Session B09: Optical Phenomena in Non-moiré Graphene Stacks

Optical Properties of Gated Bilayer Graphene Quantum Dots with Trigonal Warping

We study the effects of trigonal warping on the optical properties of gated lateral bilayer graphene (BLG)[1-5] quantum dots (QDs) where both electrons and holes are confined by applied voltages. Building upon previous work [1,2], we employ an atomistic tight-binding model to compute the single-particle QD states and analyze how trigonal warping influences the wavefunctions and energy spectrum as a function of the applied vertical electric field and quantum dot confining potential. We obtain the dipole matrix elements and analyze the impact of trigonal warping and electric field on the oscillator strengths. Furthermore, we incorporate the electron-electron interactions by computing the microscopic Coulomb matrix elements, self-energies, and electron-hole direct attraction and exchange repulsion and solve the Bethe-Salpeter equation to obtain the excitonic absorption spectrum[1,3-5]. Our results reveal an increase in the amplitude of absorption peaks for the low energy exciton states as compared to the case where trigonal warping is neglected, similar to that found in biased BLG encapsulated in hexagonal boron nitride [3-5]. Finally, we compute the absorption spectrum as a function of vertical electric field and lateral quantum dot potential that further amplifies the effects of trigonal warping on the optical properties. This has the potential to tune both bright and dark peaks in the excitonic absorption spectrum, yielding a variety of applications in quantum information [3], storage, detection [1], and voltage-tunable emission properties in the THz photon energy range [4].   

Voltage-induced Modulation of Optical Properties in Multilayer Graphene with Ionic Liquid Intercalation

We report a significant modulation of the optical properties of multilayer graphene using a simple device consisting of a multi-layer graphene/ porous membrane/ gold film stack. Applying a voltage of 3-4V drives the intercalation of anion [TFSI]^- resulting in the reversible modulation of the properties of this optically dense material. The intercalation of [TFSI]^- ions and the associated change in free carrier density leads to abrupt changes in the optical properties of this material. Raman spectra and IR imaging collected during the intercalation/deintercalation half-cycles show the emergence and disappearance of the intercalated G band mode around 1610 cm^-1. The intercalation/deintercalation is also monitored with thermal imaging via voltage-induced changes in the carrier density, complex dielectric function ε(ω), and thermal emissivity of the device.

 

1. “Dynamic Study of Intercalation/Deintercalation of Ionic Liquids in Multilayer Graphene Using an Alternating Current Raman Spectroscopy Technique”  Zhi Cai, Haley Weinstein, Indu Aravind, Ruoxi Li, Sizhe Weng, Boxin Zhang, Jonathan L. Habif, and Stephen B. Cronin. The Journal of Physical Chemistry Letters 0, 14 DOI: 10.1021/acs.jpclett.3c01686. https://pubs.acs.org/doi/epdf/10.1021/acs.jpclett.3c01686

 

2. “Harvesting Planck radiation for free-space optical communications in the long-wave infrared band”[

Haley A. Weinstein, Zhi Cai, Stephen B. Cronin, and Jonathan L. Habif, Opt. Lett. 47, 6225-6228 (2022). https://opg.optica.org/ol/fulltext.cfm?uri=ol-47-23-6225&id=522051

 

3. “Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions”

Zhi Cai, Indu Aravind, Haley Weinstein, Ruoxi Li, Jiangbin Wu, Han Wang, Jonathan Habif, and Stephen Cronin. Journal of Applied Physics. Volume 132,  095102 (2022). https://doi.org/10.1063/5.0093651   

Opacity of graphene independent of light frequency and polarization due to the topological charge of the Dirac points

The opacity of graphene is known to be approximately given by the fine-structure constant $alpha$ times $pi$. We point out the fact that the opacity is roughly independent of the frequency and polarization of the light can be attributed to the topological charge of the Dirac points. As a result, one can literally see the topological charge by naked eyes from the opacity of graphene, and moreover it implies that the fine-structure constant is topologically protected. A similar analysis suggests that 3D topological insulator thin films of any thickness also have opacity $pialpha$ in the infrared region owing to the topological surface states, indicating that one can see the surface states by naked eyes through an infrared lens. For 3D Dirac or Weyl semimetals, the optical absorption power is linear to the frequency in the infrared region, with a linearity given by the fine-structure constant and the topological charge of Weyl points.   

Nanoscale Time-resolved Spectroscopy of Electrically Gated Graphene Nanoribbons

Graphene nanoribbons (GNRs) have shown many interesting electrical and optical properties. We have developed a novel optical spectrometer capable of probing the nonlinear optical response of nanoparticles with dimensions ~10 nm or less, over a wide range of frequencies[1]. The experiments take advantage of strong nonlinearities in SrTiO3 and the ability to “write” conductive nanowires at the LaAlO3/SrTiO3 (LAO/STO) interface with ~10 nm gaps that are co-located with a GNR. We will probe GNRs individually under the influence of large electric fields (~1 MV/cm) with various geometries of electric gates that are both static and dynamic to open a bandgap in the GNR and create a single electron state hosted in the GNR.

 

[1] L. Chen, et al., Light: Science & Appl. 8, 24 (2019).    

Exploring hot-electron plasmons in graphene with nano-infrared imaging and spectroscopy

We report a comprehensive nano-infrared imaging and spectroscopy study of the hot-electron plasmons in graphene using the scattering-type scanning near-field optical microscope (s-SNOM). We found that the electrons can be heated up to 1400 K by femtosecond laser fields that are strongly enhanced by the metalized s-SNOM tip. With s-SNOM, we monitored both the plasmon interference fringes and hybrid plasmon-phonon resonance of graphene due to hot electrons. In highly-doped graphene, we found that the plasmon wavelength is smaller and the plasmon-phonon resonance becomes weaker at high electron temperatures. At the charge neutrality point, on the other hand, the plasmon-phonon resonance is stronger when electrons are hotter. Theoretical analysis indicates that the observed hot-electron responses can be understood by the competition by three major factors, namely the drop of chemical potential, the increase of electron scattering rate, and the excitation of thermal carriers at higher electron temperatures. Our work deepens the understanding of plasmonic responses of graphene at elevated electron temperatures and paves the way for future studies of hot-electron plasmons in other plasmonic media.   

Nano-infrared imaging and spectroscopy of epitaxial graphene on SiC

Epitaxial graphene on silicon carbide (SiC) is one of the most important types of graphene for technological applications due to its scalability, high quality, and potential for generating new electronic properties. In this work, we demonstrate that scattering-type scanning near-field optical microscopy (s-SNOM) is a powerful and efficient technique for characterizing epitaxial graphene due to its capability of high-resolution infrared (IR) imaging and spectroscopy. With nano-IR imaging, we were able to map microscopic graphene domains with various thicknesses and doping. With nano-IR spectroscopy, we found the mapped graphene domains have distinct IR contrasts at different spectral regions due to their interactions with the optical phonon resonance of SiC. Furthermore, we performed quantitative modeling of the nano-IR spectra, based on which we were able to estimate the Fermi energy of the different graphene domains. Our work uncovered new IR responses and surface properties of epitaxial graphene on SiC and paved the way for future large-scale applications of epitaxial graphene in wafer-scale electronics and optoelectronics.   

Chiral Spin Waves in a Dirac Fermi-Liquid with Spin-Orbit Coupling

Spin-orbit coupling (SOC) allows an external electric field to manipulate spins directly through the electron-dipole spin resonance (EDSR). This is particularly useful in characterizing the dynamics of spins in low-dimensional systems with a smaller number of carriers. In this work, we study the EDSR response of a 2D Dirac Fermi-Liquid (FL) with extrinsic SOC, e.g., graphene adsorbed on a transition metal dichalcogenide, and analyze the behavior of the uniform (q=0) collective spin excitations of the system – chiral spin waves – as they evolve with a static in-plane magnetic field. Treating the external effects as weak perturbations, the phenomenological FL theory (extended to a multi-valley system) can be used to describe the structure of the modes as well as obtain the resonant contributions to the optical conductivity.   

Optical Nonlinearities in Triangular Graphene Quantum Dots

We study theoretically interaction of short optical pulses with triangular graphene quantum dots. If such quantum dots have zigzag edges then there are in-gap degenerate edge states, the number of which depends on the size of the system. We show that the nonlinear optical response, such as high harmonic generation, of triangular quantum dots is sensitive to the initial electron population of their edge states. In general, the emission spectra of quantum dots have weak dependence on the number of occupied edge states, but if half of the edge states are initially occupied, which can be realized only in the quantum dots with even number of edge states, then the even high harmonics are strongly suppressed. The suppression is the strongest when the frequency of the pulse is well below the band gap and it is weak when the pulse frequency becomes comparable to the band gap of the system.   

Near-field Measurements of Magneto-edge-plasmon of Graphene

Surface plasmon polariton confines long-wavelength electromagnetic waves to nanometer scale on the interface of a metal and an insulator. The wide tunability and high quality-factor of polariton modes are keys to observe a plethora of quantum effects involving strong light-matter interaction. At low temperature, graphene has been shown to host plasmon polaritons with a long lifetime, which enables graphene plasmons to probe interfacial properties of neighboring materials. Under magnetic field, the plasmon, gapped by cyclotron frequency, becomes topologically nontrivial magneto-plasmon, which contributes to a gapless edge state called magneto-edge-plasmon (MEP). The low damping rate, topological protection, and non-reciprocity of MEP gives rise to a unique propagating pattern. In this talk, we discuss the observation of the magneto-edge-plasmon in both near-field scattering and nano-photocurrent signal using the state-of-the-art magnetic scanning near-field optical microscope (m-SNOM).   

Terahertz electrodynamics of tunable graphene microcavities

VdW heterostructures host a wide range of gate-tunable topological and strongly correlated quantum phenomena that fall on the THz energy scale. Capturing the THz electrodynamics of these systems could provide a unique window into their ground-state physics. However, far-field THz techniques cannot operate on microstructured samples. 

 

We present a generalizable platform for near-field on-chip THz spectroscopy that can access the electrodynamic response of gate-tunable vdW heterostructures from ~30 GHz - 1.2 THz (0.12 - 5 meV). Our results show that graphene heterostructures embedded in ultrafast circuitry act like microcavities, confining the electric field to excite coherent plasmon-polariton modes. We develop an analytical model to describe these cavity modes, which can be tuned by engineering the geometrical properties of the cavity, the dielectric environment, and the carrier density.  

 

These results illustrate the potential for this technique to operate at millikelvin temperatures and strong magnetic fields, bringing THz spectroscopy into the realm traditionally reserved for quantum transport.   

Helical boundary modes from synthetic spin in a plasmonic lattice

Artificial lattices have been used as a platform to extend the application of topological physics beyond electronic systems. Here, using the two-dimensional Lieb lattice as a prototypical example, we show that an array of disks which each support localized plasmon modes give rise to an analog of the quantum spin Hall state enforced by a synthetic time reversal symmetry. We find that an effective next-nearest-neighbor coupling mechanism intrinsic to the plasmonic disk array introduces a nontrivial Z₂ topological order and gaps out the Bloch spectrum. A faithful mapping of the plasmonic system onto a tight-binding model is developed and shown to capture its essential topological signatures. Full wave numerical simulations of graphene disks arranged in a Lieb lattice confirm the existence of propagating helical boundary modes in the nontrivial band gap.   

Electronic interactions in Dirac fluids measured by nano-terahertz spacetime metrology

Electrons in graphene near the charge neutral point form correlated Dirac fluid, which exhibits unusual behaviors including a quantum critical scattering rate, breakdown of Wiedemann-Franz law and large magnetoresistance. Here, we harnessed electron-photon quasiparticles to quantify electronic interactions in a Dirac fluid. To investigate the collective behavior of the Dirac fluid, we experimentally visualized the worldlines of surface plasmon polariton --- trajectories of polaritonic wave packets in space and in time. Our methodology is based on a newly developed spacetime metrology, which utilizes terahertz time-domain nano-imaging. This is implemented in a custom-built cryogenic Terahertz Scanning Near-field Optical Microscopy setting. Leveraging the sub-50nm spatial resolution and femtosecond temporal resolution, we visually depict the electron-photon hybrids within a strongly interacting Dirac fluid, where the dynamics of these hybrids fundamentally stem from graphene’s fine-structure constant. Our work reveals new aspects of light-matter interaction in strongly correlated quantum systems.