Research

I am broadly interested in the study and manipulation of materials that possess complex optical, electronic and mechanical properties at the nanoscale. Not only can materials with interesting physics be used to construct devices with new functionality, but the devices themselves can also be employed as a platform to probe new science.

Lithium Niobate

Lithium niobate (LiNbO3, LN) is a trigonal crystal well-known in the photonics industry for its usage in the electro-optic modulators foundational to fiber-optic communications. LN is a highly nonlinear material, with large $\chi^{(2)}$ and $\chi^{(3)}$ nonlinear optical susceptibilities, in addition to favorable electro-optic, piezoelectric, photoelastic, and ferroelectric properties. Recently, the development of thin-film lithium niobate (TFLN) in conjunction with advances in microfabrication techniques has enabled much stronger optical confinement than traditional Ti-indiffused LN waveguides, ushering in a new generation of integrated photonic devices.

A review of the material properties can be found here whereas an overview of TFLN’s application in integrated photonics can be found here.

Frequency control of single photons
Although not energy-conserving, the frequency of light can be manipulated in much the same way as other mode characteristics such as polarization, path, and time-bin. Already, light is readily frequency-multiplexed in classical telecommunications to parallelize information transfer. In quantum optics, the wide bandwidth and high dimensionality of the frequency domain could potentially lead to more compact implementations of linear optical quantum computing.

If these schemes are to be compatible with the ITU grid’s existing channel spacings, then high-fidelity, GHz-scale frequency shifts should be made readily available. Using the strong electro-optic effect inherent to LN, we have developed several devices and protocols to perform frequency shifting and beamsplitting on the scale of 10s of GHz, for both pulsed and continuous-wave light inputs. See below for representative works.

Layered Materials (pre-2020)

Layered materials are a broad class of crystals composed of stacks of monolayers held together by van der Waals interactions. In the few-layer limit these materials can exhibit markedly different properties from their bulk counterparts, forming an entire material library including semiconductors, insulators, superconductors, piezoelectrics, ferromagnets and ferroelectrics, among others.

Some examples of layered semiconductors include transition-metal dichalcogenides (TMDCs) and black phosphorus (bP).

TMDC photophysics
In contrast to conventional bulk semiconductors (Si, III-V’s), the room-temperature photophysics of TMDC monolayers is dominated by excitons (bound electron-hole pairs) instead of free carriers. By applying a voltage bias to electrostatically dope the material, we showed that even in the presence of defects the recombination pathways can be entirely radiative, as long as the photogenerated carriers are in the form of neutral excitons. This technique also enabled the first observation of isolated neutral exciton diffusion in TMDC monolayers at room temperature.

Black phosphorus optoelectronics
Black phosphorus is the most stable allotrope of elemental phosphorus, in the same way that graphite is the most stable allotrope for carbon. Like graphene, single- and few-layer sheets can be isolated from the bulk. In this limit, due to its puckered lattice geometry bP is particularly sensitive to mechanical deformation (strain). By using strain as an active knob, we showed that bP is a promising candidate for widely tunable photodetectors and LEDs in the MWIR range, where the absorption spectrum of most toxic gases of interest lie.

Actively variable-spectrum optoelectronics with black phosphorus