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MIT Center for Integrated Photonic Systems

• Leadership: Professor Rajeev J. Ram, Director

The core activity of CIPS is the development of a long-range vision for research and the development of integrated photonic devices and systems.  As an academic institution we can work openly with a variety of different organizations in developing and gathering input for our models. Whether it is performance data for new devices “in the lab,” yield data for existing manufacturing processes, planning documents, or first-hand observations of the corporate decision making process, CIPS researchers benefit greatly from the unique relationship between MIT and industry. The level of detail and intellectual rigor of the models being developed here is complemented by the high quality of data available to us. CIPS researchers are developing models of optical and electronic devices, the packages they are wrapped inside, the manufacturing processes that assemble them, the standards that define them, the market that buys them, and the policy processes that influence their deployment.

The Departments of Electrical Engineering and Computer Science, Materials Science and Engineering, Mechanical Engineering and Economics are consistently ranked as the top graduate programs in the country. Likewise, the Sloan School of Management has consistently ranked first in the nation in the areas of information technology, operations research, and supply chain management. CIPS leverages MIT’s strengths, by unifying the photonics researchers in these departments and laboratories to focus on technological developments in photonics. The combined volume of research funds in the photonics area at MIT exceeds $20 million dollars annually. The faculty and staff at MIT in photonics-related areas have included Claude Shannon (founder of information theory), Charles Townes (inventor of the laser), Robert Rediker (inventor of the semiconductor lasers), and Hermann Haus (inventor of the single-frequency semiconductor laser & ultrafast optical switch). CIPS-affiliated faculty and staff continue this tradition of excellence in areas ranging from optical network architectures, to novel optical devices, to novel photonic materials.

Rajeev J. Ram

• Title: Professor
• Department: Department of Electrical Engineering and Computer Science

Collaborators

• H. Lee, RLE
• H. I. Smith, EECS
• F. X. Kaertner, EECS
• J. Hudgings, Mt. Holyoke College
• J. L. Hoyt, EECS
• V. Stojanovic, EECS
• K. Asanovic, UC Berkeley

Graduate Students

• R. Amatya, Res. Asst., EECS
• S. Goh, Res. Asst., DMSE
• E .Kapusta, Res. Asst, EECS
• K.S.K. Lee, Res. Asst., EECS
• J. Orcutt, Res. Asst., EECS
• P. Santhanam, Res. Asst., EECS

Support Staff

• G. Brew, Administrative Assistant II

Publications

Lee, K.S. and Ram, R.J., 2009, Plastic–PDMS bonding for high pressure hydrolytically stable active microfluidics, Lab on a Chip, 9, 1618-24, 2009.

M Farzaneh, K Maize, D Lüerßen, J A Summers, P M Mayer, P E Raad, K P Pipe, A Shakouri, R J Ram and J A Hudgings, 2009, CCD-based thermoreflectance microscopy: principles and applications, J. Phys. D: Appl. Phys., 42, 143001, 2009.

Santhanam, P.. and Ram, R.J., 2010, Self-Consistent Drift–Diffusion Transport in Thermoelectrics and Implications for Measuring the Scattering Parameter, Journal of Electronic Materials, on-line, 2010.

Amatya, R.. and Ram, R.J., 2010, Solar Thermoelectric Generator for Micropower Applications, Journal of Electronic Materials, on-line April, 2010.

Orcutt, J.. and Ram, R.J., 2010, Photonic Device Layout within the Foundry CMOS Design Environment, IEEE Photonics Technology Letters, 22, 544-546, 2010.

Summers, Joseph A. and Farzaneh, Maryam and Ram, Rajeev J. and Hudgings, Janice A.. 2010, Thermal and Optical Characterization of Photonic Integrated Circuits by Thermoreflectance Microscopy. IEEE Journal of Quantum Electronics, 46, 3-10, 2010

Indium Phosphide Plasmonics

• Authors: J. A. Summers, R. J. Ram
• Sponsorship: Physical Sciences, Inc., DARPA

Surface plasmon polaritons (SPPs) are optical waves that propagate at the interface between a metal and a dielectric, and can provide high optical mode confinement for the realization of compact optical devices ^{[1] [2]} .  Because SPPs can support polarizations only where the electric field is normal to the metal-semiconductor interface, metals can be patterned onto purely dielectric waveguides to form optical polarizers, where either the TE or TM mode of the dielectric waveguide is resonantly coupled to the SPP ^{[3] [4]} .

Here, we describe the design of a compact SPP-based TE polarizer that can be easily integrated with a standard InP-based ridge waveguide (Figure 1).  The InP ridge waveguide consists of a 610-nm-thick InGaAsP core (Q=1.44), with InP upper and lower claddings, and the 8-mm-long TE polarizer is created by coating the top and sides of the ridge with a layer of Ag, allowing an SPP mode to be supported at the top Ag-InP interface.

The polarizer works by removing TM polarized light from the semiconductor ridge via resonant coupling to the TM mode of the SPP.  After light is fully coupled to the SPP, the Ag layer is abruptly terminated, allowing the light to radiate into unguided modes.  Due to the large absorption loss of the SPP-mode (~0.8dB/mm), resonant coupling from the semiconductor ridge mode is sensitive to the coupling strength and the relative propagation losses of the symmetric and antisymmetric coupled waveguide supermodes.  If the InP upper cladding thickness is varied, the coupling strength can be adjusted to achieve critical coupling, where the propagation losses of the ridge-SPP supermodes are equal.  For this waveguide geometry, critical coupling and optimal power transfer to the SPP mode occurs at an upper cladding thickness of 280 nm, enabling a compact device length of 8 mm, 20 dB polarization extinction, and 2 dB insertion loss at 1550 nm (Figure 2).



Figure 1: Schematic of the resonantly-coupled Ag-InP SPP waveguide polarizer.



Figure 2: Eigenmode expansion method simulations show that the polarizer should provide 20 dB of polarization extinction and 2 dB of insertion loss for a device length of 8 μm (λ = 1550 nm).




1. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature, vol. 424, pp. 824-830, Aug. 2003. [↩]
2. S. A. Maier and H. A. Atwater, “Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures,” Journal of Applied Physics, vol. 98 no. 011101, July 2005. [↩]
3. C.-H. Chen and L. Wang, “Design of finite-length metal-clad optical waveguide polarizer,” IEEE Journal of Quantum Electronics, vol. 34, no. 7, pp. 1089-1097, July 1998. [↩]
4. W. Johnstone, G. Stewart, T. Hart, and B. Culshaw, “Surface plasmon polaritons in thin metal films and their role in fiber optic polarizing devices,” IEEE Journal of Lightwave Technology, vol. 8, no. 4, pp. 538-544, Apr. 1990. [↩]

Wearable Thermoelectric Generators

• Authors: K. Settaluri, H. Lo, R. J. Ram
• Sponsorship: Office of Naval Research, MIT Undergraduate Research Opportunities Program

There exists great interest in the utilization of the human body as a possible power supply for running such electronics as medical monitoring devices and sensors. This project details the effective utilization of a new type of form-fitting, wearable, wristband thermoelectric heat sink, Figure 1, which relies on 2D heat-spreading for dissipating heat from the cold side of the TEG for the purposes of energy harvesting. The performance of the TEG/heat sink systems is limited by (1) the small working temperature differential between the body and ambient temperature; (2) the desire to use natural air convection cooling on the cold-side of the generator; and (3) the requirement for thin, light-weight systems that are comfortable for long-term use. The 2D heat-spreading wristbands produce 285 μW from a Bi₂Te₃ TEG module (256 thermocouples, each is 2 mm in thickness and 1 mm² in area). The large available surface area of the wristband as well as the highly thermally conductive nature of copper ensures effective cooling of the cold side and allows heat to spread and dissipate throughout the wristband, thereby enhancing the effective heat transfer coefficient. A 75-minute field test was conducted that measured the voltage output from the TEG as well as the temperature difference across the module when worn on the upper forearm. In addition, the output of the TEG was connected to a commercially available DC-DC converter, which boosted the voltage to a steady 4.15 V, shown in Figure 2, allowing for coupling with external applications such as EKG chips and pulse oximeters.



Figure 1: A model of the wearable, wristband heat-sink. The heat-sink differs from other wearable heat-sinks in that 2D heat-spreading is utilized for effective cooling of the cold side of the TEG. The TEG/heat-sink system has a maximum thickness of 5 mm.



Figure 2: A 75-minute field test of the TEG/wristband system. In addition to measuring the open circuit voltage and temperature differential across the module, we also measured the output from a commercially available DC-DC converter. In particular, the output from the DC-DC converter showed a steady 4.15 V during the duration of the test.




Thermal Energy Harvesting Light Sources

• Authors: P. Santhanam, D. J. Gray, Jr., R. J. Ram
• Sponsorship: NDSEG

We are investigating thermoelectric transport in light-emitting diodes (LEDs). Electro-luminescent cooling is a phenomenon in which the optical output power from an LED exceeds its electrical input power, resulting in a net cooling of the emitter. The physical process is outlined in Figure 1. Proposed in the 1950s ^[1] , electro-luminescent cooling has been a topic of recent theoretical interest ^{[2] [3]} , but there are no known reports that it has been experimentally observed. Additionally, this investigation could lead to practical light emitters operating with efficiencies approaching or even surpassing unity.

The Second Law of Thermodynamics places limits on the power density achievable in these high-efficiency devices. Nevertheless, LEDs designed to exploit thermoelectric transport may find practical application in lighting, spectroscopy, and communication.

Since such devices would see improved efficiency at higher ambient temperatures, this work could lead to LED designs that do not suffer from efficiency droop, a major hurdle for state-of-the-art LEDs ^[4] . If LED packaging was designed for the concentration of heat, just as in an incandescent filament, self-heating due to non-radiative recombination could raise the achievable power density at visible wavelengths to practical levels.

Remote and portable spectroscopy applications like pulse oximetry, infrared oilfield spectroscopy, and automotive emissions monitoring may benefit from such highly efficient photon sources. Drastically reduced power demands may increase the lifespan of such remote platforms, or even permit the use of ambient energy-harvesting power sources in place of batteries. The fundamental limitations on the power density of this technology do not impede this case, as the less-constricting power requirements for spectroscopic sources are set by the noise-equivalent power of the detector and the required signal-to-noise ratio. Similarly, since the power requirements for infrared optical communication are set by ambient blackbody radiation, the quality of the detector, and the desired bitrate, dim but efficient sources may prove practical.



Figure 1: Band diagram of a p-n junction depicting thermoelectric heat-exchange processes involved in electro-luminescent cooling. Entropy and energy are absorbed, in the form of phonons, by the electrons and holes as they climb their respective electrical potential energy hills. Radiative recombination results in the rejection of this entropy and energy, along with some external work, through photons.



Figure 2: Potential downhole spectroscopy system configuration. Since the efficiency and power of the emitter technology improve with heating, the emitter could be installed at the site of heat rejection for any actively cooled components.




1. J. Tauc, “The share of thermal energy taken from the surroundings in the electro-luminescent energy radiated from a p-n junction,” Czechoslovak Journal of Physics, vol. 7, no. 3, pp. 275-276, Jan. 1957. [↩]
2. O. Heikkilä, J. Oksanen, and J. Tulkki, “Ultimate limit and temperature dependency of light-emitting diode efficiency,” Journal of Applied Physics, vol. 105, pp. 093119:1-9, May 2009. [↩]
3. P. Han, K.-J. Jin, Y.-L. Zhou, H.-B. Lu, and G.-H. Yang, “Numerical designing of semiconductor structure for optothermionic refrigeration,” Journal of Applied Physics, vol. 101, pp. 014506:1-4, Jan. 2007. [↩]
4. J. Piprek, “Efficiency droop in nitride-based light-emitting diodes,” Phys. Status Solidi A, vol. 207, no. 10, pp. 2217-2225, July 2010. [↩]

CMOS Photonics

• Authors: J. S. Orcutt, H. Li, V. Stojanovic, R. J. Ram
• Sponsorship: DARPA



Figure 1: (a) Propagation loss as a function of waveguide width for wafers with and without thermal processing representative of the full electronics process. Solid fit lines estimating the confinement factor scaling for the indicated bulk material losses overlay measured data points. Inset of electric field contours show confinement change from core to cladding material for narrow waveguide widths. (b) Propagation loss as a function of wavelength and waveguide width for the thermally processed wafer.

Low propagation loss deposited waveguides are an important component to enable photonic integration in the majority of high-volume electronics processes. Polycrystalline silicon waveguides are desirable in this role as propagation losses below 10 dB/cm are achievable ^{[1] [2] [3]} using materials already common in such processes. However, previous demonstrations of low-loss poly-Si waveguides have utilized layer thicknesses of 200 nm or greater and reduced index-contrast oxynitride claddings to achieve such results. Further, the low-loss performance was not verified to withstand the high-temperature steps present in electronics processes.

To provide a suitable process integration test platform, 120-nm thick poly-Si waveguides were deposited and patterned on 300- mm wafers within the surrounding dielectric stack-up of a state-of-the-art memory process. To measure the impact of the thermal processing, the wafers were split with and without the all of the high-temperature anneals present in the full process for comparison. To minimize the impact of this thermal processing on top surface roughness, the waveguide core was first deposited at low-temperature by LPCVD to form an amorphous film and crystallized to form poly-Si with a ~950 °C 20-second anneal.

To measure the resulting waveguide loss, “paperclip” structures were patterned in which straight waveguide sections of varying propagation lengths are connected by identical input and output bends and vertical grating couplers. The resulting losses at 1550 nm as a function of waveguide width for wafers with and without thermal processing are shown in Figure 1a and are shown as a function of wavelength for the thermally processed wafers in Figure 1b. Although the waveguide loss increases slightly after thermal processing, both the as-deposited and end-of-line waveguide losses are below 10 dB/cm for 350-nm waveguide widths at 1550 nm. The narrow waveguides enable long-distance, on-chip routing to be enabled with 6.2 dB/cm loss.

1. L. Liao et al., “Optical transmission losses in polycrystalline silicon strip waveguides: effects of waveguide dimensions, thermal treatment, hydrogen passivation, and wavelength,” J. Electron. Mater. vol. 29, pp. 1380-1386, 2000. [↩]
2. Q. Fang et al., “Low loss (~6.45 dB/cm) sub-micron polycrystalline silicon waveguide integrated with efficient SiON waveguide coupler,” Opt. Express, vol. 16, pp. 6425-6432, 2008. [↩]
3. C. W. Holzwarth et al., “Localized substrate removal technique enabling strong-confinement microphotonics in bulk Si CMOS processes,” in Conference on Lasers and Electro-Optics, Technical Digest (Optical Society of America, 2008), paper CThKK5. [↩]

Nonlinear Optics in CMOS photonics

• Authors: K. K. Mehta, J. S. Orcutt, R. J. Ram
• Sponsorship: DARPA



Figure 1: (a) Through transmission of 40 µm radius ring with 200 nm coupling gap for four different in-waveguide input powers. (b) Measured through transmissions (circles) on resonance normalized to off-resonance transmission, and fits (lines) for 20 µm rings, and (c) for 40 µm rings with various coupling gaps g; along with the gap, the fitted β is labeled for each sample.

Nonlinear optics comprises a rich body of physics that could enable a number of interesting functionalities in integrated silicon photonics ^[1] . We are currently interested in the phenomenon of two-photon absorption (TPA) as a means to achieving sub-bandgap photodetection in silicon. Though TPA is a relatively weak absorption mechanism in crystalline silicon, the fact that absorbed power scales quadratically with intensity implies that its strength can be increased for device applications by concentrating a large amount of light energy in a small mode-volume resonator, for example in photonic crystal microcavities ^{[2] [3]} .

Further enhancement of TPA may be expected when polycrystalline Si is used instead of crystalline Si as the detector material, due to a larger TPA coefficient (β) resulting from mid-gap electronic states at grain boundaries. Though β has been fairly well characterized for crystalline Si ^[4] , an understanding of two-photon absorption in the polycrystalline phase is lacking. We have therefore measured β in polycrystalline Si waveguide devices by monitoring the peak through transmission dip in ring resonator devices as a function of input power and fitting the results to a model with β as a fitting parameter; data from such measurements are shown in Figure 1. These measurements indicate β = 310 ± 70 cm/GW, a value over two orders of magnitude larger than that in crystalline Si at 1550 nm and on the same order of magnitude as the value observed in amorphous silicon ^[5] . As polysilicon is widely available in advanced CMOS processes, these results suggest that high-density, integrated devices utilizing nonlinear absorption at relatively low powers should be achievable.

1. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics.” Nature Photonics, vol. 4, pp. 535-544, July 2010. [↩]
2. J. Bravo-Abad, E.P. Ippen, and M. Soljacic. “Ultrafast photodetection in an all-silicon chip enabled by two-photon absorption.” Appl. Phys. Lett., vol. 94, no. 241103, pp. 1-3, June 2009. [↩]
3. T. Tanabe, H. Sumikura, H. Taniyama, A. Shinya, and M. Notomi. “All-silicon sub-Gb/s telecom detector with low dark current and high quantum efficiency on chip,” Appl. Phys. Lett., vol. 96, no. 101103, pp. 1-3, March 2010. [↩]
4. M. Dinu, F. Quochi, and H. Garcia. “Third-order nonlinearities in silicon at telecom wavelengths,” Appl. Phys. Lett., vol. 82, pp. 2954-2956, May 2003. [↩]
5. K. Ikeda,Y. Shen and Y. Fainman. “Enhanced optical nonlinearity in amorphous silicon and its application to waveguide devices,” Opt. Express, vol. 15, 17761-17771, December 2003. [↩]

Watt-class Optoelectronic Oscillators

• Authors: W. Loh, J. Klamkin, S. M. Madison, F. J. O’Donnell, J. J. Plant, S. Yegnanarayanan, R. J. Ram, P. W. Juodawlkis

We are investigating the performance of low-noise, high-power optoelectronic oscillators (OEO) using all slab-coupled optical waveguide (SCOW) components. The optoelectronic oscillator (OEO) demonstrates the capability to generate a pristine tone through down-conversion of a modulated optical carrier into the microwave regime ^{[1] [2]} . The two primary advantages for using an optical carrier are low loss (high quality factor (Q)) and large bandwidth (50+ GHz modulation and detection). Other notable advantages of the OEO are its light weight, immunity to electromagnetic interference, and ability to generate both optical and microwave clock outputs. A schematic diagram of the SCOW based-OEO structure is shown in Figure 1. The phase noise performance of an OEO depends largely on the relative intensity noise (RIN) properties of the laser source. Figure 2 compares the SCOW external cavity laser (SCOWECL) ^[3] to a commercial external cavity semiconductor laser (RIO Orion). The SCOWECL RIN is 10-15 dB lower than the RIO Orion RIN.

High performance optoelectronic oscillators are important as low-noise sources for driving low noise modelocked lasers and as stable local oscillators (LO) for RADAR and communication applications. In modelocked lasers, the noise of the RF oscillator directly transfers to the laser through jitter present during modulation of the optical pulse. In RADAR, the reflected signal after the Doppler shift must overcome the phase noise of the master oscillator reflected off background clutter. Finally, in global positioning system (GPS) applications, the master oscillator’s stability is essential during triangulation of the receiver position. Clock phase noise results in range errors that limit the accuracy of the computed receiver position. In addition to requiring high performance, all of these applications can benefit from a low-size, -weight, and -power compact master oscillator. This is especially true during flight as SWaP becomes critically important for airborne systems compared to ground-based systems.



Figure 1: SCOW-based optoelectronic oscillator schematic comprising a SCOW external cavity laser (SCOWECL), an electro-optic intensity modulator, a phase modulator, an optical fiber delay, a SCOW photodiode, an RF phase shifter, a 3-GHz RF filter with 1.5-MHz bandwidth, and a 10-dB RF directional coupler.



Figure 2: Low-frequency SCOWECL and RIO Orion RIN. The RIN was measured in 3 separate intervals 10 Hz – 2 kHz (resolution bandwidth (B) = 10 Hz), 10 Hz – 100 kHz (B = 1 kHz), and 10 Hz – 2 MHz (B = 10 kHz). The regions of divergence occur due to integration close to DC within the resolution bandwidth of each measurement.




1. X. S. Yao and L. Maleki, “Optoelectronic microwave oscillator,” J. Opt. Soc. Am. B, vol. 13, pp. 1725-1735, 1996. [↩]
2. D. Eliyahu and L. Maleki, “Low phase noise and spurious level in multi-loop opto-electronic oscillators,” in Proc. IEEE International Frequency Control Symposium, 2003, pp. 405-410. [↩]
3. W. Loh, F. J. O’Donnell, J. J. Plant, M. A. Brattain, L. J. Missaggia, and P. W. Juodawlkis, “Packaged, high-power, narrow-linewidth slab-coupled optical waveguide external cavity laser (SCOWECL),” accepted for publication in IEEE Photon. Technol. Lett., 2011. [↩]

CNT-based Optoelectronic Device Packaging

• Authors: S.-L. Lee, H. Lo, R. J. Ram
• Sponsorship: National Science Council, Taiwan

Carbon nanotubes (CNTs) are excellent heat spreaders for packaging electronic/photonic devices due to their high thermal conductivity and good mechanical strength ^{[1] [2]} . They have been used for packaging LED chips ^{[3] [4] [5]} . However, poor adhesion between CNTs and adjacent materials may impact device yield and reliability. We proposed a platform for optoelectronic device packaging by integrating vertical aligned CNTs (VACNTs) onto patterned silicon U-grooves for achieving enhanced thermal conduction and stable mechanical properties.

Figure 1 shows the schematic of the CNT-based platform. CNTs are grown on a silicon trench to provide heat spreading for the bonded device chip. A thin dielectric layer between CNTs and silicon substrate is usually needed to eliminate current leakage to the substrate. The device chip is bonded to the platform by the bonding pads on chip surface. Figure 2 shows the calculated thermal resistance of the CNT platform by using the Finite Element Method. A silicon monoxide (SiO) film of 500-nm thick is used as the dielectric layer. The planar silicon platform with the same thickness of SiO is also simulated for comparisons. When the top open area of the U-groove covers about 87% of the device chip and the depth is about 30% of the 200-mm thick silicon substrate, the CNT platform can provide about 39% reduction in thermal resistance if the thermal conductivity of CNTs is 600 W/mK. The reduction can reach about 50% by using a deeper U-groove with taller CNTs. The enhancement on the thermal conduction by the CNT platform results not only from the use of high-thermal-conductivity VACNTs but also from the increased surface area of the low-thermal-conductance dielectric layer.

The CNT platforms were demonstrated for packaging red LEDs to have high saturation current and high manufacturing yield ^[6] .



Figure 1: Cross-sectional view of device packaging using a CNT-based platform.



Figure 2: Simulated thermal resistance versus the top open surface area of the U-groove for growing CNTs. The thermal resistance is normalized to the thermal resistance of the silicon substrate with a 500-nm thick silicon monoxide layer on the surface, 12.9 K/W. The surface area is normalized to the area of heat source, 300µm by 300 µm. The depth is normalized to the thickness of silicon substrate, 200µm.




1. S. Berber, Y. K. Kwon, and D. Tomanek, “Unusually high thermal conductivity of carbon nanotubes,” Phys. Rev. Lett., vol. 84, pp. 4613-4616, 2000. [↩]
2. P. Kim, L. Shi, A. Majumdar, and P. L. McEuen, “Thermal transport measurements of individual multiwalled nanotubes,” Phys. Rev. Lett., vol. 87, pp. 215502-215505, 2001. [↩]
3. K. Zhang, G. W. Xiao, C. K. Y. Wong, H. W. Gu, M. M. F. Yuen, P. C. H. Chan, and B. Xu, “Study on thermal interface material with carbon nanotubes and carbon black in high-brightness LED packaging with flip-chip technology,” Proc. 55^th IEEE Electronic Components and Technology Conference, pp. 60-65, Lake Buena Vista, FL, May 2005. [↩]
4. K. Zhang, Y. Chai, M. M. F. Yuen, D. G. W. Xiao, and P. C. H. Chan, “Carbon nanotube thermal interface material for high-brightness light-emitting diode cooling,” Nanotechnology, vol. 19, pp. 215706-215714, 2008. [↩]
5. M. Arik, S. E. Weaver, Jr., J. C. Carnahan, C. A.  Becker,  W. D. Gerstler, “Electronic devices and methods for making same using nanotube regions to assist in thermal heat-sinking,” US Patent, 6864571, Mar. 8,  2005. [↩]
6. S.-C. Chen, S.-L. Lee, H. Lo, Y.-J. Hung, K.-Y., Lee, C.-G. Tu, Y.-T. Pan, and R. J. Ram, “VACNT-on-silicon platform for improving heat conduction in optoelectronic packaging,” in Proc. Conference on Lasers and Electro-Optics (CLEO), Baltimore, MD., May  2011. [↩]

Continuous Microbioreactors

• Authors: K. S. Lee, R. J. Ram
• Sponsorship: National Science Foundation

For systems biology, the models are more often limited by the absence of reliable experimental data than by available computational resources. Unfortunately, there is still great difficulty in making the leap from genetic and biochemical analysis to accurate verification with conventional culture growth experiments due to variations in culture conditions. Measurements of metabolic activity through substrate and product interactions or cellular activity through fluorescent interactions are highly dependent on environmental conditions and cellular metabolic state. For such experiments to be feasible, continuous cultures ^{[1] [2]} utilizing control strategies must be developed to measure chemical concentrations, introduce chemical inputs, and remove waste. An integrated microreactor system with built-in fluid metering will enable environmental control and programmable experiments capable of generating reproducible data.

The chip shown in Figure 1 is fabricated out of a rigid plastic polycarbonate, utilizing PDMS membranes for actuation and pumping ^[3] . The fabrication process for bonding plastic-PDMS hybrid devices has been described previously ^[4] . Mixing and oxygen delivery are performed through membranes between the fluidic and actuation layers of the growth chamber sections. A growth volume of 1 mL ensures the ability to couple sampled volume to offline chemical analysis. Culture experiments are performed using E. coli strain FB21591 grown on defined media. The glucose input is separated to provide input control. As shown in Figure 2, various metabolic states are observable through continuous flow control. Cell density is directly dependent on glucose input, and acid production is proportional to cell density in chemostat mode. In turbidostat mode, cell density can be kept constant and glucose utilization can be observed, demonstrating the direct observation of overflow metabolism.



Figure 1: Schematic of the micro-continuous culture device. Eight inputs lines provide control over the chemical composition of the medium within the growth reservoir. Fluids are introduced through a peristaltic pump, which injects liquid into the growth chamber.



Figure 2: Plots of chemical concentrations in chemostat mode, demonstrating dependence of cell density on input glucose concentration. Plot of concentrations in turbidostat mode, demonstrating dependence of acetic acid on glucose concentration.




1. J. Monod, “La technique de culture continue, theorie et applications,” Annales de I’Institut Pasteur, vol. 79, pp. 390-410, 1950. [↩]
2. A. Novick and L. Szilard, “Description of the chemostat,” Science, vol. 112, pp. 715-716, 1950. [↩]
3. V. Studer, G. Hang, A. Pandolfi, M. Ortiz, W. F. Anderson, and S. R. Quake, “Scaling properties of a low-actuation pressure microfluidic valve,” J. Applied Physics, vol. 95, pp. 393-398, 2004. [↩]
4. K. S. Lee and R. J. Ram, “Plastic-PDMS bonding for high pressure hydrolytically stable active microfluidics,” Lab on a Chip, no. 9, pp. 1618-1624, 2009. [↩]

Monitoring Cell Physiology in Microbioreactors

• Authors: S. Goh, R. J. Ram
• Sponsorship: National Science Foundation, Agency for Science Technology & Research-Singapore

Mammalian cell cultures dominate the biopharmaceutical industry because their products, valuable biotherapeutics including monoclonal antibodies, vaccines and growth factors, account for 60% of the market. However, mammalian cells are also the most sensitive to changes in the culture environment, e. g., mechanical agitation, nutrient depletion, and waste byproduct accumulation. Hence, maintaining cell viability in mammalian cell cultures for an extended period of time is an important limiting factor for mammalian cell cultures, in particular Chinese Hamster Ovary (CHO) cell cultures ^[1] . Currently, state-of-the-art micro-bioreactors estimate cell density by measuring the turbidity of the culture using an Optical Density (OD) sensor ^[2] . Unfortunately, an OD sensor measures biomass, which includes live and dead cells. An online sensor that explicitly measures viable cells in a micro-bioreactor is necessary.

Dielectric spectroscopy is a promising online sensor for cell viability in micro-bioreactors ^[3] . The difference between the low frequency and high frequency capacitance measurements (ΔC = C_LF – C_HF) in the radio frequency regime gives the capacitance contributed by the cells to the total capacitance of the suspension. Due to the fact that most dead cells no longer have an intact membrane, defined as a membrane that is selectively impermeable to ions in the solution, they do not contribute to the capacitance reading (ΔC). By performing an initial calibration with live cell suspensions of different density, the ΔC per cell per mL can be determined for the particular cell line. The modeled calibration curve is shown in Figure 1. The measured capacitance of a cell culture of unknown live cell density can then be determined by comparing with the calibrated capacitance values. By combining dielectric spectroscopy with OD measurements, the percent cell viability can be utilized to optimize the yield of a mammalian cell culture in a micro-bioreactor. The first part of our project involves designing and calibrating a dielectric spectroscopy sensor for a micro-scale bioreactor, shown in Figure 2.



Figure 1: Modelled dielectric spectroscopy of CHO cells. As the live cell density increases, the ΔC reading increases linearly.



Figure 2: Picture of device made by sputtering platinum electrodes on a milled piece of polycarbonate, which forms the bottom half of the dielectric spectroscopy test chip.




1. J. Goswami, A. J. Sinskey, H. Stellar, G. N. Stephanopaulos, and D. I. C. Wang, “Apoptosis in batch cultures of Chinese hamster ovary cells,” Biotechnology and Bioengineering, vol. 62, no. 6, pp. 632-640, Mar. 1999. [↩]
2. H. L. T. Lee, P. Bocazzi, R. J. Ram, and A. J. Sinskey, “Microbioreactor arrays with integrated mixers and fluid injectors for high-throughput experimentation with pH and dissolved oxygen control,” Lab On A Chip, vol. 6, pp 1229-1235, July 2006. [↩]
3. K. Asami, “Characterization of biological cells by dielectric spectroscopy,” Journal of Non-Crystalline Solids, vol. 305, pp. 268-277, March 2002. [↩]

Solar Thermoelectric Generator for the developing world

• Authors: R. Amatya, R. J. Ram
• Sponsorship: Greater China Fund for Innovation/MISTI Global Seed Funds

With nearly 1.6 billion people living without electricity, the demand for small-scale distributed power generation is high. In developing countries, especially in rural communities, residential electricity consumption is less than 0.5kWh per day which is used for lighting and powering household appliances such as televisions, radios, and satellite receivers ^[1] . Till now, solar photovoltaics (PV) have been the prominent source for rural electrification, but the penetration has not been very rapid, primarily due to the large upfront system cost for the panels. Previously, we have designed and demonstrated a Solar Thermoelectric Generator (STEG) using concentrated sunlight (66x suns) incident on a Micro Alloyed Material Bi₂TE₃ thermoelectric module with a selective surface coating achieving 3% peak efficiency ^[2] . This approach was shown to be cost competitive with small-scale solar PV. We have demonstrated that a STEG can be integrated into solar cookers, which are widely deployed in the developing world to sterilize drinking water.

The generator produces a useful power of ~5W to directly charge small appliances such as cell phones and was used to charge a small sealed lead-acid battery. The thermal power dissipated at the cold side of the generator is used to heat water to 70C sufficient for sterilization ^[3] . The generator can also be used at night with the latent heat from cooking stoves in the rural households providing useful electricity for lighting and small electrical appliances. In collaboration with a non-profit organization, One Earth Designs (OED), we have tested the generator system in rural Western China’s Qinghai Province utilizing existing concrete/glass hybrid solar cookers and a portable solar cooker as the light concentrator. A part of a portable solar cooker called Sol-Source, the STEG has won numerous design awards including the Clinton Global Initiative Award, St. Andrews Prize for the Environment, and First Place in National Sustainable Design Competition.



Figure 1: Solar thermoelectric generator (STEG) being tested in a nomadic village in Western China with a portable solar cooker from OED.



Figure 2: Output voltage and electric power measured across a load resistance of 16 ohm (equal to internal resistance of the TE module) for STEG using a solar cooker.




1. X. Zhang, and A. Kumar, “Evaluating renewable energy-based rural electrification program in western China: Emerging problems and possible scenarios,” Renewable and Sustainable Energy Reviews, vol. 15, no. 1, 2011. [↩]
2. R. Amatya and R. J. Ram, “Solar thermoelectric generator for micropower applications,” Journal of Electronic Materials, vol. 39, no. 9, 2010. [↩]
3. J. D. Burch and K. E. Thomas, “Water disinfection for developing countries and potential for solar thermal pasteurization,” Solar Energy, vol. 64, no. 1-3, 1998. [↩]