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Blue near-field terahertz nonlinear optical technology


Recently, Angela Pizzuto and others from the Department of Physics at Brown University completed the first experimental demonstration of a scanning near-field microscope using blue light. With femtosecond pulses at 410 nanometers, the researchers generated terahertz pulses directly from bulk silicon for spatial resolution at nanoscale resolution, signals that provide spectral information not available using near-infrared excitation. They developed a new theoretical framework to explain this nonlinear interaction, making precise extraction of material parameters possible. This work establishes a possible new frontier for the study of technically relevant wide bandgap materials using scanning near-field microscopy methods.

Background

In the mid-90s, the advent of scattered scanning near-field optical microscopy (s-SNOM) changed the field of sub-wavelength optics. This technique involves coupling electromagnetic radiation to a sharp sub-wavelength metal tip and subsequently measuring the radiation scattered from that tip-sample junction in the far field. Over the past decade, this method of near-field measurements has had a significant impact in the infrared and terahertz regions of the spectrum. Aperture-based subwavelength spectroscopy methods are challenging, with incident waves becoming easier to couple with metal tips as wavelengths increase, while spatial resolution is still limited by tip size. The coupling of short-wavelength radiation to nanoscale tips is a difficult task, hindering nanoscale research on important wide-bandgap materials such as silicon and gallium nitride. These materials have been studied in near-field linear optics with excitation below the band gap. The application of nanoscale nonlinear optical methods to other materials is relatively mature, but it has not yet been realized because the application of this method to these highly correlated material systems generally requires higher energy light excitation.

Angela Pizzuto of Brown University et al. described a scanning near-field optical microscopy measurement with an incident photon energy of more than 3 eV. Using femtosecond pulses of 410 nanometers, the researchers illuminated a sharp metal atomic force microscopy (AFM) tip and induced terahertz emission from several different materials through a second-order nonlinear optical process to achieve laser terahertz emission microscopy (LTEM) with nanoscale spatial resolution. Due to the two-photon excitation above the wide direct bandgap, the high energy of the pump photons enables the strong terahertz emission of large crystalline silicon. The characteristics of laser terahertz emission microscopes have led to a greatly relaxed optical alignment requirement; Traditional linear scanning near-field light microscopy uses nanotips to limit incident waves, and the precise alignment of this focused short-wavelength radiation under the nanotips is actually challenging.

Innovative research

In experiments, nanoscale resolution can be obtained by externally coupling a small portion of macroscopic photogenerated terahertz dipoles, and researchers have achieved the use of closely focused blue light in scanning near-field optical microscopy for the first time. They obtained the first near-field laser terahertz emission microscope image of silicon and compared the results with those obtained by terahertz scanning near-field optical microscopy through elastic scattering of terahertz pulses at the tip.

Figure 1 is a schematic diagram of the laser path and scanning near-field optical microscope experimental setup. Near-infrared, blue, and terahertz beams are generated separately, where terahertz pulses are generated using conventional photoconductive antennas, and all three beams of light overlap and couple into an atomic force microscope. Scattered or emitted terahertz pulses are detected coherently on the other side by free-space electro-optical sampling.

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Figure 1 Schematic diagram of experimental setup

To illustrate the value of using laser terahertz emission microscopy in wide-bandgap materials, the researchers used silicon wafers as samples, which do not emit significant terahertz radiation under near-infrared excitation. The wafer has a small area that has been ion-implanted, and subsequent annealing activates the dopants injected in this area. In this way, the silicon wafer contains two regions with very different doping densities, with a clear boundary between them. The researchers took linear and nonlinear measurements of this boundary region and compared the results.

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Figure 2 Terahertz radiation of silicon samples. (a) Terahertz pulses. (b) The relationship between the peak-to-peak of a terahertz pulse and the average power of the pump beam

First, when pumped with ultrafast blue light, terahertz pulses are emitted both the uninjected substrate and the injected area. Figure 2a shows the THz pulse excited by blue light, demodulated at the second harmonic of the probe strike frequency. It can be observed that lightly doped substrates produce significantly more terahertz emission than heavily doped implanted areas. To better understand the terahertz production mechanism, the researchers measured the relationship between the emitted terahertz peak-to-peak and the average power of the blue pumped beam, as shown in Figure 2b. When the power is above about 2 mW, the terahertz emission intensity is less affected by the increase in blue power; In fact, once the pump flux is high enough, a significant portion of the available charge carriers will be excited by light, and any excess pump photons will be shielded by high local conductivity. As can be seen from the illustration in Figure 2b, there is a clear quadratic relationship between the amplitude of the emitted terahertz field and the pumped optical power. This suggests that the main mechanism of THz production is two-photon absorption; The carriers in the valence band absorb more than 6 eV of pump energy and are excited above the direct band gap of 4.2 eV wide well above the bulk Si. The experimental results provide new possibilities for the application of scanning near-field optical microscopy in wide bandgap materials.

The article was published in the journal Light: Science & Applications under the title “Near-field Terahertz Nonlinear Optics with Blue Light,” with Angela Pizzuto as the first author. (Source: LightScience Applications WeChat public account)

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1137-y

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