Abstract
Opto-mechanical forces result from the momentum transfer that occurs during light-matter interactions. One of the most common examples of this phenomenon is the radiation pressure that is exerted on a reflective surface upon photon reflection. For an ideal mirror, the radiation pressure is independent of the wavelength of light and depends only on the incident power. Here we consider a different regime where, for a constant input optical power, wavelength-dependent radiation pressure is observed due to coherent thin film Fabry-Perot interference effects. We perform measurements using a Si microcantilever and utilize an in-situ optical transmission technique to determine the local thickness of the cantilever and the light beam’s angle of incidence. Although Si is absorptive in the visible part of the spectrum, by exploiting the Fabry-Perot modes of the cantilever, we can determine whether momentum is transferred via reflection or absorption by tuning the incident wavelength by only ~20 nm. Finally, we demonstrate that the tunable wavelength excitation measurement can be used to separate photothermal effects and radiation pressure.
Highlights
Radiation pressure was first quantitatively described by Maxwell based on his wave-theory of electromagnetism published in 18731
For a perfectly reflecting mirror, the radiation pressure only depends on the incident optical power and is independent of wavelength
For a thin film structure, such as a microcantilever, interference effects can cause a dramatic change in the reflectivity and absorption within a short wavelength range
Summary
K0 is the spring constant of the fundamental frequency calibrated by the thermal method[23]. The tunable wavelength excitation measurement discussed here enables a method to determine whether the dominant driving mechanism of the cantilever oscillation is radiation pressure or photothermal when illuminating different regions along the cantilever To demonstrate this effect, another experiment is conducted with the external laser excitation near the base of the cantilever. For excitation near the base (Fig. 4b), the normalized cantilever oscillation amplitude is proportional to the absorption spectrum, indicating that the driving mechanism is the photothermal bending moment caused by photon absorption[24]. This conclusion is in agreement with our previous results at a single illumination wavelength[18]. Development of opto-mechanical devices and allow for future studies of the coupling between optical and thermal effects in such systems
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