Measurement of wavelength-dependent radiation pressure from photon reflection and absorption due to thin film interference

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.

Radiation pressure measurement using a macroscopic oscillator in an ambient environment

“…It can be noted that, in the presence of photothermal effects, this response function would be modified from the ideal harmonic oscillator form as described, e.g., in Refs. 30,31 . Thus, the ideal harmonic oscillator form of the frequency response function in Fig.…”

“…However, optical forces in those nanomicromechanical systems have been directly accompanied by photothermal effects due to short thermal time constants of the miniaturized resonators 6,7,[26][27][28][29] , which has required further sophisticated techniques to discern them from the radiation pressure effects. Therefore, various optical, mechanical, and thermal techniques have been developed to overcome the trade-off between the radiation pressure and the photothermal effects, such as complex resonator designs consisting of highly reflective multilayer coatings deposited on the cantilever to further increase the reflectivity 7,12 , attachment of an additional mass to increase the thermal time constant of the cantilever 13 , or other ways to separate the optical force from the photothermal effects 30,31 .…”

“…In this work, we attempted a new direction opposite to the current trends by achieving a quantitative measurement of the radiation pressure of light in an ambient environment at room temperature by utilizing a macroscopic mechanical harmonic oscillator, which is orders of magnitude heavier than oscillators in previous reports [30][31][32][33][34][35] . The experimental setup is illustrated in Fig.…”

“…Here we varied the mass and the damping constant to verify the accuracy of the harmonic oscillator model in the radiation pressure measurements. Note that our method can obviate the elaborated process to calibrate the spring constants 30,31 ,…”

Photonic Excitation of a Micromechanical Cantilever in Electrostatic Fields

“…The exciting laser beam is focused down to 5 µm on the AFM tip, which is roughly half the base of the pyramidal tip. A tip-less geometry without electric fields or photovoltage effects was used in [30]. The EOM modulation was a sinusoidal shape, periodically changing the laser power from 0 to P , which can be up to P max = 40 mW.…”