We demonstrate universal polarization transformers based on an engineered diffractive volume, which can synthesize a large set of arbitrarily-selected, complex-valued polarization scattering matrices between the polarization states at different positions within its input and output field-of-views (FOVs). This framework comprises 2D arrays of linear polarizers with diverse angles, which are positioned between isotropic diffractive layers, each containing tens of thousands of diffractive features with optimizable transmission coefficients. We demonstrate that, after its deep learning-based training, this diffractive polarization transformer could successfully implement N_i x N_o = 10,000 different spatially-encoded polarization scattering matrices with negligible error within a single diffractive volume, where N_i and N_o represent the number of pixels in the input and output FOVs, respectively. We experimentally validated this universal polarization transformation framework in the terahertz part of the spectrum by fabricating wire-grid polarizers and integrating them with 3D-printed diffractive layers to form a physical polarization transformer operating at 0.75 mm wavelength. Through this set-up, we demonstrated an all-optical polarization permutation operation of spatially-varying polarization fields, and simultaneously implemented distinct spatially-encoded polarization scattering matrices between the input and output FOVs of a compact diffractive processor that axially spans 200 wavelengths. This framework opens up new avenues for developing novel optical devices for universal polarization control, and may find various applications in, e.g., remote sensing, medical imaging, security, material inspection and machine vision.

翻译：我们展示了一种基于设计衍射体积的通用偏振变换器，该变换器能够在其输入和输出视场（FOVs）内不同位置的偏振态之间，合成大量任意选择的复值偏振散射矩阵。该框架包含具有不同角度的二维线性偏振器阵列，这些偏振器位于各向同性衍射层之间，每层包含数万个具有可优化透射系数的衍射特征。我们证明，在经过基于深度学习的训练后，该衍射偏振变换器能够在单个衍射体积内，以可忽略的误差成功实现N_i × N_o = 10,000个不同的空间编码偏振散射矩阵，其中N_i和N_o分别代表输入和输出视场中的像素数。我们通过在太赫兹频谱部分制造线栅偏振器，并将其与3D打印衍射层集成，形成工作波长为0.75 mm的物理偏振变换器，来实验验证这一通用偏振变换框架。通过该装置，我们实现了空间变化偏振场的全光偏振置换操作，并在一个轴向跨度达200个波长的紧凑型衍射处理器的输入和输出视场之间，同时实现了不同的空间编码偏振散射矩阵。该框架为开发用于通用偏振控制的新型光学器件开辟了新途径，并可能在遥感、医学成像、安保、材料检测和机器视觉等领域找到多种应用。