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In this approach , a pulsed-laser beam was used to excite the surfaces of Al alloy samples to generate ultrasound and optical spectra .
 beam, surfaces, Al alloy, optical spectra
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 beam, surfaces, Al alloy, optical spectra