All entries for Philip Reusswig

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Understanding the Role of Self-absorption on the Trapping Efficiency of Luminescent Solar Concentrators

• Authors: C. L. Mulder, B. Parker, P. D.Reusswig, M. A. Baldo
• Sponsorship: EFRC Center for Excitonics

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 = [latex]\sqrt{1-1/n_5^2}[/latex] . 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.



Figure 1: (a) A schematic representation of a luminescent solar concentrator. The trapping efficiency is defined as the fraction of photons emitted from the edge versus photons emitted from the face and edge combined. (b) Illustration of the compounding effect of self-absorption on the trapping efficiency. The green molecule is the first in the system to absorb light, but the three surrounding red dye molecules can absorb photonsthat are re-emitted. The process continues throughout the waveguide, resulting in further loss from the face.



Figure 2: The measured trapping efficiency, ηtrap of LSC with low (red dots) and high (blue dots) self-absorption. The dotted line is the theoretical trapping efficiency. This measured ηtrap is the ratio between the measured number of photons emitted from the edge and the total number of photons emitted from the face and edges.




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. [↩]

Cascaded Energy Transfer for Efficient Broadband Pumping of High-quality Micro-lasers

• Authors: C. Rotschild, P. R. Reusswig, N. Thompson, M. A. Baldo
• Sponsorship: EFRC Center for Excitonics



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 [↩]

Glass-based Luminescent Solar Concentrator

• Authors: N. J. Thompson, P. D. Reusswig, H. L. Tuller, M. A. Baldo
• Sponsorship: : EFRC Center for Excitonics, MITEI

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.



Figure 1: A schematic representation of an LSC. The LSC consists of transition metal (TM) and rare earth (RE) ion-doped glass and a diffuse back reflector. Solar radiation incident on the LSC is primarily absorbed by the TM ion and transferred to the RE ion via non-radiative energy transfer (represented by the blue arrow), the RE then emits the light that is waveguided to the edges.



Figure 2: A comparison between the solar spectrum (black) and the absorption of several transition metal ions (blue and red). The broadband absorption of the TMs will utilize a large portion of the solar spectrum, including previously under-utilized portions of the near-infrared.




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. [↩]