Abstract
Electron microscopy and electron diffraction techniques rely on electron sources. Those sources require strong electric fields to extract electrons from metals, either by the photoelectric effect, driven by multiphoton absorption of strong laser fields, or in the static field emission regime. Terahertz (THz) radiation, commonly understood to be nonionizing due to its low photon energy, is here shown to produce electron field emission. We demonstrate that a carrier-envelope phase-stable single-cycle optical field at THz frequencies interacting with a metallic microantenna can generate and accelerate ultrashort and ultrabright electron bunches into free space, and we use these electrons to excite and ionize ambient nitrogen molecules near the antenna. The associated UV emission from the gas forms a novel THz wave detector, which, in contrast with conventional photon-counting or heat-sensitive devices, is ungated and sensitive to the peak electric field in a strongly nonlinear fashion.
Highlights
Femtosecond electron emission from metals can be induced by single- or multi-photon excitation of bulk metal with femtosecond laser pulses from amplified laser systems [1,2,3] or even with pulses directly from a femtosecond oscillator by exploiting the enhancement of the laser field at a nanotip [4,5,6]
Electron emission nanotips driven by femtosecond optical fields can be controlled by the carrier-envelope phase (CEP) of the field itself [7], or by an auxiliary long-wavelength optical field at terahertz (THz) frequencies [8]
The absolute electron yield in photoemission processes varies from a few electrons per laser shot emitted from a single nanotip to approximately 105–106 electrons emitted from bulk metals irradiated by amplified femtosecond laser pulses
Summary
Femtosecond electron emission from metals can be induced by single- or multi-photon excitation of bulk metal with femtosecond laser pulses from amplified laser systems [1,2,3] or even with pulses directly from a femtosecond oscillator by exploiting the enhancement of the laser field at a nanotip [4,5,6]. Whereas multiphotondriven photoemission results in a narrow distribution of electron energies, field-driven emission results in a broad electron energy spectrum as electrons are emitted on a subcycle time scale, and subsequently accelerated by the optical field. In the latter case, the oscillation period strongly influences the energy spectrum of the electrons. The wavelength dependence of electron emission from nanotips initiated by mid-IR pulses [10] showed that longer wavelengths are favorable for acceleration of the emitted electrons by the optical field itself, with the caveat that the obtainable electron energy is limited by the extremely short range of the enhancement of the optical field [10]. The application of longer wavelength optical fields for electron emission will be favorable in terms of electron yield due to the significantly enhanced tunneling probability (lower Keldysh parameter) at lower field strengths
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