Passive plasmonic filters in metallic slot waveguides

Summary form only given. In order to modify and tune the transmission spectrum through plasmonic waveguides, waveguide-integrated passive components have to be developed. For instance, high Q-factor band-pass filters can be useful to filter a transmitted signal into different spectral bands in order to realize wavelength division multiplexing. Also passive filters can be applied for routing surface plasmon polaritons (SPPs) through a circuit of waveguides and can also play an important role for sensing applications.Here we present the numerical and experimental demonstration of plasmonic Bragg filters and resonators inside metal-insulator-metal (MIM) waveguides [1]. The presented filters and resonators are fabricated using standard top down lithography methods. The metallic slot waveguide consists of a gold/HSQ/gold layer stack. Light-SPP coupling is realized by two dry-etched subwavelength slits (I and D on Fig. 1a for injection and detection slit). The optical properties of this waveguide were experimentally assessed by studying the SPP interference caused by SPP reflection at both injection and detection slit. The filter and resonator structures are etched in the HSQ layer by ion etching (F on Fig. 1a). The optical band gap of the integrated Bragg filters is optimized by finite-difference time domain simulations and experimentally observed by transmission measurements. The optical properties are experimentally investigated as a function of the grating pitch and the number of grating periods.By introducing a defect inside the Bragg reflector, we create a resonant nanocavity. Depending on the length of the defect compared to the pitch of the Bragg grating, the resonance wavelength of the cavity can be positioned in the middle of the stop-band of the reflector, leading to a very strong resonance with high quality factor. For the reflectors, a perfect symmetric Bragg grating was used at both sides of the defect. In Fig. 1a, a schematic representation of the structur- can be found. In Fig. 1b and 1c, the measured and simulated transmission spectra of two resonators with cavity lengths of 150 and 220 nm are shown together with the transmission spectrum of the Bragg grating without defect structure, serving as a reference. The experiments only have a very weakly pronounced edge around 750 nm due to high losses inside the metal parts of the waveguide for smaller wavelengths. The low-energy edge of the optical band gap is very clearly visible and lies just below a wavelength of 900 nm (black curve). When introducing a 150 nm wide defect cavity in the middle of the 200 nm pitch Bragg grating, a resonant mode is created with a resonant wavelength close to the highenergy edge of the optical band gap, as is shown by the red curves in Fig. 6(a) and Fig. 6(b). The quality-factor of the resonance at a wavelength of 775 nm is determined to be 31, compared to 33 in the simulation. We show that the Q-factor can be increased by increasing the Bragg mirror etch depth or increasing the number of periods of the Bragg mirror. By changing the cavity length from 150 nm to a length of 220 nm, the resonance is shifted through the entire optical band gap from 775 to 870 nm, as shown by the green curves. We experimentally show that the resonance can be tuned throughout the entire optical band gap.