Infrared Pump-Probe Spectroscopy of Plasmons in Graphene and Semiconductors

Abstract

Unraveling exciting new physics in complex novel materials requires access to both material excitations and their dynamics, thus continuously pushing ultrafast pump-probe spectroscopy to its limits. However, most of the materials whose dynamics are at the center of current attention are also known to be inhomogeneous at the nanoscale. Hence, diffraction-limited optical techniques with their inherent areaaveraging character inhibit access to characteristic time scales of nanoscopic, heterogeneous systems. Circumventing the diffraction limit, scattering scanning near-field optical microscopy (s-SNOM) is a well-established technique that enables broad-band infrared spectroscopy with the nanoscale spatial resolution. In s-SNOM backscattered light from an atomic force microscope (AFM) tip reveals the local dielectric function of a sample [1]. Previous infrared s-SNOM studies were static, utilizing primarily continuous wave (CW) laser sources. Here, we extend s-SNOM by merging nano-spectroscopy with ultrafast pump-probe techniques and exemplify new capabilities with the time-resolved control of the plasmonic response of graphene and the semiconductor InAs.The goal of plasmonics is to utilize electromagnetic energy on a sub-wavelength scale in form of collective surface charge oscillations. Amongst various candidates for plasmonic media graphene stands out due to its most favorable properties: ultimate energy confinement in a monoatomic layer along with easy control over its charge density via electrostatic fields. Recently, the strong coupling between Dirac plasmons and the AFM tip of a near-field microscope has proven ideal to investigate their characteristics, so far utilized for their infrared spectroscopy [2] and real space visualization [3,4]. Another interesting material class for plasmonics are semiconductors due to mature processing technologies and carrier density control via doping.In our experiment we combine mid-infrared s-SNOM and ultrafast 100 fs, near-infrared laser excitation (Fig. 1a)) to study the time-dependent behavior of graphene [5] and InAs plasmons [6] at the nanoscale in infrared spectroscopy. For graphene we find strong pump-induced spectral changes in the infrared plasmonic response (Fig. 1b), top) around the SiO 2 substrate phonon resonances at 800 and 1125 cm -1 (Fig. 1b), bottom panel). Modeling reveals that pump-induced heating of carriers up to a temperature of 2100 K is the dominant effect. It results in an increase in Drude weight that s-SNOM is sensitive to. In

Model for quantitative tip-enhanced spectroscopy and the extraction of nanoscale-resolved optical constants

“…These novel interferometric techniques detect both amplitude and phase [12][13][14] of the probe-scattered "near-field signal", which encodes nano-scale near-field optical contrasts from the sample and transmits them to the farfield. While applications to nanoscale chemical sensing at vibrational "fingerprint" energies are obvious 9,15,16 , the utility of this instrument for fundamental nanoscale studies of correlated electron systems are equally compelling [17][18][19][20][21][22][23][24] . ANSOM employs a conductive or dielectric AFM probe as both an intense near-field source and scatterer of light into the far-field.…”