Tuning-fork based near field probe for spectral measurement, near-field microscope using the same, and spectral analysis method using near-field microscope

A near-field microscope using a tuning-fork based near-field probe for spectral measurement, the near-field probe microscope comprising:

• an optical system configured to include a plurality of beam splitters, a mirror, and a lens for transferring a laser pulse generated by a fast-pulse laser to a terahertz pulse generation device, and to include one or more beam splitters and a plurality of mirrors for transferring a part of the laser pulse generated by the fast-pulse laser to an electric field detector;
• a sample stand configured to enable a location and a height thereof to be controlled;
• a plurality of first optical components for transferring and focusing a terahertz pulse generated by the terahertz pulse generation device on a sample;
• a tuning-fork based near-field probe comprising a first electrode and a second electrode arranged apart from each other, and a wire-shaped nano-probe downward attached to a one-side end of the second electrode and configured to vibrate in a perpendicular direction with respect to the sample, wherein the nano-probe includes a shaft for receiving a terahertz pulse focused through the first optical components, an end part for allowing the focused terahertz pulse to interact with the sample and scattering a terahertz pulse, into air, which has been subjected to an interaction procedure, and a tapered region for connecting the shaft and the end part;
• an AC voltage generation device connected to the first and second electrodes of the near-field probe in order to vibrate the near-field probe;
• an electric field detector for measuring an alternating current flowing through the tuning-fork based near-field probe;
• a control device for controlling a height of the sample stand to uniformly maintain a distance between the end part of the near-field probe and the sample;
• a plurality of second optical components for focusing a terahertz pulse directly reflected from the sample and a part of a pulse scattered from the end part of the near-field probe onto the surface of the electric field detector;
• a lock-in amplifier for demodulating a signal detected through the electric field detector with a vibration frequency of the nano-probe or harmonic of the nano-probe, thereby extracting a component modulated by vibration of the nano-probe;
• a mechanical optical delay unit for measuring a transient reaction of the terahertz pulse in a time domain;
• and
• a control photoconductive (control PC) for analyzing the measured transient reaction in a frequency domain, and measuring a characteristic spectrum of the sample,
• wherein the length of the nano-probe, which is a sum of the lengths of the shaft and the tapered region is longer than the radius of a focus of a terahertz beam focused by a means for focusing light.

A spectral analysis method using a near-field microscope, the method comprising the steps of:

• (a) transferring an ultrafast laser pulse generated by a fast-pulse laser to a terahertz generation unit using a plurality of beam splitters, a mechanical chopper, a mechanical optical delay unit, and a mirror;
• (b) transferring and focusing a terahertz pulse generated by the terahertz generation device onto a sample attached on a piezo-stage;
• (c) mounting a tuning-fork based near-field probe above the sample and driving the near-field probe, the near-field probe comprising:
• a wire-shaped nano-probe downward attached to a one-side end of a second electrode of first and second electrodes arranged apart from each other, and configured to vibrate in a perpendicular direction with respect to the sample;
• and a wire-shaped knife edge attached to a one-side end of the first electrode to stabilize vibration of the nano-probe;
• (d) measuring, through an electric field detector, an electric field of a pulse wave scattered from an end part of the near-field probe and a terahertz pulse directly reflected from the sample;
• (e) demodulating a signal, which is detected through the electric field detector, with a vibration frequency of the nano-probe or a harmonic of the nano-probe through a lock-in amplifier, thereby extracting only a scattered wave of the nano-probe;
• and
• (f) recoding a harmonic component detected through the lock-in amplifier while scanning a mechanical delay line, acquiring a time-domain signal, Fourier-transforming the acquired time-domain signal, and performing an analysis in a frequency domain, wherein characteristics of the sample are measured by removing second and third pulse waves by multiple reflections, performing a Fourier transform, and performing an analysis in a frequency domain.