Xinghui Yin

Large-area 3D chiral plasmonic structures

We manufacture large-area plasmonic structures featuring 3-dimensional chirality by colloidal nanohole lithography. By varying the polar rotating speed of the samples during gold evaporation, we can fabricate spiral-type ramp nanostructures. The optical properties show chiroptical resonances in the 100 to 400 THz frequency region (750 to 3000 nm), with circular dichroism values of up to 13%. Our method offers a simple low-cost manufacturing method of cm(2)-sized chiral plasmonic templates for chiroptical applications such as stereochemical enantiomer sensors.

Interpreting Chiral Nanophotonic Spectra: The Plasmonic Born–Kuhn Model

One of the most intuitive ways to classically understand the generation of natural optical activity in chiral media is provided by the coupled oscillator model of Born and Kuhn consisting of two identical, vertically displaced, coupled oscillators. We experimentally realize and discuss its exact plasmonic analog in a system of corner-stacked gold nanorods. In particular, we analyze the arising circular dichroism and optical rotatory spectra in terms of hybridized electromagnetic modes and retardation. Specifically, we demonstrate how tuning the vertical distance between the nanorods can lead to a selective excitation of the occurring bonding and antibonding chiral plasmonic modes.

A Switchable Mid‐Infrared Plasmonic Perfect Absorber with Multispectral Thermal Imaging Capability

A switchable perfect absorber with multispectral thermal imaging capability is presented. Aluminum nanoantenna arrays above a germanium antimony telluride (GST) spacer layer and aluminum mirror provide efficient wavelength-tunable absorption in the mid-infrared. Utilizing the amorphous-to-crystalline phase transition in GST, this device offers switchable absorption with strong reflectance contrast at resonance and large phase-change-induced spectral shifts.

Active Chiral Plasmonics

Active control over the handedness of a chiral metamaterial has the potential to serve as key element for highly integrated polarization engineering approaches, polarization sensitive imaging devices, and stereo display technologies. However, this is hard to achieve as it seemingly involves the reconfiguration of the metamolecule from a left-handed into a right-handed enantiomer and vice versa. This type of mechanical actuation is intricate and usually neither monolithically realizable nor viable for high-speed applications. Here, enabled by the phase change material Ge3Sb2Te6 (GST-326), we demonstrate a tunable and switchable mid-infrared plasmonic chiral metamaterial in a proof-of-concept experiment. A large tunability range of the circular dichroism response from λ = 4.15 to 4.90 μm is achieved, and we experimentally demonstrate that the combination of a passive bias-type chiral layer with the active chiral metamaterial allows for switchable chirality, that is, the reversal of the circular dichroism sign, in a fully planar, layered design without the need for geometrical reconfiguration. Because phase change materials can be electrically and optically switched, our designs may open up a path for highly integrated mid-IR polarization engineering devices that can be modulated on ultrafast time scales.

Formation of chiral fields in a symmetric environment

Chiral fields, i. e., electromagnetic fields with nonvanishing optical chirality, can occur next to symmetric nanostructures without geometrical chirality illuminated with linearly polarized light at normal incidence. A simple dipole model is utilized to explain this behavior theoretically. Illuminated with circularly polarized light, the chiral near-fields are still dominated by the distributions found for the linear polarization but show additional features due to the optical chirality of the incident light. Rotating the angle of linear polarization introduces more subtle changes to the distribution of optical chirality. Using our findings, we propose a novel scheme to obtain chiroptical far-field response using linearly polarized light, which could be utilized for applications such as optical enantiomer sensing.

Quantitative Angle-Resolved Small-Spot Reflectance Measurements on Plasmonic Perfect Absorbers: Impedance Matching and Disorder Effects

Plasmonic devices with absorbance close to unity have emerged as essential building blocks for a multitude of technological applications ranging from trace gas detection to infrared imaging. A crucial requirement for such elements is the angle independence of the absorptive performance. In this work, we develop theoretically and verify experimentally a quantitative model for the angular behavior of plasmonic perfect absorber structures based on an optical impedance matching picture. To achieve this, we utilize a simple and elegant k-space measurement technique to record quantitative angle-resolved reflectance measurements on various perfect absorber structures. Particularly, this method allows quantitative reflectance measurements on samples where only small areas have been nanostructured, for example, by electron-beam lithography. Combining these results with extensive numerical modeling, we find that matching of both the real and imaginary parts of the optical impedance is crucial to obtain perfect absorption over a large angular range. Furthermore, we successfully apply our model to the angular dispersion of perfect absorber geometries with disordered plasmonic elements as a favorable alternative to current array-based designs.

Mid-infrared Fourier-transform spectroscopy with a high-brilliance tunable laser source: investigating sample areas down to 5 μm diameter

We demonstrate highly sensitive infrared spectroscopy of sample volumes close to the diffraction limit by coupling a femtosecond fiber-feedback optical parametric oscillator (OPO) to a conventional Fourier-transform infrared (FTIR) spectrometer. The high brilliance and long-term stable infrared radiation with 1e(2)-bandwidths up to 125 nm is easily tunable between 1.4 μm and 4.2 μm at 43 MHz repetition rate and thus enables rapid and low-noise infrared spectroscopy. We demonstrate this by measuring typical molecular vibrations in the range of 3 μm. Combined with surface-enhanced infrared spectroscopy, where the confined electromagnetic near-fields of resonantly excited metal nanoparticles are employed to enhance molecular vibrations, we realize the spectroscopic detection of a molecular monolayer of octadecanethiol. In comparison to conventional light sources and synchrotron radiation, our compact table-top OPO system features a significantly improved performance making it highly suitable for rapid analysis of minute amounts of molecular species in life science and medicine laboratories.

Beam switching and bifocal zoom lensing using active plasmonic metasurfaces

Compact nanophotonic elements exhibiting adaptable properties are essential components for the miniaturization of powerful optical technologies such as adaptive optics and spatial light modulators. While the larger counterparts typically rely on mechanical actuation, this can be undesirable in some cases on a microscopic scale due to inherent space restrictions. Here, we present a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge3Sb2Te6. In particular, we demonstrate beam switching and bifocal lensing, thus, paving the way for a plethora of active optical elements employing plasmonic metasurfaces, which follow the same design principles.

Design of plasmonic nanostructures for chiral sensing

The chiral nature of many biomolecules would make highly sensitive enantiomer detection desirable. Utilizing the concept of optical chirality in nanophotonics and plasmonics we numerically analyze and compare different structures to meet the design criteria for increased sensitivity of such sensors. We demonstrate the differences between planar structures that are easily fabricated and three-dimensional designs that are capable of higher enhancement of optical chirality. Additionally, a sensor scheme combining both enantiomers of a chiral plasmonic nanostructure is proposed and investigated.