研究目的
To study the combined influence of illumination wavelength and nanotip shape on strong-field photoemission from silver nanotips, including the effects of thermal load at different laser repetition rates.
研究成果
The research demonstrates strong-field photoemission from silver nanotips for the first time, with higher field enhancement at 400 nm compared to 800 nm due to the nanotip's shape and plasmonic properties. Numerical calculations confirm the wavelength and shape dependence. High repetition rates suppress strong-field emission due to thermal effects, highlighting the importance of laser parameters and tip engineering for optimizing photoemission in applications like ultrafast microscopy.
研究不足
The study is limited by the specific shape and material of the silver nanotips used, which may not generalize to other materials or shapes. Thermal effects at high repetition rates could lead to tip damage or altered emission properties, and the numerical modeling may not fully capture real-world complexities. The pulse duration and focus size at 400 nm were not precisely measured, introducing uncertainty in enhancement calculations.
1：Experimental Design and Method Selection:
The study investigates optical field emission from silver nanotips using femtosecond laser pulses at different wavelengths (800 nm and 400 nm) and repetition rates (1 kHz and 62 MHz). The experimental setup includes a retarding field spectrometer to measure kinetic energy spectra of emitted electrons. Numerical calculations using COMSOL Multiphysics software are employed to model the electric field enhancement based on the nanotip shape and material properties.
2：Sample Selection and Data Sources:
Silver nanotips are fabricated from 99.99% pure polycrystalline silver wire using electrochemical etching and focused ion beam (FIB) annular milling. The specific tip used has a radius of approximately 20 nm, as determined from SEM images.
3：99% pure polycrystalline silver wire using electrochemical etching and focused ion beam (FIB) annular milling. The specific tip used has a radius of approximately 20 nm, as determined from SEM images.
List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: Equipment includes femtosecond laser sources (S1 at 1 kHz, S2 at 62 MHz, both at 800 nm; second harmonic generation at 400 nm), off-axis parabolic mirrors for focusing, a retarding field spectrometer, nano-positioning translation stages (Attocube systems AG), and a Beta Barium Borate (BBO) crystal for frequency doubling. Materials include silver wire, perchloric acid, methanol, and gallium ions from FIB milling.
4：Experimental Procedures and Operational Workflow:
Laser pulses are focused onto the nanotip apex with polarization aligned along the tip axis. The tip is biased with a DC voltage (VDC) to accelerate electrons. Kinetic energy spectra are recorded by varying laser intensity. Alignment is checked using a CCD camera and cold-field emission measurements. Numerical modeling involves inputting dielectric permittivity values for silver at different wavelengths and simulating field enhancement using finite element methods.
5：Data Analysis Methods:
Cutoff energies from spectra are extracted using linear fits to plateau and cutoff regions. Intensity enhancement factors are calculated using the ponderomotive energy formula. Statistical analysis includes averaging enhancement factors and comparing results between wavelengths and repetition rates.
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