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纳米尺度下的太赫兹(THz)时域光谱与成像技术对于材料研究和器件检测等至关重要. 然而, 受限于THz波波长衍射极限的限制, 传统的远场THz时域光谱与成像技术无法提供飞秒时间尺度和纳米空间分辨的载流子浓度分布和超快动力学过程研究. 本文介绍了基于超快THz时域光谱成像技术与扫描探针技术耦合的超快THz散射型扫描近场光学显微镜系统. 利用针尖与样品表面的近场相互作用, 该近场系统已被证实可以以~60 nm横向空间分辨率的静态THz光谱实现半导体材料和器件研究, 进而获得半导体材料和器件的静态THz电导率分布情况, 而且还可以通过光激发瞬态载流子的方式, 获得半导体材料的瞬态电导率和激光THz发射超快动力学过程, 为研究材料和器件在纳米空间分辨, 超快时间分辨和THz谱学成像方面的性能提供了有力支持.Terahertz (THz) time-domain spectroscopy and imaging techniques on a nanoscale are crucial for material research, device detection, and others. However, traditional far-field THz time-domain spectroscopy faces inherent diffraction limitations, which limits the applications of carrier dynamics analysis that require femtosecond time resolution and nanoscale spatial precision. We present a scattering-type scanning near-field optical microscopy that overcomes these limitations by combining ultrafast THz time-domain spectroscopy with atomic force microscopy (AFM). The utilization of the near-field interaction between the needle tip and the sample surface is demonstrated to facilitate the study of semiconductor materials and devices by using static THz spectroscopy with a lateral spatial resolution of ~60 nm. This, in turn, enables the acquisition of static THz conductivity distributions of the semiconductor materials. Additionally, it facilitates the acquisition of transient conductivity distributions of semiconductor materials and laser THz emission ultrafast via photoexcited transient carrier kinetic processes, which provides substantial support for studying the performances of materials and devices in nanometer spatial resolution, ultrafast time resolution, and THz spectroscopic imaging. The experimental results show that the system has a signal-to-noise ratio as high as 56.34 dB in the static THz time-domain spectral mode, and can effectively extract the fifth-order harmonic signals covering the 0.2–2.2 THz frequency band with a spatial resolution of up to ~60 nm. Carrier excitation and complexation processes in topological insulators are successfully observed by optical pump-THz probe with a time resolution better than 100 fs. Imaging of SRAM samples by the system reveals differences in THz scattering intensity due to non-uniformity in doping concentration, thereby validating its potential application in nanoscale defect detection. This study not only provides an innovative means for studying the nanoscale electrical characterization of semiconductor materials and devices, but also opens a new way for applying the THz technology to interdisciplinary subjects such as nanophotonics and spintronics. In the future, by integrating the superlens technology, optimizing the probe design, and introducing deep learning algorithms, it is expected to further improve the temporal- and spatial-resolution and detection efficiency of the system.
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Keywords:
- terahertz electromagnetic wave /
- scanning near-field optical microscope /
- semiconductor materials /
- devices
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图 1 超快THz s-SNOM系统示意图, laser为光纤飞秒激光器, SHG为二次谐波转化器, HWP为半波片, OI为光隔离器, M1—M14为光学反射镜, BE为扩束器, S1为分束器, AT为发射光导天线, OAP为离轴抛物面镜, AR为接收光导天线, L1和L2为镜头
Fig. 1. Schematic diagram of the ultrafast THz s-SNOM system. Laser represents the fiber femtosecond laser, SHG represents second harmonic trigger, HWP represents half-wave plate, OI represents optical isolator, M1—M14 represent mirror, BE represents beam spreader, S1 represents semi transparent and semi reflective mirror, AT represents transmitter antenna, OAP represents off-axis parabolic mirror, AR represents receiver antenna, L1 and L2 represent lenses.
图 3 SRAM样品的高度信息 (a)表面光学照片; (b)样品的三维立体图像; (c) AFM高度形貌
Fig. 3. Height information of SRAM sample: (a) Optical photographs of the surface taken with the cantilever; (b) three-dimensional images of the sample; (c) AFM height topography of the sample.
图 4 静态THz时域光谱扫描模式示意图 (a)时域波形; (b)频域波形; (c)不同阶数信号峰峰值
Fig. 4. Static THz time-domain spectroscopy scanning pattern mapping: (a) Time-domain waveforms; (b) frequency-domain waveforms; (c) peak value of signals at different orders.
图 5 样品SRAM的THz散射强度映射及拓扑绝缘体的光泵浦-THz探测响应 (a) THz散射时域波形; (b)同一区域的THz散射强度图; (c)拓扑绝缘体的光泵浦-THz探测时域波形; (d)拓扑绝缘体光激发前THz散射强度图; (e)拓扑绝缘体光激发后散射强度图
Fig. 5. THz scattering intensity mapping of SRAM and optical pump-THz probe response of topological insulator: (a) THz scattering time-domain waveforms; (b) THz scattering intensity map of the same region; (c) optical pump-THz probe time-domain waveforms of topological insulator; (d) topological insulator morphology before photoexcitation; (e) topological insulator morphology after photoexcitation.
图 6 InAs样品THz发射信号 (a)二阶时域波形; (b)二阶频域波形; (c)高阶时域波形; (d)静态THz时域光谱散射扫描成像图; (e) AFM高度形貌
Fig. 6. THz emission signals of the InAs sample: (a) Second-order time-domain waveform; (b) second-order frequency-domain waveform; (c) higher-order time-domain waveforms; (d) static THz time-domain spectral scattering scanning imaging; (e) AFM height topography of the sample.
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