Charles W. Roberts

Optical transitions to above‐barrier quasibound states in asymmetric semiconductor heterostructures

An asymmetric semiconductor electron wave Fabry–Perot interference filter has been designed with two above‐barrier quasibound states for optical transitions. The upper state was designed to have a spatial confinement lifetime greater than three times that of the lower state (which was designed to be less than 100 fs). Such lifetime ratios and magnitudes, which are nearly impossible for below‐barrier states, satisfy the criteria required for achieving population inversion. Furthermore, the transitions were designed to have large dipole matrix elements. Absorption measurements at multiple temperatures were used to demonstrate the first bound‐to‐quasibound transitions in an asymmetric structure. The experimental energies and dipole matrix elements are in agreement with calculated values. This type of structure could represent the basis for a new room‐temperature infrared semiconductor laser.

NON UNIFORM GRATING COUPLERS FOR COUPLING OF GAUSSIAN BEAMS TO COMPACT WAVEGUIDES

Coupling to compact waveguides can have improved efficiency with use of a grating with non uniform teeth. A combined numerical and mathematical technique for designing this coupler and Finite Element Method results is presented. This paper is a preprint of the final paper which was published in the Integrated Photonics Research Technical Digest, Optical Society of America, 1994 * Currently at Integrated Photonic Systems, Inc. P.O. Box 717, Clarksburg, NJ 08510, (609) 259-1654, e-mail: lcw@intphsys.com NON UNIFORM GRATING COUPLERS FOR COUPLING OF GAUSSIAN BEAMS TO COMPACT WAVEGUIDES Lawrence C. West, Charles Roberts, and Jason Dunkel AT&T Bell Laboratories, Room 4G518, Holmdel, NJ 07733 (908) 949-8715 and Gregory Wojcik, John Mould, Jr., Weidlinger Associates, 4410 El Camino Real, Los Altos, CA 94022 (415) 949-3010 INTRODUCTION: The beams that emit from lasers are typically have a Gaussian spatial profile. Yet the field profile that emits from a uniform grating is one of a decaying exponential[1]. This fundamental mismatch prevents high efficiency (> 90%) coupling of light into compact waveguides. This limitation has been supported by the difficulty of making high precision waveguide gratings in the near infrared on semiconductor surfaces, especially when non-uniform. However, improved lithography techniques and use of practical devices in the mid-IR overcome these limitations. We discuss a method for understanding and designing grating couplers that may emit beam profiles other than a decaying exponential. Special attention is given to the Gaussian beam profile because of its importance to the laser community and it’s minimum diffraction spread. The basic technique is to vary the tooth width along the grating coupler so as to project the desired beam image. THE DESIGN METHOD: We use a numerical Finite Element Method[2] to measure the complex amplitude reflection, transmission, and scattered power coefficients for a single mode waveguide for various tooth depths, widths, and geometries. A single mode is injected into a waveguide and scattered off a single tooth within the waveguide. The resultant complex reflection and transmission coefficients into the single mode are measured. The energy difference of these and the original beam is assumed to be the scattered power. A table of these coefficients versus tooth parameters is created. For fabrication simplicity, we typically constrain ourselves to a constant depth tooth so the grating can be fabricated with a single mask and single etch. But actual lithography results, such as a change in etch hole profile for smaller teeth, can and should be taken into account by a substitution of those teeth with their respective coefficients. Figure 1: Geometry of scattering problem solved per tooth. The grating itself will be made of an ensemble of the individual teeth. The tooth spacing will remain roughly constant (this is not a chirped grating) but the tooth size will change. The spacing between teeth can be adjusted to correct for phase changes under the teeth so as to maintain a flat phase profile in the scattered beam, but this is a small percentage of the overall spacing. These Input Reflection Transmission Scattered Power GaAs Substrate Ge Waveguide tooth 0.6 m 1.75 m n = 4.0 n = 3.27 = 10.0 m 0 w reflection and transmission coefficients are calculated by subtracting the original mode as if there were no tooth and taking the remainder as a change in transmission or reflection caused by the tooth. Shown here is a chart of the coefficients for the tooth of Figure 1 with a 0.477 mm deep tooth. Of particular interest is the scattered power versus tooth width. Since the period of the grating is 3.0 micrometers to match the phase period of the waveguide mode, the maximum tooth size is 3.0 micrometers, at which point the waveguide is a smaller unconfined material. The scattered power is almost linear in tooth width with a proportionality of α. The reflection coefficient is almost a constant. We use these assumptions to derive a simple analytic form for the scattered light. A simple first order derivation of the desired tooth width can be found by assuming the grating is efficient, so that the power remaining in the grating is that which has not been scattered yet. Since the desired scattered power is proportional to the product of remaining power in the guide and the tooth width with constant α, we have the n’th tooth has a size of w p e e dz n z z z z z zc = ∫ α 2 0 2

Infrared absorption of Ge epitaxial films on a GaAs substrate

Germanium films were deposited on GaAs (100) substrates with or without an epiready surface oxide at temperatures between room temperature (RT) and 500 °C using an ultrahigh‐vacuum e‐beam deposition system. The film at 100 °C on a substrate with a surface oxide had a flat absorption curve over the wave‐number range investigated, 500–4000 cm−1, with an absorption of less than 10/cm at 1000 cm−1 (10 μm wavelength). Films deposited at RT and 50 °C on substrates with a surface oxide had comparable low absorption, but they contained an absorption peak at 830 cm−1 associated with the Ge—O bonds. Although all three films were amorphous, the films deposited at the lower temperatures were more porous. This enabled oxygen to percolate in from the atmosphere to form the Ge—O bonds. The films deposited at 150 °C and above on substrates with a surface oxide and at 100 °C with the surface oxide removed thermally in situ prior to deposition the Ge films, and the single crystal films deposited at 400 and 500 °C on oxide‐...

Nanostructure optical emitters based on quasibound electron energy levels

Abstract Given two energy states (levels) in a quantum well formed by two potential barriers of finite thickness, elementary quantum mechanics tells us that the lower energy state is more tightly bound than the upper state. This produces a longer spatial confinement lifetime in the lower state than in the upper state. This ratio of lifetimes is opposite to that needed for laser action between these states. Furthermore, the lifetime of the lower energy state must be significantly shorter than the electron scattering time for the upper state. These facts have blocked the development of lasers based on these transitions. However, in this paper we report experimental and analytical results on a versatile type of semiconductor heterostructure that overcomes these difficulties. Unlike previous devices, this structure relies on an optical transition between two states which are both above-barrier quasibound states in the ‘classical’ continuum. The oscillator strength is large and the operation of the device clearly demonstrates coherent electron wave behavior. Such structures could represent the basis for a new room-temperature infrared semiconductor laser.

OXYGEN CONTAMINATION OF LOW TEMPERATURE ULTRAHIGH VACUUM-DEPOSITED GE FILMS ON GAAS

The Fourier transform infrared absorption spectrum for the range of 500–4000 cm−1 wave numbers was measured for several Ge films deposited on GaAs using ultrahigh vacuum e‐beam deposition at various substrate temperatures ranging from room temperature (RT) to 500 °C. Spectra indicate oxygen incorporation at low deposition temperatures whether or not the native oxide was removed from the substrate prior to film deposition. Using transmission electron microscopy, we show that Ge films deposited at RT and 100 °C on a (100) GaAs surface that did not have the oxides removed are amorphous while those deposited at 100 °C with the oxide removed are crystalline, but are highly defective. Secondary ion mass spectroscopy (SIMS) measurements show that the amorphous films at RT contain more than two orders of magnitude more oxygen than the films deposited at 100 °C or a single crystal film deposited at 400 °C. Oxygen‐18 diffusion studies definitively show that the excess oxygen in the amorphous films percolates in fro...

ULTRA HIGH CONFINEMENT WAVEGUIDES FOR VERY LARGE SCALE INTEGRATED OPTICS (VLSIO) WITh THREE DIMENSIONAL PACKAGING

By use of very high index ratios between the cladding and guide, Ultra High Confinement (UHC) waveguides1 can be created, leading to compact devices and sharp bends. Furthermore, with the use of diffractive optics, these Very Large Scale Integrated Optical (VLSIO) circuits’ can be interconnected in three dimensional stacks with high density connectivity. A non-uniform grating coupler between a compact waveguide mode and a large Gaussian profile single mode beam is designed and tested. This coupler is a critica1 component for interconnects, allowing efficient coupling between various beam shapes and compact devices. The size of UHC integrated optical devices is 10 to 100 times smaller per cubic wavelength than present waveguides and resonators, resulting in much higher speed and lower power. These UHC waveguides and components are analyzed using 3D Vector Field Finite Element Methods and microwave scaled experiment.3

Quantum well mid-infrared lasers based on above-barrier transitions

A possible laser device is designed with the use of classically free quasibound electron states. An asymmetric semiconductor electron wave Fabry-Perot interference filter is designed with an upper electron state having much stronger confinement than the lower electron state. This structure also allows for direct current pumping of the upper state and rapid depletion of the lower state under the presence of a field. Spectroscopy experiments demonstrate the existence of the upper quasibound state in a test structure. This laser filter structure, designed for infrared gain with current pumping, is combined with a special injector filter for room temperature narrow energy current injection into the upper lasing state. A stack of 54 periods of this electrically pumped structure is placed within a waveguide geometry. A laser device is fabricated by etching mesa structures from 50 to 100 micrometers wide. End cleaved facets serve as reflectors for mesas from 2 to 5 mm long. Tests are performed on these devices to determine their electrical properties and suitability for lasing.

Multiple long-wavelength VCSEL arrays for low-cost WDM

Vertical-cavity surface-emitting lasers (VCSELs) emitting in the 1530-1565 nm region of flat gain in Er-doped fibers offer the potential for low-cost transmitters for wavelength division multiplexing (WDM). Methods are described to produce precisely-defined vertical-cavity surface-emitting laser arrays which: 1) efficiently utilize wafer real estate; 2) have precise and uniform wavelength distributions despite wafer thickness nonuniformity and wafer-to-wafer thickness variation; 3) are compatible with known multiplexing technologies; 4) have minimum wavelength variation with temperature. Epitaxial growth on patterned substrates with varying-size mesas has been shown to produce multiple-wavelength VCSEL arrays by Igas group at the Tokyo Institute of Technology. This can be combined with additional refinements to fine tune the wavelengths, increase yield, and to maximize VCSEL efficiency, manufacturability and performance. Multi-wavelength VCSEL arrays represent a much lower cost, more controllable alternative to distributed-feedback laser arrays for WDM sources. The difference in laser output powers can be largely compensated via use of an Er-doped fiber amplifier within the transmitter. Reports such as that by ElectroniCast point to transmitters and receivers as being the most vital WDM components, in terms of both cost and technology.

Three-dimensional packaging of very large scale integrated optics (VLSIO) for high-complexity optical systems

Optics has the fundamental capability of dramatically improving computer performance via the reduction of capacitance for intrinsic high bandwidth communications and low power usage. Yet optical devices have not displaced silicon VLSI in any measure to date. The reason is clear. When placed into systems, the optical devices have not had significantly greater performance in equally complex information processing circuits and similarly low manufacturing cost. An approach demonstrated here uses the same system integration techniques that have been successful for silicon electronics, only applied to optics. Essential for creation of very large scale integrated optics (VLSIO), with over 50,000 high speed logic gates per square centimeter, is a new class of ultra high confinement (UHC) waveguides. These waveguides are created with high index difference (as high as 4.0 to 1.0) between guide and cladding. The waveguides have been demonstrated with infrared cross sections less than 5% of a square free space wavelength. These waveguides can be manufactured today only in the mid-infrared, but the concepts should scale to the near-infrared as lithography improves. Waveguide corners have been designed and demonstrated with a bend radius of less than one free space wavelength. Resonators have been designed which have over 100 times smaller volume than VCSELs, yet efficiently inter-connected laterally in high densities. A connector to the UHC waveguides has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate. The coupler arrays can allow up to 10,000 Gaussian beam connections per square centimeter. This connectivity also has advantages for low cost three dimensional packaging for reduced cost and thermal dissipation. Experimental results on the above concepts and components are presented.

Germanium waveguide for optical interconnects in very large scale integrated optics

The Fourier Transform Infrared (FTIR) absorption spectrum for the range of 500 to 4000 cm-1 wavenumbers was measured for several Ge films deposited on GaAs using ultra high vacuum E-beam deposition at various substrate temperatures ranging from room temperature (RT) to 500 degree(s)C. Using transmission electron microscopy, we show that Ge films deposited at room temperature and 100 degree(s)C on a (100) GaAs surface that did not have the oxides removed are amorphous while those deposited at 100 degree(s)C with the oxide removed are crystalline, but are highly defective. Secondary ion mass spectroscopy (SIMS) measurements show that the amorphous films at RT contain more than two orders of magnitude more oxygen than the films deposited at 100 degree(s)C or a single crystal film deposited at 400 degree(s)C. The oxygen-18 diffusion studies definitively show that the excess oxygen in the amorphous films percolates in from the atmosphere. SIMS studies further reveal that thermally removing the GaAs substrate surface oxide or depositing a Au film on top of the Ge film has little effect on the incorporation of oxygen.