Visible light modulator by sputter-deposited lithium niobate

Thin-film lithium niobate (LN) modulators have significant potential for ultra-high-speed optical communications. Although a significant cost reduction of the modulator is needed to satisfy the growing demands of huge data communications, all the existing studies of thin-film LN modulators were fabricated using the adhesion process of bulk LN to a substrate, and it severely constrains the use case of LN modulators. In order to overcome this critical issue, we have deposited thin-film LN (006) directly on a Al2O3 sapphire (001) substrate and fabricated LN modulators by using the wafer process. Furthermore, previous studies have been largely limited to the use of infrared light (typical wavelengths λ = 1550 nm), which also constrains half-wave voltage and interaction electrode length (Vπ · L) required for modulation. Small Vπ · L is essentially important not only for small die size with lower cost but also for small voltage operation by using a complementary metal oxide semiconductor. Here, we demonstrate the modulation of red (λ = 637 nm), green (λ = 520 nm), and blue (λ = 473 nm) visible light using sputter-deposited thin-film LN; the product of the half-wave voltage and interaction electrode length (Vπ · L) was 1.9, 1.4, and 1.2 V cm, respectively: it decreased with the decreasing wavelength. Thus, LN modulators can be adopted for emerging applications of visible light communications, and wafer-level fabrication using sputter-deposited thin-film LN provides opportunities for future mass production with a much lower fabrication cost.