The visible light source is based on second harmonic generation (SHG) of a 10-ns, 1000-Hz, 1064-nm pulsed Q-switched laser (NL204-1K, Ekspla) with a nonlinear crystal LBO (Eksma Optics). The spatial mode of the SHG beam is cleaned by a single-mode optical fibre (P3-405B-FC-5, Thorlabs). The spectral bandwidth is ~2 nm after the fibre, which reduces the coherent noise. We note that a CW laser can be used for many applications of this technique, although a nanosecond pulsed laser is required as the probe light for MIR photothermal QPI.