The three-dimensional imaging of threading dislocations in GaN films was demonstrated using two-photon excitation photoluminescence. The threading dislocations were shown as dark lines. The spatial resolutions near the surface were about 0.32 and 3.2 µm for the in-plane and depth directions, respectively. The threading dislocations with a density less than 108 cm−2 were resolved, although the aberration induced by the refractive index mismatch was observed. The decrease in threading dislocation density was clearly observed by increasing the GaN film thickness. This can be considered a novel method for characterizing threading dislocations in GaN films without any destructive preparations.

[INVITED] Three-dimensional imaging of threading dislocations in GaN crystals using two-photon excitation photoluminescence

Multiphoton excitation microscopy at 730 nm and 960 nm was used to image in vivo human skin autofluorescence from the surface to a depth of approximately 200 microm. The emission spectra and fluorescence lifetime images were obtained at selected locations near the surface (0–50 microm) and at deeper depths (100–150 microm) for both excitation wavelengths. Cell borders and cell nuclei were the prominent structures observed. The spectroscopic data suggest that reduced pyridine nucleotides, NAD(P)H, are the primary source of the skin autofluorescence at 730 nm excitation. With 960 nm excitation, a two-photon fluorescence emission at 520 nm indicates the presence of a variable, position-dependent intensity component of flavoprotein. A second fluorescence emission component, which starts at 425 nm, is observed with 960-nm excitation. Such fluorescence emission at wavelengths less than half the excitation wavelength suggests an excitation process involving three or more photons. This conjecture is further confirmed by the observation of the super-quadratic dependence of the fluorescence intensity on the excitation power. Further work is required to spectroscopically identify these emitting species. This study demonstrates the use of multiphoton excitation microscopy for functional imaging of the metabolic states of in vivo human skin cells.

Multiphoton excitation fluorescence microscopy and spectroscopy of in vivo human skin

Realization and optimization of the tunable/enhanced optical properties are critical to further advancing the fields of optoelectronics, photonics, and nanoelectronics. In this context, here, we de...

Size- and Phase-Controlled Nanometer-Thick β-Ga2O3 Films with Green Photoluminescence for Optoelectronic Applications

The mixed cation compounds Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2 crystallize in the polar noncentrosymmetric space group Cc. The AAsQ2 (A = alkali metals, Q = S, Se) family features one-dimensional (1D) 1/∞[AQ2-] chains comprising corner-sharing pyramidal AQ3 units in which the packing of these chains is dependent on the alkali metals. The parallel 1/∞[AQ2-] chains interact via short As···Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As···Se interchain distance increases the band gap from 1.75 eV in γ-NaAsSe2 to 2.01 eV in Na0.35K0.65AsSe2, 2.07 eV in Na0.2K0.8AsSe2, and 2.18 eV in Na0.1K0.9AsS2. The Na1-xKxAsSe2 (x = 0.8, 0.65) compounds melt congruently at approximately 316 °C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of Na1-xKxAsSe2 (x = 0.8, 0.65, 0.5) and Na0.1K0.9AsS2 suggest that Na0.2K0.8AsSe2 and Na0.1K0.9AsS2 have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on Na0.5K0.5AsSe2 confirm its SHG response with the highest value of d33 = 22.5 pm/V (χ333(2) = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be deff = 18.9 pm/V (χeff(2) = 37.8 pm/V), which is consistent with the experimentally obtained value of deff = 16.5 pm/V (χeff(2) = 33.0 pm/V). Three-photon absorption is the dominant mechanism for the optical breakdown of the compounds under intense excitation at 1580 nm, with Na0.2K0.8AsSe2 exhibiting the highest stability.

Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors Na1-xKxAsQ2.

Large second-harmonic generation (SHG) response and broad band gap are two important and competitive parameters. It is difficult to balance them out in one material. In this work, by coupling alkali earth metal (AEM) octahedra with large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap and nonlinear optical (NLO)-active tetrahedra units, nine noncentrosymmetric (NCS) compounds, belonging to a new quaternary chalcogenide family AIB3IIC3IIIQ8VI with unique windmill-like [Mg3M3IIIQ24] (MIII = Al, Ga; Q = S, Se) units constructed by alternated [MgQ6] octahedra and [MIIIQ4] tetrahedra, are rationally designed and fabricated. The compounds show a stable structural framework but adjustable optical properties. Among them, NaMg3Ga3Se8 shows a large SHG response (∼1 × AgGaS2 (AGS)), wide band gap (in selenide) (2.77 eV), high laser-induced damage threshold (LIDT) (∼2.3 × AGS), and suitable birefringence (0.079@546 nm). It should be a potential candidate for infrared (IR) nonlinear optical (NLO) materials. The results enrich the chemical diversity of chalcogenides and open an avenue for the development of new IR NLO materials through the octahedra and tetrahedra coupled strategy.

AIB3IIC3IIIQ8VI: A New Family for the Design of Infrared Nonlinear Optical Materials by Coupling Octahedra and Tetrahedra Units.

β-Ga2O3 is a highly promising semiconductor for a deep ultraviolet (UV) emitter owing to its wide band gap, which significantly varies in the range of 4.49–4.74 eV because of its optical trirefring...

Highly Asymmetric Optical Properties of β-Ga2O3 as Probed by Linear and Nonlinear Optical Excitation Spectroscopy

The new, quaternary diamond-like semiconductor (DLS) Cu4MnGe2S7 was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu4MnGe2S7 features [Ge2S7]6- units and adopts the Cu5Si2S7 structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu2MnGeS4 with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu2MnGeS4 and Cu4MnGe2S7 compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz-Perry powder method, the second-order nonlinear optical susceptibility, χ(2), values for Cu2MnGeS4 and Cu4MnGe2S7 were estimated to be 16.9 ± 2.0 pm V-1 and 2.33 ± 0.86 pm V-1, respectively, by comparing with an optical-quality standard reference material, AgGaSe2 (AGSe). Cu2MnGeS4 was found to be phase matchable at λ = 3100 nm, whereas Cu4MnGe2S7 was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu4MnGe2S7 precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu2MnGeS4 was estimated to be ∼0.1 GW cm-2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu4MnGe2S7 could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations.

Jeong Bin Cho

Papers

Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors Na1-xKxAsQ2.

Highly Asymmetric Optical Properties of β-Ga2O3 as Probed by Linear and Nonlinear Optical Excitation Spectroscopy

Cu4MnGe2S7 and Cu2MnGeS4: two polar thiogermanates exhibiting second harmonic generation in the infrared and structures derived from hexagonal diamond

Third-Order Nonlinear Optical Response-Driven Upconversion Phosphors