Donghan Lee

Investigating single-photon dynamics is crucial for characterization of single-photon emitters. Here, we demonstrate a telecom-band quantum dot single-photon emitter integrated with a lithographically aligned double solid immersion lens structure, achieving a collection efficiency of 18(3)% at the first lens. Using two-time correlation measurements, we observed re-excitation-induced multiphoton emission arising from a long-lived, carrier-induced capture process under non-resonant excitation. Under quasi-resonant excitation, this effect was strongly suppressed, reducing the multiphoton emission probability within a 3 ns window from 0.052(1) to 0.0163(4). We further performed second-order quantum spectroscopy based on Fourier analysis of the Hong–Ou–Mandel interference, enabling direct extraction of the exciton fine-structure splitting. This work provides a direct characterization of re-excitation dynamics and coherence properties in a telecom-band single-photon emitter.

Plasmonic molecules, which are geometrically well-defined plasmonic metal nanoparticle clusters, have attracted significant attention due to their enhancement of light–matter interactions owing to a stronger electric field enhancement than that by single particles. High-resolution lithography techniques provide precise positioning of plasmonic nanoparticles, but their fabrication costs are excessively high. In this study, we propose a lithography-free, self-assembly fabrication method, termed the dual-dewetting process, which allows the control of the size and density of gold nanoparticles. This process involves depositing a gold thin film on a substrate and inducing dewetting through thermal annealing, followed by a second deposition and annealing. The method achieves a uniform distribution of particle size and density, along with increased particle density, across a 6-inch wafer. The superiority of the method is confirmed by a 30-fold increase in the signal intensity of surface-enhanced Raman scattering following the additional dewetting with an 8 nm film, compared to single dewetting alone. Our findings indicate that the dual-dewetting method provides a simple and efficient approach to enable a variety of plasmonic applications through efficient plasmonic molecule large-area fabrication.

Bacteriophages demonstrate a remarkable ability to adhere to host surfaces, thus improving their chances of reproduction. These viral entities demonstrate extreme interface properties through their highly specific and periodic peptide receptors, surpassing any manmade surface in terms of variability and adhesiveness. This intriguing quality has led to investigations into biohybrid nanostructures, wherein bacteriophages are combined with inorganic substances. Among them, cesium lead halide (CsPbI₃) perovskite quantum dots (PQDs) are promising emissive materials, with their optical characteristics being vital for the advancement of light-emitting and optoelectronic apparatuses. In this study, we explored the integration of M13 bacteriophages (phages) with CsPbI₃ PQDs. Our observations indicated that the photoluminescence of CsPbI₃ + M13 phage was amplified 7.7-fold compared to pure CsPbI₃, the lifetime of the quantum dots extended from 40.47 ns to 53.32 ns and enhanced the stability. Simulations and experimental results both demonstrate the significant role of M13 bacteriophages in achieving enhanced optical properties for PQDs. These findings confirm the significant contribution of M13 phages to enhancing the optical attributes in PQDs, laying the groundwork for innovative optoelectronic applications.

Mode conversion is crucial for coupling a light source to a desired waveguide. While traditional mode converters such as fiber Bragg gratings and long-period fiber gratings exhibit high transmission and conversion efficiency, the mode conversion of two orthogonal polarizations remains challenging. Here, we present a bidirectional metasurface mode converter that can convert the transverse electric (TE)01 or transverse magnetic (TM)01 mode to the fundamental mode (LP01) with orthogonal polarization, and vice versa. The mode converter is located on a facet of a few-mode fiber and connected to a single mode fiber. Through simulations, we find that 99.9% of the TM01 or TE01 mode is converted into the x- or y-polarized LP01 mode, and that 99.96% of the x- or y-polarized LP01 mode is converted to the TM01 or TE01 mode. Furthermore, we expect a high transmission of over 84.5% for all mode conversions, up to 88.7% for TE01 to y-polarized LP01 conversion.

High-brightness single photon sources (SPSs) are key components for practical quantum information processing systems. Although the performances of recently reported high-brightness SPSs are excellent, it remains challenging to match the emission wavelength of a quantum dot (QD) to the cavity since the high-Q cavity structures have narrow spectral bandwidths. Here, we propose a highly bright and broadband QD SPS that can be deterministically fabricated with a simple yet precise method. The optimized GaAs-polymer double solid immersion lens structure is capable of a brightness of 88% at 0.5 NA and has an operation band of 65 nm with brightness over 80% from numerical simulations. Experimentally, we achieved a brightness of 51.6{plus minus}2% and pure single photon emission [g⁽²⁾(0) = 0.029{plus minus}0.005] at the saturation. We believe that our result can pave the way to a practical high-brightness QD SPS, considering its simple QD geometry together with its low cost, precise deterministic fabrication without using expensive and complicated e-beam lithography and dry etching processes.

Hyun-Ju Ahn, Yudong Jang, and Donghan Lee. Applied Science and Convergence Technology 2022;31:60-2. https://doi.org/10.5757/ASCT.2022.31.2.60

Detecting trace amounts of explosives to ensure personal safety is important, and this is possible by using laser-based spectroscopy techniques. We performed surface-enhanced Raman scattering (SERS) using plasmonic nanogap substrates for the solution phase detection of some nitro-based compounds, taking advantage of the hot spot at the nanogap. An excitation wavelength of 785 nm with an incident power of as low as ≈0.1 mW was used to excite the nanogap substrates. Since both RDX and PETN cannot be dissolved in water, acetone was used as a solvent. TNT was dissolved in water as well as in hexane. The main SERS peaks of TNT, RDX, and PETN were clearly observed down to the order of picomolar concentration. The variations in SERS spectra observed from different explosives can be useful in distinguishing and identifying different nitro-based compounds. This result indicates that our nanogap substrates offer an effective approach for explosives identification.

The integration of single quantum dots (QDs) into a planar Fabry–Pérot microcavity has been established as a direct and viable approach to vertically steer photons emitted from the quantum emitters, resulting in a strong increase in the source brightness, which becomes particularly evident when a lens with a low numerical aperture is used. However, the spectral bandwidth of QD–microcavity structures is limited and determined by their intrinsic quality factor; these structures are, thus, not ideal for the extraction of entangled photon pairs or for studies of exciton dynamics. We have found that, when a deterministic low-index solid immersion lens is placed on top of the QD in a QD–microcavity structure, the structure exhibits an enhancement in the bandwidth to 27 nm and a source brightness of 23%. The solid immersion lens is deterministically fabricated via two-photon absorption and can be remade several times without perturbing the QD, ensuring that the QD's intrinsic properties are preserved and ensuring its long-term reliability.