Abstract By designing a plasmonic waveguide–slit structure (a nanoslit etched in a silver nanowire) on a silver substrate, an ultrahigh Purcell factor and ultralarge figure of merit (FOM) are numerically predicted. Because of the large field enhancement (>150 times the incident field) and the ultrasmall optical volume (V ≈ 2 × 10−5λ3) of the resonant mode in the metallic nanoslit, the simulations show that the Purcell factor in the system can reach up to FP = 1.68 × 105, which is more than ten times the maximum Purcell factor in previous work (by placing metallic nanoparticles on a metal surface with a nanogap). Because of the utilization of a silver substrate rather than the common dielectric substrate, the mode cutoff of the surface plasmon polariton (SPP) waveguide mode is completely eliminated, which provides a large selection range of the nanowire radii to support the resonant mode in the nanoslit. Moreover, the SPP propagation length is significantly increased by more than 30 times. As a result, an ultralarge FOM of 1.40 × 107 is obtained, which is more than 80 times the maximum FOM in previous work where the metallic nanowire is placed on or surrounded by dielectric materials.
Linear-optical logic gates have the potential to be the bases of the next-generation information technology (IT) because of the low power consumption and rapid response. This study proposes a general theoretical model to obtain the optimal solutions for linear-optical logic gates. All common logic gates (AND, OR, NOT, NAND, NOR, XOR, and XNOR) are experimentally demonstrated with one single sample structure based on ultracompact plasmonic waveguides. The measured intensity contrast ratio between the output-logic “1” and “0” states reaches 28 dB for the OR gate and 9.4 dB for the AND gate, thereby approaching the theoretical maximum of infinity and 9.5 dB, respectively. The proposed logic gates provide uniform output intensities for identical output logics when the input logics are different. The measured intensity discrepancies are below 1% for the three output-logic “1” states of the OR gate and the three output-logic “0” states of the AND gate. This phenomenon is favored in practical applications and the cascading of logic gates. The proposed universal linear-optical logic gate with maximal intensity contrast ratios may find important future applications in the field of IT.
Polarization beam splitters (PBSs) are one of the key components in the integrated photonic circuits. To increase the integration density, various complex hybrid plasmonic structures have been numerically designed to shrink the footprints of the PBSs. Here, to decrease the complexity of the small hybrid structures and the difficulty of the hybrid micro-nano fabrications, the radiation losses are utilized to experimentally demonstrate an ultra-small, broadband, and efficient PBS in a simple bending hybrid plasmonic waveguide structure. The hybrid plasmonic waveguide comprising a dielectric strip on the metal surface supports both the transverse-magnetic (TM) and transverse-electric (TE) waveguide modes. Because of the different field confinements, the TE waveguide mode has larger radiation loss than the TM waveguide mode in the bending hybrid strip waveguide. Based on the different radiation losses, the two incident waveguide modes of orthogonal polarization states are efficiently split in the proposed structure with a footprint of only about 2.2 × 2.2 μm2 on chips. Since there is no resonance or interference in the splitting process, the operation bandwidth is as broad as Δλ = 70 nm. Moreover, the utilization of the strongly confined waveguide modes instead of the bulk free-space light (with the spot size of at least a few wavelengths) as the incident source considerably increases the coupling efficiency, resulting in a low insertion loss of <3 dB.
High-sensitivity plasmonic refractive index sensors show great applications in the areas of the biomedical diagnostics, healthcare, food safety, environmental monitoring, homeland security, and chemical reaction. However, the unstable and complicated environments considerably limit their practical applications. By employing the independent double Fano resonances in a simple metallic grating, we experimentally demonstrated a self-reference plasmonic sensor, which significantly reduces the error contributions of the light intensity fluctuations in the long-distance propagation and local temperature variations at the metallic grating, and the detection accuracy is guaranteed. The numerical simulation shows that the two Fano resonances have different originations and are independent with each other. As a result, the left Fano resonance is quite sensitive to the refractive index variations above the metal surface, while the right Fano resonance is insensitive to that. Experimentally, a high figure of merit (FOM) of 31 RIU-1 and FOM* of 860 RIU-1 are realized by using the left Fano resonance. More importantly, by using the right Fano resonance as a reference signal, the influence of the light intensity fluctuations and local temperature variations are monitored and eliminated in the experiment. This simple self-reference plasmonic sensor based on the double Fano resonances may find important applications in high-sensitive and accurate sensing under the unstable and complicated environments, as well as multi-parameter sensing.