Jacob B. Khurgin

When the film thickness is reduced to a few nanometers, ultrathin metal films exhibit distinct behaviors that differ fundamentally from 3D bulk materials and atomically thin 2D films. Recently, such films were classified as transdimensional (TD) plasmonic materials, an intermediate regime between 3D and 2D materials. Here, the strong vertical quantum confinement makes the linear electromagnetic (EM) response of the film nonlocal (spatially dispersive), and the degree of nonlocality can be controlled by the film thickness. This talk reviews recent experimental and theoretical studies of the EM properties of the TD plasmonic material platform, including optical nonlinearity enhancement due to the EM response nonlocality in the epsilon-near-zero region.

The recent, rapid advances in nonlinear chipscale nanophotonics in the visible and near-infrared have been largely driven by manipulating the local dielectric environment proximate to decades-old workhorse bulk nonlinear optical materials, rather than increasing the inherent strength of their nonlinear response. While proposed decades ago, we demonstrate the first, to our knowledge, experimental realization of a new class of designer nonlinear materials that leverage the interband optical transition in asymmetric structures to provide strong second-order susceptibility, χ ⁽²⁾ . Using simple AlGaAs/GaAs coupled quantum wells operating in the near-infrared as a prototype, we observed strong second harmonic generation enhancement of 1550–775 nm over bulk controls. Extracted χ ⁽²⁾ values were as high as 2750 pm/V, which is >7× that of bulk GaAs. Furthermore, measured susceptibilities agreed well with quantum mechanical calculations of χ ⁽²⁾ using layer profiles extracted from electron microscopy. Growth interruptions were employed to improve interfacial abruptness in response to electron microscopy characterization, resulting in increased χ ⁽²⁾ toward the simulation predictions for ideal heterointerfaces. More complex layer designs showed predicted χ ⁽²⁾ up to 7 nm/V. Such materials are anticipated to find myriad applications, including entangled photon generation at telecommunications wavelengths for chipscale quantum information processing.

Plasmonics enables the miniaturization of photonic devices beyond the optical diffraction limit, yet its potential is hindered by inherently large ohmic losses. Hence, it is prudent to explore low-loss alternatives to the current mainstay of plasmonics—the noble metals. In this work, we demonstrate the potential of potassium as a plasmonic material with intrinsically low losses in the optical region. The ultra-flat, high-quality potassium film, fabricated via a rapid slipping-assisted oxide-free crystallization process, achieves measured optical damping rate down to 2.27 meV, with a measured imaginary permittivity of ~0.1 across the entire visible to near-infrared range (400–2000 nm). Near-field optical spectroscopic measurements further confirmed the reduced losses by revealing deeply subwavelength confinement of optical modes. This result overcomes the loss-confinement tradeoff existing in state-of-the-art plasmonic materials and devices, establishing a new platform for exploring extreme light–matter interactions in a variety of plasmonic systems.

Alexandrite lasers exhibit exceptional power-scaling potential at room temperature, yet this capability remains underexplored. We have demonstrated a continuous-wave (CW) alexandrite laser achieving a record output power of 20.4 W in single transverse-mode operation, surpassing all prior reports for laser diode pumped systems. The demonstrated fundamental-mode laser exhibits a high optical-to-optical efficiency of 34%, near-diffraction-limited beam quality M ² < 1.05, and exceptional stability (RMS < 0.2% over 2 hours), stemming from elaborate dynamically stable resonator optimization and excellent heat dissipation. This advance provides compelling evidence for the superior room-temperature power-scaling capability of alexandrite with excellent efficiency and exceptional beam quality .

Wigner crystallization of free electrons at room temperature has been explored theoretically for a new class of metallic ultrathin (transdimensional) materials whose properties can be controlled by their thickness. Our calculations of the melting surface, critical electron density and temperature explain consistently the experimental data reported previously. We show that by reducing the material thickness one can Wigner-crystallize free electrons at room temperature to get them pinned onto a two-dimensional triangular lattice of a supersolid inside of the crystalline material. Such a solid melts and freezes reversibly with increase and decrease of electron doping or temperature, whereby its resistivity behaves opposite to the free electron gas model predictions.

On-chip narrow linewidth microlasers with real-time wavelength tunability are highly desirable for various applications, including precision metrology, quantum technology, and coherent information processing. Although significant progress has been made by various groups in recent years [M. Li et al., Nat. Commun. 13, 5344 (2022)NCAOBW2041-172310.1038/s41467-022-33101-6; V. Snigirev et al., Nature (London) 615, 411 (2023)NATUAS0028-083610.1038/s41586-023-05724-2; X. Zhang et al., Appl. Phys. Lett. 124, 131101 (2024)APPLAB0003-695110.1063/5.0195628; S. Gundavarapu et al., Nat. Photonics 13, 60 (2019)NPAHBY1749-488510.1038/s41566-018-0313-2], realizing such microlasers, especially within compact microresonators with a small footprint, remains a challenge. In this Letter, we overcome these hurdles and demonstrate on-chip electro-optically tunable Brillouin microlasers in compact lithium niobate on insulator microdisks with diameters of 31.5 and 117.0 μm by utilizing cross-polarized stimulated Brillouin scattering configuration. A quasicontinuum band of bound shear mechanical modes inside the suspended microdisk is revealed, allowing feasible phase matching of stimulated Brillouin lasing. We achieve efficient cross-polarized optomechanical coupling and Brillouin lasing via the significant photoelastic tensors of lithium niobate (e.g., p_{41}=-0.151). This approach yields a 118-Hz short-term integral linewidth, a 0.16-Hz intrinsic linewidth, and a comparatively low threshold power of 3.15 mW. A real-time electro-optic tuning with a tuning efficiency of ∼93.1 kHz/V is also achieved, further showcasing potential of the lithium niobate on insulator platform for next-generation tunable photonic systems.

Second-order nonlinear optical materials form the basis for many essential techniques in modern photonics. However, a look at the current state of the field shows that a limited number of materials continue to dominate practical applications. While earlier studies have provided estimates for achievable second-order nonlinear susceptibility, χ ⁽²⁾ , I find it worthwhile to reconsider these estimates in light of factors like bond polarity and the Penn gap. My analysis indicates that while existing materials in longer wavelength ranges have χ ⁽²⁾ values nearing theoretical limits, there is still considerable room for improvement at shorter wavelengths. Still, the magnitude of χ ⁽²⁾ is far from the only factor defining material performance; other characteristics, especially phase matching ability, are also significant. Thus, exploring new χ ⁽²⁾ materials could remain an important area of research for the foreseeable future.