Min Namkung

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.

Z-scan and four-wave mixing characterization of semiconductor cadmium chalcogenide nanomaterials

The possible physical origin of third-order nonlinearity of cadmium chalcogenide (Te, Se, and S) semiconductor nanocrystals were discussed based on the results of both Z-scan and degenerate four-wave mixing spectroscopies at 532, 775, 800, and 1064 nm in nanosecond, picosecond, and femtosecond time scale for nonlinear photonic applications.

Time - resolved Gamma Ray spectral analysis of planetary neutron and Gamma Ray instrumentation

The current gamma ray/neutron instrumentation development effort at NASA Goddard Space Flight Center¿s Astrochemistry Laboratory aims to extend the use of neutron interrogation techniques, using a 14 MeV Pulsed Neutron Generator (PNG) combined with neutron and gamma ray detectors, to probe the surface and subsurface of planetary bodies in situ without the need to drill. One aspect of the current work includes the development of taking timed tagged event-byevent data using our custom designed software with the Canberra Lynx Digital Signal Analyzer to provide a unique three-dimensional master data set with channel/energy, time, and intensity information. Since the master data set is not limited to predetermined coincidence timing gates set for a specific nuclear process, the user is allowed the flexibility to slice the data cube in a multitude of ways without loss of information or experimental time due to the need for additional acquisition windows. Time tagged event-by-event data allows the user to isolate a particular energy line from the spectrum over a specific window in time with respect to the PNG pulse, analyze a gamma ray spectrum resulting from either neutron capture, between the burst, or inelastic scattering events, during the neutron burst, and extract data for engineering purposes to optimize timing windows to look at specific elements in different environments. In this paper, we will present the results of our experimental data using the time tagged event-by-event data analysis technique compared with non-time-gated data taken at the test facility at NASA Goddard Space Flight Center. Comparison of these data will show the advantages and validity of this method to obtain more precise, sensitive, and accurate elemental composition measurements.

Adaptation of Pixelated CdZnTe gamma-ray imaging technology for in situ planetary science applications

It is important for planetary sciences to find a gamma-ray spectrometer that is compact, light and provides good energy resolution in an energy range that varies from ∼ 30 keV to 10 MeV. Pixelated CdZnTe detector is a good candidate for this application due to its small volume, light weight and it has demonstrated good energy resolution below ∼2–3 MeV. At higher energies, charge sharing effect becomes a problem due to the size of the pixel. In this paper, the electron cloud diameter as a function of energy has been studied using the MCNPX Monte Carlo code. In the energy range studied, the electron cloud size increases linearly with energy. The effect of charge sharing in a 2×2×1.5 cm3 pixelated CdZnTe detector at 662 keV, 2.614 MeV and 7.631 MeV is shown. The anode is an 11 × 11 pixelated array with 1.72 mm pitch. The simulation reveals that the electron cloud size is about three times the size of the pixel pitch for an 8 MeV event. As the energy of the source increases, charge sharing effect is dominant and the charge cloud is collected by multiple pixels which reduces the spectral performance of the detector.

Polycrystalline mercuric iodide films for novel detector applications

Polycrystalline mercuric iodide (Hgl2) films have been in use as direct-conversion detectors for a number of applications including X-ray, gamma-ray, and charged-particle detectors. One application for particle detectors is as an anticoincidence detector used in conjunction with a large-volume scintillator employed as gamma-ray spectrometer. For space- based applications, background high-energy protons interact with the scintillator to produce spurious signals which interfere with the primary gamma-ray data. The shield detector produces a veto signal so that those detection events in the spectrometer which are due to high-energy charged particle interactions can be ignored, improving the quality of the gamma-ray spectrum collected. We are optimizing a sublimation-based film growth process to grow large-area Hgl2 active anticoincidence shield detectors directly on planar substrates. The growth process is quick and relatively inexpensive, and results in a shield detector that is just a few hundred microns in thickness, thus saving cost and valuable space and weight for the total instrument package. We present recent results from polycrystalline Hgl2 films used for charged-particle detection. These results demonstrate that a novel active anticoincidence shield based on polycrystalline mercuric iodide can act as an alternative to a scintillator/PMT design.

Development of the probing in-situ with Neutron and Gamma rays (PING) instrument for planetary science applications

This paper describes the testing of a prototype active neutron/ gamma ray instrument for use in planetary science space applications. The Probing In situ with Neutrons and Gamma rays (PING) instrument can measure the full bulk elemental composition of a planets surface over a 1 m2 area and down to 30 – 50 cm depth without the need to drill into the surface material. PING consists of three components: a pulsed neutron generator that emits 14 MeV neutrons that penetrate the surface and excite the nuclei of the planetary material; a gamma ray spectrometer that measures the energy and intensity of the gamma rays emitted by these nuclear reactions; and neutron detectors to measure the neutron moderation properties of the material. PING is tested on Earth at a unique facility near NASA/Goddard Space Flight Center where PING can be safely operated outdoors and unshielded sitting atop large (1.8m × 1.8m × .9m), well-characterized granite and basalt monuments. We will describe both this test facility and our experiments, and present gamma ray spectroscopy results that demonstrate PINGs capabilities.

Polarization-resolved cubic nonlinearity and optical power limiting of highly porous silica nanoaerogels

A highly porous silica nanoaerogel has a strong optical power limiting behavior with a negative nonlinear property. The third-order nonlinear susceptibility of silica nanoaerogels was estimated to be ~9.6 x 10^-19 m² / V² (~6.9 x 10^-11 esu) from DFWM measurements.

Slow and Fast Light Using Stimulated Rayleigh-Wing Scattering

Stimulated Rayleigh-Wing Scattering has been found to be a perfect way for group velocity controlling. The velocity of a pulse can be exactly controlled by tuning the wavelength and intensity of a strong CW beam.

Nonlinear Optical Properties of Nanostructured Supramolecular Organic Semiconductor

Resonant third-order optical susceptibility and hyperpolarizability of donor polymer in chloroform were revealed to be ~2.5 − 9.1 × 10−20 m2/V2 and ~8.6 × 10−42 m5/V2 by degenerate four-wave mixing in nanosecond scale at 532 nm, which was attributed to the resonant enhancement.