Ritesh Agarwal
Most photodetectors detect optical power by converting the total number of photons into a corresponding photocurrent. This approach utilizes only a small subset of the entire information carrying capacity of light and limits the information carrying capacity of photonic networks. Within the semiclassical picture, light-matter interactions can be designed to be extremely versatile to go beyond the simple intensity and wavelength characteristics, therefore taking the full advantage of different degrees of freedom of light, i.e., intensity, frequency, polarization and phase. There is a need to develop on-chip devices that can measure the polarization, intensity and phase gradients of light using direct photocurrent readout method for integrated photonics applications. Fabricating on-chip photonic devices that can detect complex optical modes representing different polarization and complex phase differences and gradients is not an easy task and requires an intricate interplay of symmetry and topology along with the optimization of the geometrical parameters of the device that is compatible with the underlying phase-dependent geometrical response of the material system. Hence, new advances in materials, fundamental understanding of complex light-matter interaction and innovative optical and device engineering based on symmetry and topology are required to solve this problem for making photodetectors that can directly detect the complete vectorial nature of optical beams.
We will first discuss the properties of type-II Weyl semimetals where we observed a new nonlocal photogalvanic effect that goes beyond the electric-dipole approximation [1]. We will describe how spatially inhomogeneous optical excitation along with unique symmetry, band structure and inversion, large Berry curvature and topology of Weyl semimetals produces a strong photogalvanic response. We will then extend this idea to the orbital photogalvanic effect (OPGE), which is driven by the helical phase gradient of optical beam, that is characterized by a current winding around the optical beam axis with a magnitude proportional to its quantized OAM mode number [2]. OPGE accesses different properties of the material via a more complex carrier excitation mechanism and symmetry characteristics and can lead to charge vortices in topological systems. The direct transduction of photocurrents mapped to various SAM-OAM (spin-orbital angular momentum) coupled states is engineered via nonlocal light-matter interactions that cannot be described within the electric-dipole approximation and requires a theoretical description accounting for the topology of electronic bands and light (OAM states with topological charge).
We will also discuss the nonlinear optical Hall effect in self-assembled supertwisted multilayered WS2 system formed by a screw-dislocation-driven mechanism [3]. The optical Hall current direction changed with the structural handedness of the supertwisted system, along with an unusual photon-momentum dependence of the nonlinear optical response in the moire potential. Signatures of thickness-dependent exciton-polaritons and the associated strong photon momentum-lattice interaction dependent photocurrent response were measured, which suggest a fundamentally altered light-matter interaction in 3D moire systems. These responses are analogous to OAM light interacting with non-twisted materials discussed above. Our response function theory explains the origin of the photon momentum dependent nonlinear response, revealing new observables of the system going beyond Berry curvature and other conventional band geometrical quantities. Our study seamlessly connects 2D and 3D twistronics and provides a bridge connecting the electrons and photons by overcoming their significant length scale differences in conventional systems. Finally, we will discuss our approach for the development of-chip generation, transport and detection of chiral optical modes for next-generation chiral photonic circuits.
References
[1] Ji, Zhurun; Liu, Gerui; Addison, Zachariah; Liu, Wenjing; Yu, Peng; Gao, Heng; Liu, Zheng; Rappe, Andrew M.; Kane, Charles L.; Mele, Eugene J.; Agarwal, Ritesh, "Spatially dispersive circular photogalvanic effect in a Weyl semimetal". Nat. Mater. 2019, 18 (9), 955-962. [2] Ji, Zhurun; Liu, Gerui; Krylyuk, Sergiy; Fan, Xiaopeng; Zhang, Zhifeng ; Pan, Anlian; Feng, Liang; Davydov, Albert; Agarwal, Ritesh, "Photocurrent detection of the orbital angular momentum of light". Science 2020, 368 (6492), 763-767. [3] Z. Ji, Y. Zhao, Y. Chen, Z. Zhu, Y. Wang, W. Liu, G. Modi, E. J. Mele, S. Jin and R. Agarwal, “Photon momentum driven nonlinear optical Hall effect in a supertwisted spiral lattice”, Nature, 634, 69 (2024).