Optical trapping in air

Optical trapping of absorbing mesoscopic particles in air

Trapping in air tough, since the viscosity of air is much lower than that of liquids – as a result of which particles shoot about very fast! Normal optical tweezers using the gradient forces generated by a focused Gaussian beam are often inadequate to confine particles, and collisions with air molecules are also an issue! Researchers often resort to trapping in vacuum so that air molecules do not destabilize trapped particles. We resort to something that is far more efficient in confining particles in air – that using photophoretic forces! Ever seen particles move in a sun-beam without being scattered by the wind? That is because of the photophoretic force, something that results due to unequal heating of the different parts of an absorbing particle, and the resulting exchange of momentum of colliding air molecules with the heated particle. Let us now examine this method of trapping in detail. 

Trapping mechanism 

In a conventional optical tweezers, one can trap a transparent particle in liquid medium (mostly in water) with the combination of the gradient and scattering forces which arise from the refraction and reflection of a Gaussian laser beam from a particle. Using this technique to trap a particle in air is quite difficult as the low intrinsic viscosity of air leads to much faster particle diffusion. So, a tightly focused laser beam with very high intensity would be required to trap a particle in air. Here a different kind of force, namely the Photophoretic force comes to our aid to trap a particle in air. One can get an idea about this force that if we see some gas suspended particles which are illuminated by a visible light beam (say sun light) then a particle can undergo different kind of motion which are in the direction or opposite direction of the illuminated beam, vertical movement, helical motion etc. As light is responsible for these kind of motion, this phenomenon is called Photophoresis. The word photophoresis originates form two Greek words ‘photo’ and ‘phoreo’; ‘photo’ means ‘light’ and ‘phoreo’ means ‘motion’. So, in short it means ‘light induced motion’. This phenomenon was first mentioned by Thore’ (1877), and after that Ehrenhaft (1918) studied it intensively. 

Photophoretic force arises due to the momentum exchange between a particle and gas molecules. As an example, let us consider a particle which has a certain surface temperature (Ts) and is put inside a vessel containing gas molecules having an initial temperature (Ti). When a gas molecule interacts with the particle surface and reflects back, then there is a heat exchange between them. As a result the final temperature of the gas molecule increases, which means the velocity as well as momentum also increases. So there is a change in momentum of that gas molecule and that give a force on the particle. This type of force is called the radiometric force and when light is responsible for heating the particle, then the force is called photophoretic force. Now, there is an intrinsic property of particle called thermal accommodation coefficient (α) which defines how much heat exchange occur between a particle and a gas molecule during a collision. This (α) varies from 0 to 1 and a higher value of (α) means higher heat transfer. Depending upon these two parameter (Ts) and (α), there are two types of forces, one is the Photophoretic ∆T force and the other is the Photophoretic Δα force. 

Photophoretic ∆T force 

If the accommodation coefficient is constant then the final temperature of the gas molecule solely depends on the surface temperature of the particle. Now, if an opaque highly absorbing particle is illuminated by an intense laser where the particle has finite absorptivity at that particular wavelength, then there is always high absorption at the illuminated side compared to the dark side, so that the temperature also higher. Then gas molecules on the warm side of the particle leave the surface faster than gas molecules on the cold side, and the particle experiences a force along the propagation direction of the light beam (see figure). So, this type of force is called the positive Photophoretic ∆T force. On the other hand, for a loosely absorbing and semi-transparent particle, the particle behaves like a lens system and focuses the light to the dark side. So, the dark side gets more heat and the reverse phenomenon occurs – which means that the force acts in the opposite direction (see figure). This is called the negative Photophoretic ∆T force. In terms of magnitude, this is more than 4-5 orders greater than radial pressure forces – which is easily understandable from the fact that the radiation pressure is inversely proportional to the velocity of light, whereas the photophoretic force depends on the velocity of air molecules! 

Photophoretic ∆α force 

Considering the surface temperature of the particle is constant, the force solely depends on the accommodation coefficient of the particle. For an example, let us consider a spherical particle, where the upper half has a value of accommodation coefficient α1, and the lower half has a value of α2. α1 is higher than α2. We know that higher the value of α, higher the heat transfer rate – so here the lower surface transfers more heat to the gas molecule than the upper surface. As a result, the force is directed from higher α to lower α (see figure). This type of force is called Photophoretic Δα force. 

We develop a simple experimental setup by a single laser beam of wavelength 671 nm which is focused by a lower focal length of lens (see figure) to trap absorbing particles in air using this Photophoretic force. For trapping we use toner particles and also PbS coated SiO2 particles which have high absorptivity at that particular wavelength and the size at around (8 to 12) micrometer. Here, we trap a particle in a vertical configuration i.e. particles are falling under gravity (-Z direction) within a home build sample chamber, and the laser beam travels in the +Z direction. Under this situation, the particle is axially in equilibrium when the gravity is balanced by the action of the radiation pressure force and the photophoretic force. Here, we accord a hypothesis, let the particle has two different α' s (α1, α2) and (α1 > α2) (see figure) as a result FΔα is directed from α1 to α2. Now, this FΔα has two component, one is longitudinal and the other is transverse. The photophoretic ΔT force is not of consequence in the pressure regimes we are have here – as is explained in Rohatschek paper. Thus, it is the longitudinal component of the Δα force which balances gravity to lead to axial trapping – so that in this direction we basically have optical levitation. But as the particle is trapped in 3 dimension, there should be a restoring force in radial direction. Now, due to the interaction of the transverse component of the photophoretic Δα force and gravity, the particle experiences a torque and undergoes a complex rotational motion which is responsible for radial trapping. From our experiment, we are able to find out evidences of rotational motion of the particle. 

Motivation 

The photophoretic force is obviously very effective in confining mesoscopic particles in air. So, our first job was to characterize the force by trapping an absorbing particle in air and study its Brownian motion in order to quantify the trapping force. Once we developed a robust good trapping system, it has enormous applications in environment, biological and atmospheric sciences. For example, airborne particles such as aerosols which may include soil, soot, textile particles, bacteria, spores, fungi etc, - can be simultaneously trapped and analysed by spectroscopy etc. These can have important applications in disease diagnostics and other chemical analyses. Now, we go on to some of the work we have done in this area (see figure)). 

Simultaneous measurement of mass and rotation of trapped particle in air 

We trapped a particle in air using the configuration which we described previously, and the trapped particle displayed a complex rotation as well as its typical Brownian motion. In this work, we measured a component of the radial Brownian motion of the particle using a positive-sensitive detection system and determined the power spectrum density, mean square displacement, normalized position and velocity autocorrelation functions to quantify the photophoretic force. Due to the low viscosity of air, we were able to observe the inertial effects of the particle and also measured the rotational frequency from its noise power spectral density. To the best of our knowledge, ours is the first direct characterization of particle motion induced by photophoretic forces using a very simple experimental setup. More details about this work is available here. 

Dual mode optical fiber-based tweezers for robust trapping and manipulation of absorbing particles in air 

From the previous experiment we made two important inferences. The first was that particles are trapped off-axis from the trapping beam center in the radial direction, and the second was that trap stiffness is linearly proportional to the laser intensity. So, in order to maximize the trapping force in the radial direction, it would thus be useful if we used a beam having high off-axis intensity such as that found in a higher order Hermite-Gaussian (HG) beam. This is what we achieved when we used a commercial single optical fiber where we generated the HG beam rather innovatively. Thus, we coupled a laser with a wavelength at which the fiber sustained the first two transvers modes inside it, so that we obtained a superposition of fundamental and first order Hermite-Gaussian beam modes! As expected, we were able to demonstrate that the trapping efficiency was improved by around 80% on using the Hermite-Gaussian beam. This work has promising applications for trapping and spectroscopy of aerosols in air using simple optical fiber-based traps. More details about this work is available here. 

Ongoing works 

From the previous experiment, we proved that the trap stiffness increased with laser power particles were trapped off-axis in the beam. The next step is to naturally check the trapping efficiency using single multimode fiber as it has a higher mode volume that gives higher off-axis intensity which can even be controlled by changing the input coupling angle. We have obtained some really interesting results in this! Watch out for more updates here!