Session T37: Quantum geometric advantage in opto-electronics

Cryogenic near-field photocurrent studies of gapped and twisted bilayer graphene systems

Berry curvature is analogous to magnetic field but in momentum space and is commonly present in materials with non-trivial quantum geometry. It endows Bloch electrons with transverse anomalous velocities to produce Hall-like currents even in the absence of a magnetic field.  We present various optoelectronic probing schemes of the Berry curvature in gapped and twisted bilayer graphene, employing infrared and Terahertz radiation. One tangible example is the direct observation of in situ tunable valley-selective Hall effect (VSHE), where inversion symmetry, and thus the geometric phase of electrons, is controllable by an out-of-plane electric field.

 

To take this physics further to the nanoscale, we implemented cryogenic (10K) near-field optical microscopy and near-field photocurrent mapping. This technique allows for 20-nm resolution spatial mapping of the local optical properties, collective excitations and Seebeck coefficient. Our observations allow us to relate the spatial variations of correlated electronic states to spatial variations of the twist angle. Moreover, we observe spatial anisotropies, which shed light on the microscopic mechanisms of symmetry breaking.

    

Moiré quantum geometry: from fundamentals to intelligent sensing

Overlaying two atomic layers with a slight lattice mismatch or at a small rotation angle creates a moiré superlattice with properties that are markedly modified from and at times entirely absent in the ‘parent’ materials. In a series of studies that combine experiments and theories, we have shown that moiré systems are fertile ground for tunable, substantial, rich quantum geometric properties.

First, I will introduce that the superlattice gap of twisted bilayer graphene is associated with unusual Z₂ band topology originating from the (single-state) quantum metric instead of (abelian) Berry curvature, by using K-theory, nonlocal transport, and superconducting quantum interferometry. This resolves the puzzle of flat-band superfluid weight in the magic-angle limit. Further, I will show that for graphene moiré systems the linear resonant optical responses and second-order bulk photovoltaic effects (BPVE), as manifestations of the Hermitian (double-state) metric and connection, are particularly strong and tunable in the infrared range. In the BPVE rendered by moiré quantum geometry, not only are the phase and amplitude of the photovoltage substantially dependent on the excitation light polarization, but they are also highly tunable by external electric fields. This has enabled us to utilize the BPVE as an encoder and a convolutional neural network as a decoder to achieve full-Stokes polarimetry together with wavelength and power detections simultaneously using only one single moiré device with a subwavelength footprint of merely 3 × 3 μm² .

These results not only reveal the significant roles of moiré engineered quantum geometry in strongly correlated phases and in tunable light–matter interactions but also identify a pathway for future intelligent quantum geometric sensing technologies in an extremely compact, on-chip manner across a broad spectral range.

    

Photocurrent generation within the optical gap

The macroscopic directed flow of electrons in response to oscillating electric fields, or bulk rectified photocurrent, is a non-equilibrium phenomenon allowed in systems that break spatial inversion symmetry and is of significant practical and fundamental research interest. A question that has been debated for decades is whether an electric field with a frequency that lies within the optical gap of the crystal can generate a bulk photocurrent in the clean limit. We answer this question affirmatively by demonstrating that this is possible in metals and is consistent with the fundamental principles of thermodynamics. We also show that the Berry curvature dipole is a unique mechanism that behaves as an ideal reversible and dissipationless energy conveyor between the radiation and an external circuit. We will further demonstrate rigorously that the occupation of a periodic driven system does not have a simple Fermi-Dirac distribution function of the Floquet energy, and this generically leads to a finite rectified electric current within the optical gap of a metal even when the carrier relaxation rates vanish.   

Quantum geometric induction of material phases

Quantum geometry, which is often studied through Berry curvature and quantum metric, has been found to dictate the properties of electrons in highly unusual ways. In this presentation, I will discuss how the interaction between quantum geometry and circularly polarized light can be used to control the material phase. I will provide two examples to illustrate this phenomenon. The first example is the realization of the optical chiral induction of the gyrotropic phase in the transition-metal dichalcogenide semimetal 1T-TiSe2. By shining mid-infrared circularly polarized light on 1T-TiSe2 while cooling it below the critical temperature, we are able to induce the preferential formation of one chiral domain. The chirality of this state is confirmed by measuring an out-of-plane circular photogalvanic current (CPGE), the direction of which depends on the optical induction. Our theory suggests that the generation of CPGE and chiral training arise from the interaction between quantum geometry in the chiral electronic phase and the chiral light. The second example is the helicity-dependent optical control of fully-compensated antiferromagnetic (AFM) order in 2D even-layered MnBi2Te4, a topological Axion insulator. We show that the optical control arises from the optical Axion electrodynamics, which can be visualized as a Berry curvature real space dipole.   

Geometrical nonlinear optical effects of correlated electron systems

The responses of materials to high intensity light, i.e., nonlinear optical responses, constitute a vast field of physics and engineering. One of nonlinear optical responses that are attracting a recent keen attention is a bulk photovoltaic effect called shift current which arises from a geometrical (Berry) phase of a Bloch wave function and has a close relationship to the modern theory of electric polarization [1]. While most previous studies of the bulk photovoltaic effects have focused on band insulators of noninteracting electrons, systems of correlated electrons have a potential to support a novel nonlinear functionality. In this talk, I will present novel nonlinear optical effects originating from unique excitations in correlated electron systems, including excitons in semiconductors [2], magnetic excitations in multiferroic materials [3], and phonon excitations in electron-phonon coupled systems [4].