Zhiyuan Fan

Experimental demonstration of the microscopic origin of circular dichroism in two-dimensional metamaterials

Optical activity and circular dichroism are fascinating physical phenomena originating from the interaction of light with chiral molecules or other nano objects lacking mirror symmetries in three-dimensional (3D) space. While chiral optical properties are weak in most of naturally occurring materials, they can be engineered and significantly enhanced in synthetic optical media known as chiral metamaterials, where the spatial symmetry of their building blocks is broken on a nanoscale. Although originally discovered in 3D structures, circular dichroism can also emerge in a two-dimensional (2D) metasurface. The origin of the resulting circular dichroism is rather subtle, and is related to non-radiative (Ohmic) dissipation of the constituent metamolecules. Because such dissipation occurs on a nanoscale, this effect has never been experimentally probed and visualized. Using a suite of recently developed nanoscale-measurement tools, we establish that the circular dichroism in a nanostructured metasurface occurs due to handedness-dependent Ohmic heating.

Manifestations of photon acceleration in semiconductor metasurfaces (Conference Presentation)

The metasurface under study was engineered to enhance the local fields, which is crucial for the efficiency of the nonlinear photon conversion. In our design of the metasurface, we make use of high-Q collective resonances common to regular arrays of semiconductor particles, as verified by MIR FTIR spectroscopy. We facricate 600-nm-thick silicon rectangles situated on a sapphire substrate. In order to demonstrate power- dependent blue-shifting of THG, we focused a 200 ± 30 fs MIR laser pulses centered at λ = 3.62 μm, with a variable non-destructive fluence in the 1 < F < 6 mJ/cm2 range, onto the metasurface to achieve 5 < I < 30 GW/cm2 intensity. The advantage of operating in the MIR regime is that the refractive index change scales as ∆n(t) ∝ −N(t)λ^2, which is crucial for photon acceleration; here, N(t) is the time-dependent FC density. The concept of photon-acceleration-induced spectral shifting of the nonlinearly upconverted signal in a metasurface- based semiconductor cavity is as follows. MIR photons interact with, and get trapped by, the metasurface. As FCs are generated by four-photon absorption in silicon, the resonant frequency of the metasurface blue-shifts, and the frequency of the trapped photons follows. Accelerated MIR photons then upconvert via the standard χ(3) nonlinear process, resulting in the observed blue-shifting of the third harmonic generation (THG). As a result, the spectral peak and width of the THG light generated in the metasurface can be controlled by incident fluence. The central THG wavelength can be blue-shifted by more than 30 nm, enabling harmonics generation with center frequencies of up to ≈ 3.1ω. In contrast, the same measurements performed in unstructured silicon films yield no apparent spectral modifications to THG. A common belief is that the resonant enhancement of the THG must be accompanied by spectral narrowing; in contrast, here, due to photon acceleration, we observe spectral broadening of the resulting THG spectrum by approximately 50%. We connect the observed blue shift and broadening of the THG spectrum with the time-dependent nature of the complex eigenfrequency of the mode. Qualitative agreement is reached between the experimental data and the calculations based on a coupled mode theory with the eigenfrequency ωR(t) and damping factor γR(t) being driven by the pump pulse through free carrier generation. We find photon acceleration in semiconductor metasurfaces a promising tool for active control over the frequency of light in prospective nanophotonics devices.