Ultrafast infrared nano-imaging of far-from-equilibrium carrier and vibrational dynamics

Abstract

Ultrafast infrared nano-imaging has demonstrated access to ultrafast carrier dynamics on the nanoscale in semiconductor, correlated-electron, or polaritonic materials. However, mostly limited to short-lived transient states, the contrast obtained has remained insufficient to probe important long-lived excitations, which arise from many-body interactions induced by strong perturbation among carriers, lattice phonons, or molecular vibrations. Here, we demonstrate ultrafast infrared nano-imaging based on excitation modulation and sideband detection to characterize electron and vibration dynamics with nano- to micro-second lifetimes. As an exemplary application to quantum materials, in phase-resolved ultrafast nano-imaging of the photoinduced insulator-to-metal transition in vanadium dioxide, a distinct transient nano-domain behavior is quantified. In another application to lead halide perovskites, transient vibrational nano-FTIR spatially resolves the excited-state polaron-cation coupling underlying the photovoltaic response. These examples show how heterodyne pump-probe nano-spectroscopy with low-repetition excitation extends ultrafast infrared nano-imaging to probe elementary processes in quantum and molecular materials in space and time.

Nanoscale heterogeneity of ultrafast many-body carrier dynamics in triple cation perovskites

“…We then spectrally resolve the transient absorption signal, acquired for a constant injected carrier density of n 0 = 1.7 × 10 19 cm −3 , but for variable pump-probe delays with results shown in Fig. 3 c. We note that the spectrally narrow feature at ~1720 cm −1 arises from CN anti-symmetric vibration of the FA cation 20 , 40 , 46 . The narrowing of the transient absorption spectra (Fig.…”

“…We implement ultrafast heterodyne infrared (IR) nano-imaging as developed recently 40 to probe the spatio-temporal evolution of the ultrafast carrier dynamics. As shown in Fig.…”

“…The repetition rate of 1 MHz ensures complete relaxation of the photoinduced carriers between subsequent laser shots. In our home-built ultrafast s -SNOM system, the collinear pump and probe pulses are directed to the apex of a metallic tip (Arrow-NCPt, NanoAndMore USA) in an atomic force microscope (AFM; Innova, Bruker) operating in tapping mode at the frequency ω t ~ 250 kHz 40 .…”

“…The near-field pump-probe signal Δ E NF is selectively detected by sideband lock-in detection at 2 ω t ± Ω M . Based on the two-phase measurement, the background-free near-field pump-probe amplitude A HPP ( T ) is isolated as a function of pump-probe delay T , with spatial resolution of ~50 nm limited by the radius of the tip apex 40 . Typically 1 μm × 1 μm images are acquired at 64 × 64 pixels with a scan rate of 0.2 Hz.…”

Subcycle Mid-Infrared Electric-Field-Driven Scanning Tunneling Microscopy with a Time Resolution Higher Than 30 fs

“…However, since the mechanism of this system is based on the interference between two pulses of 6 fs width, unlike other laser-combined STM systems, its range of application is limited. Recently, scanning near-field optical microscopy (SNOM) techniques with a time resolution of 100 fs level have been reported. − Since SNOM measures permittivity of materials, we can obtain optically important local information. However, it is difficult to directly measure transient LDOSs.…”