Electron beam probing optical properties of a nanoparticle

Applied Nano-optics

In the Applied Nano-optics group, we focus on controlling and manipulating light using novel optical nanomaterials, such as metals, semiconductors, and two-dimensional materials.
Transmission electron microscopy for nanophotonics

Research in nanophotonics has benefited tremendously from the ability to measure optical properties in the electromagnetic near-field of the nanostructured sample (e.g., the meta-atom described in the Metasurface section below). Perhaps slightly counter-intuitive, one of the most powerful near-field techniques involves using electrons rather than photons to probe the sample. In particular, a technique called electron energy-loss spectroscopy (EELS), which is performed in a transmission electron microscope (TEM), provides the optical near-field properties of the sample with an impressive spatial resolution down to few nanometers. EELS measures the inelastic (energy loss) scattering events suffered by the electron as it interacts with the sample. This signal is directly related to the electromagnetic field in and around the sample. We use EELS performed in a TEM (in particular, the FEI Titans available at DTU CEN) to gain unique insight into the optical properties of nanomaterials, which is otherwise unattainable. 

Adaptive optics with reconfigurable metasurfaces

Metamaterials are a new palette of materials with engineered optical properties not found in nature. While natural materials are composed of atoms, metamaterials are constructed from meta-atoms. These meta-atoms have sizes smaller than the wavelength of visible light and are typically made of metallic or dielectric nanostructures. By controlling the composition and morphology of the meta-atom, the properties of the metamaterial can be designed. The two-dimensional analogue of metamaterials are called metasurfaces. Metasurfaces are made of only a single plane of meta-atoms and are therefore typically very thin (less than few hundreds of nm). Even though they are so thin, metasurfaces can still manipulate the phase, amplitude or polarization of the incident light. These exotic properties have unlocked the possibility of creating flat optics, which refers to making extremely thin optical components, such as lenses or high-resolution colorized surfaces. Our research is focused on exploring new meta-atom designs, which can be reconfigured after fabrication, such as to realize novel adaptive optical metasurfaces (see also this newsletter for the perspective of our research).

Selected recent publications

A. F. Cihan, A. G. Curto, S. Raza, P. G. Kik, and M. L. Brongersma, “Silicon Mie resonators for highly directional light emission from monolayer MoS2”, Nature Photonics 12, 284 (2018).

X. Zhu, M. K. Hedayati, S. Raza, U. Levy, N. A. Mortensen, and A. Kristensen, “Digital resonant laser printing: Bridging nanophotonic science and consumer products”, Nano Today 19, 7 (2018).

A. L. Holsteen, S. Raza, P. Fan, P. G. Kik, and M. L. Brongersma, “Purcell effect for active tuning of light scattering from semiconductor optical antennas”, Science 358, 1407 (2017).

S. Raza, M. Esfandyarpour, A. L. Koh, N. A. Mortensen, M. L. Brongersma, and S. I. Bozhevolnyi, “Electron energy-loss spectroscopy of branched gap plasmon resonators”, Nature Communications 7, 13790 (2016).

Contact

Søren Raza
Assistant Professor
DTU Physics
+4526 99 99 14