Abstract
At terahertz (THz) frequencies, scattering-type scanning near-field optical microscopy (s-SNOM) based on continuous wave sources mostly relies on cryogenic and bulky detectors, which represents a major constraint for its practical application. Here, we devise a THz s-SNOM system that provides both amplitude and phase contrast and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a quantum cascade laser that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique. We demonstrate its performance by probing a phonon-polariton-resonant CsBr crystal and doped black phosphorus flakes.
Highlights
Terahertz (THz) frequency electromagnetic waves (30–300 μm wavelength) can resonantly interact with fundamental excitations of molecules and solids and offer an ideal tool for the optical characterization of emerging low-dimensional materials and biological-systems
We use the radiation source, a THz quantum cascade laser (QCL) [13], for illuminating the tip and for detecting the radiation field scattered from the tip, avoiding the use of bulky THz cryogenic detectors
The inherent stability of QCLs against optical feedback [16] has been recently exploited in a number of self-mixing interferometry (SMI) configurations, providing an interesting method to control the emission of THz QCLs by reconfigurable photo-generated anisotropic metamaterials [17], to trace the free carrier distribution in a semiconductor target [18], and to map the real and imaginary refractive index of polymeric materials [12], for example
Summary
Terahertz (THz) frequency electromagnetic waves (30–300 μm wavelength) can resonantly interact with fundamental excitations of molecules and solids and offer an ideal tool for the optical characterization of emerging low-dimensional materials and biological-systems. Scattering-type scanning near-field optical microscopy (s-SNOM) displayed an exceptional potential for nanoscale imaging of material properties [1] as has been demonstrated at 2.5 THz [2]. We demonstrate amplitude- and phaseresolved background-free SD-s-SNOM imaging with 60-70 nm spatial resolution comparable to the scattering tip size, providing a key step forward to make THz nanoscopy a widely used tool. Our approach, based on a simple 2-parameter fitting of self-mixing interferograms, outperforms by far all previously reported attempts based on the use of self-mixing and either attaining incoherent near-field s-SNOM imaging, or using a rather complex 6-parameter fitting procedure for amplitude-like and phase-like images at diffraction-limited resolutions (> 100 μm) [12]
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