## 2 Materials and Methods  
Ag films were deposited by vacuum evaporation onto the hypotenuse surface of right-angle quartz prism (\(25\times 35\) cm\({}^{2}\)) at room temperature. The evaporation was performed in a diffusion-pumped vacuum deposition system (VPC-260, ULVAC, Chigasaki, Japan) using 99.9% pure Ag wires on a tungsten boat at a pressure of \(4.0\times 10^{-3}\) Pa and a deposition rate of 0.30 nm/s to obtain 53-nm-thick film. A conducting wire was attached to the deposition surface using silver paste. The incident angle dependence of the SPP resonance spectra of Ag in air was measured in the Kretschmann configuration (by attenuated total reflection) by varying the incident angle using a spectrophotometer (SolidSpec-3700 DUV, Shimadzu, Kyoto, Japan) in the variable-angle absolute reflectance measurement mode. For the measurement of reflection spectra and _E_-field modulation spectra in water, the Ag-deposited prism was placed in a self-made triangular prism-shaped glass cell fixed on a rotating stage with all but the hypotenuse surface in close contact with the glass cell. A sodium chloride aqueous solution with a concentration of \(1.0\times 10^{-4}\) mol/L was poured into the space between the prism hypotenuse surface and the glass cell, and a Pt plate electrode was placed at 5 mm from the Ag film. For the measurement of the electromodulation spectra, we adopted a reflection angle within the prism of \(70.3^{\circ}\) (\(\theta=70.3^{\circ}\)). The reason for setting the aqueous solution concentration to \(1.0\times 10^{-4}\) mol/L is that the Ag film reacts with the aqueous solution and deteriorates when an \(E\) field is applied. The experimental setup is shown in Figure 1. The white light from a Xe lamp (EQ-99 X, Energeticj, Wilmington, MA, USA) was irradiated to the prism and was made incident on the Ag thin film from within the prism. Then the total reflected light is transferred through an optical fiber bundle to an imaging spectrometer (SpectraPro 300i, Acton, IN, USA) with a liquid nitrogen cooled Charge Coupled Device (CCD) (SPEC-10 System, Princeton Instruments, Trenton, NJ, USA). The reflected light spectra _R\({}_{p}\)_ and _R\({}_{s}\)_ of p-polarized and s-polarized light were measured by adjusting the angle of the polarizing plate for the reflectance ratio _R\({}_{p}\)_/_R\({}_{s}\)_ to be obtained. By switching a flipping mirror in the spectrometer to the other exit port, the totally reflected and spectrally dispersed light was sent to a multichannel lock-in amplifier (7210, Signal Recovery, Edinburgh, UK) for electromodulation spectroscopy. At the focal plane of the exit port, 128 monochromatized components of light were focused onto a bundled fiber array of 100 \(\upmu\)m core/110 \(\upmu\)m clad fibers arranged in a 16 \(\times\) 128 rectangle, and were sent to 128 avalanche photododes (APDs, S5343, Hamamatsu Photonics, Hamamatsu, Japan) through 128 fibers bundled with 16 vertical fibers together for detection. The signals from 128 APDs were pre-amplified and detected with the 128-channel lock-in  
Figure 1: Experimental setup with a cooled Spectrometric CCDs for measurement of reflection spectra and with a multi lock-in amplifier for measurement of electromodulation spectra \(\Delta R\) using the function generator. \(R\) for normalization of \(\Delta R\) was measured by intensity-modulating the reflected probe with a chopper (not shown). amplifier. Thus, a tiny change in the reflected light spectrum synchronized with a specific modulation frequency of the AC \(E\) field applied to the sample was efficiently detected at 128 wavelengths simultaneously [2; 3]. The function generator was connected to a lead wire attached to the Ag thin film deposited on the prism. The AC \(E\) field with amplitude of 0.4 V, 0.8 V, 1.2 V, 1.6 V, and 2.0 V at frequencies \(f\) of 222 Hz, 333 Hz, and 444 Hz were applied between the Ag film and the Pt electrode.