Luminescent Solar Concentrators (LSCs) aim to reduce the cost of solar electricity by using an inexpensive collector to concentrate solar radiation without mechanical tracking (Figure 1a) ^[1] . Ideally, the dyes re-emit the absorbed light into waveguide modes that are coupled to solar cells attached to the edges of the collector (black arrows). However, some photons are always lost, re-emitted through the face of the LSC, and coupled out of the waveguide (grey arrows). The trapping efficiency, η_trap, is defined as the fraction of photons emitted from the edge versus photons emitted from the face and edge combined. Assuming the dyes emit their photons isotropically, η_trap is given by η_trap = . If the refractive index of the waveguide n_s is ~ 1.7, then 20% of the re-emitted photons are lost.

Such surface losses become compounded if photons trapped in the waveguide and travelling towards the edges are re-absorbed by other dye molecules in the waveguide; see Figure 1b. When such re-absorbed photons are re-emitted, the LSC will suffer more confinement losses, and the cycle repeats. As the number of re-absorption events increases, the overall efficiency of LSCs drops exponentially.

In this work, we aim to quantify the contribution of self-absorption to the surface losses of an LSC. We vary the amount of self-absorption by tuning the dye concentration of ALq₃ and DCJTB in the waveguide (n_s = 1.7). Figure 2 shows preliminary results for η_trap of an LSC with high and low self-absorption. The LSC with low self-absorption has η_trap of 78%, which approaches the theoretical η_trap of 81%. The slightly lower measured value of η_trap is likely to be explained by scattering losses from the LSC surface.

1. J. S. Batchelder, A.H. Zewail, and T. Cole, “Luminescent solar concentrators. 2: Experimental and theoretical analysis of their possible efficiencies,”Applied Optics, vol. 20, pp. 3733-3754, Nov. 1981.

Figure 1: Illustration of ray tracing under (a) normal (spontaneous) and (b) lasing (stimulated) conditions within an LSC. (c) Typical laser transfer characteristic, showing the dramatic change in efficiency above the threshold.

Many on-chip optical applications, including spectroscopy ^[1] ; sensing ^{[2] [3]} ; nonlinear optics ^{[4] [5] [6]} ; and optical communications require high-finesse ^{[7] [8]} , high-quality factor (high-Q) micro-lasers. Such lasers must be exceptionally transparent at their emission wavelength. But if high-Q micro-lasers exhibit correspondingly weak absorption at the pump wavelengths, they are challenging to excite. Here we demonstrate micro-ring lasers that exhibit Q > 5.2 × 10⁶ and a finesse of > 1.8 × 10⁴ with a direct-illumination, non-resonant pump.  The micro-rings are coated with a combination of three organic dyes. This cascaded combination of near and ultimately far field energy transfer reduces material-losses by a factor of more than 10⁴, transforming incoherent light to coherent light with high quantum-efficiency. The operating principle established here is general and can enable fully integrated on-chip, high-finesse micro-lasers without the complications of coupled pump and emitter resonators ^[9] .

We are now working on lasing luminance solar concentrators (LSC) or solar powered lasers based on the cascaded energy concept. Above threshold, all the fundamental properties of an LSC improve. Specifically, (i) the brightness of the lasing LSC can be orders of magnitude larger than conventional solar concentrators; (ii) stimulated emission enhances the photoluminescent efficiency; (iii) it increases the trapping efficiency of the LSC; (iv) and stimulated emission decreases self-absorption. A solar powered laser also, in essence, converts a portion of the incoherent solar spectrum into a coherent source. This conversion enables the solar powered laser light to be frequency converted in nonlinear crystals, allowing harvesting of more of the solar spectrum via efficient upconversion and downconversion for high efficiency photovoltaics. Figure 1 shows ray tracing of a below- and an above-threshold LSC.

1. C. Y. Chao, W. Fung, and L. J. Guo, “Polymer microring resonators for biochemical sensing applications,” IEEE J. of Sel. Top. Quantum Electron, vol. 12, no. 1, pp. 134-142, Jan. 2006.

2. [1]    F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, and S. Arnold, “Protein detection by optical shift of a resonant microcavity,” Applied Physics Letters, vol. 80, no. 21,  pp. 4057-4059, April 2002.

3. A. Serpenguzel, S. Arnold, and G. Griffel, “Excitation of resonances of microspheres on an optical fiber,” Opt. Lett., vol. 20, no. 7, pp. 654-656, Apr. 1995
4. R. K. Chang, and A. J. Campillo, Optical Processes in Microcavities. Singapore: World Scientific, 1996.
5. F. Treussart, V. S. Ilchenko, J. F. Roch, J. Hare, V. Lefevre-Seguin, J. M. Raimond, and S. Haroche, “Evidence of intrinsic Kerr bistability of high-Q microsphere resonators in superfluid helium,”  Eur. Phys. J. D., vol. 1, pp. 235-238, Jan. 1998.
6. S. M. Spillane, T. J. Kippenberg, and K. J. Vahala, “Ultra-low threshold Raman laser using a spherical dielectric microcavity,”  Nature, vol. 415, pp. 621-623, Feb. 2002.
7. B. E. Little, S. T. Chu, P. P. Absil, J. V. Hryniewicz, F. G. Johnson, F. Seiferth, D. Gill, V. Van, O. King, and M. Trakalo, “Very high-order microring resonator filters for WDM applications,”  IEEE Photon. Technol. Lett., vol. 16, no. 10, pp. 2263-2265, Oct. 2004.
8. M. Ferrera, L. Razzari, D. Duchesne, R. Morandotti, Z. Yang, M. Liscidini, J. E. Sipe, S. Chu, B. Little, and D. J. Moss, “Low power continuous-wave nonlinear optics in doped silica glass integrated waveguide structures,” Nature Photonics, vol. 2, pp. 737-740, Nov. 2008.
9. Rotschild, C., Tomes, M., Mendoza, H., Andrew, T. L., Swager, T. M., Carmon, T. and Baldo, M. A., “Cascaded Energy Transfer for Efficient Broad-Band Pumping of High-Quality, Micro-Lasers,” Advanced Materials, vol. 23, 2011

Organic photovoltaics (OPVs) are promising low-cost solar cells: they can be stacked in multi-junctions, and they are compatible with roll-to-roll processing. But as a solar cell’s installation costs are proportional to the area it covers, OPVs’ low efficiencies presently bar their widespread adoption. A significant source of loss in OPVs is the recombination of charges at the donor-acceptor interface: excited electrons combine with holes, returning the system to its ground state, rather than powering an external load. We therefore need to reduce the recombination rates in organic photovoltaics. We consider doing so by taking advantage of spin-disallowed transitions.

Excited states in OPVs come in two flavors of spin: singlets and triplets. Since the ground state is almost always a singlet, quantum mechanical rules prevent triplet excited states from relaxing, so triplets have longer lifetimes—i.e., lower recombination rates. In the absence of spin mixing processes, an OPV that produces electron-hole pairs in the triplet state should be more efficient than an OPV that produces singlets.

To prepare excited states in either the singlet or triplet state, we made a heterojunction solar cell with PTCBI (which produces singlets when excited) and pentacene (which produces triplets when excited ^[1] ). The spectral dependence of optical absorption in the two materials allows us to produce mostly triplets or singlets by exciting the device with 635-nm or 532-nm light, respectively. At room temperature the two show the same behavior; we are now examining the devices at much lower temperatures and under a magnetic field, where the mixing rate between the two states may be low enough to reveal the difference between the two.

1. J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Appl. Phys. Lett., vol. 95, p. 033301, July 2009.

Photovoltaic solar concentrators aim to increase the electrical power obtained from solar cells.  Conventional solar concentrators track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain.  High optical concentrations without excess heating in a stationary system can be achieved with a luminescent solar concentrator (LSC) ^[1] . The LSC consists of a dye dispersed in a transparent waveguide.  Incident light is absorbed by the dye and then reemitted into a waveguide mode. The energy difference between absorption and emission prevents reabsorption of light by the dye, isolating the photon population in the waveguide. The performance of LSCs has been limited by two factors: self-absorption losses and a scarcity of dyes that emit efficiently in the infrared region. We have previously made significant progress on the problem of self-absorption losses ^[2] . Now we address operation in the infrared region.

Neodymium (Nd³⁺) and ytterbium (Yb³⁺) are nearly the optimal infrared LSC materials: inexpensive, abundant, efficient, and spectrally well-matched to high-performance silicon solar cells. These rare earth ions are natural three or four-level systems, therefore reasonably transparent to their own radiation and capable of generating high optical concentrations. Neodymium’s and ytterbium’s disadvantage is their relatively poor absorption overlap with the visible spectrum, meaning that the rare earth ions will require sensitization in the visible spectrum. Transition metals (TMs) can efficiently transfer energy to Nd and Yb ions, a process depicted in Figure 1. With a selection of TMs, broad sensitization of the both the visible and near infrared regions of the solar spectrum is possible, as shown in Figure 2. Finally, these ions can be combined with a high-throughput and chemically robust glass-making process for low cost and stable LSCs. Sensitized neodymium and ytterbium should enable LSCs matched to silicon.

1. W. H. Weber and J. Lambe, “Luminescent greenhouse collector for solar radiation,” Applied Optics, vol. 15, no. 10, pp. 2299-2300, Oct. 1976.
2. M. J. Currie, J. K. Mapel, T. D. Heidel, S. Goffri, and M. A. Baldo, “High-efficiency organic solar concentrators for photovoltaics,” Science, vol. 321, no. 5886, pp. 226 -228, July 2008.

Semiconductor quantum dots (QDs) are electronically-quantized systems with promising applications in optoelectronic devices ^[1] . A key aspect of such systems is the fine control of optical transitions in the synthesis process ^[2] . These QDs are predominantly used in thin-film arrangement, deposited by spin casting or dip coating. Single QD patterning is one of the major challenges to designing a system that takes advantage of individual properties of QDs. Here we present a template self-assembly technique to control the position of individual QDs through electron-beam lithography (EBL). This optimized top-down lithographic process is a step towards the integration of individual QDs in optoelectronics systems for industrial applications.

The fabrication process of templated QDs is illustrated in Figure 1a. A poly(methylmethacrylate) (PMMA) resist was spin coated on a silicon substrate, followed by the fabrication of a mask through EBL. The size of the resulted PMMA templates was minimized by varying development temperature ^{[3] [4]} . Figure 1b shows the optimized PMMA patterning, with minimum template (i.e., hole) size of 8 nm for development at 6 °C. After defining the PMMA templates, a solution of QDs (6-nm-diameter CdSe) was spin casted and the remaining resist was removed by dissolution in acetone. This process resulted in QD clusters attached on the substrate. By optimizing the QD solution concentration, resist thickness, and feature size, we fabricated clusters with 1 to 10 QDs. One figure of merit in this process is the pattern yield, which is the ratio of yielded structures to the patterned templates. Figure 2 shows QD clusters with 87% pattern yield, with an average of 3 QDs in each cluster. Control of QD placement will be further optimized and integrated into photonic devices.

1. A. P. Alivisatos, “Semiconductor clusters, nanocrystals, and quantum dots,” Science, vol. 271. no. 5251, pp. 933-937, Feb. 1996.
2. S. A. Empedocles, D. J. Norris, and M. G. Bawendi, “Photoluminescence spectroscopy of single CdSe Nanocrystallite quantum dots,” Phys. Rev. Lett. vol. 77, pp. 3873-3876, Oct. 1996.
3. W. Hu, K. Sarveswaran, M. Lieberman, and G. H. Bernstein, “Sub-10 nm electron beam lithography using cold development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 22, pp. 1711-1716, June 2004.
4. B. Cord, J. Lutkenhaus, and K. K. Berggren, “Optimal temperature for development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 25, pp. 2013-2016, Dec. 2007.

Figure 1: Scanning electron micrographs with uniform array of micropillars. The micropillars have the same height of 17 µm and pitch of 25 µm, while the diameters of pillars are (a) 6 µm, (b) 11 µm, and (c) 16 µm, respectively.

The mechanism of critical heat flux (CHF) is commonly attributed to two limits during boiling behavior: 1) the hydrodynamic limit due to Helmholtz instability and 2) the capillary limit determined by surface wettability ^[1] .  In recent years, a significant amount of research has been focused on CHF enhancement by utilizing micro/nanostructured surfaces to improve wettability ^{[2] [3] [4]} , with CHF of ~200 W/cm² being demonstrated ^[4] .  While most works are focused on making small structure sizes to improve surface wettability, the effect of this roughness-augmented wettability on CHF is poorly understood.  The limit of CHF enhancement with roughness-augmented wettability, where hydrodynamic instability becomes the dominant mechanism for CHF, has not been investigated.  In addition, boiling on nanostructured surfaces suffers from the requirement of high superheat due to bubble geometries closer to the homogeneous nucleation limit.  As a result, the heat transfer coefficient (HTC) on nanostructured surfaces is sacrificed ^[3] , which impairs the heat removal capability especially for applications demanding small temperature difference.

In this study, micro/nanopillar arrays are fabricated with a series of pitch and diameter size, as shown in Figure 1.  The sizes of pillar are designed to ensure that bubbles in the Cassie state, where vapor bubbles are suspended on the pillars, are energetically favorable such that bubble detachment is enhanced.  The series of sizes of the structured arrays generate various capillary forces, which allow the study on the mechanism for CHF and the limit of CHF enhancement with roughness-augmented wettability.  Furthermore, the investigation on surface roughness, where hydrodynamic instability dominants, gives the optimal size of structures for CHF enhancement and explores the feasibility of heterogeneous bubble nucleation on surfaces with proper structure geometry to reduce superheat.

1. S. G. Liter and M. Kaviany, “Pool-boiling CHF enhancement by modulated porous-layer coating: theory and experiment,” Int. J. Heat Mass Transfer, vol. 44, pp. 4287–4311, 2001.
2. C. Li and G. P. Peterson, “Parametric study of pool boiling on horizontal highly conductive microporous coated surfaces,” J. Heat Transfer, vol. 129, pp. 1465-1475, 2007.
3. S. Kim, H. D. Kim, H. Kim, H. S. Ahn, H. Jo, J. Kim, and M. H. Kim, “Effects of nano-fluid and surfaces with nano structure on the increase of CHF,” Exp. Therm Fluid Sci., vol. 34, pp. 487–495, 2010.
4. R. Chen, M.-C. Lu, V. Srinivasan, Z. Wang, H. H. Cho, and A. Majumdar, “Nanowires for enhanced boiling heat transfer,” Nano Lett., vol. 9, no. 2, pp. 548-553, 2009.

There is a need for microscale vacuum pumps that can be readily integrated with other MEMS and electronic components at the chip-scale level. Vacuum pumps exhibit favorable scaling and are promising for a variety of applications such as portable mass spectrometers ^[1] and vacuum amplifiers. This project aims to develop the technology for a micro-fabricated electron-impact-ionizer pump.  The micropump consists of a field-emission electron source that is an array of double-gated isolated vertically aligned carbon nanotubes (VA-CNTs), an electron-impact-ionization region, and an ion implantation getter, as shown in Figure 1. The pump works as follows: first, electrons are field-emitted from the VA-CNT array; then, the electrons are accelerated at a bias voltage that maximizes the probability of collision with neutral gas molecules, this way achieving ionization by fragmentation of the molecules; finally, ions are implanted into the getter.

In a double-gated field-emitter array, the first gate (extractor) is used to modulate the tunneling of electrons out of the tip, while the second gate (focus) is biased at a lower voltage than the first gate to focus the emitted electrons and to collect the back-streaming ions, thus protecting the tip ^[2] . As part of this work, we designed and fabricated single-gated isolated VA-CNT field-emission arrays, shown in Figure 2(a), to quantify the effectiveness of the field emitter-extractor diode to enhance the electric field on the emitter tip (i.e., estimate the extractor field factor), through experiments and simulations using the commercial software COMSOL. Figure 2(b) shows the solution of electric field using the same geometry of the device we fabricated. Each emitter has a 15-nm tip radius and 2-µm height with a 1-µm aperture from a single gate. From the simulation results we obtain an extractor field factor of 7.35×10⁵V/cm. Figure 2(c) is the experimental FN plot of an array of ~10,000 single-gated emitters. From the slope of the plot we estimate a field factor of 7.8×10⁵V/cm, which is in good agreement with the prediction of the extractor field factor from the COMSOL simulation.

1. K. H. Gilchrist, C. A. Bower, M. R Lueck, J. R. Piascik, B. R. Stoner, S. Natarajan, C. B. Parker, and J. T. Glass, “A novel ion source and detector for a miniature mass spectrometer,” IEEE Sensors, pp. 1372-1375, Oct. 2007.
2. L. –Y. Chen, L. F. Velásquez-García, X. Wang, K. Cheung, K. Teo, and A.-I. Akinwande, “Design, fabrication and characterization of double-gated vertically aligned carbon nanofiber field emitter arrays,” in Vacuum Nanoelectronics Conference, 2007, pp. 82-83.

This project aims to develop the technology for field-enabled low-power portable vacuum sources that can be made cheaply and reliably, opening the doors to exciting applications such as portable mass spectrometers and high-performance sensors for inertial navigation. Our micropump uses arrays of isolated vertically aligned carbon nanotubes (VA-CNTs) to field-ionize the background gas, that is, to quantum tunnel electrons from the outer shell of neutral gas molecules due to the presence of a very high electrostatic field near the VA-CNT tip (Figure 1). Field strength of at least 10⁸V/cm is needed to field-ionize gases ^[1] .  The ions are then implanted in a non-evaporative getter structure biased at a high negative voltage, hence obtaining vacuum. The field ionization micropump that we are developing is designed to work at pressures as high as 30 Torr.  Our fabricated field ionizer, shown in Figure 2, is composed of arrays of VA-CNTs surrounded by a ring of VA-CNTs. The central VA-CNT of each unit enhances the electric field to achieve field ionization, while the high-transparency ring increases the flux of neutral molecules to the ionization region. VA-CNTs are ideal for field ionization because of their high aspect ratio, which enables low-voltage field ionization and their inherent chemical and mechanical robustness.  Unlike electron impact ionizers ^[2] , the field enhancer of a field ionizer is biased at a higher voltage than the gate.  Therefore, the ions it creates do not stream back to the field enhancers, which results in enhanced reliability.  The getter will be biased at a lower potential with respect to the gate to attract and implant the positive ions. Current research efforts include optimization of the fabrication of the devices and experimental characterization as ionizers and pumps.

1. R. Gomer, Field Emissions and Field Ionization, New York: Springer-Verlag, Dec. 1992.
2. L.-Y. Chen and A. I. Akinwande, “Aperture-collimated double-gated silicon field emitter arrays,” IEEE Transactions on Electron Devices, vol. 54,  no. 3, pp. 601-608, Mar. 2007.

With continuously growing energy demands, new alternative energy solutions become essential. In order to achieve sustainability, efficient conversion and storage of solar energy are imperative ^{[1] [2]} . Photoelectrolysis utilizes solar energy to evolve hydrogen and oxygen from water, thereby enabling energy storage via chemical means. In this work, photoelectrodes are being investigated; they offer high conversion efficiency, long-term stability, and low cost. The focus is initially on semiconducting metal oxides in which the energy band, defect, and micro-structure are tuned to optimize optical absorption, charge transport, and reduced overpotentials. For high efficiency, a cobalt-based oxidation catalyst ^[3] is implemented at the photoelectrode. The electro-deposition kinetics of this catalyst are studied as part of this project to allow further insights into the catalysis mechanism.

1. N. S. Lewis and D. G. Nocera, “Powering the planet: Chemical challenges in solar energy utilization,” Proc. Natl. Acad. Sci. USA., vol. 103, pp. 15729-15735, 2006.
2. R. van de Krol, Y. Liang, and J. Schoonman, “Solar hydrogen production with nanostructured metal oxides,” J. Mater. Chem., vol. 18, pp. 2311-2320, 2008.
3. M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co²⁺,” Science, vol. 321, pp 1072-1075, 2008.

Solid oxide fuel cells (SOFCs) directly convert chemical to electrical energy with high efficiency and can operate using a wide variety of fuels from hydrogen to kerosene and gasified coal.  Many of the more advanced materials in SOFCs experience significant changes in oxygen content, or oxygen stoichiometry, during operation, resulting in changes in volume and elastic properties termed chemomechanics.  This chemical expansion, analogous to temperature-induced thermal expansion, is oxygen nonstoichiometry-induced and can have a negative impact on SOFC performance. In this project, we are studying the chemical expansion coefficient, elastic properties, and oxygen stoichiometry of thin film and bulk SOFC materials. Thin films are of particular interest since they allow for control of strain and increase the surface-to-volume ratio, particularly important for electrode performance.  The chemomechanical properties are being investigated using high temperature, atmosphere controlled nanoindentation, high resolution x-ray diffraction, dilatometry, impedance spectroscopy, and thermo-gravimetry techniques.