Integrated Photonics Group

Research Directions

· Ferroelectric thin films for guided wave nonlinear optics

· Artificial structures for nonlinear optical processes

· Fiber optic mesh networks

· Production and new applications of entangled photons

· THz generation and spectroscopy

RESEARCH AREAS

Growth and Patterning of Ferroelectric Thin Films

During the course of our work on LiNbO₃ thin film growth, it has become apparent that precursor volatility-vs.-stability is a major factor in the low growth rate of thin films of complex oxides – and that the stability of alkoxide precursors makes them superior to those of diketonates. However, our CBE studies of common alkoxide precursors of LiNbO₃ show that the precursor decomposition cycle is auto-catalytic: the precursor ligands themselves generate water upon decomposition and this water, in turn, reacts further to form volatile monomers of Li and Nb that desorb from the surface, precluding high growth rates. We are therefore exploring precursors re-designed such that the ligands themselves no longer contain the elements of water, as well as engineering molecules that, on decomposing, remove the ligand intact from the growth surface before dehydration can take place. The first category of improved precursors includes those where ethoxy ligands are replaced by the chelating ligand dimethylaminoethanol, DMAE. The introduction of chelating ligands serves two purposes: 1) the DMAE ligand does not generate water upon decomposition, thus blocking the detrimental auto-catalytic cycle, and 2) the DMAE ligand may bind the central Nb atom at two positions, yielding monomeric species that are significantly more volatile than the currently used Nb₂(OEt)₁₀ dimer.

We have synthesized, purified, and spectroscopically characterized multi-gram amounts of three new precursors containing varying numbers of ethoxy and DMAE ligands: Nb(OEt)₄(DMAE) Nb(OEt)₃(DMAE)₂, and Nb(OEt)₂(DMAE)₃. Film growth studies utilizing these new precursors will reveal whether these species decompose by mechanisms fundamentally different than those exhibited by the conventional Nb precursor, Nb₂(OEt)₁₀. If our studies reveal that film formation takes place by the simple superposition of the independent decompositions of the different ligand types, then the amount of water generated at the surface can be modulated by varying the number of ethoxy ligands delivered to the substrate. A precise compromise between growth rate and film crystallinity could thus be attained.

Unfortunately, the extreme chemical inertness of ferroelectrics severely limits the available suite of processing tools essentially to lithography, thermal diffusion of index-elevating dopants, and field–assisted reversal of ferroelectric dipole domains. Wet and dry etching rates of LiNbO₃ are extremely low (< 1mm/h) and invariably result in unacceptably rough surfaces. We have recently developed, in collaboration with Structure Materials Industries (SMI) of Piscataway NJ, a method for patterning high index contrast, high aspect ratio features in LiNbO₃. The technique uses chemical vapor deposition (CVD) to deposit an amorphous LiNbO₃ film on a LiNbO₃ substrate. The film can be easily patterned using conventional photolithography and standard wet (10% HF ~ 1min.) or dry etching (RIE) techniques. The patterned film undergoes solid state regrowth to crystalline LiNbO₃ when heated to 900^o C for ~ 1h. The figure to the right shows a series of Ti-doped ridge waveguides after annealing to crystallinity. The index profile of the patterned film can be modified by the addition of dopants via alkoxide precursors such as [Ti(O-Pr)₄]_n . Thick films can be rapidly grown (~ 5 mm/ h). We have confirmed that Ti-doped films guide light. We have also recently fabricated a ridge waveguide Mach-Zhender modulator in a 4 mm patternable LiNbO₃ film, from which we observed electro-optic modulation, indicating that the film has oriented ferroelectric domains. More detailed characterization is currently underway.

Artificial structures for nonlinear optical processes

We are exploring applications of the nonlinear optical equivalent of photonic crystals – a two-dimensionally modulated 2^nd order nonlinearity. This lattice has a unique spatio-spectral characteristic that removes the collinearity requirement for quasi-phase matching, creating a new class of all-optical functions. We have used this 2-D phase matching to produce simultaneous wavelength interchange of data between two optical carriers at different wavelengths.

Guided Wave Entangled Photon Source

A new generation of optical communication – which truly deserves the name photonics – is beginning to emerge. One manifestation is the entanglement of photons. Entanglement is the non-classical correlation between separate quantum mechanical systems. A number of intriguing functions with obvious applications to optical communications and computation have been demonstrated. These include quantum teleportation (transferring the quantum state of one photon to another), entanglement swapping (teleportation of an entangled state), quantum dense coding, as well as quantum cryptography (in which two remote parties generate a common secret key which is immune to eavesdropping). These functions are beginning to show applications which blur the line between communications and computation (e.g., complex quantum computation).

Entangled photon pairs can be produced by the process of spontaneous parametric down conversion (w₃ à w₁ + w₂) in 2^nd order nonlinear materials such as LiNbO₃. Entangled pair production to date has been limited to demonstrations in bulk material or performed collinearly with the pump light. We are exploring the use of new waveguide geometries and/or the 2-D nonlinear photonic lattices mentioned above to extract spatially separated but entangled daughter photons. Channel waveguides provide spatial mode (and therefore, momentum) filtering, control of optical confinement, and compatability with fiber optics. In addition, guided wave optics provides a platform on which to integrate the linear optical operations needed for entanglement applications, including coherent coupling (beam splitting), spectral filtering, and multi-GHz modulation of amplitude, phase, and polarization control.

A Ferroelectric Engineered for THz Generation

The inherently large 2^nd order nonlinearities inherent in LiNbO₃ (and its sister ferroelectrics), together with the ability to quasi-phase match pumps and products with periodic poling, make these materials ideal candidates for optically pumped terahertz generation via difference frequency mixing (DFM) or optical rectification (OR). Their major drawback is that, unpumped, these materials absorb strongly in the biologically important THz frequency regime (~0.5 – 3 THz). As a consequence, THz generation has favored surface interactions by way of reflection or diffraction. We are developing what we believe is a promising solution: a dual-waveguide heterostructure, consisting of an optically pumped waveguide gain region surrounded by a transparent THz waveguide. When properly phase matched, two pump beams will produce a guided mode in the THz regime by difference frequency mixing.

Analysis of Fiber Optic Networks

Advances in photonic devices and circuits as described above will likely move fiber optic networks toward higher degrees of interconnectivity and transparency. As these networks evolve from a point-to-point communications toward mesh topologies, new capacity provisioning and restoration schemes need to be developed. Analysis of these requirements and protocols is typically performed by computer simulation, which is slow for large networks (thus limiting the number of variants which can be tested) and provides no general guidelines for optimization of network performance parameters. To remedy this situation, we have begun by deriving analytic relations describing – and allowing us to compare – the capacity requirements and elapsed times of several mesh restoration schemes.

Mesh networks have emerged as an attractive topology for optical transport networks and new restoration schemes have been suggested to provide full survivability against single link failures. We have derived analytically the pre-provisioned capacity requirements of two mesh restoration schemes, link restoration (LR), and path-disjoint restoration (PRd). This analytic description combined with the network global expectation model (NetGEM) developed by Korotky allows for the quick evaluation of the capacity expenditure (CAPEX) for a wide range of mesh topologies utilizing these restoration schemes. We found that the CAPEX for PRd is lower than that for LR and the difference between the two increases as the network size increases. It is generally believed, however. that the CAPEX advantage of PRd is offset by the higher operational expenditure (OPEX), in particular a larger mean time to restore (MTTR) a single failed link. We have more recently formalized this tradeoff by analytically deriving the MTTR for PRd in planar mesh networks as a function of the network parameters. The analysis quantifies the tradeoff between capacity provisioning and the delay involved in restoration and will prove useful in evaluating the relative benefits of different switch architectures and signaling protocols.

EXPERIMENTAL FACILITIES

Instrumentation in our Integrated Optics Laboratory includes:

· Guided wave integrated optic characterization test bed

· Dual ion beam sputtering system for metal and dielectric thin film deposition

· Diffusion furnaces

· Fixed and tunable, visible and near IR, laser sources

· 3W fiber amplifier

· Programmable high voltage poling station for patterned poling of wafers to 3”

· Monochromators and Optical Spectrum analyzers

· Fluorescence excitation and standard absorption spectroscopy.

· Microwave test bench with a 50 GHz network analyzer

COLLABORATING PARTNERS

· Professor Thomas F. Kuech, Dept. of Chemical & Biological Engineering, University of Wisconsin – Madison

· Dr. Aref Chowdhury, Bell Laboratories, Murray Hill NJ

· CatherineRice, Structured Materials Industries, Piscataway NJ

· Steve Korotky, Bell Laboratories, Crawford Hill NJ

STUDENTS

Current

· Manish Bhardwaj, Electrical & Computer Engineering

· Chad Staus, Electrical & Computer Engineering

Alumni

· Dr. Niraj Agrawal, formerly of Bell Labs & HHI, now CEO of Lightmaze (Germany).

· Professor Kent Choquette, Dept of Electrical Engineering, University of Illinois – Champagne – Urbana.

· Dr. C. H. Huang, Wavesplitter Technologies, Fremont CA.

· Dr. Kraisan Songwatana, NYNEX Science and Technology Asia Ltd.

· Dr. Emily True

· Dr. Aref Chowdhury, Bell Laboratories, Murray Hill NJ

· Dr. Douglas Gill, Bell Laboratories, Murray Hill NJ

MAJOR PUBLICATIONS, BOOK CHAPTERS

Refereed Publications

• M. Bhardwaj, L. McCaughan, S. K. Korotky,” Simulation and modeling of the restoration time for path based restoration schemes in planar mesh networks,” accepted for publication in the Journal of Optical Networking (’06).
• M. Bhardwaj, L. McCaughan, S. K. Korotky, I. Saniee, “Analytical description of shared restoration capacity for mesh networks,” Journal of Optical Networking, 4, 130 (2005).
• C. Staus, R. Suess, L. McCaughan, “Laser-induced fracturing: an alternative to mechanical polishing and patterning of LiNbO3 integrated optic chips,” IEEE J. Lightwave Technology, 22, 1327 (’04).
• V. Joshkin, K. Dovidenko, S. Oktyabrsky, D. Saulys, T. Kuech, and L.McCaughan, “New Methods for Fabricating Patterned Lithium Niobate for Photonic Applications,” Journal of Crystal Growth Vol.259, 273-278 (2003).
• Aref Chowdhury, Chad Staus, Brian F. Boland, Thomas F. Kuech and Leon McCaughan, “ Experimental Demonstration of 1535-1555 nm Simultaneous Optical Wavelength Interchange with a Nonlinear Photonic Crystal,” Opt. Lett., 26, 1353 (2001).
• Chowdhury, L. McCaughan, Figure of merit for near-velocity-matched traveling wave modulators,” Optics Letters, 26,1317 (’01).
• Chowdhury, S. C. Hagness and L. McCaughan, “Simultaneous optical wavelength interchange with a two-dimensional second-order nonlinear photonic crystal,” Opt. Lett., 25, 832-834 (2000).

· Aref Chowdhury and Leon McCaughan, “Optical Multiplication Using a Bisected Intersecting Waveguide,” J. Lightwave Technol. 18, 688 (2000).

· Aref Chowdhury and Leon McCaughan, “Continuously-Phase Matched M-Waveguides for Second-Order Nonlinear Upconversion,” Photonics Technol. Lett. 12, 486 (2000).

• V. A. Joshkin, S. R. Oktyabrsky, P. Moran, D. Saulys, T. F. Kuech, and L. McCaughan, “Growth of Oriented Lithium Niobate on Silicon by Alternating Gas Flow Chemical Beam Epitaxy with Metalorganic Precursors Appl. Phys. Lett., vol. 76, pp. 2125-2127, 2000.

· D. Saulys, V. Joshkin, M. Khoudiakov, T. F. Kuech, A. B. Ellis, and L. McCaughan, “An Examination of the Surface Decomposition Chemistry of Lithium Niobate Precursors under High Vacuum Conditions,” J. Crystal Growth 217, 287 (2000).

· Paulson, A. B. Ellis, L. McCaughan, B. Hawkins, Jingxi Sun, T. F. Kuech, “Demonstration of near-field scanning photoreflectance spectroscopy,” Applied Physics Letters,  v 77, p 1943-5 , (2000).

· S. Jaloviar, Jia-Ling Lin, Feng Liu, V. Zielasek, L. McCaughan, and M. Lagally, “Step-induced Optical Anisotropy of Vicinal Si(001),” Phys. Rev. Letters, 82, 791-794 (1999).

· E. Rudkevich, F. Liu, D. Savage, T. Kuech, L. McCaughan, and M. Lagally, "Hydrogen-induced Si Surface Segregation on Ge-covered Si(001)," Phys. Rev. Letters, 81, 3467-70 (1998).

· E. Rudkevich, D. Saulys, D. Gaines, T. F. Kuech, L. McCaughan, "Adsorption and decomposition studies of t-butyl silane on Si(001) 2x1 surfaces using FTIR-ATR spectroscopy," Surface Science, 383, 69 (1997).

· H. Huang and L.McCaughan, "980 pumped Er-doped LiNbO₃ waveguide amplifiers: a comparison with 1484 nm pumping," IEEE J. of Selected Topics in Quantum Electronics, 2, 367 (1997).

· H. Huang and L. McCaughan, "Polarization-dependent Enhancement of Population Inversion and of Green Upconversion in Er:LiNbO3 by Yb Co-doping," IEEE Photonics Technol. Lett., 9, 599 (1997).

· H. Huang and L. McCaughan, "Photorefractive-damage-resistant Er-indiffused MgO:LiNbO₃ ZnO-waveguide amplifiers and lasers," Electronics-Letters.vol.33, p.1639-40.. (1997).

· E. Rudkevich, D. Savage, W. Cai, J. C. Bean, J. S. Sullivan, S. Nayak, T.F. Kuech, L. McCaughan, M. G. Lagally, "Extended spectral range Fourier transform infrared attenuated total reflection spectroscopy on Si surfaces using a novel Si coated Ge attenuated total reflection prism," J. Vacuum Sci Technol. A 15(4), 2153 (1997).

· Gill, J. Wright, and L. McCaughan, "Spectroscopic site determination in erbium-doped lithium niobate,” Physical Review B 53, 2334 (1996).

· H. Huang and L. McCaughan, “Er-diffused Ti:LiNbO₃ Channel Waveguide Optical Amplifiers Pumped at 980 nm,” Electronics Lett. 32, 215 (1996).

· V. White, R. Ghodssi, C. Herdey, D. Denton, and L. McCaughan, " Use of photo-sensitive polyimide for deep x-ray lithography," Appl. Phys. Lett. 66, 2072 (1995).

· V. White, R. Ghodssi, G. Fish, C. Herdey, H. Liu, D. Denton, and L. McCaughan, " A new method for producing graded index PMMA waveguides," IEEE Photonics Technol. Lett. 7, 772 ('95).

· C. H. Huang, D. Gill, L. McCaughan, "Evaluation of absorption and emission cross sections of Er-doped LiNbO₃ for application to integrated optic amplifiers, " IEEE J. Lightwave Technol. 12, 803 (1994).

· Gill, J.C. Wright, and L. McCaughan, "Site characterization of rare-earth doped LiNbO₃ using total site selective spectroscopy," Appl. Phys. Lett., 64, 2483 (1994).

· Gill, A. Judy, L. McCaughan, and J.C. Wright, "A Method for the Local Incorporation of Er into LiNbO₃ Guided Wave Optic Devices by Ti Co-diffusion, Appl. Phys. Lett., 60, 1067 (1992).

· K.D. Choquette, L. McCaughan, D.K. Misemer, J. E. Potts, and G.D.Vernstrom, "Tunable Photoluminescence of Uniformly Doped Short-period GaAs Doping Superlattices," J. Appl. Physics, 71, 2805 (1992).

· K.D. Choquette, D.K. Misemer, and L. McCaughan, "Electronic Structure of Short-period n-p GaAs Doping Superlattices," Phys. Rev. B, 43, 7040 (1991).

· D. M. Gill, L. McCaughan, and Niraj Agrawal, "A New Mechanism for Controlling the Optical Properties of Intersecting Waveguides: Fractionally Doping the Intersection Region," J. Quantum Electronics, 27, 588 (1991).

· M. True and L. McCaughan, “Large nonresonant light-induced refractive-index changes in thin films of amorphous arsenic sulfide,” Optics Lett. 16, 458 (’91).

· D. Gill, N. Agrawal and L. McCaughan, "Reducing Radiative Loss in Intersecting Waveguides by Fractionally Doping the Intersection Region," IEEE Photonics Technology Lett., 2, 887 (1990).

· K.D. Choquette, L. McCaughan, and D.K. Misemer, "Third-order Optical Susceptibility in Short-period GaAs Doping Superlattices," J. Appl. Phys., 66, 4387 (1989).

· N. Agrawal and L. McCaughan, "Radiation Losses in Intersecting Optical Waveguides," J. Appl. Phys., 65 (12) 4509 (1989).

· N. Agrawal and L. McCaughan, "Low-loss TiLiNbO₃ Intersecting Waveguides," Appl. Phys. Lett., 54, 1669 (1989).

· K.D. Choquette, L. McCaughan, and W.K. Smith, "Improved Optical Switching Extinction in Three-Electrode Ti:LiNbO₃ Directional Couplers," Appl. Phys. Lett. 51, 2097 (1987).

· L. McCaughan, N. Agrawal, and G.A. Bogert, "Novel Physical Effects in Intersecting Waveguides," Appl. Phys. Lett., 51, 1389 (1987).

· L. McCaughan and K.D. Choquette, "Ti Concentration Inhomogeneities in Ti:LiNbO₃ Waveguides," Optical Lett., 12, 567 (1987).

· N. Agrawal, L. McCaughan, and S.R. Seshadri, "A Multiple Scattering Analysis of Intersecting Waveguides," J. Appl. Phys. 62, 2187 (1987).

· S.K. Korotky and L. McCaughan, "Control of Section Symmetry in Reverse-Directional Coupler Switches," Electronics Lett., 22, 1222, (1986).

· L. McCaughan and S.K. Korotky, "Three-Electrode Ti:LiNbO₃ Optical Switch," IEEE J. Lightwave Tech., LT-4, 1324-1328, (1986).

· L. McCaughan and K.D. Choquette, "Crosstalk in Ti:LiNbO₃ Directional Coupler Switches," IEEE J. Quant. Elect., QE-22, 947-951 (1986).

· L. McCaughan and G.A. Bogert, "Integrated Optical 4 x 4 Ti:LiNbO₃ Crossbar Switch Array," Appl. Phys. Lett., 47, 348 (1985).

· E.J. Murphy, T.C. Rice, L. McCaughan, and G. T. Harvey, "Permanent Attachment of Single Mode Fiber Arrays to Waveguides," IEEE J. Lightwave Tech., 3, 795 (1985).

· L. McCaughan, "Long Wavelength Ti-doped LiNbO₃ Directional Coupler Optical Switches and Switch Arrays," invited, Optical Engineering, 24, 241 (1985).

· E.E. Bergmann, L. McCaughan, and J.E. Watson, "Coupling of Intersecting Ti:LiNbO₃ Diffused Waveguides," Appl. Optics, 23, 3000 (1984).

· L. McCaughan, "Low Loss Polarization-Independent Electro-Optic Switches at 1.3 µm Wavelength," IEEE J. Lightwave Tech., 2, 51 (1984).

· L. McCaughan and E.E. Bergmann, "Index Distribution of Optical Waveguides from their Mode Profile," IEEE J. Lightwave Tech., 1, 241 (1983).

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