REVIEW ● OPEN ACCESS                                                                              Read More

Ridong Wang, Shen Xu, Yanan Yue, Xinwei Wang

1. Introduction

Laser-assisted manufacturing (LAM) is a method that applies the instantaneous heating capability of laser with a focused beam to materials that are difficult to process by mechanical machining alone. This method has been applied in various manufacturing processes, including drilling, cutting, turning, peening, nanoimprinting, additive manufacturing, scanning probing microscope, etc. All these processes involve intensive heating, which indicates that the heating level/material’s thermal response is critical for understanding the physics behind the laser-assisted manufacturing, and for providing feedback for manufacturing optimization. Prof. Ridong Wang from Tianjin University, China, Prof. Shen Xu from Shanghai Univerisity of Engineering Science, China, Prof. Yanan Yue from Wuhan University, China, and Prof. Xinwei Wang from Iowa State University, America, wrote a review "Thermal behavior of materials in laser-assisted extreme manufacturing: Raman-based novel characterization" in IJEM. In this article, the authors have introduced the research background, systematically discussed the Rama-based temperature probing, the application in near-field laser manufacturing and transient response characterization, and the outlook of Raman-based temperature probing in LAM processes.

2. Background

Raman spectroscopy, which is widely used for structure characterization, can also be used for temperature probing. Temperature variation of the tested sample will affect the properties of its Raman peaks when other measuring conditions are well controlled. To be specific, take a Stokes Raman peak as an example, its Raman wave number red shifts, intensity decreases, and linewidth broadens as the temperature goes higher. In a certain temperature range of approximate 50 degrees variation, the changing rates/temperature coefficients of these three properties could be safely assumed constant. Thus, the temperature can be determined based on a linear relation. That is, Raman spectroscopy provides a novel way to measuring temperature during LAM, and can provide unique and unprecedented knowledge about the physics involved in LAM.

3.  Recent Advances

Recent advances in Raman-based thermal probing have been divided into two sections: Raman-based thermal probing in near-field laser manufacturing, and Raman-based thermal probing in transient response characterization. In near-field laser manufacturing, the distance between the laser-focusing feature and material of processing is very small, even less than 10 nm. In addition, the laser focal spot is in the same order of size. As the Raman signal is related to the temperature and structure of material, near-field Raman-based thermal probing, which is a non-contact technique with high resolution, can be used under such situation as shown in Fig. 1. In addition, as shown in Fig. 2, the Raman-uncovered temperature and stress field can be used to study the effect of various physical parameters, including laser power, polarization, wavelength, the focal level, and pulse width. For samples with structure variation in space, this structure variation must be taken into serious consideration in scanning Raman for stress and temperature measurement. And the mapping results based on Raman intensity variation, wavenumber shift, and linewidth broadening consist of conjugated thermal, stress, and near-field focusing effects at a 20 nm resolution. Besides, as the structure variation can also cause Raman intensity and wavenumber change, the asymmetries of Raman scattering along one scanning direction, and between two scanning directions should be considered. Figure 3 shows a data construction method to eliminate these asymmetries. In transient thermal response characterization, shown in Fig. 4, a technique named Energy Transport-state resolved Raman (ET-Raman) has been developed for probing thermal transport in 2D materials. In this technique, different energy transport states in both space and time domains are constructed to probe a materials’ thermal response for different situations. Although the ET-Raman technique was initially focused on thermal transport study, it provides a novel way to probing a material’s thermal response under ultrafast laser heating. Additionally, Raman pump-probe spectroscopy, stimulated Raman scattering and coherent anti-Stokes Raman scattering also show high potential in characterizing the transient thermal response of materials in LAM processes. 

Figure 1. Experimental setup for tip-induced laser heating and temperature measurement based on Raman spectrum. Reprinted with permission. Copyright (2011) American Chemical Society.

Figure 2. Nanoscale mapping for near-field heating under 1210 nm particles. Reprinted with permission. Copyright (2013) Public Library of Science.

Figure 3. Schematic of the scanning Raman system used to detect the Raman signal of (a) silicon substrate and (b) MoSe2 nanosheet. Reprinted with permission. Copyright (2017) The Optical Society (OSA).

Figure 4. The schematic for the physical principle of five-state picosecond ET-Raman technique. (a) The generation, diffusion of hot carrier, and electron-hole recombination. (b, c) Two sub-states in ps laser heating under 50´ and 100´ objective lenses. (d-f) Three sub-states in CW laser heating under 20´, 50´, and 100´ objective lenses. Reprinted with permission. Copyright (2018) Royal Society of Chemisty.

4. Perspectives 

In LAM, if the target material is not Raman active (e.g. metals), a Raman active material can be placed in the manufacturing region for temperature probing. For instance, some high temperature-resistant materials, like diamond nanoparticles and graphene can be used for very efficient Raman signal excitation and ultrafast probing due to the sensor’s extremely small thermal inertia. Additionally, in many LAM processes, the temperature variation of materials is very fast (even in picoseconds). In such situations, Raman techniques with a picosecond/femtosecond pulsed laser can be used to explore the transient thermal response of materials and the possible structure variation during such short heating process. As the laser spot size is controllable, this technique can also realize a high spatial resolution. In summary, the Raman technique is a very promising one for probing the thermal response of materials with very high resolution in time and space domains.

5.  About the Authors

Dr. Ridong Wang is an Associate Professor of Measuring and Controlling Technologies and Instruments in School of Precision Instruments and Optoelectronics Engineering at Tianjin University. He earned his B.S. (2011) and M.S. (2014) in Measuring and Controlling Technologies and Instruments at Tianjin University. He received his Ph.D. in Mechanical Engineering at Iowa State University in August 2019, and received the Research Excellence Award at Iowa State University. His research covers the thermal transport of 2D materials and the application of 2D materials in biosensors. He has published more than 20 journal articles in Advanced Science, Carbon, Nanoscale, ACS Photonics, etc. He can be reached via Email: rdwang@tju.edu.cn

Dr. Shen Xu is an associate professor of Shanghai University of Engineering Science. She has received her bachelor's degree from East China University of Science and Technology, a master's degree from Fudan University, and a doctorate degree from Iowa State University. Her research interests are mainly focused on the fundamental research of energy transfer in micro/nanoscale, heat transfer behavior of new materials and complex nanostructures, and multiphysics in the coupling of optical and temperature fields. She holds the position of Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, participated in one project National Natural Science Foundation of China, published 38 SCI papers in peer-reviewed journals such as Nanoscale, Optical Express, Optical Letters, Nanotechnology, which have been cited more than 300 times, published a monograph, and has one patent under review. She also serves as peer-reviewer for several international journals.

Yanan Yue is a Professor in School of Power and Mechanical Engineering at Wuhan University, China. He obtained his Bachelor’s degree from Wuhan University, China in 2007, and his PhD in Mechanical Engineering from Iowa State University in 2011. His research interests include thermophysical properties, laser based thermal characterization techniques etc. He has published more than 60 papers in peer-reviewed journals with citation more than 1300 times, and serves in several scientific committees, such as Young Scholars Committee of China Society of Heat and Mass Transfer, and China Electricity Education Association-Sub-division Energy and Power Engineering. He can be reached via Email: yyue@whu.edu.cn 

Dr. Xinwei Wang is a full professor at Iowa State University (http://web.me.iastate.edu/wang). He obtained his Ph.D. from the School of Mechanical Engineering, Purdue University in 2001, and had his M.S. (1996) and B.S. (1994) from the University of Science and Technology of China. Over the past 18 years, he has led his laboratory to develop novel technologies for micro/nanoscale thermal characterization, study ultrafast-laser material interaction, investigate light-structure coupling, and probe energy transport in various materials down the sub-nm scale. His current work focuses on energy transport in macromolecules, 2D atomic layer materials, and atomic scale interface phonon energy transport. He has published more than 150 papers in highly-visible journals. He received the inaugural Viskanta Fellow Award of Purdue University in recognition of his pioneering and independent work in thermal sciences. He is the recipient of the 2014 Mid-career Award for Research of Iowa State University (ISU) and 2018 ISU Award for Outstanding Achievement in Research. He is the Fellow of American Society of Mechanical Engineers (ASME) and Associate Fellow of American Institute of Aeronautics and Astronautics (AIAA). He is the Senior Editor of International Journal of Thermophysics and Journal of Laser Applications.