Femtosecond Laser Assisted Field Emission Spectroscopy Of A Single Quantum Dot

Field Electron Emission (FEE) is one of the earliest tool for surface science characterization and occurs when a strong negative electric field is applied. Field Ion Emission, obtained with opposite bias voltage, was even the first process allowing the direct observation of individual atoms in 1955, more than 25 years before the scanning tunneling microscope. FEE is at the heart of modern characterization instruments in nanoscience such as scanning electron microscopy, transmission electron microscopy or far field scanning tunneling microscopy. Recently, several experiments on sharp metallic nano-tips irradiated by femtosecond laser have shown the great potential of femtosecond laser assisted FEE (FLAFEE) to access the ultra-fast electronic dynamics of nano-objects. We recently developed a FLAFEE experiments with a Ti:Sa ultra -fast laser oscillator (14 fs pulse duration, 80 MHz repetition rate, centered at 800 nm of wavelength). The pulse duration was measured with a frequency resolved optical gating (FROG). The laser is focused with an off -axis parabolic mirror in a 5 gm waist at the apex of a tungsten nanotip set to a negative voltage (100's of Volts) and located inside an ultra-high vacuum chamber (5x10 4° Ton). We captured the emitted electrons with a microchannel plate (MCP), a phosphor screen and a camera to get the spatial distribution, and a retarding field analyzer to characterize the electron energy distribution. In this experiment, individual quantum dots with a diameter in the nanometer range have been grown in situ on a tungsten nano-tip. By combining femtosecond laser irradiation and a negative static electric field on this quantum dot, we performed the first observation of the ultrafast electron dynamics by FLAFEE of the discrete energy levels. Clear distinct features are seen in the electron energy distribution compared to metallic nano-tips. A model including the non-equilibrium electron distribution in the metallic tip and the resonant tunneling barrier of the quantum dot is proposed to explain our data. From this model, an effective two-photon cross section above 200 000 GM (Goppert-Mayer) has been deduced, an order of magnitude higher than the highest quantum dot cross sections found in the literature. We attribute this increase to the presence of the tungsten nanotip. These results open up a new path for the characterization of nanometric and molecular systems in the ultrafast regime.