Wavelength-Selective control of Atomic Scale Au Contacts

We demonstrate wavelength-selective control of atomic motion in a mechanically controllable Au break junction. Excitation at $\lambda_{\mathrm{form}}\simeq 530~{\rm nm}$ drives gap closure and metallic bridge formation, whereas excitation at $\lambda_{\mathrm{rup}}\simeq 407~{\rm nm}$ drives neck thinning, bridge rupture, and subsequent gap opening. Unlike conventional optical switching in metallic contacts, where illumination primarily acts via thermal expansion, the present experiment reveals oppositely directed atomic drift at different wavelengths. Time-resolved conductance traces allow us to distinguish two dynamical regimes. In the tunneling regime, exponential conductance transients measure the drift velocity of the gap coordinate for both gap closure and gap opening. In the metallic regime, the Sharvin relation converts linear $\sqrt{G/G_0}$ transients into radial neck-growth and neck-thinning velocities of comparable magnitude. These results establish optically selected atomic drift as a mechanism for reversible control of metallic nanocontacts and provide a quantitative route to follow plasmon-assisted atomic rearrangements in real time.