Phase retardation is a cornerstone of modern optics, yet, at mid-infrared (mid-IR) frequencies, it remains a major challenge due to the scarcity of simultaneously transparent and birefringent crystals. Most materials resonantly absorb due to lattice vibrations occurring at mid-IR frequencies, and natural birefringence is weak, calling for hundreds of microns to millimeters-thick phase retarders for sufficient polarization rotation. Here, we demonstrate mid-IR phase retardation with flakes of α-MoO₃ that are more than ten times thinner than the operational wavelength, achieving 90 degrees polarization rotation within one micrometer of material. We report conversion ratios above 50% in reflection or transmission mode, and wavelength tunability by several micrometers. Our results showcase that exfoliated flakes of low-dimensional crystals can serve as a platform for mid-IR miniaturized integrated low-loss polarization control.

相位延迟是现代光学的基石，然而，在中红外 (mid-IR) 频率下，由于同时透明和双折射晶体的稀缺，它仍然是一个重大挑战。大多数材料由于中红外频率发生的晶格振动而产生共振吸收，并且自然双折射很弱，需要数百微米到毫米厚的相位延迟器来实现足够的偏振旋转。在这里，我们展示了使用比工作波长薄十倍以上的α -MoO ₃薄片实现的中红外相位延迟，在一微米材料内实现 90 度偏振旋转。我们报告称，反射或透射模式下的转换率超过 50%，波长可调性可达数微米。我们的结果表明，低维晶体的剥离薄片可以作为中红外小型化集成低损耗偏振控制的平台。