Fig. 1. Plasmonic pumping experiment and photoinduced near-field optical response in Hg_0.81Cd_0.19Te.

(A) Nano-optical spectroscopy was carried out by means of apertureless scattering-type near-field optical microscope based on a metallic tip of an atomic-force microscope illuminated with broadband IR light and photoexcited with a femtosecond fiber laser operating in the conventional telecommunications window of the Er-doped optical fiber (1530 to 1565 nm) (23); high-fluence data in Fig. 3C were obtained using a free-space amplified laser system (see Materials and Methods). Strong field enhancement and gradient in the sample-tip nanocavity enable large light-matter momentum transfer (on the order of inverse tip radius) and, thus, the excitation and nanospectroscopy of large-momentum collective modes (27), overcoming the long-wavelength restriction of conventional IR optics. (B and C) Photon-energy dependence of the amplitude (B) and phase (C) of the normalized transient near-field signal demodulated at the second harmonic of the tip tapping frequency (O2: O, optical; 2, second harmonic). The spectra are plotted for various pump-probe delays from 0 to 12.2 ps and a pump pulse energy of 1 nJ. The dashed and dotted lines indicate the location of the primary and secondary plasmon-polaritonic feature, respectively. Solid black vertical lines indicate the time delays at which data were obtained.