Integrated photonics is a powerful platform that can improve the performance and stability of optical systems, while providing low-cost, small-footprint and scalable alternatives to implementations based on free-space optics. While great progress has been made on the development of low-loss integrated photonics platforms at telecom wavelengths, visible wavelength range has received less attention. Yet, many applications utilize visible or near-visible light, including those in optical imaging, optogenetics, and quantum science and technology. Here we demonstrate an ultra-low loss integrated visible photonics platform based on thin film lithium niobate on insulator. Our waveguides feature ultra-low propagation loss of 6 dB/m, while our microring resonators have an intrinsic quality factor of 11 million, both measured at 637 nm wavelength. Additionally, we demonstrate an on-chip visible intensity modulator with an electro-optic bandwidth of 10 GHz, limited by the detector used. The ultra-low loss devices demonstrated in this work, together with the strong secondand third-order nonlinearities in lithium niobate, open up new opportunities for creating novel passive, and active devices for frequency metrology and quantum information processing in the visible spectrum range.
I. 1. INTRODUCTIONLow-loss, active and integrated photonic platform operating at visible wavelengths is of great interest for applications ranging from quantum optics and metrology to bio-sensing and bio-medicine. For example, alkali and alkaline earth metals such as rubidium, cesium, calcium and sodium, the key elements for modern precision optical frequency metrology [1-3], magnetometry [4-6] and quantum computation [7-10], have their atomic transitions in visible and near-visible spectrum range. In addition, integrated photonic circuits at visible wavelengths found their way into the fields such as optogenetics [11,12] and bio-sensing [13][14][15]. Furthermore, visible wavelength light is used for quantum state preparation [16], manipulation and read out of color centers [17], quantum dots [18,19] and various quantum emitters in 2d materials [20,21]. Driven by these applications, several materials have been investigated as candidates for visible photonics platform, including SiO 2 [22,23], Si 3 N 4 [24-27], diamond [28-32], TiO 2 [33] and AlN [34]. With exception of AlN, all of these platforms are electro-optically passive and do not allow for fast control of optical signals. Here we show that lithium niobate (LN) is a promising integrated platform for visible photonics, owing to its wide transparency window (400 nm -5000 nm), and large electro-optic coefficient, ∼30 times larger than that of AlN, and strong optical nonlinearity [35]. Our work builds on recently developed thin film lithium niobate (TFLN) substrates [36] and the novel fabrication method [37] which enabled realization of high-performance electro-optical (EO) modulators [38][39][40] and Kerr and EO frequency combs [41,42] in telecom wavelength range (1500-1650nm). TFLN platform has...