Solar cells with broadband absorption and large acceptance angle are demonstrated by using two-dimensional core-shell structures, which are composed of silicon oxide shells and silicon cores. This study considers structure parameters such as core/shell thickness and periodicity. Finite difference time domain calculation (FDTD) is used in the simulation.  As Figure 1 shows, our core-shell structure is built of Si and SiOx with a feature size much smaller than the wavelength of light.  The core can be either amorphous or polycrystalline silicon, and the shell is silicon dioxide. Four structural parameters are considered: outer radius (r1), inner radius (r2), height (h), and periodicity (a) or symmetry. By tuning these four parameters, we are able to generate an antireflection effect that covers the entire silicon absorption spectrum.

Figure 2 (a) highlights the superior antireflective property of the core-shell layer from our study. An enhancement of absorption is observed between wavelengths of 400-1200 nm. Strong antireflection is achieved as a 0.1-um-thick core-shell layer is applied. The reflectivity drops significantly by 20% at wavelength λ=600 nm and 25% at λ=900 nm. Figure 2 (b) shows that the reflectivity reduces as the periodicities decrease. A huge drop of 20% in reflectivity is observed at wavelength of 0.7 um as the periodicities change from 0.7 um to 0.3 um. The reflectivity within this range is uniformly low. We note that the broadband absorption spectra are not sensitive to the light incident angles. The large acceptant angle is contributed by the continuous varied refractive indexes. Hence, by using the core-shell structure, we improve the efficiency of solar cells by minimizing the loss caused by surface reflection. The easy fabrication of the core-shell structure enables large-scale fabrication of highly efficient solar cells.

1. P. T. Lin, Y. Yi, X. Duan, and L. C. Kimerling, “Simulation and fabrication of two-dimensional core-shell structures for broadband solar cells absorption,” presented at MRS Fall Meeting & Exhibit, Boston, J7.20, 2011.

Two-dimensional photonic crystals (PhCs) are fabricated using dual-beam focused ion beam (FIB) in Er3+-TeO2 thin films and demonstrate broadband enhancement of PL emission at near Infrared (NIR). As Figure 1 shows, highly uniformed patterns with smooth surfaces and pattern resolution better than hundred nanometers are achieved. PhCs arrays with photonic lattice constants ranging from 350 nm to 1700 nm are explored to optimize the PL extraction efficiency. Strong photoluminescence around 1530 nm is observed using 488-532 nm laser pump. A confocal microscope with spectrometer is used to capture the broadband PL signals from individual PhC arrays.

The emission enhancement factor and spectral dependent extraction ratio were analyzed to find the interaction between PL emission and PhC structures. Figure 2 (a) shows that when the PhC structures are optimized, 1500 µm-1560 µm broadband PL is successfully converted between the PL in-plane and out-of-plane emission. As in Figure 2(b), a 60 % enhancement of surface extraction efficiency is achieved when PhC with periodicity a=800 nm is applied. When photonic lattice constants a are smaller than the critical periodicity of 600 nm, the PL light becomes confined inside the thin film layer. Two-dimensional finite difference time domain (FDTD) simulation explains the experimentally observed anisotropic PL enhancement as due to the photonic band gap. The broadband PL enhancement enables Er3+-TeO2 PhCs thin film as a potential light source for three- dimensional integrated photonic circuits.

1.  P. T. Lin, M. Vanhoutte, N. S. Patel, V. Singh, J. Hu, Y. Cai, R. Camacho-Aguilera, J. Michel, L. C. Kimerling, and Anu Agarwal, “Engineering broadband and anisotropic photoluminescence emission from rare earth doped tellurite thin film photonic crystals,” Optics Express, vol. 20, no. 3, pp. 2124-2135, 2012.

We have experimentally demonstrated a monolithically integrated ultra-compact optical isolator on silicon. This device allows a significant device footprint reduction from the centimeter or millimeter scale down to the 10s-of-micrometers level. As a first experimentally demonstrated monolithic nonreciprocal optical component on silicon, this device can also be developed into a variety of integrated nonreciprocal photonic devices including optical circulators, modulators, or switches, which opens a new dimension of functionality for silicon photonics.

The optical isolator, as shown in Figure 1, has been fabricated on silicon on insulator (SOI) substrate with part of the SiO₂ top cladding layer etched away to expose the underlying silicon waveguide. Then a magneto-optical oxide thin film layer was deposited to fill this etched region. When an in-plane magnetic field is applied perpendicular to the light propagation direction in the exposed resonator region shown in the figure, a nonreciprocal magneto-optical phase shift is observed for TM polarized light in the resonator. The effective indices between forward and backward propagating TM mode lights are different, and the resonance wavelengths are non-degenerate. This effect is demonstrated in a sketch of the transmission spectra of forward and backward propagating TM polarized light near resonant wavelengths, shown in Figure 2 (a). The different resonant wavelengths allow different transparencies of the device at the given wavelength range, enabling the device to operate as an optical isolator. Experimental demonstration of the device performance was carried out by using an Y₃Fe₅O₁₂ buffered Ce₁Y₂Fe₅O₁₂ polycrystalline thin film as the magneto-optical oxide layer, fabricated by pulsed laser deposition ^[1] . The experimental and theoretical TM mode transmission spectra with oppositely applied magnetic fields near the resonance wavelength of 1549.5 nm are shown in Figure 2 (a). This device achieved an isolation ratio of 19.5±2.9 dB, an insertion loss of 18.8±1.1 dB, and a 10 dB isolation bandwidth of 1.6 GHz. We are now investigating multiple resonator configurations for enhanced optical isolation and tunable bandwidth that arises due to the coupling regime, as shown in Figure 2 (b). Tunability range is 100 nm in wavelength ^[2] .

1. L. Bi, J. Hu, P. Jiang, D. H. Kim, G. F. Dionne, L. C. Kimerling, and C. A. Ross, “On-chip optical isolation in monolithically integrated non-reciprocal optical resonators,” Nature Photonics, vol. 5, p. 758, 2011.
2. M. Onbasli, J. Hu, L. Bi, G. F. Dionne, and C. A. Ross, “Cascaded magneto-optical ring resonator structures for tunable faraday rotation and reduced isolator footprint,” presented at American Physical Society Meeting, Boston, MA, 2012.

Collaborators

• Harry Atwater, Caltech
• Mark Brongersma, Stanford
• Luca DalNegro, Boston University
• Clara Dimas, Masdar Institute of Technology
• Chris Doerr, Acacia
• Charles Fine, MIT
• Eugene Fitzgerald, MIT
• Eric Ippen, MIT
• Tomoyuki Izuhara, Enablence
• Franz Kaertner, MIT
• Thomas Koch, Arizona
• Inna Kozinsky, Bosch
• Michal Lipson, Cornell University
• Igor Luzinov, Clemson University
• Andrea Melloni, Politec. di Milano
• Yukiya Miyachi, Fujifilm
• Daisuke Okamoto, NEC
• Shintaro Okamoto, Toshiba Semi.
• Dennis Prather, Delaware
• Kathleen Richardson, Clemson
• Marco Romagnoli, PhotonIC Corp
• Marco Stefancich, Masdar Institute
• Kazumi Wada, University of Tokyo
• Alice White, A-L Bell Laboratories
• Chee Wei Wong, Columbia

Postdoctoral Associates

• Lirong Broderick
• Pao-Tai Lin
• Jianwei Mu (Postdoctoral Fellow)
• Lin Zhang

Graduate Students

• Brian Albert (DMSE)
• Yan Cai (DMSE)
• Rodolfo Camacho-Aguilera (DMSE)
• Zhaohong Han  (DMSE)
• Corentin Monmeyran  (DMSE)
• Neil Patel  (DMSE)
• Brian Pearson (MechE)
• Vivek Raghunathan (DMSE)
• Vivek Singh (DMSE)
• Michiel Vanhoutte (DMSE)
• Wei Yu (DMSE)

Research Staff

• Anuradha Agarwal
• Jurgen Michel

Support Staff

• Lisa Sinclair, Administrative Assistant II

Publications

Wang, J. F., T. Zens, J. J. Hu, P. Becla, L. C. Kimerling and A. M. Agarwal, “Monolithically Integrated, Resonant-Cavity-Enhanced Dual-Band Mid-Infrared Photodetector on Silicon,” Applied Physics Letters, 100, 21 (2012).

Stefancich, M., A. Zayan, M. Chiesa, S. Rampino, D. Roncati, L. C. Kimerling and J. Michel, “Single Element Spectral Splitting Solar Concentrator for Multiple Cells Cpv System,” Optics Express, 20, 8 (2012).

Singh, V., J. J. Hu, A. M. Agarwal and L. C. Kimerling, “Integrated Optical Sensors,” IEEE Photonics Journal, 4, 2 (2012).

Raghunathan, V., T. Izuhara, J. Michel and L. Kimerling, “Stability of Polymer-Dielectric Bi-Layers for Athermal Silicon Photonics,” Optics Express, 20, 14 (2012).

McComber, K. A., X. M. Duan, J. F. Liu, J. Michel and L. C. Kimerling, “Single-Crystal Germanium Growth on Amorphous Silicon,” Advanced Functional Materials, 22, 5 (2012).

Lin, P. T., M. Vanhoutte, N. S. Patel, V. Singh, J. J. Hu, Y. Cai, R. Camacho-Aguilera, J. Michel, L. C. Kimerling and A. Agarwal, “Engineering Broadband and Anisotropic Photoluminescence Emission from Rare Earth Doped Tellurite Thin Film Photonic Crystals,” Optics Express, 20, 3 (2012).

Canciamilla, A., F. Morichetti, S. Grillanda, P. Velha, M. Sorel, V. Singh, A. Agarwal, L. C. Kimerling and A. Melloni, “Photo-Induced Trimming of Chalcogenide-Assisted Silicon Waveguides,” Optics Express, 20, 14 (2012).

Camacho-Aguilera, R. E., Y. Cai, N. Patel, J. T. Bessette, M. Romagnoli, L. C. Kimerling and J. Michel, “An Electrically Pumped Germanium Laser,” Optics Express, 20, 10 (2012).

Zens, T., P. Becla, A. M. Agarwal, L. C. Kimerling and A. Drehman, “Long Wavelength Infrared Detection Using Amorphous Insb and Inas0.3sb0.7,” Journal of Crystal Growth, 334, 1 (2011).

Sheng, X., J. F. Liu, I. Kozinsky, A. M. Agarwal, J. Michel and L. C. Kimerling, “Design and Non-Lithographic Fabrication of Light Trapping Structures for Thin Film Silicon Solar Cells,” Advanced Materials, 23, 7 (2011).

Sheng, X., L. Z. Broderick, J. J. Hu, L. Yang, A. Eshed, E. A. Fitzgerald, J. Michel and L. C. Kimerling, “Design and Fabrication of High-Index-Contrast Self-Assembled Texture for Light Extraction Enhancement in LEDs,” Optics Express, 19, 14 (2011).

Canciamilla, A., S. Grillanda, F. Morichetti, C. Ferrari, J. J. Hu, J. D. Musgraves, K. Richardson, A. Agarwal, L. C. Kimerling and A. Melloni, “Photo-Induced Trimming of Coupled Ring-Resonator Filters and Delay Lines in As2s3 Chalcogenide Glass,” Optics Letters, 36, 20 (2011).

Bi, L., J. J. Hu, P. Jiang, D. H. Kim, G. F. Dionne, L. C. Kimerling and C. A. Ross, “On-Chip Optical Isolation in Monolithically Integrated Non-Reciprocal Optical Resonators,” Nature Photonics, 5, 12 (2011).

Ahn, D., L. C. Kimerling and J. Michel, “Efficient Evanescent Wave Coupling Conditions for Waveguide-Integrated Thin-Film Si/Ge Photodetectors on Silicon-on-Insulator/Germanium-on-Insulator Substrates,” Journal of Applied Physics, 110, 8 (2011).