Dr. Harshal Agrawal Profile

Dr. Harshal Agrawal

at AMOLF

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Publications (2)

Proceedings Article | 1 April 2020 Presentation

Strong directional CdSe/ZnS core-shell quantum dot emission for tetracene/Si singlet-triplet down-conversion photovoltaics (Conference Presentation)

Tom Veeken, Benjamin Daiber, Harshal Agrawal, Albert Polman, Bruno Ehrler

Proceedings Volume 11366, 113660K (2020) https://doi.org/10.1117/12.2555007

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In this work, we investigate a tetracene/Si singlet-triplet down-conversion solar cell geometry in which we control the directional emission of quantum dots (QDs). In this system, photons in the visible range (450-550 nm) excite high-energy singlet-excitons in tetracene that rapidly convert into two triplet-excitons at about half the energy. The triplet-exciton energy is transferred to the QDs that subsequently emit at 1000-1100 nm. A significant loss channel is the QD emission that is directed upwards, so anisotropic downward emission into the underlying Si cell is essential. Here, we demonstrate directional light emission of CdSe/ZnS core-shell quantum dots (QDs) coupled to Si Mie resonators fabricated on a Si solar cell. By varying the shape and size of the Mie resonator, interference of the electric and magnetic multipolar modes supported by the resonator is controlled. Placing the QDs in the near-field of the resonator enables efficient coupling of the QD transition dipole with these multipolar modes. Using numerical FDTD calculations we show that the QD emission is efficiently directed into the solar cell. We fabricate nanostructures on a Si substrate using electron-beam lithography and reactive-ion etching. Using soft-stamp imprinting we selectively print CdSe/ZnS QDs (peak emission 800 nm) on top of the nanostructures. We then map the QD emission anisotropy for different Mie resonator sizes, using photoluminescence spectroscopy. Photoluminescence lifetimes show a systematic increase from 7 ns to 17 ns, for Si nanocylinder diameter from 200 to 425 nm (cylinder height 125 nm), consistent with the varying nanostructure resonances as found in FDTD simulations. The anisotropic downward emission demonstrated in this work of QDs coupled to Si nanostructures can enhance the efficiency of a tetracene/Si down-conversion system. Moreover, this work can impact a broad range of other applications in which directional emission is relevant, in solid-state lighting, integrated optics, and photovoltaics.

Proceedings Article | 1 April 2020 Presentation

Phase-resolved surface plasmon scattering probed by cathodoluminescence holography (Conference Presentation)

Albert Polman, Nick Schilder, Harshal Agrawal, Erik Garnett

Proceedings Volume 11345, 113450R (2020) https://doi.org/10.1117/12.2556240

KEYWORDS: Spiral phase plates, Scattering, Laser scattering, Holography, Surface plasmons, Plasmonics, Plasmons, Spectroscopy, Electron beams, Spatial resolution

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Cathodoluminescence spectroscopy (CL) is a unique technique to probe optical modes at the nanoscale. The electric field surrounding a 10-30 keV electron beam dynamically polarizes matter, creating optical excitations over the 0-10 eV spectral range, that are then detected in the far field. CL can measure the dispersion of plasmonic and dielectric nanostructures at deep-subwavelength spatial resolution. So far, CL has probed the angle-dependent spectrum and polarization of nanoscale emitters. However, detecting the phase of the emitted plasmon scattering wavefronts has remained elusive. Here, we introduce Fourier-transform CL holography as a method to determine the far-field phase distribution of scattered plasmonic fields. To do so, we measure the interference between two fields: (1) the electron-induced CL emitted by a plasmonic nanoscatterer and (2) a broadband reference field created by transition radiation induced by the same electron. From the angular interference patterns we directly reconstruct angle-resolved phase and intensity distributions. Taking the 6 (x-y-z) plasmonic electric and magnetic dipoles as a complete orthogonal set of scatterers we directly derive from the amplitude and phase data the relative strength and phase of all scattering dipoles, as they are excited by electron-beam. We investigate the resonant scattering of 30 keV electron-beam excited surface plasmon polaritons (SPPs) off single-crystalline Ag nanocubes and find dominant scattering from the z-oriented electric dipole plasmon. In contrast, SPP scattering from nanoscale holes made in a Ag film induces an in-plane x-oriented electric dipole with the concomitant y-oriented magnetic dipole; both interfering in the far field creating a strongly beamed plasmon scattering distribution. Using a newly developed CL energy-momentum spectroscopy configuration we derive the phase of scattered fields as a function of frequency. The data are fully consistent with the plasmon polariton dispersion and the pi phase flip across the scattering resonance is directly observed in the measured phase fronts. Fourier-transform CL holography opens up a new world of coherent light scattering and surface wave studies at nanoscale spatial resolution. It also opens up novel ways to investigate the temporal and spatial coherence of electron beam wavefonts and addresses fundamental questions regarding the collapse of the electron wavefunction as it excites surface plasmons.

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