  The terahertz (THz) frequency range, bridging the gap between microwave and
infrared frequencies, presents unparalleled opportunities for advanced imaging,
sensing, communications, and spectroscopy applications. Terahertz photonics, in
analogy with microwave photonics, is a promising solution to address the
critical challenges in THz technologies through optical methods. Despite its
vast potential, key technical challenges remain in effectively interfacing THz
signals with the optical domain, especially THz-optic modulation and optical
generation of THz waves. Here, we address these challenges using a monolithic
integrated photonic chip designed to support efficient bidirectional
interaction between THz and optical waves. Leveraging the significant
second-order optical nonlinearity and strong optical and THz confinement in a
thin-film lithium niobate on quartz platform, the chip supports both efficient
THz-optic modulation and continuous THz wave generation at up to 500 GHz. The
THz-optic modulator features a radio frequency (RF) half-wave voltage of 8V at
500 GHz, representing more than an order of magnitude reduction in modulation
power consumption from previous works. The measured continuous wave THz
generation efficiency of 4.8*10-6 /W at 500 GHz also marks a tenfold
improvement over existing tunable THz generation devices based on lithium
niobate. We further leverage the coherent nature of the optical THz generation
process and mature optical modulation techniques to realize high-speed
electro-THz modulation at frequencies up to 35 GHz. The chip-scale THz-photonic
platform paves the way for more compact, efficient, and cost-effective THz
systems with potential applications in THz communications, remote sensing, and
spectroscopy.
