Tailor-made unitary operations using dielectric metasurfaces.

Qubit operation belonging to unitary transformation is the fundamental operation to realize quantum computing and information processing. Here, we show that the complex and flexible light-matter interaction between dielectric metasurfaces and incident light can be used to perform arbitrary U(2) operations. By incorporating both coherent spatial-mode operation together with two polarizations on a single metasurface, we further extend the discussion to single-photon two-qubit U(4) operations. We believe the efficient usage of metasurfaces as a potential compact platform can simplify optical qubit operation from bulky systems into conceptually subwavelength elements.

Characterization of thermal bump due to surface plasmon resonance.

Surface plasmon resonance (SPR) has found wide applications in sensing down to molecular level due to its extreme sensitivity to change of dielectric properties. An unavoidable effect in SPR is surface deformation (thermal bump) due to local heating by incident laser light used in SPR. In addition, changes in the reflection phase from the metal film used in SPR could also contribute to the SPR signal, and thus proper handling of the SPR signal is very important in order to broaden the potential applications of SPR. Here we report a simple Fabry Perot (FP) interference technique for measuring, simultaneously, the thermal bump height as well as the reflection phase shift of gold film used in SPR. We find that the shift of the FP signal is dominated by the effect of the thermal bump while it is small for the effect of the reflection phase shift due to change of dielectric property of the metal. To support our experimental results, we have also performed model simulation for the SPR system and obtain good agreement with the experiment. As both amplitude and phase can be measured, our method could lead to better characterization of SPR and can also be applied to the study of active metasurfaces under external excitation.

Characterization of free-standing 1D photonic crystals using an effective medium approach.

Photonic crystals (PCs) are usually fabricated on bulk substrates which break the symmetry of the PC system for incidence from either side of the PCs. Here we report the fabrication of a free-standing 1D layered dielectric PC by using a two-beam holographic interference method. The free-standing PC exhibits distinct photonic bandgaps as well as Fabry-Perot oscillations in the photonic bands. Furthermore, we show that the PC can be modeled by an effective medium approach and obtain the reflection phase for the photonic bands of the PC. We have also performed full-wave simulations for the PC and obtained very good agreement with the experiment. The free-standing PC enables a better comparison between experiment and simulation, and importantly, it is flexible enabling new applications for PCs.

Correction of numerical aperture effect on reflection phase measurement using a thick-gap Fabry-Perot etalon.

We propose a method for the measurement of the reflection phase using a thick-gap Fabry-Perot (FP) etalon interferometry technique with correction for the numerical aperture effect of the optical setup. The setup is first calibrated using a known sample by comparing the reflectance from a two-beam interference model for the FP etalon with experimental data. We then apply the correction to a sample of interest and obtain the reflection phase of the sample. Our method can be used to measure the reflection phase of a small sample and could lead to practical applications in optical characterization of metamaterials. Moreover, the principle of our approach could be generalized to other systems in the correction of numerical aperture effect due to microscopic objectives.

Measurement of reflection phase using thick-gap Fabry-Perot etalon.

We report measurement of the reflection phase of a dielectric (glass)/titanium (Ti) surface in the visible wavelength using a thick-gap Fabry-Perot (FP) interferometry technique. Using a two-beam interference model for the reflection peaks and troughs of the FP etalon, we obtain the air-gap spacing of the etalon and, more importantly, the reflection phase of the etalon substrate. We find systematic dependence of the as-measured reflection phase on the air-gap spacing due to the numerical aperture effect of the measuring objective. However, the relative reflection phase of Ti with respect to glass is independent of the air-gap spacing. As a demonstration of our approach in the optical characterization of small metamaterial samples, we also measure the reflection phase of a micron-sized 2D Au sawtooth nanoarray. The experiment is in good agreement with the model simulation.

Polarization coincidence images from metasurfaces with HOM-type interference.

Metasurfaces provide a promising route for structuring light and generating holograms with designed amplitude, phase, and polarization profiles, leading to a versatile platform for integrating and constructing optical components beyond the conventional ones. At the same time, incorporating coincidence in imaging allows a high signal-to-noise ratio for imaging in very low light levels. As beneficial from the recent development in both metasurfaces and single-photon avalanche diode (SPAD) cameras, we combine the polarization-sensitive capability of metasurfaces with Hong-Ou-Mandel (HOM)-type interference in generating images with tailor-made two-photon interference and polarization coincidence signatures. By using orthogonal linear-polarized photons as incidence, correlated, anticorrelated, and uncorrelated polarization coincidence features can be observed within the same image from the pairwise second-order coherence statistics across different pixels of the image. Our work adds polarization to the demonstrated amplitude and phase sensitivity in the domain of "HOM microscopy" and can be useful for biological and security applications.