Fig. 1. Au-covered Si gratings used for narrowband NIR photodetection. (a) Schematic of the proposed Au/Si nanojunction. Surface localized plasmonic resonance supported by the Au/Si gratings is plotted with an overlap to the gratings, which is expected to improve the surface sensing. The in situ photoelectric conversion concept is also indicated as a unique property in this Au/Si nanojunction platform. (b) Schematic of the energy band structure of the Au/Si Schottky junction indicating two photodetection mechanisms. (c) Photograph of the fabricated PD integrated with microfluidics. (d) An AFM image of the fabricated Au/Si gratings with a period of $1\mu \mathrm{m}$. (e) Numerical simulation of the electrical field distribution at a wavelength of $\lambda =1550\mathrm{nm}$ at an incident angle of 3 deg. Grating period $P=1000\mathrm{nm}$, width $W=400\mathrm{nm}$, height $H=50\mathrm{nm}$, thickness of Au film $t_{\mathrm{Au}}=50\mathrm{nm}$. (f) The s-SNOM mapping results.

Fig. 2. Optical characterization of Au/Si gratings. (a) Simulated absorption spectra for $P=900$, 1100, 1300, 1500, and 1700 nm. $H=28\mathrm{nm}$, $W=550\mathrm{nm}$, and $t_{\mathrm{Au}}=60\mathrm{nm}$. (b) Maximum amplitude variation of a normalized Lorentz-type on-resonance spectrum with different FWHMs, assuming a same resonance shift of 2 nm. The insets show the cases of $\mathrm{FWHM}=5$ and 150 nm. (c), (d) Calculated and normalized measured absorption spectra for $H=20$, 40, and 80 nm. $P=900\mathrm{nm}$, $W=500\mathrm{nm}$, and $t_{\mathrm{Au}}=60\mathrm{nm}$. (e), (f) Calculated and measured absorption spectra at different incidence angles. $P=900\mathrm{nm}$, $H=28\mathrm{nm}$, $W=550\mathrm{nm}$, and $t_{\mathrm{Au}}=60\mathrm{nm}$. (g) Calculated absorption spectra mimicking the wavelength-scanning measurement process in the cases of different passband linewidth settings of the AOTF. $P=900\mathrm{nm}$, $H=28\mathrm{nm}$, $W=477\mathrm{nm}$, and $t_{\mathrm{Au}}=60\mathrm{nm}$. The incident angle is 45 deg. (h) Measured absorption spectra with both an NKT supercontinuum laser source (12 nm linewidth) and a Santec tunable laser (0.03 nm linewidth).

Fig. 3. Optoelectronic characterization of the proposed narrowband PDs. (a) Power-dependent photocurrent of the on-resonance device. The incident beam (launched by a 1342 nm laser) was fixed at an incident angle of 28.7 deg. The inset is the output $I-V$ curves measured under dark and light conditions. (b) Time-dependent photoelectric responses under different laser illuminations. The red line is the current, and the blue line is the driving signal. The inset shows the absorption spectra of Au/Si gratings, planar Au/glass at normal incidence, and a 45 deg incident angle for comparison. (c) Simulated and measured light absorption spectra of the associated Au/Si gratings at normal incidence and a 45 deg incident angle. (d) The responsivity of the device shown in (c).

Fig. 4. Sensing performance of the narrowband plasmonic PD. (a) Calculated absorption spectra in the case of bulk sensing. $P=900\mathrm{nm}$, $W=477\mathrm{nm}$, $H=28\mathrm{nm}$, and $t_{\mathrm{Au}}=60\mathrm{nm}$. (b) Resonance shift and absorption change of the results in (a) in the wavelength and amplitude interrogations, respectively. (c) Real-time output currents of two sensors with $H=20\mathrm{n}\mathrm{m}$ (black line) and 80 nm (red line) in glucose solution sensing. (d) Calculated absorption spectra in the case of surface sensing. (e) Resonance shift and absorption change of the results in (c) in the wavelength and amplitude interrogations, respectively. (f) Calculated absorption change of two sensors with $H=20\mathrm{nm}$ (blue dots) and 80 nm (red dots) in surface sensing.

Fig. 5. Rabbit IgG detection experiment. (a) Schematic of plasmonic sensor surface modification and rabbit IgG detection process. (b) Real-time current output from the sensor with the rabbit IgG solution injection at three concentrations in sequence. The inset shows the noise level during the measurement.