Bio

I am an experimental and applied physicist, focusing on extreme light-matter interaction at the nanoscale. I am currently working with Prof. Tony F. Heinz as a postdoctoral researcher in the Department of Applied Physics at Stanford University. Before my current position, I obtained my Ph.D. in Materials Science and Engineering from Stanford University in 2022 under the guidance of Prof. Mark L. Brongersma and my B.Sc. in Physics from Peking University in 2016.

My research concentrates on developing platforms with state-of-the-art tailored (optically resonant) nanostructures to achieve improved control over the photon-electron interaction at the nanoscale. This immediately allows us to create novel photonic and optoelectronic device concepts by coupling free-space lights into a series of well-engineered quantized optical modes and co-engineering electronic and optical components together. We therefore foresee a system-level revolution in industry enabled by nanotechnology. On the other hand, by providing a non-trivial and tunable optical, electrical, and mechanical nano-environment, this platform also fundamentally functions as a versatile tool and offers a new degree of freedom to better probe, study, and control various quantum properties and excitations in solids, especially those enhanced ones in low-dimensional materials. This will ultimately lead us to have a clearer understanding of unconventional phenomena in quantum materials and start to utilize them in a more controllable way.

Professional Education

  • Doctor of Philosophy, Stanford University, MATSC-PHD (2022)
  • Bachelor of Science, Peking University, Physics (2016)

Stanford Advisors

Contact

  • Academic
    qitongli@stanford.edu
    University - Scholar Department: Applied Physics Position: Postdoctoral Scholar

Additional Info

  • Mail Code: 4045

All Publications

  • Optoelectronic metadevices. Science (New York, N.Y.) Ha, S. T., Li, Q., Yang, J. K., Demir, H. V., Brongersma, M. L., Kuznetsov, A. I. 2024; 386 (6725): eadm7442

    Abstract

    Metasurfaces have introduced new opportunities in photonic design by offering unprecedented, nanoscale control over optical wavefronts. These artificially structured layers have largely been used to passively manipulate the flow of light by controlling its phase, amplitude, and polarization. However, they can also dynamically modulate these quantities and manipulate fundamental light absorption and emission processes. These valuable traits can extend their application domain to chipscale optoelectronics and conceptually new optical sources, displays, spatial light modulators, photodetectors, solar cells, and imaging systems. New opportunities and challenges have also emerged in the materials and device integration with existing technologies. This Review aims to consolidate the current research landscape and provide perspectives on metasurface capabilities specific to optoelectronic devices, giving new direction to future research and development efforts in academia and industry.

    View details for DOI 10.1126/science.adm7442

    View details for PubMedID 39607937

  • Direct Exfoliation of Nanoribbons from Bulk van der Waals Crystals. Small (Weinheim an der Bergstrasse, Germany) Saunders, A. P., Chen, V., Wang, J., Li, Q., Johnson, A. C., McKeown-Green, A. S., Zeng, H. J., Mac, T. K., Trinh, M. T., Heinz, T. F., Pop, E., Liu, F. 2024: e2403504

    Abstract

    Confinement of monolayers into quasi-1D atomically thin nanoribbons could lead to novel quantum phenomena beyond those achieved in their bulk and monolayer counterparts. However, current experimental availability of nanoribbon species beyond graphene is limited to bottom-up synthesis or lithographic patterning. In this study, a versatile and direct approach is introduced to exfoliate bulk van der Waals crystals as nanoribbons. Akin to the Scotch tape exfoliation method for producing monolayers, this technique provides convenient access to a wide range of nanoribbons derived from their corresponding bulk crystals, including MoS2, WS2, MoSe2, WSe2, MoTe2, WTe2, ReS2, and hBN. The nanoribbons are predominantly monolayer, single-crystalline, parallel-aligned, flat, andexhibit high aspect ratios. The role of confinement, strain, and edge configuration of these nanoribbons is observed in their electrical, magnetic, and optical properties. This versatile exfoliation technique provides a universal route for producing a variety of nanoribbon materials and supports the study of their fundamental properties and potential applications.

    View details for DOI 10.1002/smll.202403504

    View details for PubMedID 39140377

  • Temperature-Dependent Excitonic Light Manipulation with Atomically Thin Optical Elements. Nano letters Guarneri, L., Li, Q., Bauer, T., Song, J. H., Saunders, A. P., Liu, F., Brongersma, M. L., van de Groep, J. 2024

    Abstract

    Monolayer 2D semiconductors, such as WS2, exhibit uniquely strong light-matter interactions due to exciton resonances that enable atomically thin optical elements. Similar to geometry-dependent plasmon and Mie resonances, these intrinsic material resonances offer coherent and tunable light scattering. Thus far, the impact of the excitons' temporal dynamics on the performance of such excitonic metasurfaces remains unexplored. Here, we show how the excitonic decay rates dictate the focusing efficiency of an atomically thin lens carved directly out of exfoliated monolayer WS2. By isolating the coherent exciton radiation from the incoherent background in the focus of the lens, we obtain a direct measure of the role of exciton radiation in wavefront shaping. Furthermore, we investigate the influence of exciton-phonon scattering by characterizing the focusing efficiency as a function of temperature, demonstrating an increased optical efficiency at cryogenic temperatures. Our results provide valuable insights into the role of excitonic light scattering in 2D nanophotonic devices.

    View details for DOI 10.1021/acs.nanolett.4c00694

    View details for PubMedID 38578061

  • Terahertz Radiation of Plasmonic Hot Carriers Taghinejad, M., Xia, C., Hrton, M., Lee, K., Kim, A. S., Li, Q., Guzelturk, B., Kalousek, R., Xu, F., Cai, W., Lindenberg, A. M., Brongersma, M. L., Lin, S. Y., Adibi, A., Scherer, A. SPIE-INT SOC OPTICAL ENGINEERING. 2024

    View details for DOI 10.1117/12.3010182

    View details for Web of Science ID 001209319500001

  • Determining hot-carrier transport dynamics from terahertz emission. Science (New York, N.Y.) Taghinejad, M., Xia, C., Hrton, M., Lee, K. T., Kim, A. S., Li, Q., Guzelturk, B., Kalousek, R., Xu, F., Cai, W., Lindenberg, A. M., Brongersma, M. L. 2023; 382 (6668): 299-305

    Abstract

    Understanding the ultrafast excitation and transport dynamics of plasmon-driven hot carriers is critical to the development of optoelectronics, photochemistry, and solar-energy harvesting. However, the ultrashort time and length scales associated with the behavior of these highly out-of-equilibrium carriers have impaired experimental verification of ab initio quantum theories. Here, we present an approach to studying plasmonic hot-carrier dynamics that analyzes the temporal waveform of coherent terahertz bursts radiated by photo-ejected hot carriers from designer nano-antennas with a broken symmetry. For ballistic carriers ejected from gold antennas, we find an ~11-femtosecond timescale composed of the plasmon lifetime and ballistic transport time. Polarization- and phase-sensitive detection of terahertz fields further grant direct access to their ballistic transport trajectory. Our approach opens explorations of ultrafast carrier dynamics in optically excited nanostructures.

    View details for DOI 10.1126/science.adj5612

    View details for PubMedID 37856614

  • Impact of substrates and quantum effects on exciton line shapes of 2D semiconductors at room temperature. Nanophotonics (Berlin, Germany) van de Groep, J., Li, Q., Song, J. H., Kik, P. G., Brongersma, M. L. 2023; 12 (16): 3291-3300

    Abstract

    Exciton resonances in monolayer transition-metal dichalcogenides (TMDs) provide exceptionally strong light-matter interaction at room temperature. Their spectral line shape is critical in the design of a myriad of optoelectronic devices, ranging from solar cells to quantum information processing. However, disorder resulting from static inhomogeneities and dynamical fluctuations can significantly impact the line shape. Many recent works experimentally evaluate the optical properties of TMD monolayers placed on a substrate and the line shape is typically linked directly to the material's quality. Here, we highlight that the interference of the substrate and TMD reflections can strongly influence the line shape. We further show how basic, room-temperature reflection measurements allow investigation of the quantum mechanical exciton dynamics by systematically controlling the substrate reflection with index-matching oils. By removing the substrate contribution with properly chosen oil, we can extract the excitonic decay rates including the quantum mechanical dephasing rate. The results provide valuable guidance for the engineering of exciton line shapes in layered nanophotonic systems.

    View details for DOI 10.1515/nanoph-2023-0193

    View details for PubMedID 39634142

    View details for PubMedCentralID PMC11501823

  • A Purcell-enabled monolayer semiconductor free-space optical modulator NATURE PHOTONICS Li, Q., Song, J., Xu, F., van de Groep, J., Hong, J., Daus, A., Lee, Y., Johnson, A. C., Pop, E., Liu, F., Brongersma, M. L. 2023

    View details for DOI 10.1038/s41566-023-01250-9

    View details for Web of Science ID 001031418200001

  • Controlling Valley-Specific Light Emission from Monolayer MoS2 with Achiral Dielectric Metasurfaces. Nano letters Liu, Y., Lau, S. C., Cheng, W., Johnson, A., Li, Q., Simmerman, E., Karni, O., Hu, J., Liu, F., Brongersma, M. L., Heinz, T. F., Dionne, J. A. 2023

    Abstract

    Excitons in two-dimensional transition metal dichalcogenides have a valley degree of freedom that can be optically manipulated for quantum information processing. Here, we integrate MoS2 monolayers with achiral silicon disk array metasurfaces to enhance and control valley-specific absorption and emission. Through the coupling to the metasurface electric and magnetic Mie modes, the intensity and lifetime of the emission of neutral excitons, trions, and defect bound excitons can be enhanced and shortened, respectively, while the spectral shape can be modified. Additionally, the degree of polarization (DOP) of exciton and trion emission from the valley can be symmetrically enhanced at 100 K. The DOP increase is attributed to both the metasurface-enhanced chiral absorption of light and the metasurface-enhanced exciton emission from the Purcell effect. Combining Si-compatible photonic design with large-scale 2D materials integration, our work makes an important step toward on-chip valleytronic applications approaching room-temperature operation.

    View details for DOI 10.1021/acs.nanolett.3c01630

    View details for PubMedID 37347949

  • Impact of substrates and quantum effects on exciton line shapes of 2D semiconductors at room temperature NANOPHOTONICS van de Groep, J., Li, Q., Song, J., Kik, P. G., Brongersma, M. L. 2023

    View details for DOI 10.1515/nanoph-2023-0193

    View details for Web of Science ID 001015007800001

  • Quantitative phase contrast imaging with a nonlocal angle-selective metasurface. Nature communications Ji, A., Song, J. H., Li, Q., Xu, F., Tsai, C. T., Tiberio, R. C., Cui, B., Lalanne, P., Kik, P. G., Miller, D. A., Brongersma, M. L. 2022; 13 (1): 7848

    Abstract

    Phase contrast microscopy has played a central role in the development of modern biology, geology, and nanotechnology. It can visualize the structure of translucent objects that remains hidden in regular optical microscopes. The optical layout of a phase contrast microscope is based on a 4 f image processing setup and has essentially remained unchanged since its invention by Zernike in the early 1930s. Here, we propose a conceptually new approach to phase contrast imaging that harnesses the non-local optical response of a guided-mode-resonator metasurface. We highlight its benefits and demonstrate the imaging of various phase objects, including biological cells, polymeric nanostructures, and transparent metasurfaces. Our results showcase that the addition of this non-local metasurface to a conventional microscope enables quantitative phase contrast imaging with a 0.02π phase accuracy. At a high level, this work adds to the growing body of research aimed at the use of metasurfaces for analog optical computing.

    View details for DOI 10.1038/s41467-022-34197-6

    View details for PubMedID 36543788

    View details for PubMedCentralID PMC9772391

  • Metasurface optofluidics for dynamic control of light fields. Nature nanotechnology Li, Q., van de Groep, J., White, A. K., Song, J., Longwell, S. A., Fordyce, P. M., Quake, S. R., Kik, P. G., Brongersma, M. L. 2022

    Abstract

    The ability to manipulate light and liquids on integrated optofluidics chips has spurred a myriad of important developments in biology, medicine, chemistry and display technologies. Here we show how the convergence of optofluidics and metasurface optics can lead to conceptually new platforms for the dynamic control of light fields. We first demonstrate metasurface building blocks that display an extreme sensitivity in their scattering properties to their dielectric environment. These blocks are then used to create metasurface-based flat optics inside microfluidic channels where liquids with different refractive indices can be directed to manipulate their optical behaviour. We demonstrate the intensity and spectral tuning of metasurface colour pixels as well as on-demand optical elements. We finally demonstrate automated control in an integrated meta-optofluidic platform to open up new display functions. Combined with large-scale microfluidic integration, our dynamic-metasurface flat-optics platform could open up the possibility of dynamic display, imaging, holography and sensing applications.

    View details for DOI 10.1038/s41565-022-01197-y

    View details for PubMedID 36163507

  • Fundamental Limitations of Huygens' Metasurfaces for Optical Beam Shaping LASER & PHOTONICS REVIEWS Gigli, C., Li, Q., Chavel, P., Leo, G., Brongersma, M. L., Lalanne, P. 2021

    View details for DOI 10.1002/lpor.202000448

    View details for Web of Science ID 000660500800001

  • Structural color from a coupled nanowire pair beyond the bonding and antibonding model OPTICA Li, Q., Wu, T., van de Groep, J., Lalanne, P., Brongersma, M. L. 2021; 8 (4): 464-470

    View details for DOI 10.1364/OPTICA.418888

    View details for Web of Science ID 000642200300006

  • Nanoelectromechanical modulation of a strongly-coupled plasmonic dimer. Nature communications Song, J., Raza, S., van de Groep, J., Kang, J., Li, Q., Kik, P. G., Brongersma, M. L. 2021; 12 (1): 48

    Abstract

    The ability of two nearly-touching plasmonic nanoparticles to squeeze light into a nanometer gap has provided a myriad of fundamental insights into light-matter interaction. In this work, we construct a nanoelectromechanical system (NEMS) that capitalizes on the unique, singular behavior that arises at sub-nanometer particle-spacings to create an electro-optical modulator. Using in situ electron energy loss spectroscopy in a transmission electron microscope, we map the spectral and spatial changes in the plasmonic modes as they hybridize and evolve from a weak to a strong coupling regime. In the strongly-coupled regime, we observe a very large mechanical tunability (~250meV/nm) of the bonding-dipole plasmon resonance of the dimer at ~1nm gap spacing, right before detrimental quantum effects set in. We leverage our findings to realize a prototype NEMS light-intensity modulator operating at ~10MHz and with a power consumption of only 4 fJ/bit.

    View details for DOI 10.1038/s41467-020-20273-2

    View details for PubMedID 33397929

  • Quantitative Phase Contrast Imaging using Guided-mode Resonator Devices Ji, A., Song, J., Li, Q., Kik, P. G., Miller, D. B., Brongersma, M. L., IEEE IEEE. 2021

    View details for Web of Science ID 000831479803156

  • Exciton Resonance Tuning in Atomically-Thin Optical Elements van de Groep, J., Song, J., Li, Q., Celano, U., Kik, P. G., Brongersma, M. L., IEEE IEEE. 2021

    View details for Web of Science ID 000831479802173

  • Exciton resonance tuning of an atomically thin lens NATURE PHOTONICS van de Groep, J., Song, J., Celano, U., Li, Q., Kik, P. G., Brongersma, M. L. 2020

    View details for DOI 10.1038/s41566-020-0624-y

    View details for Web of Science ID 000529001600001

  • Transparent multispectral photodetectors mimicking the human visual system. Nature communications Li, Q., van de Groep, J., Wang, Y., Kik, P. G., Brongersma, M. L. 2019; 10 (1): 4982

    Abstract

    Compact and lightweight photodetection elements play a critical role in the newly emerging augmented reality, wearable and sensing technologies. In these technologies, devices are preferred to be transparent to form an optical interface between a viewer and the outside world. For this reason, it is of great value to create detection platforms that are imperceptible to the human eye directly onto transparent substrates. Semiconductor nanowires (NWs) make ideal photodetectors as their optical resonances enable parsing of the multi-dimensional information carried by light. Unfortunately, these optical resonances also give rise to strong, undesired light scattering. In this work, we illustrate how a new optical resonance arising from the radiative coupling between arrayed silicon NWs can be harnessed to remove reflections from dielectric interfaces while affording spectro-polarimetric detection. The demonstrated transparent photodetector concept opens up promising platforms for transparent substrates as the base for opto-electronic devices and in situoptical measurement systems.

    View details for DOI 10.1038/s41467-019-12899-8

    View details for PubMedID 31676782

  • Spin-Switched Three-Dimensional Full-Color Scenes Based on a Dielectric Meta-hologram ACS PHOTONICS Feng, H., Li, Q., Wan, W., Song, J., Gong, Q., Brongersma, M. L., Li, Y. 2019; 6 (11): 2910–16

    View details for DOI 10.1021/acsphotonics.9b01017

    View details for Web of Science ID 000499742000037

  • Reversible and selective ion intercalation through the top surface of few-layer MoS2. Nature communications Zhang, J., Yang, A., Wu, X., van de Groep, J., Tang, P., Li, S., Liu, B., Shi, F., Wan, J., Li, Q., Sun, Y., Lu, Z., Zheng, X., Zhou, G., Wu, C., Zhang, S., Brongersma, M. L., Li, J., Cui, Y. 2018; 9 (1): 5289

    Abstract

    Electrochemical intercalation of ions into the van der Waals gap of two-dimensional (2D) layered materials is a promising low-temperature synthesis strategy to tune their physical and chemical properties. It is widely believed that ions prefer intercalation into the van der Waals gap through the edges of the 2D flake, which generally causes wrinkling and distortion. Here we demonstrate that the ions can also intercalate through the top surface of few-layer MoS2 and this type of intercalation is more reversible and stable compared to the intercalation through the edges. Density functional theory calculations show that this intercalation is enabled by the existence of natural defects in exfoliated MoS2 flakes. Furthermore, we reveal that sealed-edge MoS2 allows intercalation of small alkali metal ions (e.g., Li+ and Na+) and rejects large ions (e.g., K+). These findings imply potential applications in developing functional 2D-material-based devices with high tunability and ion selectivity.

    View details for PubMedID 30538249

  • Order and Disorder Embedded in a Spectrally Interleaved Metasurface ACS PHOTONICS Yannai, M., Maguid, E., Faerman, A., Li, Q., Song, J., Kleiner, V., Brongersma, M. L., Hasman, E. 2018; 5 (12): 4764–68

    View details for DOI 10.1021/acsphotonics.8b01138

    View details for Web of Science ID 000454463000007

  • Spectrally interleaved topologies using geometric phase metasurfaces OPTICS EXPRESS Yannai, M., Maguid, E., Faerman, A., Li, Q., Song, J., Kleiner, V., Brongersma, M. L., Hasman, E. 2018; 26 (23): 31031–38

    Abstract

    Metasurfaces facilitate the interleaving of multiple topologies in an ultra-thin photonic system. Here, we report on the spectral interleaving of topological states of light using a geometric phase metasurface. We realize that a dielectric spectrally interleaved metasurface generates multiple interleaved vortex beams at different wavelengths. By harnessing the space-variant polarization manipulations that are enabled by the geometric phase mechanism, a vectorial vortex array is implemented. The presented interleaved topologies concept can greatly enhance the functionality of advanced microscopy and communication systems.

    View details for DOI 10.1364/OE.26.031031

    View details for Web of Science ID 000449972600116

    View details for PubMedID 30469990