Coviretinopathy: COVID-19-induced VEGF-dependent retinopathy.

Compact and efficient control of laser emission is an essential feature in diverse scientific and technological contexts, including laser ranging (, ), high-resolution spectroscopy (, ), optical communications (–), and three-dimensional (3D) printing (). Since the mid-1970s (), tremendous efforts have gone into the development of external cavity diode laser (ECL) designs with a broad range of configurations optimized for wavelength tunability and spectral purity (–). Early proposals relied predominantly on mechanical tuning, which limited speed and stability. To enable faster, voltage-controlled actuation, electro-optic (EO) crystals such as lithium niobate () and lithium tantalate () were later introduced as tuning elements (). However, modulators made from these materials were bulky and required a grating in addition to the modulator to simultaneously achieve single-mode operation and high-speed control over the laser wavelength (). These multicomponent systems were prone to multiple reflections and misalignment over time, ultimately affecting the long-term stability of the entire system. The lack of compact modulators put a roadblock to reducing the system footprint while maintaining fast and single-mode operation. Recent advances in integrated photonic platforms have begun to propose solutions using on-chip EO and piezoelectric modulators that enable ultrafast tunable lasers (–, ). However, similar capabilities have not been achieved for free-space laser systems, where the intracavity light propagates in free space rather than in a waveguide (). In contrast to integrated photonic lasers, these systems would be immediately applicable to free-space applications such as imaging (–), ranging (, –), diffractive optics, and light projection (–). They are also attractive for applications where propagation in fibers or along waveguides can lead to uncontrolled dispersion, nonlinearities, and temperature fluctuations (–). Optical metasurfaces, i.e., planar arrays of subwavelength elements, condense the functionalities of multiple bulky and complex optical components into a single, engineered 2D surface (, , –). Their compactness makes them particularly important in applications requiring a small footprint, along with improved long-term robustness and stability. In most applications, metasurfaces are used downstream of the laser, where they manipulate the space-time characteristics of the light with minimal component overhead, simplifying an optical setup (). In this case, they do not affect the lasing process.