  Femtosecond-fast and nanometre-size pulses of electrons are emerging as
unique probes for ultrafast dynamics at the nanoscale. Presently, such pulses
are achievable only in highly sophisticated ultrafast electron microscopes or
equally complex setups involving few-cycle-pulsed lasers with stable
carrier-envelope phase (CEP) and nanotip probes. Here, we show that the
generation of femtosecond pulses of nanoscale tunnelling electrons can be
achieved in any ultrafast optical laboratory, using any (deep-UV to mid-IR)
femtosecond laser in combination with photosensitive asymmetric nanogap (PAN)
diodes fabricated via easy-to-scale adhesion lithography. The dominant
mechanism producing tunnelling electrons in PANs is strong-field emission,
which is easily achievable without CEP locking or external bias voltage. We
employ PANs to demonstrate ultrafast nanoscopy of metal-halide perovskite
quantum dots immobilised inside a 10-nm Al/Au nanogap and to characterise laser
pulses across the entire optical region (266-6700 nm). Short electron pulses in
PANs open the way towards scalable on-chip femtosecond electron measurements
and novel design approaches for integrated ultrafast sensing nanodevices.
