Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction

Abstract

Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photoassisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron–lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband terahertz (THz) pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photoinduced hot electron tunneling and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction.

Real-Space Sampling of Terahertz Waveforms Under Scanning Tunneling Microscope

“…Notably, the divergence observed in PES relative to the EOS result is more significant than that in TDFES. This distinction can be attributed to the different regions where the two methods are applied and the potential influence of the incident visible light pulse in PES, whose result is varied with the laser strength . Specifically, PES waveforms were measured by using photoemission current from the sample side, which may be susceptible to thermal effects from the sample.…”

“…This distinction can be attributed to the different regions where the two methods are applied and the potential influence of the incident visible light pulse in PES, whose result is varied with the laser strength. 31 Specifically, PES waveforms were measured by using photoemission current from the sample side, which may be susceptible to thermal effects from the sample. Nevertheless, it has been demonstrated that relying solely on the sample photoemission current is sufficient for retrieving the near-field waveforms.…”

Optoelectronic Readout of Single Er Adatom’s Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9 K)

“…Hence, the electronic population at the tungsten tip at the Fermi level energy is modulated via the formation of hot electrons. 36 Meanwhile, free carriers and free excitons are simultaneously created at various energies, ranging from 1.55 eV (800 nm) to 1.03 eV (1200 nm) within the Si(100) conduction band, specifically from the top of the Si π* orbital band to the lower edge of the conduction band. This transient electronic population change may significantly affect the charge state of the STM junction and hence the shape of the tunnel barrier.…”

“…When the laser wavelength is tuned from 800 to 1200 nm, the irradiated zone that surrounds the STM junction promotes the formation of a large number of free charge carriers in the silicon surface as well as hot electrons in the STM tip. Hence, the electronic population at the tungsten tip at the Fermi level energy is modulated via the formation of hot electrons . Meanwhile, free carriers and free excitons are simultaneously created at various energies, ranging from 1.55 eV (800 nm) to 1.03 eV (1200 nm) within the Si(100) conduction band, specifically from the top of the Si π* orbital band to the lower edge of the conduction band.…”

Ultrafast Dynamics Revealed with Time-Resolved Scanning Tunneling Microscopy: A Review

“…Moreover, the coherent manipulation of light–matter interactions can now occur with atomic accuracy, which may provide an alternate approach to switch the quantum state population of a qubit. Even though TR-STM is currently a sophisticated technique only available in a limited number of research laboratories, recent advances in technology have made TR-STM more accessible and user-friendly. ,,,,,− Because of its versatility, we highly anticipate its future as a widely used technique that is accessible to users in a diverse array of fields.…”