Why Do I Have These Vertical Wrinkles Under Chin After Neck Lift?

Three weeks post surgery is still very early for you to judge the results of your surgery - be patient and follow up with your surgeon

The vertical wrinkles suggest that skin laxity still exists after your lift. It doesn't seem likely that blue light will shrink the skin to your liking. Photos would help but a secndary lift might be the solution to further tightening.
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The cause of these wrinkles is not for certain, but may be secondary to the skin being stretched to provide exposure to the anterior neck. When the skin heals it rebounds causing these wrinkles. You can try a variety of modalities from Botox to the platysma muscles to relax the underlying muscles to radio frequency skin tightening treatments to this wrinkled area, but unfortunately I do not think these wrinkles will go away. I would discuss your situation and options with your surgeon.