@article{lin_ghaffari_gu_2025, title={Chiral Metasurfaces with Multidimensional Tunability for Optical Chiral States}, volume={10}, url={https://doi.org/10.1002/adom.202501859}, DOI={10.1002/adom.202501859}, abstractNote={Abstract Chiroptics has gained significant interest due to its fundamental science and advanced applications in optical communications, quantum processing, and sensing. While optical bound states in the continuum (BICs) provide a powerful framework to generate optical chiral states, existing BIC platforms often suffer from limited tunability. In this study, a novel metasurface featuring chiral structures atop a thin film layer is demonstrated, supporting strong near‐ and far‐field chirality, characterized using the localized optical chirality formula and the generalized chiroptics theory. By preserving or breaking the rotational symmetry, the transition between the BIC‐associated vortex center and circularly polarized states is investigated. Notably, the upward and downward far‐field radiation remains identical, independent of geometric symmetry. More importantly, through in situ rotation of the upper‐layer structure within the meta‐atom, precise, all‐directional control over C points in momentum space around the Γ point is achieved, forming a Möbius‐like band. These findings provide a new strategy for tuning optical chiral states in a compact form, with potential implications for chiral sensing, structured light generation, and next‐generation optoelectronic devices.}, journal={Advanced Optical Materials}, author={Lin, Dayang and Ghaffari, Abbas and Gu, Qing}, year={2025}, month={Oct} }
 @article{ghaffari_kashani_li_gkoupidenis_riehn_gu_2025, title={Plasmonic Double-Hole Bull’s Eye Nanoantenna for Far-Field Polarization Control}, volume={5}, url={https://doi.org/10.1021/acsnanoscienceau.5c00031}, DOI={10.1021/acsnanoscienceau.5c00031}, abstractNote={Plasmonic polarization conversion offers significant advantages over conventional methods, including a smaller device footprint and easier integration into photonic circuits. In this work, we numerically and experimentally investigate the polarization conversion properties of a plasmonic double-hole structure surrounded by circular nanograting, i.e., a bull's eye antenna. Using a combination of polarimetric imaging via back focal plane (BFP) microscopy and Stokes parameter analysis, we demonstrate the functionality of our structure as a miniature on-chip polarization converter. Our results show that this nanostructure enables complex polarization transformations, including converting linear to circular polarization and vice versa. Polarization conversion efficiency is found to be dependent on the periodicity of the circular gratings and is particularly pronounced in the central region of Fourier space. Moreover, strong asymmetric scattering leads to distinctive patterns in the Stokes parameters across various incident polarization states. This work provides insights into the plasmonic manipulation of light polarization at the nanoscale with potential applications in miniature on-chip polarization convertors, polarization-controlled emitters, and advanced sensing technologies.}, journal={ACS Nanoscience Au}, author={Ghaffari, Abbas and Kashani, Somayeh and Li, Jiazhen and Gkoupidenis, Paschalis and Riehn, Robert and Gu, Qing}, year={2025}, month={May} }
 @article{ghaffari_abbas_gu_2024, title={Plasmon near-field coupling and universal scaling behavior in shifted-core coaxial nano-cavity pair}, volume={4}, url={https://doi.org/10.1364/OE.516604}, DOI={10.1364/OE.516604}, abstractNote={We computationally and analytically investigate the plasmon near-field coupling phenomenon and the associated universal scaling behavior in a pair of coupled shifted-core coaxial nano-cavities. Each nano-cavity is composed of an InGaAsP gain medium sandwiched between a silver (Ag) core and an Ag shell. The evanescent coupling between the cavities lifts the degeneracy of the cut-off free transverse electromagnetic (TEM) like mode. The mode splitting of the supermodes is intensified by shifting the metal core position, which induces symmetry breaking. This coupling phenomenon is explained with spring-capacitor analogy and circuit analysis. The numerical simulation results reveal an exponential decay in the fractional plasmon wavelength relative to the ratio of gap distance and core shifting distance, which aligns with the plasmon ruler equation. In addition, by shifting the Ag cores in both cavities toward the center of the coupled structure, the electromagnetic field becomes strongly localized in nanoscale regions (hotspots) in the gain medium between the cavities, thus achieving extreme plasmonic nanofocusing. Utilizing this nanofocusing effect, we propose a refractive index sensor by placing a fluidic channel between the two cavities in close vicinity to the hotspots and reaching the highest sensitivity of ∼700nm/RIU.}, journal={Optics Express}, author={Ghaffari, Abbas and Abbas, Farhat and Gu, Qing}, year={2024}, month={Apr} }
 @article{ghaffari_kashani_do_weninger_riehn_2023, title={A nanophotonic interferometer}, volume={34}, url={https://doi.org/10.1088/1361-6528/acb443}, DOI={10.1088/1361-6528/acb443}, abstractNote={Abstract The transmission of light through sub-wavelength apertures (zero-mode waveguides, ZMW) in metal films is well-explored. It introduces both an amplitude modulation as well as a phase shift to the oscillating electromagnetic field. We propose a nanophotonic interferometer by bringing two ZMW (∼100 nm diameter) in proximity and monitoring the distribution of transmitted light in the back-focal plane of collecting microscope objective (1.3 N.A.). We demonstrate that both an asymmetry induced by the binding of a quantum dot in one of the two ZMW, as well as an asymmetry in ZMW diameter yield qualitatively similar transmission patterns. We find that the complex pattern can be quantified through a scalar measure of asymmetry along the symmetry axis of the aperture pair. In a combined experimental and computational exploration of detectors with differing ZMW diameters, we find that the scalar asymmetry is a monotonous function of the diameter difference of the two apertures, and that the scalar asymmetry measure is higher if the sample is slightly displaced from the focal plane of the collecting microscope objective. An optimization of the detector geometry determined that the maximum response is achieved at an aperture separation that is comparable to the wavelength on the exit side of the sensor. For small separations of apertures, on the order of a quarter of the wavelength and less, the signal is strongly polarization dependent, while for larger separations, on the order of the wavelength or larger, the signal becomes essentially polarization-independent.}, number={18}, journal={Nanotechnology}, author={Ghaffari, Abbas and Kashani, Somayeh and Do, Kevin and Weninger, Keith and Riehn, Robert}, year={2023}, month={Jan} }
 @article{kashani_rech_liu_baustert_ghaffari_angunawela_xiong_dinku_you_graham_et al._2023, title={Exciton Binding Energy in Organic Polymers: Experimental Considerations and Tuning Prospects}, volume={12}, DOI={10.1002/aenm.202302837}, abstractNote={Abstract Discrepancies in reported values of exciton binding energy (E b ) for organic semiconductors (OSs) necessitate a comprehensive study. Traditionally, E b is defined as the difference between the transport gap (E t ) and the optical gap (E opt ). Here, the E b values of PBnDT‐TAZ polymer variants are determined using two commonly employed methods: a combination of ultraviolet photoemission spectroscopy and low‐energy inverse photoemission spectroscopy (UPS‐LEIPS) and solid‐state cyclic voltammetry (CV). E b values obtained by UPS‐LEIPS show low dispersion and no clear correlation with the polymer structure and thedielectric properties. In contrast, CV reveals a larger dispersion (200 meV‐1 eV) and an apparent qualitative E b ‐molecular structure correlation, as the lowest E b values are observed for oligo‐ethylene glycol side chains. This discrepancy is discussed by examining the implications of the traditional definition of E b . Additionally, the impact of both intrinsic and extrinsic factors contributing to the derived experimental values of E t is discussed. The differences in intrinsic and extrinsic factors highlight the context‐dependent nature of measurement when drawing global conclusions. Notably, the observed E b trend derived from CV is not intrinsic to the pure materials but likely linked to electrolyte swelling and associated changes in dielectric environment, suggesting that high‐efficiency single‐material organic photovoltaics with low E b may be possible via high dielectric materials.}, journal={Advanced Energy Materials}, author={Kashani, Somayeh and Rech, Jeromy James and Liu, Tuo and Baustert, Kyle and Ghaffari, Abbas and Angunawela, Indunil and Xiong, Yuan and Dinku, Abay and You, Wei and Graham, Kenneth and et al.}, year={2023}, month={Dec} }
 @article{esfehani_do_kashani_weninger_riehn_2022, title={A nanophotonic interferometer for small particle detection}, DOI={10.1117/12.2634318}, abstractNote={The transmission of light through sub-wavelength apertures (zero-mode waveguides, ZMW) in metal films is wellexplored. It introduces both an amplitude modulation as well as a phase shift to the transmitted oscillating electromagnetic field. We propose a nanophotonic interferometer by bringing two zero-mode waveguides in proximity and monitoring the distribution of light in the back-focal plane of the collecting microscope objective. We demonstrate that both an asymmetry induced by the binding of a quantum dot in one of the two ZMW, as well as a asymmetry in ZMW diameter yield qualitatively similar deflection patterns. Using ZMW pairs with diameter asymmetries, we find that the complex pattern of the transmitted light can be quantified through a scalar measure of asymmetry along the symmetry axis of the aperture pair. We find that this scalar asymmetry is a monotonous function of the diameter difference of the two apertures.}, journal={INTERFEROMETRY XXI}, author={Esfehani, Abbas Ghaffari and Do, Kevin and Kashani, Somayeh and Weninger, Keith and Riehn, Robert}, year={2022}, month={Sep} }