Download Research Projects in Photonics

 Research Projects in Photonics Engineering Science, Material Science, Physics and Electrical and Computer Engineering Amr S. Helmy Group ECE @ U of T A.S. Helmy Group ECE @ U of T Optofluidic Platform for “Fingerprinting” Bio‐Materials using Raman Spectroscopy Our fingers have fingerprints that are unique to each one of us. But did you know that every piece of material also have unique fingerprints of their own as well? Raman Spectroscopy is a powerful optical technique that can detect “fingerprints” of materials through detecting the different colours of light scattered from the material. In addition, through the changes of the intensity, bandwidth, and colours of these weakly scattered light, different properties of the materials can also be determined and quantified. However, Raman scattering signals are inherently weak, even the sensitivity of a commercialized research grade system is only suitable to detect scattering signals from solid samples. Diluted aqueous solutions, suspensions and other materials that live in liquid state, including most biological samples such as DNA, live cells, proteins are typically not detectable using conventional Raman systems and techniques. Recently, our group has demonstrated a novel technique to improve the sensitivity of the Raman systems for liquid solutions. This technique utilizes a small size optofluidic device to increase the generation of the Raman scattering light as well as their collection efficiency. With this device, the sensitivity is improved by 1000 times compared to that of conventional Raman spectroscopy. This sensitivity improvement enabled us to detect detailed structures of nanoparticles with the size of a few nanometers1, particle interactions with different agents2, and conformation of specific DNA structures3. In this project, we will investigate the use of a chip based optofluidic platform for use in Raman spectroscopy. Students in this project will design, fabricate and characterize different designs of optofluidic chips to obtain and optimize the signal sensitivity, compatibilities, and usability. Students will learn and apply lithography techniques to fabricate different designs of optofluidic chips using the newly built cleanroom at the top floor of the Bahen Center for Information Technology. Students will also learn to use the advanced research grade Raman system in the Characterization lab in the Sandford Flaming building and test the final optofluidic chips using the system. Figure 2: Fingerprints of DNA Frayed Wires.3 Figure 1: Example of Optofluidic Setup 1. J. S. W. Mak, A. A. Farah, F. Chen, A. S. Helmy, ACS NANO, 5 (2011) 3823‐3830 2. F. Eftekhari, A. Lee, E. Kumacheva, A. S. Helmy, Optics Letters (In Press) Doc. ID.151763 3. R. M. Abu‐Ghazalah, J. Irizar, A. S. Helmy, R. B. Macgregor Jr., Biophysical Chemistry 147 (2010) 123‐129 http://photonics.light.utoronto.ca/helmy/ [email protected] A.S. Helmy Group ECE @ U of T Assessing the performance of photonic integrated circuits Photonic integrated circuits are formed of photonic devices fabricated together on a single semiconductor
substrate. The integration leads to advancements in functionality and decrease in manufacturing cost,
paralleling the evolution of the silicon IC industry [1]. A fundamental photonic component is the optical
waveguide, which is commonly formed of a rectangular ridge in the surface of the semiconductor (see
Figure 1). Light may travel below this ridge with minimal leakage to the surroundings, and therefore
provides the mechanism for optical signal transmission [2]. Another photonic component is the coupler
(see Figure 2), which allows an optical signal generated off-chip to be transferred to the waveguide with
minimal power loss via an optical fibre. The coupler in Figure 2 is a Bragg grating, which is formed of
periodic ridges in the surface perpendicular to the transport of optical power in the ridge [3]. Optical
fibres are positioned above these gratings to transmit and collect optical power to and from the chip.
Other basic components include optical filters, which is formed of a ring-shaped waveguide in Figure 2,
and acts as a notch filter with potentially very narrow frequency selection and extinction ratio.
Figure 1: Scanning Electron Microscope (SEM) image of a ridge waveguide formed on a semiconductor substrate. Figure 2: Illustration of a simple Photonic Integrated Circuit, featuring a ring filter and grating couplers with optical fibres
Our group is in the process of developing grating coupler and ring filter tools for a novel photonic
substrate. The characterization of fabricated samples involves careful lens and fibre alignment with
micrometer precision. Measurement of transmitted power variation with variation in optical frequency
yields performance parameters of waveguides, couplers and filters. Through processing of the data,
different sources of loss may be isolated and quantified. We seek an undergraduate student to assist in the
assembly of a bench-top set-up and to design and carry out these measurements. The student will then be
involved in the feedback of these results to iterate the device designs.
[1]
R. G. Beausoleil, “Large-scale integrated photonics for high-performance interconnects,” ACM Journal on Emerging
Technologies in Computing Systems, vol. 7, Article 6, 2011.
[2]
G. Lifante, Integrated Photonics Fundamentals, “Chapter 3: Theory of Integrated Optic Waveguides,” Wiley, 2003.
[3]
G. Maire et al., “High efficiency silicon nitride surface grating couplers,” Optics Express, vol. 16, p. 328, 2008.
http://photonics.light.utoronto.ca/helmy/ [email protected] A.S.HelmyGroup
ECE@UofT
DesignofNanoscalePhotonicDevicesThatUseMetals(Plasmonic)
The overall speed of central processing units (CPU) for computers are reaching conventional limits, not due
to the stop in Moore’s Law of scaling down transistor size, as there are still a few generation nodes to go
before physical limits are reached, but as a result of the bottleneck in communications between individual
transistors and also between different cores of multi-core CPUs. As the size of metallic interconnect wires
shrink, a host of parasitics such as increased resistivity, capacitance, and inductance will inhibit electron
conduction and cause cross-talk, thus jeopardizing the ability to improve the circuit speed expected from
miniaturization. One way to mitigate this “interconnect problem” is to move from electrical to optical
interconnects, which does not suffer from the above mentioned parasitics. For global interconnects between
different cores of a CPU, the optical communications medium can be silicon waveguides with sub-micron
cross-section that guide light pulses to carry the data. For local interconnects between close-by active
electronic devices, the smaller physical space available means that plasmonic waveguides must be used,
which are metallic-based structures for confining light below the sub-diffraction limit, to tens of nanometers
or less in size.
In either case, ultra-small optoelectronic devices must be in place to implement various routing functions
within the optical interconnection system, such as multiplexing, modulation, and filtering. Recently, a
terabit-per-second switching fabric for optical interconnects has been demonstrated using hetero-integration
of plasmonics and silicon photonics [1]. However, each optoelectronic device within this system still
occupies a chip area on the order of at least tens of microns. This may be quite small, but it still represents a
significant size mismatch compared to electronic circuitry. To create waveguides on the order of modern
transistor sizes, the plasmonic slot waveguide (PSW) is a promising candidate. Recently, our research group
has demonstrated high efficiency coupling into and out of a PSW from a silicon waveguide [2], and studies
on X-junctions of PSWs have shown great promise as a building block for practical nanoscale photonic
devices [3].
In this project, we will investigate networks of the PSW X-junction building block, such as 2-by-2 and 3-by3 structures, and design functional plasmonic devices based on the unique waveguiding properties of these
networks. The design process will be facilitated by using an analytical model developed in the group to
quickly resolve transmission characteristics in the PSW networks [4], and also the commercial photonics
simulation software programs, Lumerical FDTD (Finite-Difference Time-Domain) and MODE Solutions. If
successful designs with desirable functionalities result from the project and time-permitting, microfabrication of the devices will be carried out in the Emerging Communications Technology Institute (ECTI)
cleanrooms and also in a University of Western Ontario facility, including the use of state-of-the-art e-beam
lithography and focused ion beam systems.
Fig. 1 – Scanning Electron Microscope image of
silicon wire to PSW coupling platform [2]
Fig. 2 – PSW feedback structure with broadband filter response [4]
[1] N. Pleros et al., “Tb/s switching fabrics for optical interconnects using heterointegration of plasmonics and silicon photonics: The FP7 PLATON
approach,” 23rd Annual Meeting IEEE Photonics Soc. (2010).
[2] B. Lau, M. Swillam and A. S. Helmy, “Hybrid orthogonal junctions: wideband plasmonic slot-silicon waveguide couplers,” Opt. Exp., vol. 18, pp
27048 - 27059 (2010).
[3] E. Feigenbaum and M. Orenstein, “Perfect 4-way splitting in nano plasmonic X-junctions,” Opt. Exp., vol. 15, no. 26, pp 17948 - 17953 (2007).
[4] M. Swillam and A. S. Helmy, “Filter Response of Feedback Plasmonic Junctions,” Integrated Photonics Research, Silicon and Nanophotonics Nanophotonics: Plasmonics and Applications II (2011).
http://photonics.light.utoronto.ca/helmy/
[email protected]
A.S.HelmyGroup
ECE@UofT
CharacterizingthePerformanceofSemiconductorBraggLasersdiodes
At present, lasers have become an important part of our lives. Various applications
ranging from laser pointers and blue-ray players to optical communication transceivers
and medical diagnostics benefit from diode lasers. The frequency range of these laser
sources of light is however, limited by natural properties of the materials used to make
them. One of the methods to extend the frequency range of light sources is to use
nonlinear frequency conversion to convert one laser color into another color.
Recently, our group demonstrated Bragg reflection waveguide lasers1,2. These diode
lasers are not similar to any other diode laser, as they can enable efficient frequency
conversion inside the laser cavity. We have shown for the first time that frequency (or
laser light conversion) conversion takes place in such devices in the form of self-pumped
parametric fluorescence3.
In the present project, we are going to investigate various aspects of the laser diodes
fabricated inside the group. The summer student will be involved in characterizing ring
and straight GaAs/InGaAs lasers. These tests will include light-current-voltage (L-I-V)
characterization and frequency domain analysis. Also the student may have the
possibility of working in the ECTI cleanroom to contribute in parts of the fabrication
process such as the metal deposition for patterning the electrodes. During this project, the
student will have the opportunity to work with equipments including the optical spectrum
analyzer, laser characterization setup, four point measurement system, and the electron
evaporator.
Schematic of the ridge waveguide BRW diode laser
L-I curves for lasers of 500 μm (solid curve), 580 μm (dashed
curve), and 970 μm (dashed-dotted curve) cavity lengths
1. B. J. Bijlani and A. S. Helmy, “Bragg reflection waveguide diode lasers,” Optics Letters, Vol.
34, No. 23, pp. 3734-3736, 2009.
2. C.Z. Tong, B.J. Bijlani, S. Alali, and A.S. Helmy, “Characteristics of Edge-Emitting Bragg
Reflection Waveguide Lasers,” IEEE Journal of Quantum Electronics, Vol. 46, No. 11, pp.
1605-1610, 2010.
3. B.J. Bijlani, P. Abolghasem, P. Reijnders, A.S. Helmy, “Intracavity parametric fluorescence in
diode lasers,” Proceedings of Lasers and Electro-Optics (CLEO), 2011.
http://photonics.light.utoronto.ca/helmy/
[email protected]
A.S. Helmy Group ECE @ U of T Designing Novel Waveguides for Photonic Integrated Circuits
Optical waveguides are key elements in many photonic circuits with applications in optical
communication and computation, on-chip spectroscopy, optical interconnects and diode lasers.
Conventional optical waveguides rely on total internal reflection between a high refractive index
core and low refractive index cladding surrounding the core. An example of such waveguides is
optical fiber made of glass (silicon dioxide) which has revolutionized long haul communications
in present day optical communications.
Optical signals can also be transferred from one location to another using photonic bandgap
structures. In this class of waveguides, the requirement of having a high index core surrounded
by low index cladding is not required. In fact, the optical signal can be transferred in a photonic
bandgap waveguide with hallow core. An important class of photonic bandgap devices for planar
photonic circuits is Bragg reflection waveguides (BRWs). In BRWs, optical signal is propagated
between two highly reflective mirror referred to as transverse Bragg mirrors. A schematic of a
BRW is shown below:
Bragg reflection waveguides have been investigated in our group for applications in optical
frequency mixing where two optical signals provided by available laser sources are mixed
together to generate a new light at a desired frequency. This approach of light generation which
is referred to as “parametric light generation” enables the generation of optical signals which
cannot be accessed by conventional lasers.
The aim of this project is to become familiar with Bragg reflection waveguides, to design them
and to investigate various properties of these waveguides particularly their dispersion
properties. It is desired to investigate a new class of BRWs referred to as asymmetric Bragg
reflection waveguides where the two Bragg reflectors are not identical. The project additionally
involves some experimental work in optical labs in characterization of some of the fabricated
BRWs for measuring various parameters including propagation loss, facet reflection and beam
coupling factor.
[1]
P. Abolghasem, J. Han, D. P. Kang, B. J. Bijlani and A. S. Helmy, “Monolithic photonics using second-order
optical nonlinearities in multi-layer core Bragg reflection waveguides,” IEEE J. Sel. Top. Quantum Electron.,
invited, 2011.
[2]
B. J. Bijlani and A. S. Helmy. Bragg reection waveguide diode lasers. Opt. Lett., 34(23):3734-3736, 2009.
http://photonics.light.utoronto.ca/helmy/ [email protected] A.S. Helmy Group ECE @ U of T Producing photon pairs using semiconductor waveguides for applications in
quantum information processing
The nature of light, particle or wave, has been a debate since Issac Newton and Christiaan Huygens in
the 17th century. While the wave theory seemed to dominate in the late 1800’s when James Clerk Maxwell
formulated his famous equations, the particle explanation revived with the birth of Quantum Mechanics in 19001.
In over 110 years’ development since then, the “classical” explanation of light based on electro-magnetic theory
has reshaped our world using fibre-optical communications, optical holography, optical information processing,
etc. On the other hand, the “quantum” theory of light, which explains light as particles called “photons”, is
currently leading us into the era of quantum information science, with much more powerful computers and more
secure communications2. A large class of quantum information processing and quantum computing requires
photons with certain properties, such as entanglement.
To date, the most widely used photon sources are probably the photon pairs generated by spontaneous
parametric down conversion (SPDC) using second order nonlinear optical crystals such as lithium niobate
(LiNO3) and potassium titanyl phosphate (KTiOPO4). SPDC is an extremely weak process in which a high
frequency pump photon is annihilated and a pair of low frequency photons (signal and idler) is generated
simultaneously, usually with a probability in the order of 10-10. Photon sources for future practical quantum
information processing out of labs, however, needs to be more efficient, and should be integrated with pump
lasers on the same chip to achieve large scale, on-chip information processing3. Therefore, III-V semiconductor
GaAs/AlGaAs waveguide devices have become promising candidates for these photon sources, because laser
diodes can be readily fabricated in this material system. Recently, our group have developed a platform based on
AlGaAs Bragg reflection waveguides which shows efficient frequency conversion4. We showed theoretically, if
photon pairs with different polarizations are generated at the same wavelength, polarization entangled photons
can be produced without a complicated setup5.
In this project, we will use second harmonic generation (SHG), the reverse process of SPDC, to test the
wavelength conversion of nonlinear waveguides, and investigate the effect of waveguide ridge width. Students in
this project will learn the techniques about optical alignments, waveguide coupling, linear and nonlinear property
measurements. Students will also learn the fundamentals of waveguide theory and nonlinear wavelength
conversion.
Fig.1 waveguide structure and the modal distributions;
Fig. 2 Cross section SEM of a typical waveguide.
[1] For example, E. Hecht, Optics, 4th edition, Addison Wesley Publishing Company, 1997.
[2] M. A. Nielsen, I. L. Chuang, Quantum Computation and Quantum Information, Cambridge University Press, 2004.
[3] S. Tanzilli, A. Martin, F. Kaiser, et al, Laser & Photon. Rev. 6, 115 (2012).
[4] P. Abolghasem, J. B. Han, B. J. Bijlani, and A. S. Helmy, Opt. Express 18, 12681 (2010).
[5] D. Kang and A. S. Helmy, submitted.
http://photonics.light.utoronto.ca/helmy/ [email protected]