Ultrafast nanoimaging of electronic order parameters

In Chapter5, we demonstrated a combined 530 fs temporal and 5 nm spatial imaging resolution with a contrast tailored to map the structural order parameter of the NC/IC phase transition in 1T-TaS₂. While the DF imaging approach is unique in itsstructuralimaging capabilities, there are a number of related techniques that already are or may in the near future be able to spatially probe the ultrafastelectroniccomponent of correlated phase transitions.

A recent demonstration of those capabilities uses inelastic electron-light scattering to conduct photon-induced near-field electron microscopy (PINEM) of an individual VO₂ nanowire in an UTEM (Fig.6.7A and B, top panel) (168). An ultrashort gating laser pulse induces an optical near-field around the nanowire (Fig.6.7B, center panel). As a consequence, the electron beam undergoes inelastic scattering into a number of well-defined energy sidebands towards the gain and loss sides of the spectrum. Specifically, the near-field intensity encodes the dielectric function of the material, leading to a PINEM contrast between the insulating room-temperature and metallic high-temperature phases below and above 340 K, respectively.

When a second pump laser pulse is introduced, the temporal response of the dielectric function can be probed in a stroboscopic experiment (Fig.6.7C and D). The gating pulse is in overlap with the electron probe pulse at all times and enables the near-field interaction. The polarization of the pump pulse is chosen such as to minimize interaction with the electron pulse (Fig.6.7B, bottom panel). Advantageously, the temporal resolution is determined by the 50 fs duration of the optical gating and pump pulses—not by the duration of the electron pulse as it is usually the case. However, the effective electron current yielding contrast is reduced by the ratio of gating and electron pulse duration. The spatial resolution of the energy-filtered PINEM maps is on the order of 20 nm (168).

6.2 Ultrafast nanoimaging using UTEM

A

C

B

D

Figure 6.7: Two-color PINEM probing of electronic dynamics in VO₂.(A) Rendering of the VO2nanowire geometry on a silicon nitride membrane. The polarization of the 800 nm gating pulse P1 (red) and of the 400 nm pump pulse P2 (blue) is indicated. (B) Top: BF micrograph of the nanowire with the polarization of both pulses indicated. Center: PINEM map of the nanowire formed by energy-filtering electrons that have been gain-scattered due to interaction with the gating pulse P1. Bottom: PINEM map due to interaction solely with the pump pulse P2. No inelastically scattered intensity is visible by choice of the pulse polarization. (C) PINEM line scan across the nanowire as indicated by the blue box in B for two temporal delaysΔ𝑡between optical pump and electron probe pulse. The gating pulse is in overlap with the probe pulse at all times. The response of the specimen to the optical excitation is reflected by the different PINEM intensities before and after time-zero.

(D) Spatially integrated PINEM signal as a function of temporal delay. The observed decay and recovery can be described using a biexponential model. Reprinted from Ref. (168). Licensed underCC BY 4.0.

In scanning probe approaches, the optical near-field scattered by an oscillating nanometric tip is used to form images of the free-carrier response of the specimen (scanning near-field optical microscopy or SNOM). Ultrafast variants of this technique have been successfully applied to map the MIT of small VO₂ crystals and nanowires (43,44). In the experiment in Ref. (44), ultrashort pump and probe laser pulses are focused onto the scanning tip with a polarization parallel to the shaft. Figure6.8B schematically shows the scanning of the tip from the substrate

Figure 6.8: Scanning near-field optical microscopy of electronic dynamics in VO₂. (A) Left: Change in tip-scattered intensityΔ𝑠₂as a function of the position on the specimen (see B) and the temporal delayΔ𝑡 between optical pump and probe pulses. The specific response varies strongly across the crystal. The filled areas in some of the traces indicate an overshoot and fast relaxation after time-zero. Right: Height profile of the crystal (black) and integral of the filled areas (colors corresponding to the left panel). The temporal and spatial resolution is indicated by a Gaussian in both panels. (B) Schematic of the tip scanning across the crystal, starting from the substrate. At each point, a delay curve (see A) is acquired. (C) Four delay curves as a function of pump fluence illustrate different regimes of photoinduced dynamics. Reprinted with permission from Ref. (44).

Copyright 2016 American Chemical Society.

onto a VO₂crystal, acquiring a temporal delay curve at every point (Fig.6.8A). Depending on the pump fluence and the position on the crystal, behavior ranging from a quickly decaying electronic excitation to a photoinduced transition into the metallic phase is observed (Fig.6.8C).

The spatial resolution of this technique is limited by the 10 to 20 nm radius of the tip apex, while the few-100 fs temporal resolution is determined by the optical pulse duration.

There is one major difference between this kind of SNOM-based nanoimaging and the DF-based nanoimaging technique introduced in the present thesis: While the DF approach uses far-field excitation of the full specimen structure, the near-field excitation in the time-resolved SNOM experiment in Fig.6.8is scanned with the tip. Thus, the obtained specimen response is local and allows for insights into the specimen properties as modified, e.g., by local strain between VO₂ and the substrate (44). However, ultrafast SNOM can also be combined with

6.2 Ultrafast nanoimaging using UTEM

far-field excitation of the region of interest. In Ref. (43), this is realized by polarizing the pump pulses perpendicularly to the tip shaft.

To the author’s knowledge, there is no published experiment demonstrating ultrafast nanoimag-ing of the electronic component of a phase transition in 1T-TaS₂. Still, in a static SNOM ex-periment, the transition between the C and NC phase has been investigated (46). Interestingly, a 450 nm broad spatial transition region between C and NC phase regions was observed in the experiment. Supported by Ginzburg-Landau theory, this behavior has been interpreted in terms of a non-equilibrium CDW order across the NC/C phase boundary which gives rise to the observed gradual transition in metallicity. On a microscopic level, the transition is suggested to involve a change in size of the C-type domains making up the NC phase. While a large number of other non-equilibrium states is known in 1T-TaS₂(section3.5), this would be the only one accessible without using ultrashort stimuli.

Finally, time-resolved photoemission electron microscopy (PEEM) (51,54) has a great poten-tial for representing an electronic counterpart to our structure-sensitive technique. In a number of experiments, PEEM has demonstrated its capabilities of imaging the dynamics of charge carriers after optical excitation (32,53,55). These instruments achieve down to 50 nm spatial and 30 fs temporal resolution (55). With the correction of chromatic and spherical aberrations, few-nm resolution can be obtained in static PEEMs (381,382).

Most intriguingly, state-of-the-art time-resolved PEEMs are also momentum microscopes that can switch between real-space and reciprocal-space imaging (383,384). And the analogy to TEMs does not end there: By inserting an aperture into the image plane, band structure information can be extracted from a selected area of the specimen. Conversely, an aperture in the BFP can be used to control the contrast in real-space images. Additionally, energy filtering can be used to only take photoelectrons into account that were emitted from certain binding energies. Both filtering techniques combined would perspectively allow for tailoring the contrast of ultrafast DF PEEM images to precisely map the temporal evolution of CDW gaps in the band structure of 1T-TaS₂(cf. Fig.3.7).