Abstract
Scattering-type scanning near-field optical microscopy (s-SNOM) allows spectroscopic imaging with spatial resolution below the diffraction limit. With suitable light sources, s-SNOM is instrumental in numerous discoveries at the nanoscale. So far, the light sources have been limited to continuous wave or high-repetition-rate pulsed lasers. Low-repetition-rate pulsed sources cannot be used, due to the limitation of the lock-in detection mechanism that is required for current s-SNOM techniques. Here, we report a near-field signal extraction method that enables low-repetition-rate pulsed light sources. The method correlates scattering signals from pulses with the mechanical phases of the oscillating s-SNOM probe to obtain near-field signal, by-passing the apparent restriction imposed by the Nyquist–Shannon sampling theorem on the repetition rate. The method shall enable s-SNOM with low-repetition-rate pulses with high-peak-powers, such as femtosecond laser amplifiers, to facilitate investigations of strong light–matter interactions and nonlinear processes at the nanoscale.
Highlights
Scattering-type scanning near-field optical microscopy (s-SNOM) allows spectroscopic imaging with spatial resolution below the diffraction limit
In s-SNOM, an atomic force microscope (AFM) drives a sharp metallic or metal-coated tip in tapping mode or intermittent contact mode to scan over the sample of interest; an external light source is coupled to the tip apex and the scattered light from the tip with sample underneath is collected, converted into electric signal by an optical detector
The signal is routed to a lock-in amplifier, where the non-fundamental harmonic demodulations can be extracted with the tip oscillation frequency as the reference frequency
Summary
Scattering-type scanning near-field optical microscopy (s-SNOM) allows spectroscopic imaging with spatial resolution below the diffraction limit. The method shall enable s-SNOM with low-repetition-rate pulses with high-peak-powers, such as femtosecond laser amplifiers, to facilitate investigations of strong light–matter interactions and nonlinear processes at the nanoscale. To extract near-field signal with lock-in detection, the light radiation has to be either continuous or pulsed at a much higher repetition rate than the mechanical oscillation frequency of the tip[24,25,26]. At the lower limit of the critical sampling rate with lock-in detection, additional considerations have to be taken into account, such as synchronization of the detector sampling rate[25] or tip oscillation frequency[29] with the pulsed laser source to maintain the near-field contrast. The high-peak power of solid-state femtosecond amplifier is suitable for frequency conversions through nonlinear processes, such as the difference frequency generation of mid-infrared radiation[30]. This new detection technique will enable operations of s-SNOM with a range of pulsed laser systems
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