The sections below describe the research areas and major contributions by Professor Dagli and his group over the span of his career. The numbering of the representative publications corresponds to the numbering in his publication list.
Professor Dagli made the pioneering contribution to novel beam propagation methods (BPMs) based on implicit and explicit finite difference algorithms [26], [41]. He extended the initial paraxial BPM to take into account the wide angle and vector nature of electromagnetic wave propagation [72]. These are the most efficient BPM that exist today, and resulted in commercial software packages making BPM available for every day engineering applications. All the commercial software packages use the Finite Difference BPM developed first by Dagli’s group. Many other research contributions by other researchers are based this work which is widely cited. The accuracy of these CAD tools were verified by carefully designed experiments, such as novel integrated waveguide mirrors [29] and beam splitters [56].
His experimental and theoretical work on novel guided wave components continued over time. Recent examples are submicron etched beam splitters [215] and substrate removed waveguides [110]. Etched beam splitters are used in ultra compact ring resonators in the micro resonator effort described below. Substrate removed waveguides provide a novel platform for advanced components with superior properties. This technology allows processing both sides of an epilayer which generates a great deal of flexibility in device design and fabrication. It can be also be used to generate very high index contrast waveguides with the excellent electro optic properties of compound semiconductors. Extensive studies in the design and fabrication of such waveguides resulted in very low loss optical nanowires with excellent electro-optic properties [203]. As further examples of the novel features of this technology are narrowband tunable filters without gratings [129].
Representative publications
[26] Y. Chung and N. Dagli, "An Assessment of Finite Difference Beam Propagation Method, "IEEE J. Quantum Electron., vol. QE-26, No. 8, pp. 1335-1339, Aug. 1990.
[41] Y. Chung and Nadir Dagli, "Analysis of z-invariant and z-variant Semiconductor Rib Waveguides by Explicit Finite Difference Beam Propagation Method with Non-uniform Mesh Configuration," IEEE J. Quantum Electron., vol. QE-27, No. 10, pp. 2296-2305, Oct. 1991.
[72] Y. Chung and N. Dagli, "A Wide Angle Propagation Technique using an Explicit Finite Difference Scheme," IEEE Photonics Technology Lett., vol. 6, No. 4, pp.540-542, April 1994.
[29] Y. Chung and N. Dagli, "Analysis of Integrated Optical Corner Reflector Using Finite Difference Beam Propagation Method," IEEE Photonics Technology Lett., vol. 3, No. 2, pp. 150-152, February 1991.
[56] Y. Chung, Ralph Spickermann, Bruce Young and N. Dagli, "A Low Loss Beam Splitter with an Optimized Waveguide Structure" IEEE Photonics Technology Lett., vol. 4, No. 9, pp.1009-1011, September 1992.
[215] Byungchae Kim and Nadir Dagli, “Submicron Etched Beam Splitters Based on Total Internal Reflection in GaAs/AlGaAs Waveguides,” IEEE/OSA J. of Lightwave Technology, vol. LT-28, 2010.
[110] S. R. Sakamoto, C. Ozturk, Y. T. Byun, J. Ko, N. Dagli, “Low Loss Substrate-Removed (SURE) Optical Waveguides in GaAs/AlGaAs Epitaxial Layers Embedded in Organic Polymers”, IEEE Photonics Technology Lett., vol. 10, No. 7, pp.985-987, July 1998.
[203] JaeHyuk Shin, Yu-Chia Chang and Nadir Dagli, ”Propagation loss study of very compact GaAs/AlGaAs substrate removed waveguides,” Opt. Express vol. 17, No. 5, pp. 3390-3395, March 2, 2009.
[129] Cem Ozturk, Andrew Huntington, Atilla Aydinli, Young Tae Byun and Nadir Dagli, "Filtering Characteristics of Hybrid Integrated Polymer and Compound Semiconductor Waveguides", J. of Lightwave Technology, vol. LT-20, pp. 1530-1536, 2002.
Professor Dagli also made significant contributions to compound semiconductor electro-optic modulators. He used novel slow wave electrodes on unintentionally doped GaAs/AlGaAs epitaxial layers. This allowed the realization of very low optical and microwave loss devices, hence traveling wave modulators with superb properties could be fabricated. Using such electrodes it was possible to velocity match microwave and optical signals without any dispersion at least up to 40 GHz, which was the limit of experiments [77]. Furthermore, microwave loss of the electrode was kept very low without sacrificing the electric field strength, and the electrode characteristic impedance was kept very close to 50 Ω [140]. This resulted in chirp free electro-optic modulators with electrical bandwidths in excess of 40 GHz [118], [102]. Polarization independent versions of these modulators were also demonstrated [99]. Another very significant contribution was the identification of the bandwidth limiting factor as the matching of the phase velocities rather than the group velocities [102]. This had a very significant impact on semiconductor electro-optic modulators, since material dispersion is significant and phase and group velocities could be very different.
Novel modulators relying on substrate removal and polymer integration techniques have been developed as a continuation of this work. Using substrate removal and polymer integration true push pull, low drive voltage modulators were fabricated [121]. Using only bulk GaAs material drive voltages as low as 3.7 Vcm were demonstrated for 1 cm long electrode [161]. This is a factor of 2 t0 3 times better than LiNbO3. Incorporating the novel slow wave electrodes developed earlier 35 GHz electrical to electrical bandwidth versions of these devices were also fabricated [173]. Recently substrate removed compound semiconductor nanowires were incorporated in electro-optic modulators. This work resulted in 0.3 V drive voltage Mach-Zehnder electro-optic modulators in bulk GaAs material with 7 mm long electrodes [187]. This is a world record for any optical modulator.
Professor Dagli also edited a book on high speed photonic devices. He contributed a chapter on III-V compound semiconductor electro-optic modulators [164].
Recently professor Dagli started working on modulator based photonic signal processing. One example is a photonic analog-to-digital converter [211].
Representative publications
[77] Ralph Spickermann and Nadir Dagli, "Experimental Analysis of Millimeter Wave Coplanar Waveguide Slow Wave Structures on GaAs, "IEEE Trans. Microwave Theory Tech., vol. MTT-42, No. 10, pp.1918-1924, October 1994.
[140] J. H. Shin, C. Ozturk, S. R. Sakamoto, Y. J. Chiu and N. Dagli, “Novel T-Rail Electrodes for Substrate Removed Low-Voltage, High-Speed GaAs/AlGaAs Electro-optic Modulators, “ IEEE Trans. Microwave Theory Tech. vol. MTT-53, No. 2, pp. 636 – 643, February 2005.
[118] Nadir Dagli," Wide Bandwidth Lasers and Modulators for RF Photonics," IEEE Trans. Microwave Theory Tech., vol. MTT-47, No. 7, pp. 1151-1171, July 1999.
[102] R. Spickermann, S. R. Sakamoto and N. Dagli,"GaAs/AlGaAs Traveling Wave Electrooptic Modulators," Optoelectronic Integrated Circuits Conference, Paper 33, SPIE International Symposium on Optoelectronics'97, San Jose, CA, February 8-14, 1997.
[99] R. Spickermann, M. Peters and N. Dagli,"A Polarization Independent GaAs/AlGaAs Electrooptic Modulator,"IEEE J. Quantum Electron., vol. QE-32, No. 5, pp. 764-769, May 1996.
[121] S. R. Sakamoto, A. Jackson and N. Dagli, "Substrate Removed GaAs/AlGaAs Modulator," IEEE Photonics Technology Lett., vol. 11, No. 10, pp. 1244-1246, October 1999.
[161] Jae Hyuk Shin, Shaomin Wu and Nadir Dagli, " Bulk undoped GaAs-AlGaAs substrate-removed electrooptic modulators with 3.7-V-cm drive voltage at 1.55 μm, " IEEE Photonics Technology Letters, vol.18, no. 21, pp. 2251-2253, Nov. 2006.
[173] JaeHyuk Shin, Shaomin Wu, and Nadir Dagli, "35 GHz Bandwidth, 5 V-cm Drive Voltage, Bulk GaAs Substrate Removed Electro Optic Modulators" IEEE Photonics Technology Lett., vol. 19, No. 18, pp. 1362-1364, September 15, 2007.
[187] JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli, “0.3 V Drive Voltage GaAs/AlGaAs Substrate Removed Mach-Zehnder Intensity Modulators” Appl. Phys. Lett., vol.92, 201103, 2008.
[164] High Speed Photonic Devices, Taylor and Francis, edited by Nadir Dagli, 2007.
[211] Chris H. Sarantos and Nadir Dagli, “A photonic analog-to-digital converter based on an unbalanced Mach-Zehnder quantizer“, Optics Express, vol. 18, No. 14, pp. 14598-14603, 23 June 2010.
Professor Dagli also worked on micro ring resonators. Part of this effort is carried out on silicon on insulator material system. This work resulted in high Q racetrack resonators that can be thermo-optically tunable with very low powers [141], [152], [155]. Recently very compact ring resonators and ring lasers in compound semiconductors were demonstrated using total internal reflection mirrors [151], [166]. Further significant size reduction was achieved using etched beam splitters in addition to waveguide mirrors. Such resonators were fabricated both in GaAs/AlGaAs and InP/InGaAsP material systems [202], [213]. Electrically tunable very compact resonators incorporating semiconductor optical amplifiers at 1.5 μm showed free spectral ranges as large as 14.5 nm [213]. Much larger free spectral ranges were also demonstrated using substrate removed resonators [130].
Another research thrust was on novel application of coupled ring resonators. It is shown that using only two coupled rings 100 % reflection can be obtained [156]. This is the simplest reflector arrangement using ring resonators and forms an alternative to reflecting gratings. This approach does not require any submicron gratings and can be tunable. It is also shown that such reflectors can be used in the fabrication of widely tunable lasers [150]. Efforts in this direction recently resulted in widely tunable lasers [210].
Representative publications
[141] I. Kiyat, A. Aydinli, N. Dagli, "High-Q silicon-on-insulator optical rib waveguide racetrack resonators," Optics Express, vol. 13, No. 6, 21 March 2005.
[152] I. Kiyat, A. Aydinli, N. Dagli. “Polarization Characteristics of Compact SOI Rib Waveguide Racetrack Resonators” IEEE Photonics Technology Letters, vol.17, no.10, pp. 2098-3000, Oct. 2005.
[155] I. Kiyat, A. Aydinli, N. Dagli “Low-power thermooptical tuning of SOI resonator switch” IEEE Photonics Technology Letters, vol.18, no.2, pp. 364-366, Jan. 2006.
[151] Doo Gun Kim, Jae Hyuk Shin, Cem Ozturk, Jong Chang Yi, Youngchul Chung and Nadir Dagli, “Total Internal Reflection Mirror based InGaAsP Ring Resonators Integrated with Optical Amplifiers” IEEE Photonics Technology Letters, vol.17, no.9, pp. 1899-1901, Sept. 2005.
[166] Doo Gun Kim, Jae Hyuk Shin, Cem Ozturk, Jong Chang Yi, Youngchul Chung and Nadir Dagli, ”Rectangular Ring Lasers based on Total Reflection Mirrors and Three Waveguide Couplers, “IEEE Photonics Technology Letters, vol.19, no. 5, pp. 306-308, 1 March 2007.
[202] Byungchae Kim, Yu-Chia Chang, Nadir Dagli,” Compact ring resonators using conventional waveguides, etched beam splitters and total internal reflection mirrors.” 2009 Conference on Optical Fiber Communication (OFC 2009), Paper OWV6, San Diego, CA, 22-26 March 2009.
[213] Byungchae Kim and Nadir Dagli, “Compact Bandstop Filters with Semiconductor Optical Amplifier, Etched Beam Splitters and Total Internal Reflection Mirrors,” Integrated Photonics Research, Silicon and Nano Photonics Conference Proceedings, Paper ITuC-6, Monterey, California, July 25-28, 2010.
[130] Cem Ozturk, Yi-Jen Chiu and Nadir Dagli, "Polymer/compound semiconductor hybrid micro resonators with very wide free spectral range", IEEE/LEOS 2002 Annual Meeting, Paper ThN-2, Glasgow, UK, November 11- 14, 2002.
[156] Youngchul Chung, Doo-Gun Kim, and Nadir Dagli, "Reflection Properties of Coupled-Ring Reflectors,"J. of Lightwave Technology vol. 24, No. 4, pp. 1865-1874, April, 2006.
[150] Youngchul Chung, Doo-Gun Kim, and Nadir Dagli, “Widely Tunable Coupled-Ring Reflector Laser Diode” IEEE Photonics Technology Letters, vol.17, no.9, pp. 1773-1775, Sept. 2005.
[210] Suhyun Kim, Young Tae Byun, Doo-Gun Kim, Nadir Dagli, and Youngchul Chung, “Widely Tunable Coupled-ring Reflector Laser Diode Consisting of Square Ring Resonators,” Journal of the Optical Society of Korea, Vol. 14, No. 1, pp. 38-41, March 2010.
Professor Dagli’s work on quantum structures was on three different areas. The first was on electron waveguides and electron waveguide based devices [27]. This was an experimental and theoretical study which resulted in the first ever current switching and modulation in electron waveguides based on interference effects [51], [79]. The second area was on the calculations of the optical properties of quantum wires and quantum wire arrays [80], [52]. The third area is on the experimental determination of the electro-optic properties of self assembled quantum dots [185], [212]. This work demonstrated significantly enhanced electro-optic coefficients in such quantum dots.
Representative publications
[27] N. Dagli, G. Snider, J. Waldman and E. Hu, "An Electron Wave Directional Coupler and its Analysis," J. Appl. Phys., vol. 69, No. 1, pp. 1047-1051, 15 Jan. 1991.
[51] M. Thomas, N. Dagli, J. Waldman, A. Gossard, E. Yuh, E. Gwinn, R. Muller and P. Maker, "Current Switching and Modulation Based on Electron Interference in Electron Waveguides: A Zero Gap Electron Wave Coupler," 50th Annual Device Research Conference, Paper II-B, Cambridge, MA June 22-24, 1992 also in IEEE Trans. Electron Devices, vol. ED-39, No. 11, pp. 2643-2644.
[79] N. Dagli, M. Thomas, M. Rao, M. Sundaram and A. Gossard," Electron Waveguide Y Branches as Current Switches," Proceedings of the Second International Symposium on Quantum Confinement Physics and Applications, pp. 87-103, Electrochemical Society Inc., 1994.
[80] J. C. Yi and N. Dagli," Finite Element Analysis of Valence Band Structure and Optical Properties of Quantum Wire Arrays on Vicinal Substrates," IEEE J. Quantum Electron., vol. QE-31, No. 2, pp. 208- 218, February 1995.
[52] J. C. Yi and N. Dagli, "Optical Properties of Serpentine Superlattices on GaAs Vicinal Substrates for Quantum Wire Laser Applications," Appl. Phys. Lett.vol. 61, No. 2, pp. 219-221, July 13, 1992.
[185] I. B. Akca, A. Dana, A. Aydinli, M. Rossetti, L. Li, A. Fiore, and N. Dagli, "Electro-optic and electro-absorption characterization of InAs quantum dot waveguides," Opt. Express vol. 16, No. 5, pp. 3439-3444, March 3, 2008.
[212] JaeHyuk Shin, Hyochul Kim, Pierre M. Petroff, and Nadir Dagli, “Enhanced Electro-Optic Phase Modulation in InGaAs Quantum Posts at 1500 nm, “IEEE J. Quantum Electron., vol. QE-46, No. 7, pp. 1127-1131, July 2010.
Professor Dagli worked early on his carrier on high speed transistors with negative output resistance. He was the first to predict negative output resistance in heterojunction bipolar transistors due to transit delay effects [12]. He also proposed a unipolar transistor with negative output resistance that can work well into millimeter wave frequencies [20].
Representative publications
[12] N. Dagli, "Physical Origin of the Negative Output Resistance of Heterojunction Bipolar Transistors," IEEE Electron Device Letters, vol. EDL-9, No. 3,pp. 113-115, March 1988.
[20] N. Dagli, "A Unipolar Transistor with Negative Output Resistance, "Solid State Electronics, vol. 33, No. 7, pp. 831-836, July 1990.
Part of professor Dagli’s modeling efforts was on optical interconnect modeling. This work resulted in better understanding of the wave propagation on conductors with cross sectional dimensions at the order of the skin depth [21]. The results of this analysis was verified experimentally [40] and a software tool was developed to calculate the properties of such interconnects accurately [44].
Representative publications
[21] G.L. Matthaei, K. Kiziloglu, N. Dagli and S.I. Long, "The Nature of Charges, Currents and Fields in and about Conductors Having Cross-Sectional Dimensions of the Order of a Skin Depth," IEEE Trans. Microwave Theory Tech., vol. MTT-38, No. 8, pp. 1031-1036, Aug. 1990.
[40] K. Kiziloglu, N. Dagli, G.L. Matthaei and S.I. Long, "Experimental Analysis of Transmission Line Parameters in High-Speed GaAs Digital Circuit Interconnects" IEEE Trans. Microwave Theory Tech., vol. MTT-39, No. 8, pp. 1361-1367, Aug. 1991.
[44] G.L. Matthaei, G. Chinn, C. Plott and N. Dagli, "A Simplified Means for Computation of Interconnect Distributed Capacitances and Inductances," IEEE Trans. Computer-Aided Design, vol. 11, No. 4, pp. 513-524, April 1992.