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254 results on '"Rowell, Nelson"'

3. Realization of a Laser Cooled Microwave Power Standard (conference digest)

Author

Paulusse, David C., Rowell, Nelson L., and Michaud, Alain

Subjects
Physics - Instrumentation and Detectors
Abstract

We demonstrate the feasibility of a novel microwave power standard based on the electromagnetic interaction with cold atoms. Under the effect of the radiation, the internal state populations will undergo a Rabi oscillation. The measurement of its frequency will allow the determination of the electromagnetic field strength., Comment: 2 pages, 1 figure

Published
2005
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4. Microwave Power Standard using Cold Atoms

Author

Paulusse, David C., Rowell, Nelson L., and Michaud, Alain

Subjects
Physics - Instrumentation and Detectors
Abstract

We discuss how the observation of Rabi flopping oscillations in a laser cooled atomic sample could be used as a microwave power standard. The rubidium atoms are first trapped in a standard MOT, then optically pumped, and dropped. As they enter the interaction region, a resonant pulsed microwave field is applied. Following the interaction lasting up to 10 ms, a probe laser beam is turned on and the fluorescence measures the population inversion., Comment: 1 page, 2 figures

Published
2005

5. Accuracy of an Atomic Microwave Power Standard (Conference digest)

Author

Paulusse, David C., Rowell, Nelson L., and Michaud, Alain

Subjects
Physics - Instrumentation and Detectors
Abstract

We have studied the accuracy of the atomic microwave power standard. The atoms are cooled and kept in a Magneto-Optical Trap (MOT), then dropped through a terminated transmission line (a rectangular, R-70 type, waveguide). The measurement of the internal atomic state allows an accurate determination of the transmitted microwave power., Comment: 2 pages, 3 figures

Published
2005

6. Realization of a Laser Cooled Microwave Power Standard

Author

Paulusse, David C., Rowell, Nelson L., and Michaud, Alain

Subjects
Physics - Atomic Physics, Physics - Instrumentation and Detectors
Abstract

We demonstrate the feasibility of a novel microwave power standard based on the electromagnetic interaction with laser-cooled atoms. Under the effect of the radiation, the internal state populations undergo a Rabi flopping oscillation. Measurement of the oscillation frequency allows the determination of the electromagnetic field strength. The measurements are made over a dynamic range of 18 dB and the standard deviation of the measurements is about 0.3%. The field strength was linear with the Rabi frequency within the accuracy of our measurements., Comment: Presented at the Conference on Precision Electromagnetic Measurements (CPEM 2002), Ottawa, ON. Canada, 16-21 June 2002

Published
2005

7. Accuracy of an Atomic Microwave Power Standard

Author

Paulusse, David C., Rowell, Nelson L., and Michaud, Alain

Subjects
Physics - Atomic Physics, Physics - Instrumentation and Detectors
Abstract

We have built an atomic microwave power standard based on the electromagnetic interaction with laser-cooled atoms. The atoms traversed a waveguide transmission line, and under the effect of the radiation, the internal state populations underwent a Rabi flopping oscillation. Measurement of the oscillation frequency allowed the determination of the incident microwave power. As many of 60 oscillations were observed over a dynamic range of 20 dB and the standard deviation of the measurements was about 0.02%. The measured frequency was compared to a calculated one and an agreement of 1.3% with an uncertainty of 5% (rectangular) was found., Comment: 5 figures

Published
2005

11. Transformations of Magic-Size Clusters via Precursor Compound Cation Exchange at Room Temperature

Author

He, Li, Luan, Chaoran, Liu, Shangpu, Chen, Meng, Rowell, Nelson, Wang, Ze, Li, Yang, Zhang, Chunchun, Lu, Jiao, Zhang, Meng, Liang, Bin, and Yu, Kui

Subjects
Temperature, chemical reactions, General Chemistry, Biochemistry, Catalysis, Zinc, mixtures, Colloid and Surface Chemistry, colloids, Cations, cadmium selenide, cadmium sulfide, Cadmium, Oleic Acid
Abstract

The transformation of colloidal semiconductor magic-size clusters (MSCs) from zinc to cadmium chalcogenide (ZnE to CdE) at low temperatures has received scant attention. Here, we report the first room-temperature evolution of CdE MSCs from ZnE samples and our interpretation of the transformation pathway. We show that when prenucleation stage samples of ZnE are mixed with cadmium oleate (Cd(OA)₂), CdE MSCs evolve; without this mixing, ZnE MSCs develop. When ZnE MSCs and Cd(OA)₂ are mixed, CdE MSCs also form. We propose that Cd(OA)2 reacts with the precursor compounds (PCs) of the ZnE MSCs but not directly with the ZnE MSCs. The cation exchange reaction transforms the ZnE PCs into CdE PCs, from which CdE MSCs develop. Our findings suggest that in reactions that lead to the production of binary ME quantum dots, the E precursor dominates the formation of binary ME PCs (M = Zn or Cd) to have similar stoichiometry. The present study provides a much more profound view of the formation and transformation mechanisms of the ME PCs.

Published
2022

14. Manipulating Reaction Intermediates to Aqueous‐Phase ZnSe Magic‐Size Clusters and Quantum Dots at Room Temperature

Author

Li, Yang, Zhang, Meng, He, Li, Rowell, Nelson, Kreouzis, Theo, Zhang, Chunchun, Wang, Shanling, Luan, Chaoran, Chen, Xiaoqin, Zhang, Sijie, and Yu, Kui

Subjects
General Medicine, General Chemistry, Catalysis
Abstract

It is not resolved which model describes better the aqueous-phase nucleation and growth of semiconductor quantum dots (QDs), the classical one-step one or the nonclassical multi-step one. Here, we design a room-temperature reaction to trap reaction intermediates in the prenucleation stage of ZnSe QDs (as a model system). We show that the trapped intermediate can transform to magic-size clusters (MSCs) via intra-molecular reorganization and can fragment to enable the growth of QDs. The MSCs exhibit a sharp optical absorption peaking at 299 nm, labelled MSC-299. The intermediate, the precursor compound (PC-299) of MSC-299, is optically transparent at 299 nm and to longer wavelengths. This intermediate forms in various Zn and Se reaction systems. The present study provides unambiguous evidence that the nonclassical and classical pathways are both necessary to explain the nucleation and growth of aqueous-phase QDs, with the former pathway favored more by high reaction concentrations.

Published
2022

20. Inside Back Cover: Transformation Pathway from CdSe Magic‐Size Clusters with Absorption Doublets at 373/393 nm to Clusters at 434/460 nm (Angew. Chem. Int. Ed. 37/2021)

Author

Zhu, Jinming, primary, Cao, Zhaopeng, additional, Zhu, Yongcheng, additional, Rowell, Nelson, additional, Li, Yan, additional, Wang, Shanling, additional, Zhang, Chunchun, additional, Jiang, Gang, additional, Zhang, Meng, additional, Zeng, Jianrong, additional, and Yu, Kui, additional

Published
2021
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21. Innenrücktitelbild: Transformation Pathway from CdSe Magic‐Size Clusters with Absorption Doublets at 373/393 nm to Clusters at 434/460 nm (Angew. Chem. 37/2021)

Author

Zhu, Jinming, primary, Cao, Zhaopeng, additional, Zhu, Yongcheng, additional, Rowell, Nelson, additional, Li, Yan, additional, Wang, Shanling, additional, Zhang, Chunchun, additional, Jiang, Gang, additional, Zhang, Meng, additional, Zeng, Jianrong, additional, and Yu, Kui, additional

Published
2021
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25. Germanium nanocrystal luminescence: spectral and spatial variations

Author

Rowell, Nelson L. and Lockwood, David J.

Abstract

We have investigated the intense infrared photoluminescence (PL) from several dozen molecular-beam-epitaxy Si₁₋ₓGeₓ layer samples. For one sample the PL was imaged with an infrared camera with the emission integrated from 650 to 1000 meV. This sample was a SiGe/Si multiple quantum well (MQW) structure, with 40 Si₀.₇Ge₀.₃ layers 7.6 nm thick separated by 20 nm of Si on a Si(001) substrate. For excitation at normal incidence, the PL was brightest at the excitation point, but similarly intense at the sample edge. With focused excitation, the edge emission spot had the same diameter as the emission at the excitation spot. Thus the emission was much more collimated in the MQW waveguide than normally in bulk luminescence. Analysis of the luminescence spectral linewidth was performed to estimate the lateral dimensions of the Ge nanocrystals, values which were compared with plan-view TEM results.

Published
2020

26. Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs

Author

Ximing Ding, Moumanis, Khalid, Dubowski, Jan J., Tay, Lilin, and Rowell, Nelson L.

Subjects
Monomolecular films -- Optical properties, Gallium arsenide -- Optical properties, Thiols -- Optical properties, Photoluminescence -- Research, Physics
Abstract

An attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs is presented. A strong infrared (IR) signal is measured for all the investigated GaAs-thiol interfaces.

Published
2006

27. Ophthalmic Drops with Nanoparticles Derived from a Natural Product for Treating Age-Related Macular Degeneration

Author

Xin, Guang, primary, Zhang, Ming, additional, Zhong, Zhihui, additional, Tang, Li, additional, Feng, Yuliang, additional, Wei, Zeliang, additional, Li, Shiyi, additional, Li, Youping, additional, Zhang, Junhua, additional, Zhang, Boli, additional, Zhang, Meng, additional, Rowell, Nelson, additional, Chen, Zhen, additional, Niu, Hai, additional, Yu, Kui, additional, and Huang, Wen, additional

Published
2020
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28. Transformation Pathways in Colloidal CdTeSe Magic‐Size Clusters.

Author

Yang, Yusha, Li, Yang, Luan, Chaoran, Rowell, Nelson, Wang, Shanling, Zhang, Chunchun, Huang, Wen, Chen, Xiaoqin, and Yu, Kui

Subjects
OPTICAL resonance, LIGHT absorption, ISOMERS, QUANTUM dots, MONOMERS
Abstract

A rarely studied transformation in colloidal ternary magic‐size clusters (MSCs) is addressed. We report the first observation of the transformation from ternary CdTeSe MSC‐399 to MSC‐422, which occurs at room temperature. These two MSC types display sharp optical absorption resonances at 399 and 422 nm, respectively, and are related in that they are quasi isomers, together with their counterpart precursor compounds (PCs). Binary CdTe and CdSe samples were prepared in the prenucleation stage also called the induction period (IP). After they were mixed and placed in a mixture of toluene and octylamine, the transformation was found to take place and to be assisted by the addition of the CdSe IP sample. A binary IP sample contains corresponding binary PCs and monomers (Mo) and fragments (Fr). We argue that the transformation pathway is enabled by the corresponding ternary PCs, involving the substitution reaction, namely CdTeSe PC‐399 + CdSe (Mo/Fr)‐1 ⇒ CdTeSe PC‐422 + CdSe (Mo/Fr)‐2. The present study provides an in‐depth understanding of the formation characteristics of the MSCs. [ABSTRACT FROM AUTHOR]

Published
2022
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38. Fourier-transform radiation thermometry: measurements and uncertainties

Author

Dufour, Pierre C., Rowell, Nelson L., and Steele, Alan G.

Subjects
Fourier transformations -- Research, Temperature measurements -- Analysis, Optical measurements -- Methods, Astronomy, Physics
Abstract

A Fourier-transform radiometer is used to measure blackbody temperatures in the 500-1000-K range. The measurements involve collecting mid-infrared spectra at two known reference temperatures and one unknown temperature. The accuracy of the interpolation measurement technique is discussed, and the effects of the uncertainty in the temperature reference points, the voltage ratio measurement, and the wavelength accuracy are described. Temperature accuracy at the 0.5% level has been achieved; the main uncertainty component is caused by the interferometer drift. Directions to reach 100-mk accuracy levels have been identified.

Published
1998

40. Emission from strained germanium nanocrystals

Author

Rowell, Nelson L. and Lockwood, David J.

Abstract

We analyse the intense photoluminescence (PL) observed at energies from 600 to 1500 meV for many molecular beam epitaxy grown Si1-xGex epitaxial layers. We show that the unexplained broad PL peak is due to self-assembled Ge nanocrystals (NCs) within the SiGe layers. The NCs are assumed lattice matched to the SiGe in the vertical, growth direction. As the Ge-fraction in the SiGe layer increases, the vertical strain in the NCs changes from compressive to tensile at x ~ 0.36, lowering the NC band gap (BG) below that of bulk Ge. We analyse PL results for 64 samples exhibiting this broad PL peak by examining how it follows the strained Ge BG for x from 0.05 to 0.53. The PL is resolvable as two narrower peaks separated by the TO phonon energy for Ge. Strain and confinement shifted NC bound exciton energies calculated numerically agree well with the measured ones.

Published
2018

47. Chapter 4: Light Emission from Germanium Nanostructures.

Author

Rowell, Nelson L. and Lockwood, David J.

Abstract

This Chapter discusses the phenomena associated with germanium (Ge) nanocrystals emitting near infrared radiation under optical excitation. We describe how the emission properties are influenced by various effects, including those of strain and particle confinement, as well as excitation mechanisms. Two example systems are discussed, namely one of isolated Ge quantum dots and another of Ge nanocrystals coherently imbedded in SiGe alloy layers, where both systems were grown by molecular beam epitaxy (MBE) on Si substrates. For the Ge dot ensembles, we show how particle size information can be derived from the emission spectrum. For the Ge nanocrystals, the emission spectra are analyzed for the effects of strain and particle confinement over a wide range of Ge fractions in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape, which were a 1.4 nm thickness in the MBE growth direction and a 7 nm lateral dimension. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures. [ABSTRACT FROM AUTHOR]

Published
2021
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