Dr. Lan Jiang Profile

Dr. Lan Jiang

at Missouri Univ of Science and Technology

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Publications (4)

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Proceedings Article | 12 April 2005 Paper

Fundamentals of energy cascade during ultrashort laser-material interactions

Hai-Lung Tsai, Lan Jiang

Proceedings Volume 5713, (2005) https://doi.org/10.1117/12.589461

KEYWORDS: Electrons, Laser ablation, Ionization, Dielectrics, Picosecond phenomena, Absorption, Reflectivity, Laser damage threshold, Gold, Metals

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During an ultrashort laser pulse, numerous photons are emitted in a very short period of time leading to very high peak power. The photons can excite free electrons in the material to very high temperatures (heating) or strip bound electrons from the atoms (ionization). In ultrashort laser heating there is a time lag between the electron heating and the lattice heating. The two-temperature model has been proposed to calculate the electron temperature and the lattice temperature and the related damage threshold for metals. On the other hand, ablation models based on impact ionization and photoionization have been proposed to predict material removal rates for semiconductors and dielectrics. However, in existing heating or ablation models, some critical thermal and optical properties of the material are assumed to be time, space, and fluence independent or the estimations are limited to temperatures much lower than the Fermi temperature. In this paper, the quantum theories are employed to calculate the free electron heating, free electron relaxation time, and the temporal and spatial dependent thermal and optical material properties. The improved two-temperature model is used to predict damage fluences of gold thin films. The new ablation model based on the Fokker-Planck equation can predict ablation depth and crater shape of semiconductors and dielectrics. The predicted results are in good agreement with experimental data.

Proceedings Article | 8 October 2004 Paper

Prediction of damage threshold fluences for metal films by an ultrashort laser pulse

Lan Jiang, Nnu George, Hai-Lung Tsai

Proceedings Volume 5662, (2004) https://doi.org/10.1117/12.596574

KEYWORDS: Laser damage threshold, Gold, Metals, Reflectivity, Picosecond phenomena, Electron transport, Absorption, Ultrafast phenomena, Tellurium, Pulsed laser operation

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At laser intensities near the threshold fluence, the electron temperatures in metals heated by an ultrashort pulse can be comparable to the Fermi temperature. As the existing approximations for material properties used in the two-temperature models are limited to electron temperatures that are much lower than the Fermi temperature, the models are suitable only for low fluences. This paper extends the existing estimations for optical and thermal properties to high electron temperatures by the following improvements: (1) using the Fermi-Dirac distribution, the heat capacity of metal free electrons is calculated; (2) the free electron relaxation time and electron coductivity are determined by using a quantum model derived from the Boltzmann transportation equation for dense plasma; and (3) the free electron heating and interband transition are both taken into account using a modified Drude model with quantum adjustments to calculate the reflectivity and the absorption coefficient. The proposed two-temperature model is employed to calculate the heating process of thin metal films until melting occurs, which is assumed to be the initiation of damage. The predicted damage threshold fluences for 200 nm gold film using the proposed model are in good agreement with published experimental data. The damage threshold fluence as a function of pulse duration is also studied.

Proceedings Article | 8 October 2004 Paper

A plasma model with quantum treatments for femtosecond laser ablation of glasses

Lan Jiang, Hai-Lung Tsai

Proceedings Volume 5662, (2004) https://doi.org/10.1117/12.596576

KEYWORDS: Laser ablation, Femtosecond phenomena, Glasses, Plasma, Absorption, Ionization, Optical properties, Laser damage threshold, Reflectivity, Silica

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In this article, a plasma model with quantum treatments is proposed to predict ablation threshold, depth, and crater shape in femtosecond laser ablation of glasses at peak intensities on the order of 10¹³ ~ 10¹⁴ W/cm². Impact ionization and photoionization are the two major competing mechanisms considered for plasma generation using the flux-doubling model. Using a modified free electron plasma model, the proposed model considers the time and space dependent optical properties of ionized glass. The quantum treatment based on the Fermi-Dirac distribution is employed to investigate the free electron heating. The free electron relaxation time is calculated by using a quantum model derived from the Boltzmann transport equation. The predicted threshold fluence and ablation depth for barium aluminum borosilicate and fused silica are in excellent agreement with published experimental data. The model greatly improves the prediction precision of ablation depth and can predict the crater shape in femtosecond ablation of glasses. Some interesting phenomena observed experimentally, such as the bottom of the ablation crater by a femtosecond Gaussian beam could be rather flat under special ablation conditions, are well explained by the proposed model.

Proceedings Article | 3 October 2001 Paper

Robust modular product family design

Lan Jiang, Venkat Allada

Proceedings Volume 4565, (2001) https://doi.org/10.1117/12.443116

KEYWORDS: Product engineering, Fluctuations and noise, Control systems, Curium, Interference (communication), Intelligence systems, Design for manufacturability, Manufacturing, Digital signal processing, Signal processing

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This paper presents a modified Taguchi methodology to improve the robustness of modular product families against changes in customer requirements. The general research questions posed in this paper are: (1) How to effectively design a product family (PF) that is robust enough to accommodate future customer requirements. (2) How far into the future should designers look to design a robust product family? An example of a simplified vacuum product family is used to illustrate our methodology. In the example, customer requirements are selected as signal factors; future changes of customer requirements are selected as noise factors; an index called quality characteristic (QC) is set to evaluate the product vacuum family; and the module instance matrix (M) is selected as control factor. Initially a relation between the objective function (QC) and the control factor (M) is established, and then the feasible M space is systemically explored using a simplex method to determine the optimum M and the corresponding QC values. Next, various noise levels at different time points are introduced into the system. For each noise level, the optimal values of M and QC are computed and plotted on a QC-chart. The tunable time period of the control factor (the module matrix, M) is computed using the QC-chart. The tunable time period represents the maximum time for which a given control factor can be used to satisfy current and future customer needs. Finally, a robustness index is used to break up the tunable time period into suitable time periods that designers should consider while designing product families.

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