical response of the photodetectors made from the SiGeSn material system to longer wavelengths. The study suggests the QCD response can indeed significantly extend the spectral range beyond that of the photodiodes and photoconductors made from the same group-IV system for a wide variety of applications in imaging, sensing, lidar, and space-and-fiber communications.

Design of a Si-based lattice-matched room-temperature GeSn/GeSiSn multi-quantum-well mid-infrared laser diode.

This paper presents modeling and simulation of a silicon-based group IV semiconductor injection laser diode in which the active region has a multiple quantum well structure formed with Ge(0.9)Sn(0.1) quantum wells separated by Ge(0.75)Si(0.1)Sn(0.15) barriers. These alloy compositions were chosen to satisfy three conditions simultaneously: a direct band gap for Ge(0.9)Sn(0.1), type-I band alignment between Ge(0.9)Sn(0.1) and Ge(0.75)Si(0.1)Sn(0.15,) and a lattice match between wells and barriers. This match ensures that the entire structure can be grown strain free upon a relaxed Ge(0.75)Si(0.1)Sn(0.15) buffer on a silicon substrate - a CMOS compatible process. Detailed analysis is performed for the type I band offsets, carrier lifetime, optical confinement, and modal gain. The carrier lifetime is found to be dominated by the spontaneous radiative process rather than the Auger process. The modal gain has a rather sensitive dependence on the number of quantum wells in the active region. The proposed laser is predicted to operate at 2.3 µm in the mid infrared at room temperature.

High efficiency thin-film crystalline Si/Ge tandem solar cell.

We propose and simulate a photovoltaic solar cell comprised of Si and Ge pn junctions in tandem. With an anti-reflection film at the front surface, we have shown that optimal solar cells favor a thin Si layer and a thick Ge layer with a thin tunnel hetero-diode placed in between. We predict efficiency ranging from 19% to 28% for AM1.5G solar irradiance concentrated from 1 approximately 1000 Suns for a cell with a total thickness approximately 100 microm.

Infrared waveguiding in Si(1-x-y)Ge(x)C(y) upon silicon.

Polarization-independent waveguiding at 1.32 and 1.54 microm was observed in single-crystal S(1-x-y)Ge(x)C(y) grown nearly lattice matched upon Si(100) by chemical vapor deposition. Losses were <5 dB/cm at 1.54 microm. Experiments indicate that the band gap of three SiGeC alloy waveguides was in the 0.93-0.99-eV range, in agreement with theory.

Silicon optical waveguides with buried-CoSi(2) cladding layers.

We report a new Si waveguide that has a buried, metallike cladding. Infrared light propagates in a crystal Si layer atop a 50-nm film of buried CoSi(2) formed by implantation. Experiments in a 20-microm Si structure at the 1.3-microm wavelength show propagation losses below 2.5 dB/cm for TE(0) and TM(0). Results agree with theory. We also constructed two vertically integrated Si slab waveguides bounded below by CoSi(2) cladding layers.

Electro-optic Fabry-Perot pixels for phase-dominant spatial light modulators.

A theoretical analysis of phase enhancement by resonant Fabry-Perot picture elements in III-V semiconductor spatial light modulators (SLM's) is presented. For 90% reflecting electrodes, a phase modulation of 0.7pi rad is found in transmission when the electro-optic input phase is 0.06pi rad. Implementation of this resonant phase-dominant SLM in a 1.5-microm-thick AlGaAs/GaAs multiple quantum well (MQW) structure is proposed. Field effects and carrier-induced electro-optic effects are suggested for the MQW's.

N x N and 1 x N switching with chiral nematic liquid crystals.

New designs are presented for 4 x 4, 1 x 4, and 1 x 16 electrooptical switches that utilize chiral nematics.

Optical waveguiding in a single-crystal layer of germanium silicon grown on silicon.

Low-loss waveguiding at lambda = 1.3 microm has been observed in a partially strained, 10-microm-thick, single-crystal layer of Ge(0.1)Si(0.9) grown by chemical-vapor deposition upon an intrinsic (100) silicon substrate. The TM-mode propagation loss in the multimode planar guide was 1.9 dB/cm.

Silicon antiresonant reflecting optical waveguides.

A new silicon waveguide called ARROW-C is proposed and analyzed at the 1.3-microm wavelength. The silicon structure uses two buried layers of SiO(2) or beta-SiC with a thickness of 14-23 nm to permit optical tunneling for leakymode propagation. A propagation loss of ~0.5 dB/cm is predicted for the TE(0) or the TM(0) mode in a 5-microm-thick Sicore layer. Novel silicon ARROW-A and ARROW-B guides are also discussed. The former uses a pair of buried GeSi layers; the latter employs buried SiO(2) and GeSi films.

Carrier refraction in quantum well waveguides.

Carrier-induced refractive index changes in forward-biased InGaAs-InAlAs quantum well waveguides are calculated using a Kramers-Kronig transformation of Bar-Joseph's experimental absorption spectra [Phys. Rev. Lett. 59, 1357 (1987)]. At the l.65-,microm wavelength where the material is nominally transparent, an index change of -0.06 is found for an injection of 6 x 10(17) electrons/cm(3) . A quantum well waveguide 2 x 2 reversed-Deltabeta directional coupler switch with an active length of 480 microm is proposed.

Optical emission from impurities within an epitaxial-silicon optical waveguide.

Beryllium pairs form an isoelectronic complex in crystalline silicon that can bind an exciton. At sufficiently low temperatures, this bound exciton radiates in a narrow line near the wavelength lambda = 1.15 microm. We report the observation of optical confinement of this bound-exciton emission from beryllium impurities introduced, by ion implantation, into an epitaxial-silicon optical waveguide.

Optical waveguides in LiTaO(3) formed by proton exchange.

We describe the fabrication of optical waveguides in LiTaO(3) using proton exchange in hot benzoic acid. All specimens analyzed were X cut. An increase in surface extraordinary index of ~0.14 has been observed for samples exchanged for 3 h at a melt temperature of 249 degrees C. No change was observed in the ordinary index. Waveguide fabrication was facilitated by further baking the samples for an extended period of time at a temperature hotter than the acid melt. The latter technique also served to minimize the surface damage that is characteristic of acid baths exceeding roughly 4 h. The depth of proton exchange in X-cut LiTaO(3) appears to be considerably less than would be the case for X-cut LiNbO(3) under the same conditions.

Calcite 2 x 2 optical bypass switch controlled by liquid-crystal cells.

Three blocks of birefringent calcite and two 6-V twisted nematic liquid-crystal cells have been used to make a 2 x 2 optical bypass switch for series-type fiber-optic networks. Optical cross-talk levels of -32 dB have been obtained in both states. The insertion loss was 3 dB in a nonoptimized switch.

Electrooptic 4x4 matrix switch for multimode fiber-optic systems.

A new fiber-optic switch for multimode fiber networks has been developed. This voltage-controlled device can switch light from any of four input fibers to any of four output fibers. Four input/output connections are made simultaneously, and the matrix has twenty-four such states. No moving parts are needed because the switching is accomplished with an electrooptic effect in nematic liquid crystals. Low levels of optical crosstalk, ~40 dB below the output signal level, are obtained with a unique two-stage architecture. The design, construction, and observed performance of this optical switch are described.

Hybrid fiber-optic sensors using liquid-crystal light modulators and piezoceramic transducers.

A new technique is described for linking remote sensors by multimode optical fibers. It combines two well-known technologies and is compatible with multimode optical multiplexing. At the sensing location, the output of a transducer (such as a piezoceramic hydrophone) is connected directly to a low-voltage liquid-crystal light modulator that is coupled to fiber-optic transmission lines by graded-index rod lenses. Transducer voltages as low as 10 microV produce intensity modulation of the fiber light. An optical hydrophone based on the technique demonstrated a minimum detectable pressure of 39 dB re 1 [equiation] at 500 Hz. The device performance was evaluated over the 8-45 degrees C temperature range. Sensitivity was found to decrease linearly by 0.2 dB/ degrees C over that range. The technique provides a means by which rugged nonmechanical sensors can be constructed that are suitable for incorporation into practical fiber-optic sensor systems.

Low-cross-talk 2 x 2 optical switch.

An electro-optical 2 x 2 switch for fiber-optic applications has been developed. Optical cross talk is reduced significantly by inserting two 2 x 1 switches in each optical path. A compact structure is used to contain the four 2 x 1's that are required. We observed -27-dB cross talk and 2.5-dB insertion loss in both switching states. The device is controlled by a 5-V twisted-nematic liquid-crystal cell. Multimode, unpolarized light is switched.