Imaging the nanoscale phase separation in vanadium dioxide thin films at terahertz frequencies

Abstract

Vanadium dioxide (VO2) is a material that undergoes an insulator–metal transition upon heating above 340 K. It remains debated as to whether this electronic transition is driven by a corresponding structural transition or by strong electron–electron correlations. Here, we use apertureless scattering near-field optical microscopy to compare nanoscale images of the transition in VO2 thin films acquired at both mid-infrared and terahertz frequencies, using a home-built terahertz near-field microscope. We observe a much more gradual transition when THz frequencies are utilized as a probe, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2, which account for a continuous temperature dependence of the optical response of the VO2 in the insulating state.

Terahertz response of monolayer and few-layer WTe_2 at the nanoscale

“…THz scanning-type near-field optical microscope (THz-SNOM) [23]. Both the AFM scanner and focusing optics of our apparatus (Fig.1a) are situated in an ultra-high vacuum (UHV) compartment.…”

“…The wavelength of THz waves is of the order of ~300 µm and conventional diffraction-limited methods are inadequate for interrogating the THz response of WTe2 microcrystals. In order to overcome the diffraction limit in THz, we utilize a scattering-type THz scanning near-field optical microscope (THz-SNOM) [23] [24] [25] [26] [27]. This technique is a hybrid of an atomic force microscope (AFM) with a pulsed THz source.…”

“…THz-SNOMs are being successfully applied to an expanding list of materials and interesting problems. For example, THz-SNOM methods have provided insights into nano-scale studies of electronic phase separation in the vicinity of the insulator-to-metal transition in VO2 [23], the plasmonic response of graphene [26] [28], free carrier distributions in nanodevices [29] [30], and phonon resonances in multiferroic materials [31]. Here we report on near-field nano-optical experiments in THz range for WTe2 conducted at cryogenic temperature.…”

“…The tapping of the tip modulates the near-field signal at ~70 kHz. We demodulated the amplitude of the tip-scattered electric field at the first ( 1 ) and the second ( 2 ) harmonics of the tip tapping frequency to suppress the undesired far-field background [23] [33]. Figure 1|Schematic of nano-THz experiments on multi-terraced crystals of WTe2.…”

Photo-induced terahertz near-field dynamics of graphene/InAs heterostructures

“…The implementation of scattering-type scanning near-field optical microscopy at THz frequencies (THz-s-SNOM) with sub 100 nm spatial resolution was first demonstrated over fifteen years ago [1,2]. Recent developments have revealed its powerful capabilities for performing nano-spectroscopies without artefacts [3], probing the local carrier density in semiconductors with ultra-sensitivity [4,5] and imaging phase transitions of strongly correlated materials at elevated temperatures [6]. With recent developments in ultrafast pulse based THz near-field techniques [6], one natural extension to the existing imaging technique is to perform nanoscopy in the ultrafast time domain, where a conventional optical-pump THz-probe scheme can be utilized using high repetition rate THz pulses as the probe [7].…”