Our lab's research focuses on the study of light-matter interaction and optoelectronic phenomena in nanostructures – structures that are comparable or much smaller than the wavelength of light. Using state of the art nanofabrication technology, we develop photonic elements that can be applied for data communications, new color generation, ultimate sensors and quantum technology. Some of the topics currently being investigated include:
Lithium niobate nanophotonics
Single-crystal lithium niobate (LiNbO3, LN) is an excellent nonlinear optical material that is widely used in telecommunications and nonlinear optics. Conventional LN components, however are realized in weakly-perturbed LN waveguides with very low index contrast (Δn < 0.02) and large device dimensions. An integrated LN nanophotonic platform, which combines the unique material properties of LN with the superior light confinement in wavelength scale optical waveguides and cavities, could overcome these limitations and enable efficient, low-cost and highly-integrated nonlinear optical systems.
Based on thin-film LN substrates and standard nanofabrication approaches, we have developed nanophotonic LN devices with ultra-high quality factor (~ 10,000,000) and extremely low propagation loss of 3 dB/m [Optica 4(2017)]. The high-confinement and low-loss lithium niobate photonic circuit could enable a range of electro-optic and nonlinear-optic applications ranging from telecommunications to quantum information processing. Check out Harvard Press Release
Ultra-fast electro-optic modulators
High-performance electro-optic modulators that convert electrical signals into optical domain at high speeds, are key components for optical fiber communications and many other optoelectronic applications. Traditional LN modulators, as the most widely used platform for decades, however have remained in their discrete form due to the lack of appropriate nanofabrication techniques, resulting in significant trade-offs in key modulator metrics. In particular, these modulators cannot directly use the electrical signals from CMOS circuits because of their high driving voltage (> 3.5 V). They require special electrical amplifiers that are prohibitively expensive and power-consuming for next-generation fiber networks.
In our lab, we take advantage of our low-loss and high-confinement LN nanophotonic platform to develop ultra-high performance integrated LN modulators. By combining the superior material properties of LN and microfabrication-enabled integrability, we can realize chip-scale LN modulators that are dramatically smaller, cost orders of magnitude less power, and transmit data at much higher rates. Such systems are ideal for future optical fiber networks for long-distance and data center communications. See more in our paper featured in Nature.
One of the biggest challenges in developing integrated photonic circuits — which use light rather than electrons to transport information — is to control the momentum of light. More specifically, during the wavelength conversion process, colors of light travel at different speeds through a material, but in order for light to be converted between colors, it needs to have the same momentum. In this project, we use nano-engineered meta-structures on top of lithium niobate waveguides to assist the phase matching condition during the nonlinear optical process. In contrast to traditional phase-matching approaches, the devices shown in this work do not need to satisfy the phase-matching requirement, and can convert light in a broad color range.
The converter relies on a metasurface, consisting of an array of silicon nanostructures, integrated into a lithium niobate waveguide. The light passes through waveguide, interacting with the nanostructures along the way. The array of nanostructures act like a TV antenna — receiving the optical signal, manipulating its momentum and re-emitting it back into the waveguide. Check out our paper on Nature Communications and Harvard Press Release!
Our earlier work uses a similar metasurface structure to convert the polarization of light with high efficiency within a very compact area. Find out more in our paper on Nature Nanotechnology!
Efficient wavelength conversion
Second order (χ(2)) nonlinear optical processes, including second harmonic generation (SHG), sum/difference frequency generation (SFG/DFG), and parametric down conversion, not only are crucial for accessing new spectral ranges in classical optics, but also act as key resources for non-classical light generation in quantum information processing. In this project, we utilize our thin-film LN nanowaveguides to design devices that can efficiently convert the color of light from near-infrared to visible. The strong light confinement in our waveguides can boost the efficiency of this process by orders of magnitude compared with conventional weakly-confined devices. The top-down fabrication method we use also allows us to precisely engineer the dispersion properties and phase-matching conditions. Such devices are promising for on-chip quantum wavelength conversion at the single-photon level.