Professor and Associate Head,
Department of Physics, Engineering Physics and Astronomy
Stirling Hall Room 371
Tel: (613) 533-6804
GRADUATE STUDENT OPENING
I am currently looking to add a new Ph.D. student to my group in September 2014. This student could work in one of two areas: 1) The development of theoretical and computation models of the nonlinear response of graphene and a variety of semiconductor nanostructures to intense terahertz pulses or 2) Quantum nonlinear optics in coupled photonic crystal micro-cavities.
All of my research lies in the area of theoretical condensed-matter physics and nonlinear and quantum optics. This work ranges from pure to applied physics and often involves collaboration with experimentalists. The following is a brief overview of some of my current research interests.
Quantum Optics in Photonic Crystals
It is well known that the spontaneous emission of atoms, molecules, and quantum dots can be drastically affected by the environment in which they are placed. For example, the rate of spontaneous emission of an atom in vacuum will be very different from that of an atom in a small metallic cavity with dimensions on the order of the wavelength of the emitted radiation. Similarly, if an atom is located within the bandgap of a photonic crystal -- i.e. in the frequency range where there are no propagating electromagnetic modes in the structure – then the spontaneous emission can be completely suppressed. Conversely, the rate of spontaneous emission can be greatly enhanced if the atom is placed at the location of a defect in the photonic crystal, where the mode density is very high. This last effect is called the Purcell effect.
We are currently researching the interesting dynamics of spontaneous emission in photonic crystals with multiple, coupled-cavity defect states. We use a projected Green tensor approach to calculate the local density of states and spontaneous emission dynamics. We have found that the radiation dynamics can be very interesting in such structures. In particular, the spontaneous emission rate can be either enhanced or diminished due to quantum-path interference effects in the structure. This is demonstrated in the plot to the left, where we plot the local density of states for a double-cavity structure showing a large peak in the defect near x=-2 but a dip in the defect near x=+3. This approach makes it much easier to understand the basic physics of multiple-defect structures and has possible implications for quantum-computing applications of photonic crystals. More recently, we have developed a projected master equation approach to quantum optics for multiple photons in multi-cavity photonic crystal systems in the presence of quantum dots. Until recently, such calculations were prohibitive due to the complexity of the system. However our new projective technique  make a wide variety of complex structure amenable to investigation.
Optical and Terahertz Response of Semiconductor Nanostructures
Another area of research that I am currently pursuing involves the theoretical and computational investigation of coherent electron dynamics in semiconductor superlattices and other nanostructures. This work is progressing in a number of directions. One of the main thrusts involves modeling the nonlinear interaction of light with semiconductor nanostructures. Of particular interest are the dynamics of electrons that have been set into motion by shining ultra-short ( < 100 fs) pulses of laser light on the nanostructures. One important system that we are investigating is a biased semiconductor superlattice (BSSL). In a photo-excited BSSL, the excitons (electron-hole pairs) undergo an oscillatory motion called Bloch oscillations (BO). Studying the dynamics of BO in superlattices is proving very instructive in trying to understand electron and exciton dynamics and relaxation in nanostructures on a very fast time scales (< 1ps) -- a topic that is becoming more important as the size of semiconductor devices go down and their speed goes up. It is also an excellent system to test many-body theories of nonlinear optics of semiconductors. A more immediately practical goal of this work is to lay the groundwork for the development of the first compact, tunable THz laser. “T-ray” lasers, which emit radiation that lies between the microwave and infrared portions of the spectrum, are becoming important tools in materials research and in biological and mechanical imaging. We have recently shown that THz gain in suitably excited BSSLs should be comparable to current quantum cascade lasers.
We are currently developing theoretical formalisms and computational models to treat the response of optically-pumped semiconductor nanostructures and graphene to intense Terahertz fields. Some of the formalisms employ an excitonic basis, while others employ an electron-hole basis. All of the models treat the terahertz field non-pertubatively.
Dynamic Localization in Waveguide Arrays
A number of researchers have shown in recent years that when a sinusoidal ac electric field of precisely the correct amplitude is applied to electrons in a periodic potential, the electron wave packets become localized in space. This effect is called dynamic localization. It was quickly realized that the effect of dynamic localization does not occur for general bandstructures, but only for nearest-neighbour tight-binding bands. We have recently shown that there is a general class of discontinuous ac fields for which dynamic localization will occur for any bandstructure. We have found that such fields must have discontinuities at all changes in sign and have developed a scheme for constructing such fields. The tolerance to smoothing of these discontinuities is rather large, but a sinusoidal field is far from being the best continuous field to achieve dynamic localization.
Apart from its intrinsic importance to the better understanding of transport in ac fields, this work has important implication with respect to the ac conductivity and nonlinear optical response of superlattices, including excitonic effects. Finally, this work has an important analog in the area of waveguide optics. It turns out that the mathematics involved in modeling electron dynamics in semiconductor superlattices is very similar to that required to model the propagation of optical beams down arrays of coupled optical waveguides. We have found that under the right conditions one can observe “Bloch oscillations” of the beam between the waveguides as it propagates down the waveguides. More importantly, by curving the waveguide arrays in carefully specified ways, one can observe the analog of the dynamic localization of electrons that can occur in superlattice in ac electric fields. The plots at the left show the measured and calculated beam intensity in a particular curved waveguide array as a function of propagation distance, v, and transverse direction, u. The transmission properties of these waveguide arrays as a function of wavelength can be tailored over a wide range by choosing the curvature profile. This, and related phenomena have potential in the design of optical filters, multiplexing devices and all-optical switches for fibre-optic communications systems.
Current Group Members
¨ Ibraheem Al-Naib (Postdoctoral Fellow) firstname.lastname@example.org
¨ Mohsen Kamandar (Ph.D.) email@example.com
¨ Nicole Day (M.Sc.) firstname.lastname@example.org
¨ Fredrik Sy (M.Sc.) email@example.com
¨ Riley McGouran (M.Sc.) firstname.lastname@example.org
¨ Sean Doutre (M.Sc., 2013)
¨ Andrew Parks (M.Sc., 2011)
¨ John Simpson-Porco (Undergraduate Summer Researcher 2010)
¨ Dawei Wang (Ph.D., 2008)
¨ Arvin Reza (M.Sc., 2008)
¨ Dr. David Fussell (Postdoctoral Fellow, 2005-2007)
¨ Lijun Yang (M.Sc., 2001; Ph.D., 2005)
¨ Julius Wan (Ph.D., 2005)
¨ Dr. Aizhen Zhang (Postdoctoral Fellow, 2002-2004)
¨ Jean-Marc Lachaine (M.Sc., 2002)
¨ Martin Laforest (Undergraduate Summer Researcher 2002)
¨ Michael Sawler (M.Sc., 2001)
¨ Richard Wassenaar (Undergraduate Summer Researcher 1999)
¨ More than 30 undergraduate thesis students…
I am currently part of an NSERC strategic grant studying high-field THz characterization of transport phenomena in graphene and semiconductor nanostructure with experimentalists D. Cooke (McGill), T. Osaki (INRS) and R. Morandotti (INRS). My group at Queen’s will be involved in developing theoretical models of the response of graphene and semiconductor quantum wires, wells, dots and superlattices to intense terahertz fields and applying those models to the experimental systems investigated at McGill and INRS.
I have collaborated and continue to collaborate with a number of top experimental and theoretical groups from around the world. In addition, I have collaborated with and done consulting for a number of companies in the field of optical communications.
Much of my work involves a strong interaction with experimentalists and I have collaborated with the following internationally-renowned experimentalists:
¨ Profs. Tsuneyuki Ozaki and Roberto Morandotti: Terahertz response of graphene and Metamaterials, INRS, Montreal
¨ Prof. David Cooke: Terahertz response of graphene and semiconductor nanostructures, McGill University
¨ Prof. Karl Leo: Ultrafast optics of semiconductors, Technical University of Dresden, Germany.
¨ Prof. Andrew Weiner: Ultrafast optics of semiconductors, Purdue University, U.S.A.
¨ Dr. Jagdeep Shah: Ultrafast optics of semiconductors, Bell Labs, U.S.A..
¨ Prof. Emilio Mendez: Optical properties of superlattices, I.B.M. Yorktown, U.S.A.
¨ Prof. Horst Stormer (Nobel Laureate 1998): Quantum transport, Bell Labs, U.S.A.
¨ Prof. Stewart Aitchison: Dynamic localization in coupled optical waveguide arrays, U. of Toronto
¨ Prof. Hartmut Roskos: Terahertz radiation gain in semiconductor superlattices, Gothe Universitat, Frankfurt, Germany
Quantum optics and cavity QED in photonic crystals
Nonlinear quantum optics in coupled photonic crystal cavities
The coherent dynamics and coherent control of excitons excited by ultra-short optical pulses in semiconductor nanostructures
The response of semiconductor nanostructures to intense THz fields
The response of graphene to intense THz fields.
Treating Phase-Space filling and exchange effects using an excitonic approach to ultrafast carrier dynamics in semiconductor nanostructures
SELECTED PUBLICATIONS (past 10 years)
1. D. G. Cooke, P. Uhd Jepsen, Jun Yan Lek, Yeng Ming Lam, F. Sy and M. M. Dignam, Picosecond Dynamics of Internal Exciton Transitions in CdSe Nanorods, (Submitted to Physical Review Letters)
2. Ibraheem Al-Naib, Gargi Sharma, Marc M. Dignam, Hassan Hafez, Akram Ibrahim, David G. Cooke, Tsuneyuki Ozaki, and Roberto Morandotti, Effect of Local Field-Enhancement on the Nonlinear Terahertz Response of a Silicon-based Metamaterial, Phys. Rev. B 88, 195203 (2013).
3. A. M. Parks, M. M. Dignam, and D. Wang, Excitonic Analysis of Many-Body Effects on the 1s-2p Intraband Transition in Semiconductor Systems, Phys. Rev. B 87, 205306 (2013).
4. A. Joushaghani, R. Iyer, J. K. S. Poon, J. Stewart Aitchison, C. M. de Sterke, J. Wan and M. M. Dignam, Generalized Exact Dynamic Localization in Curved Coupled Optical Waveguide Arrays, Phys. Rev. Lett. 109, 103901 (2012).
5. M. M. Dignam and M. Kamandar Dezfouli, Photon–Quantum-dot Dynamics in Coupled-Cavity Photonic Crystal Slabs, Physical Review A, 85, 013809 (2012).
6. Peijun Yao, A. Reza, C. Van Vlack, M.M. Dignam and S. Hughes, Ultrahigh Purcell Factors in Slow-Light Metamaterial Waveguides, Physical Review B 80, 195106 (2009).
7. A. Joushaghani, R. Iyer, J.K.S. Poon, J.S. Aitchison, C.M. de Sterke, J. Wan and M.M. Dignam, Quasi-Bloch Oscillations in Curved Coupled Optical Waveguides, Phys. Rev. Lett. 103, 143903 (2009).
8. Dawei Wang and Marc M. Dignam, Excitonic Approach to the Ultrafast Optical Response of Semiconductor Quantum Wells, Phys. Rev. B 79, 165320 (2009).
9. A. Reza, M.M. Dignam and S. Hughes, Can Light be Stopped in Realistic Metamaterials?, Nature 455, E-10 (2008).
11. A. Lisauskas, M.M. Dignam, N.V. Demarina, Ernst Mohler, and H. Roskos, Examining the Terahertz Signal from a Photoexcited Biased Semiconductor Superlattice for Evidence of Gain, Appl. Phys. Lett. 93, 021122 (2008).
13. Dawei Wang, Aizhen Zhang, Lijun Yang, and M. M. Dignam, Tunable Terahertz Amplification Using Excitonic States in Optically-Excited Biased Semiconductor Superlattices, Phys. Rev. B 77, 115307 (2008).
15. Dawei Wang, Margaret Hawton and Marc M. Dignam, Excitonic Approach to the Ultrafast Optical Response of Semiconductors, Phys. Rev. B 76, 115311 (2007).
20. M. M. Dignam, D. P. Fussell, M. J. Steel, R. C. McPhedran, and C. M. de Sterke, Spontaneous Emission Suppression via Quantum Path Interference in Coupled Microcavities, Phys Rev Lett. 96, 103902 (2006).
28. Ben Rosam, Lijun Yang, Karl Leo, and M. M. Dignam, Terahertz Generation by Difference Frequency Mixing of Excitonic Wannier-Stark Ladder States in Biased Semiconductor Superlattices, Appl. Phys. Lett 85, 4612 (2004).
30. Lijun Yang, Ben Rosam, Jean-Marc Lachaine, Karl Leo, and M. M. Dignam, Intraband Polarization and THz Emission in Biased Semiconductor Superlattices with Full Excitonic Basis, Phys. Rev. B 69, 165310 (2004).
34. Aizhen Zhang, Lijun Yang, and M. M. Dignam, Influence of Excitonic Effects on Dynamic Localization in Semiconductor Superlattices in Combined dc and ac Electric Fields, Phys. Rev. B 67, 205318 (2003).
35. M. M. Dignam and M. Hawton, Intraband Polarization as the Source of Degenerate Four-Wave Mixing Signals in Asymmetric Semiconductor Quantum Well Structures, Phys. Rev. B 67, 035329 (2003).
Member’s of Dignam Research Group in Bold