Solid Immersion (SI) microscopy is a modern imaging modality that overcomes the Abbe diffraction limit and offers novel applications in various branches of visible, infrared, terahertz, and millimeter-wave optics. Despite the widespread use, SI microscopy usually results in qualitative imaging. Indeed, it presents only the raw distributions (in the image plane) of the backscattered field intensity, while unlocking the information about the physical properties of an imaged object, such as its complex refractive index (RI) distribution, requires resolving the inverse problem and remains a daunting task. In this paper, a method for resolving the SI microscopy inverse problem is developed, capable of reconstructing the RI distribution at the object imaging plane with subwavelength spatial resolution, while performing only intensity measurements. The sample RI is retrieved via minimization of the error function that characterizes discrepancy between the experimental data and the predictions of analytical model. This model incorporates all the key features of the electromagnetic-wave interaction with the SI lens and an imaged object, including contributions of the evanescent and ordinary-reflected waves, as well as effects of light polarization and wide beam aperture. The model is verified numerically, using the finite-element frequency-domain method, and experimentally, using the in-house reflection-mode continuous-wave terahertz SI microscope. Spatial distributions of the terahertz RIs of different low-absorbing optical materials and highly absorbing biological objects were studied and compared to a priori known data to demonstrate the potential of the novel SI microscopy modality. Given the linear nature of the Maxwell’s equations, the developed method can be applied for subwavelength-resolution SI microscopy at other spectral ranges.

中文翻译：

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通过折射率分布重建的定量超分辨率固体浸没显微镜

固体浸没 (SI) 显微镜是一种现代成像方式，它克服了阿贝衍射极限，并在可见光、红外、太赫兹和毫米波光学的各个分支中提供了新的应用。尽管广泛使用，SI 显微镜通常会导致定性成像。实际上，它仅呈现反向散射场强度的原始分布（在图像平面中），同时解锁有关成像对象物理特性的信息，例如其复折射率 (RI) 分布，需要解决逆问题和仍然是一项艰巨的任务。在本文中，开发了一种解决 SI 显微镜逆问题的方法，能够以亚波长空间分辨率重建物体成像平面上的 RI 分布，同时仅执行强度测量。通过最小化表征实验数据和分析模型预测之间差异的误差函数来检索样本 RI。该模型结合了电磁波与 SI 透镜和成像物体相互作用的所有关键特征，包括渐逝波和普通反射波的贡献，以及光偏振和宽光束孔径的影响。该模型使用有限元频域方法进行数值验证，并使用内部反射模式连续波太赫兹 SI 显微镜进行实验验证。研究了不同低吸收光学材料和高吸收生物物体的太赫兹 RI 的空间分布，并与先验已知数据证明了新型 SI 显微镜模式的潜力。鉴于麦克斯韦方程组的线性特性，所开发的方法可应用于其他光谱范围内的亚波长分辨率 SI 显微镜。