JaeTae Seo

Size-dependent composition and molar extinction coefficient of PbSe semiconductor nanocrystals.

Atomic compositions and molar extinction coefficients of PbSe semiconductor nanocrystals were determined by atomic absorption spectrometry, UV-vis-NIR spectrophotometry, and transmission electron microscopy. The Pb/Se atomic ratio was found to be size-dependent with a systematic excess of Pb atoms in the PbSe nanocrystal system. Experimental results indicated that the individual PbSe nanocrystal was nonstoichiometric, consisting of a PbSe core and an extra layer of Pb atoms. For these nonstoichiometric PbSe semiconductor nanocrystals, we proposed a new computational approach to calculate the total number of Pb and Se atoms in different sized particles. This calculation played a key role on the accurate determination of the strongly size-dependent extinction coefficient, which followed a power law with an exponent of approximately 2.5.

Stability study of PbSe semiconductor nanocrystals over concentration, size, atmosphere, and light exposure.

Infrared-emitting PbSe nanocrystals are of increasing interest in both fundamental research and technical application. However, the practical applications are greatly limited by their poor stability. In this work, absorption and photoluminescence spectra of PbSe nanocrystals were utilized to observe the stability of PbSe nanocrystals over several conventional factors, that is, particle concentration, particle size, temperature, light exposure, contacting atmosphere, and storage forms (solution or solid powder). Both absorption and luminescence spectra of PbSe nanocrystals exposed to air showed dependence on particle concentration, size, and light exposure, which caused large and quick blue-shifts in the optical spectra. This air-contacted instability arising from the destructive oxidation and subsequent collision-induced decomposition was kinetically dominated and differed from the traditional thought that smaller particles with lower concentrations shrank fast. The photoluminescence emission intensity of the PbSe nanocrystal solution under ultraviolet (UV) exposure in air increased first and then decreased slowly; without UV irradiation, the emission intensity monotonously decreased over time. However, if stored under nitrogen, no obvious changes in absorption and photoluminescence spectra of the PbSe nanocrystals were observed even under UV exposure or upon being heated up to 100 degrees C.

Slope efficiency and tunability of a Cr2+: Cd0.85Mn0.15Te mid-infrared laser

Abstract Efficient and widely tunable mid-infrared laser operation from Cr 2+ : Cd 0.85 Mn 0.15 Te was demonstrated. The pulsed Cr 2+ : Cd 0.85 Mn 0.15 Te laser was tuned using a quartz birefringent filter in a spectral range from 2300–2600 nm. Free running laser action centered at ∼2660 nm was achieved using a set of mid-infrared laser mirrors on CaF 2 substrates. The maximum slope efficiency obtained for the free-running laser using an uncoated crystal was 44.2%.

Distortions in Z-scan spectroscopy

The sample imperfection induced distortions in the closed Z-scan curve are analyzed. It is found that both the magnitudes of the peak and valley transmittances and the shape of the curve are distorted significantly by the inhomogeneous thickness, the nonuniform nonlinearity, and the curved surfaces of the sample. The results suggest that the I-scan method is a more practical choice for the measurement of the third-order nonlinear susceptibility of an inhomogeneous sample.

Large pure refractive nonlinearity of nanostructure silica aerogel

The nonlinear refraction coefficient of silica aerogel was estimated to be ∼−1.5×10−15 m2/W (∼−3.67×10−9 esu) with a signal-beam z-scan spectroscopy. The third-order nonlinear refraction coefficient of nanostructure silica aerogel was almost five orders larger than the nonlinear refraction (χ3) of bulk material. The large nonlinear refraction with high nonlinear figure of merit is an ideal optical property for nonlinear optical applications.

Spectroscopic studies of the visible and infrared luminescence from Er doped GaN

Abstract The visible and infrared luminescence of erbium doped gallium nitride prepared by metal-organic molecular beam epitaxy (MOMBE) and solid-source molecular beam epitaxy (SSMBE) were investigated as a function of excitation wavelength and temperature. Both samples exhibited 1.54 μm Er 3+ photoluminescence (PL), but only GaN:Er (SSMBE) showed visible PL lines at 537 and 558 nm. Excitation wavelength dependent PL measurements revealed the existence of multiple Er sites leading to an inhomogeneous line broadening of the Er 3+ intra-4f PL under above-gap pumping. A significant narrowing of the green Er 3+ PL lines was observed when pumping resonantly into an intra-4f transition. This observation suggests that a specific class of Er 3+ ions was selectively excited. A temperature dependent study of the PL intensity ratio and lifetime of the green Er 3+ lines revealed that the two excited states 2 H 11/2 and 4 S 3/2 are thermally coupled. Considering this thermal coupling and assuming that non-radiative decay is negligibly small at low temperatures, the green luminescence efficiency at room temperature was estimated to be near unity.

Time-resolved and temperature-dependent photoluminescence of ternary and quaternary nanocrystals of CuInS2 with ZnS capping and cation exchange

Time-resolved and temperature-dependent photoluminescence (PL) spectroscopy of ternary compound copper indium disulfide (CuInS2, or CIS) core materials, CIS/ZnS coreshells, and quaternary compound ZnCuInS2 (ZnCIS) revealed their optical properties with spectral, temporal, and thermal characteristics, which were closely linked to surface-related recombination, and shallow or deep defect-related donor-acceptor transitions. The PL peaks of semiconductor nanocrystals (SNCs) with sizes near Bohr radius displayed at ∼775 nm for CIS, ∼605 nm for CIS/ZnS, and ∼611 nm for ZnCIS. The spectral blue shift and spectral narrowing with CIS/ZnS and ZnCIS are assigned to the increased spatial confinement and surface regularity with the etching of core materials. Both the shorter lifetime at surface-trapped states or interface states and the longer lifetime at intrinsic defect-related states of CIS, CIS/ZnS, and ZnCIS SNCs were widely distributed across the entire PL spectral region. The surface or interface-trapped electr...

Large Resonant Third-order Optical Nonlinearity of CdSe Nanocrystal Quantum Dots

Resonant third-order nonlinear optical susceptibility and hyperpolarizability of CdSe nanocrystal quantum dots were revealed to be ~2.6×10 -20 - 2.7×10 -19 m 2 /V 2 and ~2.2×10 -40 m 5 /V 2 by using nanosecond degenerate four-wave mixing at 532 nm. The large nonlinearity of the CdSe nanocrystals is attributed to the resonant excitation and multiple nonlinear optical processes.

Third-order Optical Nonlinearities of Singlewall Carbon Nanotubes for Nonlinear Transmission Limiting Application

Third-order nonlinear susceptibility of single wall carbon nanotubes thin film was measured to be ~1.4u10 -16 m 2 /V 2 . The nonlinear transmission limiting threshold of carbon SWNT was ~20 MW/cm 2 with visible and nanosecond laser excitation.

Hybrid optical materials of plasmon-coupled CdSe/ZnS coreshells for photonic applications

A hybrid optical nanostructure of plasmon-coupled SQDs was developed for photonic applications. The coupling distances between the mono-layers of Au nanoparticles with a surface concentration of ~9.18 × 10-4 nm-2 and CdSe/ZnS SQDs with that of ~3.7 × 10-3 nm-2 were controlled by PMMA plasma etching. Time-resolved spectroscopy of plasmon-coupled SQDs revealed a strong shortening of the longest lifetime and ~9-fold PL enhancement. Polarization-resolved PL spectroscopy displayed linear polarization and depolarization at near- and far-field plasmon-coupling, respectively. The physical origin of PL enhancement could be attributable to both the large local field enhancement and the fast resonant energy transfer.