A study of photophoretic trapping exploiting motional resonances of trapped particles induced by wideband excitation

Abstract

Photophoretic forces confine particles by a combination of levitation and active trapping. To generate the latter, the component of photophoretic force transverse to gravity generates a torque, which leads to complex rotation of the particle, thereby generating a restoring force. This force has not been understood quantitatively. In this paper, we study this component of photophoretic force by trapping a single absorbing particle using a loosely focused Gaussian laser beam and modulating the trap-center spatially using a superposition of multiple sinusoidal frequencies applied to the trapping beam. We vary the modulation amplitude in proportion to the excitation frequency so as to compensate for the reduced particle response at higher frequencies and thus obtain an almost constant response of the particle over the entire excitation range. We use the amplitude and phase response to determine the resonance frequency and the trap stiffness—which we study as a function of laser power and the intensity experienced by trapped particles of different sizes. We observe that, while the trapping stiffness is proportional to laser power, the optical intensity experienced by a particle at different powers is the same. This is reasonable in order to facilitate optical levitation but implies that the trap stiffness actually depends on an “effective intensity,” which may be a combination of the particle accommodation coefficient and the laser power.

Comparison of the efficiency of photophoretic trapping with different types of transverse beam profiles created using optical fiber

“…But, there exists no literature on how this efficiency of photophoretic force will change when we change the trapping beam profile. Earlier in our work, we have shown that micron sized absorbing particles can be trapped stably in a freespace gaussian beam 7 as well as in HG01 mode 8 . The photophoretic force has two components, one due to the difference in thermal accommodation coefficient across the surface of the particle, the FΔα force and another due to the uneven temperature distribution across the surface of the particle, the FΔT force 9 .…”

Ultrastable Three-Dimensional Photophoretic Trap in Air Facilitated by a Single Multimode Fiber

“…Since the light intensity is responsible for balancing gravity, a certain mass requires a specific laser intensity value for trapping, which is invariant, irrespective of the laser power. In our earlier work, 4 we have shown that changing the laser power translates the trapped particle to region where the intensity is the same. As a result, particles may be manipulated by changing both the laser intensity, as well as translating the laser beam.…”

“…Since the light intensity is responsible for balancing gravity, a certain mass requires a specific laser intensity value for trapping, which is invariant, irrespective of the laser power. In our earlier work, we have shown that changing the laser power translates the trapped particle to region where the intensity is the same. As a result, particles may be manipulated by changing both the laser intensity, as well as translating the laser beam. − In addition, the large magnitude of such forces promises a large number of very promising and diverse applicationsranging from three-dimensional displays to giant manipulation of particles to spectroscopy of aerosols − and to atmospheric and bioaerosol science. − …”

Trapping multiple absorbing particles in air using an optical fiber by photophoretic forces

“…The particle sizes at each focal length are determined using the methodology described in Ref. [11].…”

“…The photophoretic force is based on two types of forcestypically referred to in the literature as F ∆T and F ∆α [12]. Particles are confined in the axial direction due to F ∆T which balances gravity (F g ), while radially, a restoring force is generated by the helical motion of particles caused by the transverse photophoretic body force (F ∆α ), which applies a torque on particles due to its interaction with gravity [10,11]. Since it is basically the photophoretic ∆T force that balances gravity to cause axial trapping (or levitation), we determine this force using the multiphysics simulation software COMSOL.…”