Shifts in plasmon resonance due to charging of a nanodisk array in argon plasma

Abstract

A method for generating charge-induced plasmonic shifts, using argon plasma to charge nanoparticle arrays, is presented. Particles develop a negative charge, due to enhanced collisions with high-temperature electrons, in low-temperature plasmas. The negative charge generated causes a blue shift in the localized surface plasmon resonance. The dynamics of the shift were recorded and discussed. This effect could be used as a real-time method for studying the dynamics for charging in plasma. Plasmonics deals with optically excited, oscillations of electrons at the interface between a metal and a dielectric, 1 and this phenomenon has driven the development of many optical devices. 2-9 Surface plasmons excited in metal structures, confined at the nanoscale in three dimensions (i.e., nanoparticles, nanodisks, etc.), are known as localized surface plasmons. At certain wavelengths, maximum coupling from electromagnetic waves to localized surface plasmons can be achieved and these are known as the localized surface plasmon resonances (LSPRs) of the structure. LSPRs of metal nanostructures depend on properties such as the size, 10,11 shape, 12-14 metal material, 15 surrounding dielectric material, [16][17][18][19][20][21][22] and the charge of the metal. [23][24][25][26][27][28][29][30] Active plasmonic devices can be realized by externally tuning the LSPR of nanostructures. Previously, chemical/electrochemical charging was used to actively tune the LSPR of arrays of both silver 24,25,30 and gold 23,26,27,29 nanostructures; however, this process is relatively slow. In this study, a rapid shift in the LSPR of an array of gold nanodisks was induced by surrounding the particles with a low-temperature argon plasma. This shift can be explained by the charging effect of the plasma. [31][32][33][34] Charging by the plasma takes place in only seconds, where chemical/electrochemical charging can take several minutes 24 to hours. 27 This method allows real-time monitoring of the charging effect induced by low-pressure plasmas and could be utilized for photonics applications based on the LSPR shift.In the experiment, the optical extinction spectrum of an array of gold nanodisks was measured while generating plasma. An array of gold nanodisks (diameters of 120 nm, thicknesses of 30 nm, and a periodicity of 300 nm) was fabricated on glass via electron-beam lithography. 35 A custom vacuum chamber was designed to include two flat, parallel windows to allow transmission of a probe light (Fig. 1(a)). The nanodisk array was placed inside the vacuum chamber where it was attached to one of the flat windows. The probe light, ejected from the input optical fiber and collimated by a lens, traveled through the vacuum chamber and the sample. The output light was collected by the output lens/optical fiber and delivered to an optical spectrometer. A vacuum system was used to evacuate the chamber to a base pressure of 150 mTorr. Argon was introduced into the chamber using a mass flow controller. The argon gas was introduced at a flow rate of 6 SCCM ...

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