Raman optical activity (ROA) is a powerful tool for identifying the absolute conformational information and behavior of chiral molecules in aqueous solutions, but suffers from low sensitivity. Here we report our development of a silicon nanodisk array that tailors a chiral field to significantly increase the interaction between the excitation light and chiral molecules via exploiting a dark mode. Specifically, we used the array with pairs of chemical and biological enantiomers to show >100x enhanced chiral light-molecule interaction with negligible artifacts for ROA measurements. Our silicon nanodisk array opens a cost-effective way for conformational analysis of trace chiral molecules.
Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for vibrational spectroscopy, but is compromised by its low reproducibility, uniformity, biocompatibility, and durability. This is because it depends on “hot spots” for high signal enhancement. Here we report our experimental demonstration of a plasmon-free nanostructure composed of a two-dimensional array of porous carbon nanowires as a SERS substrate for highly sensitive, biocompatible, and reproducible SERS. Specifically, the substrate provides not only high signal enhancement, but also high reproducibility and fluorescence quenching capability. We experimentally demonstrated these excellent properties with various molecules such as rhodamine 6G (R6G), β-lactoglobulin, and glucose.
Erbium-doped waveguide-integrated lasers (EDWLs) play an increasingly important role in optical interconnects, optical communication, and biochemical sensing, due to their advantages of tunable spectral bandwidths, narrow linewidths, and large output powers. However, compared with the near-infrared (near-IR) band, the study of mid-infrared (mid-IR) EDWLs is still in its infancy. In this paper, we theoretically studied an EDWL at a wavelength of 3.6 μm. The model is based on a mid-IR suspended membrane silicon waveguide microring resonator integrated with an Erbium-doped chalcogenide glass thin layer. The designed laser could be fabricated with the combination of a CMOS-compatible silicon chip and Erbium-doped chalcogenide glass deposition through a post-fabrication process, making it be possible for high-volume and low-cost fabrication. We numerically calculated output characteristics of the laser by solving rate equations and a beam propagation equation. Specifically, after optimizing coupling coefficients of the resonator, the output power of the laser can reach 0.25 μW. To further increase the laser slope efficiency, we designed a vertical Fabry- Perot cavity to increase the pump power intensity. Simulated results showed that the laser slope efficiency could be improved by a factor of six. Our study is expected to open an avenue to develop on-chip mid-IR lasers for exploring intriguing on-chip mid-IR applications in biochemical sensing, LiDAR, and nonlinear optics.
Here, we theoretically study a novel multi-mode modulator based on a double-layer graphene-on-silicon waveguide. Our method is based on the exploration of tunable interaction between patterned graphene nanoribbons (GNs) on the surface of a silicon waveguide and zeroth transverse electric (TE0) and first transverse electric (TE1) modes in the waveguide. By adjusting the Fermi level of graphene sheets in the double-layer graphene-on-silicon waveguide, we could simultaneously and separately obtain about pi phase shifts of the TE0 and TE1 modes in the same waveguide. Our study is expected to open an avenue to develop high-density MDM photonics integrated circuits for tera-scale optical interconnects.
Germanium photonic integrated circuits (PICs) have attracted great attention for developing mid-infrared nonlinear optics devices. However, it is challenging to develop a mode division multiplexing germanium PICs due to the strong mode dependence of the group velocity dispersion (GVD) in a germanium waveguide. We design a novel germanium convex waveguide, which has almost the same GVD curve for TE0 and TE1 modes. Flat GVD curves from -1450 ps/nm/km to -850 ps/nm/km are theoretically obtained within a spectral region from 2 µm to 2.5 µm. Our study is expected to open an avenue for exploring unprecedented MDM nonlinear applications.
The chiroptical effect is a property that describes distinct response of matter to light with opposite handedness, which is extensively utilized in stereochemistry, analytical chemistry, metamaterials, and spin photonics. Conventionally, metallic nanostructures have been harnessed to generate a strong chiroptical effect with the assistance of surface plasmon resonance, but they usually suffer from low energy efficiency and large photothermal heat generation due to the high ohmic loss of metallic materials, which severely restricts their practical applications. Here we present a dielectric spiral nanoflower with a giant chiroptical effect produced by magnetic resonance. We theoretically predicted the giant chiroptical effect of the spiral nanoflower by numerical simulations and analyzed its underlying physics by combination of a multipole expansion method. Based on the theoretical design, we experimentally fabricated the spiral nanoflower and demonstrated its strong chiroptical effect by characterizing its circular intensity difference (CID). The largest-to-date CID of 35% is demonstrated. The magnetic quadrupole interference within the spiral nanoflower was also clarified by experimentally tailoring its magnetic quadrupole interference. Our work is expected to overcome the limitation of conventional metallic platforms and pave the way toward the development of various highly efficient and thermostable chiroptical devices and applications.
Mid-infrared (MIR) resonators with high quality (Q) factors play crucial roles in a variety of applications in nonlinear optics, lasing, biochemical sensing, and spectroscopy by virtue of their features of long photon lifetime as well as strong field confinement and enhancement. Previously, such devices have been mainly studied on silicon integration platforms while the development of high-Q germanium resonators is still in its infancy due to quality limitations of current germanium integration platforms. Compared with silicon, germanium possesses a number of advantages for MIR applications, such as a wider transparency window (2 - 15 µm), a higher refractive index (~4), and a higher third-order nonlinear susceptibility. Here we present our experimental demonstration of two types of MIR high-Q germanium resonators, namely, a microring resonator and a photonic crystal nanobeam cavity. A maximum Q factor of ~57,000 is experimentally realized, which is the highest to date on germanium platforms. Moreover, we demonstrate a monolithic integration of the high-Q germanium resonators with suspended-membrane waveguides and focusing subwavelength grating couplers. Our resonators pave a new avenue for the study of on-chip light-germanium interactions and development of on-chip MIR applications in sensing and spectroscopy.
We review our recent work on waveguide grating couplers, including an apodized grating coupler with engineered
coupling strength to achieve Gaussian-like output profile, which greatly improves the fiber-chip coupling efficiency. We
will also discuss a new class of grating couplers involving the use of sub-wavelength nanostructures to engineer the
optical properties. Effective medium theory can be used in the design of sub-wavelength structures, which, when
properly engineered, can offer broadband coupling and polarization independence. Other applications of waveguide
gratings, for example bi-wavelength two dimensional gratings coupler for (de-)multiplexing two different wavelengths,
fiber-waveguide hybrid lasers and mid-infrared grating couplers on silicon-on-sapphire wafer will also be briefly
discussed.
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