A Novel Heat Transfer Model for Ultrashort Laser Heating on Metals

Dehee Kim, Chemical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, Sartaj S. Ghai, ExxonMobil Upstream Research Company, 3120 Buffalo Speedway, Houston, TX 77098, and Myung S. Jhon, Department of Chemical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213.

To overcome the technological limits of conventional magnetic recording systems, the heat assisted magnetic recording (HAMR) technology is being developed [1]. The HAMR technology utilizes the phenomenon of magnetic material property variation by tuning the temperature. Heating by a laser beam on the disk reduces the magnetic coercivity, and thus dramatically increases the aerial density. In this system, a nanoscale heat transfer process is initiated by laser heating, and the energy transport affects physical properties of the magnetic media as well as the stability and reliability of the system. Therefore, it is necessary to accurately estimate the heat conduction in the sub-continuum regime.

In addition to the HAMR application, similar concept was recently applied for micro-/nano-fabrication and material processing of thin metallic films as femtosecond laser heating has made technical innovation [2-4]. The thermal behavior of energy carriers, more specifically, electrons and phonons play a vital role in the theoretical analysis of sub-continuum thermal transport in metals.

The Boltzmann transport equation (BTE, represented in phase space) is used for the analysis by calculating the probability distribution function of both energy carriers (electron and phonon) inside the metals. These two BTEs for electrons and phonons should be solved simultaneously to examine microscale heat transfer in metals. The treatment of energy carriers as quasi-particles is simple yet powerful methodology, nonetheless, the formulation of BTEs becomes computationally intensive to solve. Furthermore, the coupling between the two BTEs for energy transport is still not understood very well, which creates difficulties in performing a full scale BTE simulation. The lattice Boltzmann method (LBM) is applied to complement BTE in this paper [5]. LBM is a technique that can be applied to solve the BTE in a simplified manner on lattices while maintaining the accuracy, as well as reducing the heavy computation tasks [5]. In our previous works [6, 7], LBM was developed and successfully applied to simulate the heat conduction of phonons. This method led us to develop a new LBM model to solve the energy transport of both electrons and phonons in metals. Here, we examine a thin gold film heating via ultrashort laser pulse and studied the electron-phonon coupling phenomenon [4].

For accurate prediction of the heat conduction in metals it is important to couple the two lattice Boltzmann equations properly. The Taylor series expansion and least squares based LBM (TLLBM) was adopted to couple two subsystems possessing different length and time scales [8]. The laser-induced heating problem was simulated successfully for the first time for metals using LBM. The numerical result is comparable to the widely used two-temperature models [4]. Although our LBM simulation has been validated only on gold films, it can be applied to other metallic films as long as the material properties are given. The material damage threshold by the hot thermal energy can be estimated using our present work for metals as well as for semiconductors and dielectric materials as shown in our previous work. Our LBM model can be a powerful tool in the investigation of thermal budget for the HAMR system, as laser-induced heating could affect the stability of the system including the substrate, lubricant film, and the air bearing.


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