Professional Education

  • Doctor Rerum Naturium, Albert Ludwigs Universitat Freiburg (2014)
  • Diplom, Albert Ludwigs Universitat Freiburg (2009)
  • Vordiplom, Ruprecht Karl Universitat Heidelberg (2006)

Stanford Advisors

All Publications

  • Improving Quantum Yield of Upconverting Nanoparticles in Aqueous Media via Emission Sensitization NANO LETTERS Wisser, M. D., Fischer, S., Siefe, C., Alivisatos, A., Salleo, A., Dionne, J. A. 2018; 18 (4): 2689–95


    We demonstrate a facile method to improve upconversion quantum yields in Yb,Er-based nanoparticles via emission dye-sensitization. Using the commercially available dye ATTO 542, chosen for its high radiative rate and significant spectral overlap with the green emission of Er3+, we decorate the surfaces of sub-25 nm hexagonal-phase Na(Y/Gd/Lu)0.8F4:Yb0.18Er0.02 upconverting nanoparticles with varying dye concentrations. Upconversion photoluminescence and absorption spectroscopy provide experimental confirmation of energy transfer to and emission from the dye molecules. Upconversion quantum yield is observed to increase with dye sensitization, with the highest enhancement measured for the smallest particles investigated (10.9 nm in diameter); specifically, these dye-decorated particles are more than 2× brighter than are unmodified, organic-soluble nanoparticles and more than 10× brighter than are water-soluble nanoparticles. We also observe 3× lifetime reductions with dye adsorption, confirming the quantum yield enhancement to result from the high radiative rate of the dye. The approach detailed in this work is widely implementable, renders the nanoparticles water-soluble, and most significantly improves sub-15 nm nanoparticles, making our method especially attractive for biological imaging applications.

    View details for DOI 10.1021/acs.nanolett.8b00634

    View details for Web of Science ID 000430155900070

    View details for PubMedID 29589449

  • Soft thermal nanoimprint of PMMA doped with upconverter nanoparticles MICROELECTRONIC ENGINEERING Hauser, H., Herter, B., Hofmann, C. M., Hoehn, O., Kuebler, V., Fischer, S., Wolf, S., Fasold, S., van Veggel, F. M., Goldschmidt, J., Blaesi, B. 2018; 187: 154–59
  • Optical nanoprobes for biomedical applications: shining a light on upconverting and near-infrared emitting nanoparticles for imaging, thermal sensing, and photodynamic therapy J. Mater. Chem. B Hemmer, E., Acosta-Mora, P., Méndez-Ramos, J., Fischer, S. 2017: 4365-4392

    View details for DOI 10.1039/C7TB00403F

  • Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF 4 Nanocrystals Induced by Precursor Injection Rate JACS Fischer, S., Swabeck, J., Alivisatos, A. 2017: 12325-12332

    View details for DOI 10.1021/jacs.7b07496

  • Comparative analysis of upconversion efficiencies in fluoride materials for photovoltaic application SOLAR ENERGY MATERIALS AND SOLAR CELLS Favilla, E., Cittadino, G., Veronesi, S., Tonelli, M., Fischer, S., Goldschmidt, J. C., Cassanho, A., Jenssen, H. P. 2016; 157: 415-421
  • Precise tuning of surface quenching for luminescence enhancement in core-shell lanthanide-doped nanocrystals. Nano letters Fischer, S., Bronstein, N. D., Swabeck, J. K., Chan, E. M., Alivisatos, A. P. 2016: -?


    Lanthanide-doped nanocrystals are of particular interest for the research community not only due to their ability to shape light by downshifting, quantum cutting, and upconversion but also because novel optical properties can be found by the precise engineering of core-shell nanocrystals. Because of the large surface area-to-volume ratio of nanocrystals, the luminescence is typically suppressed by surface quenching. Here, we demonstrate a mechanism that exploits surface quenching processes to improve the luminescence of our core-shell lanthanide-doped nanocrystals. By carefully tuning the shell thickness of inert β-NaLuF4 around β-NaYF4 nanocrystals doped with Yb(3+) and Er(3+), we unravel the relationship between quantum yield and shell thickness, and quantify surface quenching rates for the relevant Er(3+) and Yb(3+) energy levels. This enhanced understanding of the system's dynamics allowed us to design nanocrystals with a surface quenching-assisted mechanism for bright NIR to NIR downshifting with a distinctive efficiency peak for an optimized shell thickness.

    View details for PubMedID 27726405

  • Enhancing Quantum Yield via Local Symmetry Distortion in Lanthanide-Based Upconverting Nanoparticles ACS PHOTONICS Wisser, M. D., Fischer, S., Maurer, P. C., Bronstein, N. D., Chu, S., Alivisatos, A. P., Salleo, A., Dionne, J. A. 2016; 3 (8): 1523-1530
  • Upconversion in a Bragg structure: photonic effects of a modified local density of states and irradiance on luminescence and upconversion quantum yield OPTICS EXPRESS Hofmann, C. L., Herter, B., Fischer, S., Gutmann, J., Goldschmidt, J. C. 2016; 24 (13): 14895-14914


    In this paper, we present a comprehensive simulation-based analysis of the two photonic effects of a Bragg stack - a modified local density of photon states (LDOS) and an enhanced local irradiance - on the upconversion (UC) luminescence and quantum yield of the upconverter β-NaYF4 doped with 25% Er3+. The investigated Bragg stack consists of alternating layers of TiO2 and Poly(methylmethacrylate), the latter containing upconverter nanoparticles. Using experimentally determined input parameters, the photonic effects are first simulated separately and subsequently coupled in a rate equation model, describing the dynamics of the UC processes within β-NaYF4:25% Er3+. With this integrated simulation model, the Bragg stack design is optimized to maximize either the UC quantum yield (UCQY) or UC luminescence. We find that in an optimized Bragg stack, due to the modified LDOS, the maximum UCQY is enhanced from 14% to 16%, compared to an unstructured layer of upconverter material. Additionally, this maximum UCQY can already be reached at an incident irradiance as low as 100 W/m2. With a Bragg stack design that maximizes UC luminescence, enhancement factors of up to 480 of the UC luminescence can be reached.

    View details for DOI 10.1364/OE.24.014895

    View details for Web of Science ID 000381759800108

    View details for PubMedID 27410641

  • Enhanced upconversion quantum yield near spherical gold nanoparticles - a comprehensive simulation based analysis OPTICS EXPRESS Fischer, S., Kumar, D., Hallermann, F., von Plessen, G., Goldschmidt, J. C. 2016; 24 (6): A460-A475


    Photon upconversion is promising for many applications. However, the potential of lanthanide doped upconverter materials is typically limited by low absorption coefficients and low upconversion quantum yields (UCQY) under practical irradiance of the excitation. Modifying the photonic environment can strongly enhance the spontaneous emission and therefore also the upconversion luminescence. Additionally, the non-linear nature of the upconversion processes can be exploited by an increased local optical field introduced by photonic or plasmonic structures. In combination, both processes may lead to a strong enhancement of the UCQY at simultaneously lower incident irradiances. Here, we use a comprehensive 3D computation-based approach to investigate how absorption, upconversion luminescence, and UCQY of an upconverter are altered in the vicinity of spherical gold nanoparticles (GNPs). We use Mie theory and electrodynamic theory to compute the properties of GNPs. The parameters obtained in these calculations were used as input parameters in a rate equation model of the upconverter β-NaYF4: 20% Er3+. We consider different diameters of the GNP and determine the behavior of the system as a function of the incident irradiance. Whether the UCQY is increased or actually decreased depends heavily on the position of the upconverter in respect to the GNP. Whereas the upconversion luminescence enhancement reaches a maximum around a distance of 35 nm to the surface of the GNP, we observe strong quenching of the UCQY for distances <40 nm and a UCQY maximum around 125 to 150 nm, in the case of a 300 nm diameter GNP. Hence, the upconverter material needs to be placed at different positions, depending on whether absorption, upconversion luminescence, or UCQY should be maximized. At the optimum position, we determine a maximum UCQY enhancement of 117% for a 300 nm diameter GNP at a low incident irradiance of 0.01 W/cm2. As the irradiance increases, the maximum UCQY enhancement decreases to 20% at 1 W/cm2. However, this UCQY enhancement translates into a significant improvement of the UCQY from 12.0% to 14.4% absolute.

    View details for DOI 10.1364/OE.24.00A460

    View details for Web of Science ID 000373395700003

    View details for PubMedID 27136867

  • Upconverting core-shell nanocrystals with high quantum yield under low irradiance: On the role of isotropic and thick shells JOURNAL OF APPLIED PHYSICS Fischer, S., Johnson, N. J., Pichaandi, J., Goldschmidt, J. C., van Veggel, F. C. 2015; 118 (19)

    View details for DOI 10.1063/1.4936119

    View details for Web of Science ID 000367722400005

  • Enhanced energy conversion of up-conversion solar cells by the integration of compound parabolic concentrating optics SOLAR ENERGY MATERIALS AND SOLAR CELLS Arnaoutakis, G. E., Marques-Hueso, J., Ivaturi, A., Fischer, S., Goldschmidt, J. C., Kraemer, K. W., Richards, B. S. 2015; 140: 217-223
  • Record efficient upconverter solar cell devices with optimized bifacial silicon solar cells and monocrystalline BaY2F8:30% Er3+ upconverter SOLAR ENERGY MATERIALS AND SOLAR CELLS Fischer, S., Favilla, E., Tonelli, M., Goldschmidt, J. C. 2015; 136: 127-134
  • Upconversion for Photovoltaics - a Review of Materials, Devices and Concepts for Performance Enhancement ADVANCED OPTICAL MATERIALS Goldschmidt, J. C., Fischer, S. 2015; 3 (4): 510-535
  • Relation between Excitation Power Density and Er3+ Doping Yielding the Highest Absolute Upconversion Quantum Yield JOURNAL OF PHYSICAL CHEMISTRY C Fischer, S., Froehlich, B., Kraemer, K. W., Goldschmidt, J. C. 2014; 118 (51): 30106-30114

    View details for DOI 10.1021/jp510209x

    View details for Web of Science ID 000347360200074

  • Bifacial n-type silicon solar cells for upconversion applications SOLAR ENERGY MATERIALS AND SOLAR CELLS Ruediger, M., Fischer, S., Frank, J., Ivaturi, A., Richards, B. S., Kraemer, K. W., Hermle, M., Goldschmidt, J. C. 2014; 128: 57-68
  • Upconversion quantum yield of Er3+-doped beta-NaYE4 and Gd2O2S: The effects of host lattice, Er3+ doping, and excitation spectrum bandwidth JOURNAL OF LUMINESCENCE Fischer, S., Martin-Rodriguez, R., Froehlich, B., Kraemer, K. W., Meijerink, A., Goldschmidt, J. C. 2014; 153: 281-287
  • Enhanced up-conversion for photovoltaics via concentrating integrated optics OPTICS EXPRESS Arnaoutakis, G. E., Marques-Hueso, J., Ivaturi, A., Kraemer, K. W., Fischer, S., Goldschmidt, J. C., Richards, B. S. 2014; 22 (5): A452-A464


    Concentrating optics are integrated into up-conversion photovoltaic (UC-PV) devices to independently concentrate sub-band-gap photons on the up-conversion layer, without affecting the full solar concentration on the overlying solar cell. The UC-PV devices consist of silicon solar cells optimized for up-conversion, coupled with tapered and parabolic dielectric concentrators, and hexagonal sodium yttrium fluoride (β-NaYF₄) up-converter doped with 25% trivalent erbium (Er³⁺). A normalized external quantum efficiency of 1.75x10⁻² cm²/W and 3.38x10⁻² cm²/W was obtained for the UC-PV device utilizing tapered and parabolic concentrators respectively. Although low to moderate concentration was shown to maximize UC, higher concentration lead to saturation and reduced external quantum efficiency. The presented work highlights some of the implications associated with the development of UC-PV devices and designates a substantial step for integration in concentrating PV.

    View details for DOI 10.1364/OE.22.00A452

    View details for Web of Science ID 000333579200028

    View details for PubMedID 24922255

  • Absolute upconversion quantum yield of beta-NaYF4 doped with Er3+ and external quantum efficiency of upconverter solar cell devices under broad-band excitation considering spectral mismatch corrections SOLAR ENERGY MATERIALS AND SOLAR CELLS Fischer, S., Froehlich, B., STEINKEMPER, H., Kraemer, K. W., GOLDSCHMIDT, J. C. 2014; 122: 197-207
  • Upconverter Silicon Solar Cell Devices for Efficient Utilization of Sub-Band-Gap Photons Under Concentrated Solar Radiation IEEE JOURNAL OF PHOTOVOLTAICS Fischer, S., Ivaturi, A., Froehlich, B., Ruediger, M., Richter, A., Kraemer, K. W., Richards, B. S., Goldschmidt, J. C. 2014; 4 (1): 183-189
  • Increased upconversion quantum yield in photonic structures due to local field enhancement and modification of the local density of states - a simulation-based analysis OPTICS EXPRESS Herter, B., Wolf, S., Fischer, S., Gutmann, J., Blaesi, B., Goldschmidt, J. C. 2013; 21 (18): A883-A900
  • Highly Efficient IR to NIR Upconversion in Gd2O2S: Er3+ for Photovoltaic Applications CHEMISTRY OF MATERIALS Martin-Rodriguez, R., Fischer, S., Aruna, I., Froehlich, B., Kraemer, K. W., Goldschmidt, J. C., Richards, B. S., Meijerink, A. 2013; 25 (9): 1912-1921

    View details for DOI 10.1021/cm4005745

    View details for Web of Science ID 000319184600060

  • Plasmon enhanced upconversion luminescence near gold nanoparticles - simulation and analysis of the interactions: Errata OPTICS EXPRESS Fischer, S., Hallermann, F., Eichelkraut, T., von Plessen, G., Kraemer, K. W., Biner, D., Steinkemper, H., Hermle, M., Goldschmidt, J. C. 2013; 21 (9): 10606-10611


    The procedure used in our previous publication [Opt. Express 20, 271, (2012)] to calculate how coupling to a spherical gold nanoparticle changes the upconversion luminescence of Er(3+) ions contained several errors. The errors are corrected here.

    View details for DOI 10.1364/OE.21.010606

    View details for Web of Science ID 000318906500048

    View details for PubMedID 23669916

  • Increasing Upconversion by Plasmon Resonance in Metal Nanoparticles-A Combined Simulation Analysis IEEE JOURNAL OF PHOTOVOLTAICS Goldschmidt, J. C., Fischer, S., Steinkemper, H., Hallermann, F., von Plessen, G., Kraemer, K. W., Biner, D., Hermle, M. 2012; 2 (2): 134-140
  • Plasmon enhanced upconversion luminescence near gold nanoparticles-simulation and analysis of the interactions OPTICS EXPRESS Fischer, S., Hallermann, F., Eichelkraut, T., von Plessen, G., Kraemer, K. W., Biner, D., Steinkemper, H., Hermle, M., Goldschmidt, J. C. 2012; 20 (1): 271-282


    We investigate plasmon resonances in gold nanoparticles to enhance the quantum yield of upconverting materials. For this purpose, we use a rate equation model that describes the upconversion of trivalent erbium based upconverters. Changes of the optical field acting on the upconverter and the changes to the transition probabilities of the upconverter in the proximity of a gold nanoparticle are calculated using Mie theory and exact electrodynamic theory respectively. With this data, the influence on the luminescence of the upconverter is determined using the rate equation model. The results show that upconversion luminescence can be increased in the proximity of a spherical gold nanoparticle due to the change in the optical field and the modification of the transition rates.

    View details for Web of Science ID 000300082100044

    View details for PubMedID 22274350

  • Optical Simulation of Bifacial Solar Cells PROCEEDINGS OF THE 2ND INTERNATIONAL CONFERENCE ON CRYSTALLINE SILICON PHOTOVOLTAICS (SILICONPV 2012) Frank, J., Ruediger, M., Fischer, S., GOLDSCHMIDT, J. C., Hermle, M. 2012; 27: 300-305
  • Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients JOURNAL OF APPLIED PHYSICS Fischer, S., Steinkemper, H., Loeper, P., Hermle, M., Goldschmidt, J. C. 2012; 111 (1)

    View details for DOI 10.1063/1.3674319

    View details for Web of Science ID 000299127200009

  • Experimental analysis of upconversion with both coherent monochromatic irradiation and broad spectrum illumination SOLAR ENERGY MATERIALS AND SOLAR CELLS Goldschmidt, J. C., Fischer, S., Loeper, P., Kraemer, K. W., Biner, D., Hermle, M., Glunz, S. W. 2011; 95 (7): 1960-1963
  • Enhancement of silicon solar cell efficiency by upconversion: Optical and electrical characterization JOURNAL OF APPLIED PHYSICS Fischer, S., GOLDSCHMIDT, J. C., Loeper, P., Bauer, G. H., Brueggemann, R., Kraemer, K., Biner, D., Hermle, M., Glunz, S. W. 2010; 108 (4)

    View details for DOI 10.1063/1.3478742

    View details for Web of Science ID 000281857100144

  • Advanced Upconverter Systems with Spectral and Geometric Concentration for high Upconversion Efficiencies COMMAD: 2008 CONFERENCE ON OPTOELECTRONIC AND MICROELECTRONIC MATERIALS & DEVICES Goldschmidt, J. C., Loeper, P., Fischer, S., Janz, S., Peters, M., Glunz, S. W., Willeke, G., Lifshitz, E., Kraemer, K., Biner, D. 2008: 307-311