Sang-Hyuk Park

The temperature-dependent photoluminescence of CsPbBr₃/SiO₂ and CsPbI₃/SiO₂ nanocrystals was investigated to understand the thermal stability of SiO₂ encapsulation. At increased temperature, intensity quenching, linewidth broadening, energy level shift, and decay dynamics were evaluated as quantified parameters. Comprehensive analysis of these parameters supports that CsPbI₃/SiO₂ nanocrystals show a stronger interaction with phonons compared with CsPbBr₃/SiO₂ nanocrystals. Despite SiO₂ encapsulation, we conclude that trapping states are still present and the degree of localization can be characterized in terms of short-lived decay time and thermal activation energy.

Achieving efficient optical coupling between the emission from perovskite quantum dots (PQDs) and photonic integrated elements requires ultranarrow linewidths and highly directional emission. These are challenging goals at room temperature due to the broad and isotropic nature of perovskite emission. Here, we demonstrate ultranarrow‐linewidth emission from CsPbBr ₃ PQDs at room temperature, in both spontaneous and stimulated regimes, by coupling to state‐of‐the‐art open‐access curved dielectric cavities under continuous wave excitation. The emission is confined to a single transverse electromagnetic mode of the cavity, achieving a remarkably narrow linewidth of 0.2 nm, ≈100× narrower than free‐space emission in both the emission regime. Single‐mode lasing from a small number of PQDs is observed, yielding a quality factor of ≈2590, among the highest reported for single‐mode lasing. The open‐access design enables precise tuning of cavity length and selective coupling of emitters in their native state, overcoming the limitations associated with closed and fixed‐length vertical‐cavity surface emitting laser geometries. The geometry's low divergence and tunability provide an efficient route for integrating perovskite emitters with on‐chip photonic circuits, advancing their use in quantum and optoelectronic technologies.

We have successfully produced an ultrathin freely suspended GO film, which is a biomimetic structure inspired by the transparent dragonfly wing structure. Based on a colloidal self-assembly process over a large area, solvent evaporation was applied within a limited opening geometry. The free-standing GO film shows a significant enhancement of the nonlinear optical absorption, where saturable absorption and photoinduced absorption were observed at dramatically decreased excitation fluence compared with other work on GO films dispersed on substrates. Surprisingly, we also found that free-standing GO films are beneficial for compressing femtosecond pulses around 800 nm. Using a frequency-resolved optical gating as well as an open aperture Z-scan method, the origin was found to be associated with two effects. While the pulse shortening results from saturable absorption, the chirp effect is also suppressed due to the presence of an inflection point around 800 nm in the refractive index spectrum of free-standing GO film.

We have estimated the intrinsic complex refractive index spectrum of a CsPbBr₃ nanocrystal. With various dilute solutions of CsPbBr₃ nanocrystals dissolved in toluene, effective refractive indices were measured at two different wavelengths using Michelson interferometry. Given the effective absorption spectrum of the solution, a full spectrum of the effective refractive index was also obtained through the Kramers–Krönig relations. Based on the Maxwell–Garnett model in the effective medium approximation, the real and imaginary spectrum of the complex refractive index was estimated for the CsPbBr₃ nanocrystal, and the dominant inaccuracy was attributed to the size inhomogeneity.

We present an effective protocol for extracting localization energy from WSe₂ monolayers, based on temperature-dependent spectral variation of photoluminescence (PL), and observe time-sequential changes over a long period of time. We found that even freshly exfoliated WSe₂ exhibits deviation from the Varshni formula upon exposure to air, while the temperature-dependent PL lineshapes of h-BN encapsulated WSe₂ match well with delocalized features for as long as two years of the aging period. Our findings support the prospect of h-BN encapsulation for monoatomic transition metal dichalcogenides to meet the longevity condition, which is a prerequisite for practical components in the electronics industry.

We report on new THz electromagnetic emission mechanism from deformational coupling of acoustic (AC) phonons with electrons in the propagation medium of non-polar Si. The epicenters of the AC phonon pulses are the surface and interface of a GaP transducer layer whose thickness (d) is varied in nanoscale from 16 to 45 nm. The propagating AC pulses locally modulate the bandgap, which in turn generates a train of electric field pulses, inducing an abrupt drift motion at the depletion edge of Si. The fairly time-delayed THz bursts, centered at different times ( t 1 T H z , t 2 T H z , and t 3 T H z ), are concurrently emitted only when a series of AC pulses reach the point of the depletion edge of Si, even without any piezoelectricity. The analysis on the observed peak emission amplitudes is consistent with calculations based on the combined effects of mobile charge carrier density and AC-phonon–induced local deformation, which recapitulates the role of deformational potential coupling in THz wave emission in a formulatively distinct manner from piezoelectric counterpart.

Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells with low cost and eco-friendly characteristics are attractive as future sources of electricity generation, but low conversion efficiency remains an issue. To improve conversion efficiency, a method of inserting intermediate layers between the CZTSSe absorber film and the Mo back contact is used to suppress the formation of MoSe₂ and decomposition of CZTSSe. Among the candidates for the intermediate layer, graphene oxide (GO) and reduced GO have excellent properties, including high-charge mobility and low processing cost. Depending on the type of GO, the solar cell parameters, such as fill factor (FF), were enhanced. Thus, the conversion efficiency of 6.3% was achieved using the chemically reduced GO intermediate layer with significantly improved FF.