Relaxation of the electro-optic response in thin-film lithium niobate modulators.

Thin-film lithium niobate (TFLN) is a promising electro-optic (EO) photonics platform with high modulation bandwidth, low drive voltage, and low optical loss. However, EO modulation in TFLN is known to relax on long timescales. Instead, thermo-optic heaters are often used for stable biasing, but heaters incur challenges with cross-talk, high power, and low bandwidth. Here, we characterize the low-frequency (1 mHz to 1 MHz) EO response of TFLN modulators, investigate the root cause of EO relaxation and demonstrate methods to improve bias stability. We show that relaxation-related effects can enhance EO modulation across a frequency band spanning 1kHz to 20kHz in our devices - a counter-intuitive result that can confound measurement of half-wave voltage (V π) in TFLN modulators. We also show that EO relaxation can be slowed by more than 104-fold through control of the LN-metal interface and annealing, offering progress toward lifetime-stable EO biasing. Such robust EO biasing would enable applications for TFLN devices where cross-talk, power, and bias bandwidth are critical, such as quantum devices, high-density integrated photonics, and communications.

On-chip electro-optic frequency shifters and beam splitters.

Efficient frequency shifting and beam splitting are important for a wide range of applications, including atomic physics1,2, microwave photonics3-6, optical communication7,8 and photonic quantum computing9-14. However, realizing gigahertz-scale frequency shifts with high efficiency, low loss and tunability-in particular using a miniature and scalable device-is challenging because it requires efficient and controllable nonlinear processes. Existing approaches based on acousto-optics6,15-17, all-optical wave mixing10,13,18-22 and electro-optics23-27 are either limited to low efficiencies or frequencies, or are bulky. Furthermore, most approaches are not bi-directional, which renders them unsuitable for frequency beam splitters. Here we demonstrate electro-optic frequency shifters that are controlled using only continuous and single-tone microwaves. This is accomplished by engineering the density of states of, and coupling between, optical modes in ultralow-loss waveguides and resonators in lithium niobate nanophotonics28. Our devices, consisting of two coupled ring-resonators, provide frequency shifts as high as 28 gigahertz with an on-chip conversion efficiency of approximately 90 per cent. Importantly, the devices can be reconfigured as tunable frequency-domain beam splitters. We also demonstrate a non-blocking and efficient swap of information between two frequency channels with one of the devices. Finally, we propose and demonstrate a scheme for cascaded frequency shifting that allows shifts of 119.2 gigahertz using a 29.8 gigahertz continuous and single-tone microwave signal. Our devices could become building blocks for future high-speed and large-scale classical information processors7,29 as well as emerging frequency-domain photonic quantum computers9,11,14.

Flexible graphene photodetectors for wearable fitness monitoring.

Wearable health and wellness trackers based on optical detection are promising candidates for public health uses due to their noninvasive tracking of vital health signs. However, so far, the use of rigid technologies hindered the ultimate performance and form factor of the wearable. Here, we demonstrate a new class of flexible and transparent wearables based on graphene sensitized with semiconducting quantum dots (GQD). We show several prototype wearable devices that are able to monitor vital health signs noninvasively, including heart rate, arterial oxygen saturation (SpO2), and respiratory rate. Operation with ambient light is demonstrated, offering low-power consumption. Moreover, using heterogeneous integration of a flexible ultraviolet (UV)-sensitive photodetector with a near-field communication circuit board allows wireless communication and power transfer between the photodetectors and a smartphone, offering battery-free operation. This technology paves the way toward seamlessly integrated wearables, and empowers the user through wireless probing of the UV index.