Surface plasmon polaritons and surface phonon polaritons, thanks to their high spatial confinement, present new opportunities to enhance light-matter interaction in 2D materials. Furthermore, they are enabling the development of novel photonic devices like superlenses, subwavelength metamaterials, and others.
A versatile optical imaging and spectroscopy tool with nanometer spatial resolution and wide spectral coverage is necessary to obtain in-situ characterization of such polaritonic excitations across different applications.
Using s-SNOM for Nanoscale Imaging
s-SNOM provides a unique way to selectively excite and locally detect electronic and vibrational resonances in real space through a non-invasive near-field light-matter interaction.
整个中红外光谱范围内纳米级成像和光谱的独特功能由Anasys nanoIR3-s Broadband system(2.5 µm - 15 µm / 4000 - 670 cm-1）。This is achieved by coupling with a super-bright broadband light source, based upon a femtosecond OPO/DFG laser.
This laser source can also alter its linewidth for imaging (narrow-linewidth mode and a spectral resolution of 4 cm-1) and spectroscopy (broad-linewidth mode >200 cm-1), while featuring high laser power and wide spectral range.
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Figure 1.Optical permittivity of hBN in the mid-infrared.1
By studying the surface phonon polaritons dispersion of hexagonal boron nitride (hBN), the potential of this technique can be demonstrated. hBN has a hyperbolic optical permittivity near its two Reststrahlen bandsin the mid-infrared region。
Optical permittivity, with resonances at 760 cm-1for out-of-plane (εz) and 1370 cm-1for in-plane (εt), has been plotted(see Figure 1)。Nanoir3-S宽带系统成像的实验示意图如图2所示。IR光紧密集中在AFM探针上，并在尖端顶点启动HBN表面声子极性。
The excited surface phonon polaritons form a standing wave pattern, propagating out along the surface and reflecting from the sample edges. In Figure 3, an example of Nano-FTIR spectrum collected at different locations on the hBN sample is shown.
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Figure 2.Schematics of a nanoIR3-s Broadband system consisting of a super broadband mid-IR laser source and a compact nano-FTIR microscope.
Applications of NanoIR3-s Broadband System
ThenanoIR3-s Broadband system当以狭窄的线宽模式进行成像时，可以在靶向波长处获得表面声子极性子的高分辨率2D纳米影像学。超过670–4000厘米的全表面声子polaritons频谱-1can be gathered at a targeted sample location with nanometer resolution when operating in broad-linewidth mode for spectroscopy. Spatiospectral nanoimaging of the full surface phonon polariton dispersion is given in a single measurement by collecting spectrum at a variety of locations.
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Figure 3.Near-field optical spectrum on a thin hBN nanoflake shows a systematic spectral shift varying with distances to the edge
Figure 4 depicts near-field images collected in narrow-linewidth mode on a hBN nanoflake. By raster scanning the sample at a specific wavelength, AFM height (Figure 4a) and a near-field image at the selected wavelength are gathered at the same time. IR near-field images at differing laser wavelengths at a tuning step of 10 cm-1如图4B-W所示。
A standing-wave pattern along the sample edge in each image can be seen across the range. The standing-wave pattern systematically varying with laser frequency and distance from the sample edge can also be observed. Simply by doubling the fringe period, the polariton wavelength λp can be obtained.
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图4。Near-field optical images on a hBN nanoflake at different wavelengths under narrow linewidth mode. Each image is 1.5 µm x 1.5 µm with 10 nm pixel spacing: a) AFM height; b-w) near-field images at different wavenumbers, showing a systematic variation of surface phonon polariton (SPP) waves pattern.
Spectroscopy under broad-linewidth mode is shown in Figure 5. Spatiospectral nanoimaging of the full surface phonon polariton dispersion can be plotted as a 3D datacube by acquiring spectra at a range of locations. Figure 5a shows the spatiospectral nanoimaging by plotting a stack of spectra in a waterfall manner, with individual spectrum, as in Figure 3.
Each spectrum of spectral resolution (3 cm-1）的收购时间为一分钟，线扫描像素间距为15 nm，总测量时间约为2.5小时。图5中的时尚图，以> 200 cm的宽线宽模式获得-1spectral linewidth, display the full s-SNOM amplitude and phase response of hBN, with the inclusion of the optical phonon at 1370 cm-1and surface phonon polariton waves ranging between 1370–1550 cm-1。
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图5。HBN纳米薄片上的时尚纳米影像：（a）光谱阵列沿着虚线的白线在宽线宽模式下收集；通过以垂直轴的位置以瀑布的方式绘制频谱的堆栈，分别为幅度（b）和相（c）创建了时尚纳米影像图。The spatio-spectral scan shows the completesurface phonon polariton整个范围内的频率响应，具有高光谱分辨率（3厘米-1）。线扫描像素间距为15 nm，每个频谱的获取时间为1分钟，总测量时间为〜2.5小时
结论
The combination of imaging with broadband spectroscopy (and spatio-spectral imaging) enabled by thenanoIR3-s Broadband system提供了一个强大的工具，用于在2D材料中具有纳米空间分辨率，广泛的光谱覆盖范围以及高信号到噪声的2D材料中表面声子极性子和表面声子极性子的时尚纳米影像。亚博网站下载
参考
Caldwell, J. D.,et al.（2014）。Sub-diffraction volume-confined polaritons in the natural hyperbolic material: hexagonal boron nitride.Nat. Comms.5，5221。
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces, who has acquired Anasys Instruments. For more information on this source, please visit布鲁克纳米表面。