Chirality-dependent optical dipole potential

Seyedeh Hamideh Kazemi and Mohammad Mahmoudi

In this paper, we show the possibility of spatially separating two opposite enantiomers of chiral molecules, using an optical dipole potential. Because of the broken mirror symmetry of effective potential, chiral molecules have a cyclic three-level Δ-configuration structure. Irradiation of these molecules with three femtosecond laser pulses gives rise to an enantiomer-dependent optical force. Interestingly, considerable differences in the direction of the force felt by the enantiomers have been shown to cause the chirality-dependent optical dipole potential which stably captures only one enantiomeric form. Moreover, the proposed scheme provides a complete control over what kind of molecules, the left- or right-handed ones, can be selectively trapped. Note that we have analyzed the optical force, and specifically the trapping effect, by considering the full interaction Hamiltonian, including both rotating and counter-rotating terms.

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Study of adhesivity of surfaces using rotational optical tweezers

Rahul V R, Dhanush Bhatt, Anand Dev Ranjan and Basudev Roy

Optical tweezers are powerful tools for high resolution study of surface properties. Such experiments are traditionally performed by studying the active or the brownian fluctuation of trapped particles in the X, Y and Z directions. Here we find that employing the fourth dimension, rotation, allows for sensitive and fast probing of the surface, and happen when birefringent microparticles are applied with circularly polarized light, thus called the Rotational Optical Tweezers. When the trapped birefringent microparticle is far away from the surface, the rotation rate is dependent only on the laser power. However, we find that if one traps close to a surface, the rotation rate goes to zero even at finite tweezers laser powers for some specific type of substrates. We suspect this to be due to the interaction between the substrate and the birefringent particle, keeping in mind that the faxen correction for this mode of rotation cannot increase beyond 1.2 times. We use this to probe some surfaces and find that there is no binding for hydrophobic ones but hydrophilic ones particularly tend to show a laser power threshold to start rotating. We calculate that the threshold energy of the tweezers is consistent with the Van der Waals potential energy, when the mode of interaction with the surface is purely physical. We also find that for chitosan, the mode of interaction is possibly different from Van der Waals. Further, we place the particle on the threshold and observe "stick-slip" kind of rotational behaviour.

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Numeric corrections to the proximity-force approximation for lateral Casimir forces

Fanglin Bao and Kezhang Shi

We report a numeric investigation on the proximity-force approximation (PFA) for lateral Casimir forces between a sphere and a grating. Near-unity force correlations are found between the approximated force and the exact values, due to geometric effects. A minimal model yields a best-fit expression of the numeric correction to the PFA, for gratings in the dilute limit. Our results are not restricted to specific material of the sphere, and allows simple estimation of Casimir interactions for micro-scale spheres, and thus shall be useful in relevant experimental and engineering Casimir applications.

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Bohr's 'Light and Life' revisited

H M Nussenzveig

I revisit Niels Bohr's famous 1932 'Light and Life' lecture, confronting it with current knowledge. Topics covered include: life origin and evolution, quantum mechanics and life, brain and mind, consciousness and free will, and light as a tool for biology, with special emphasis on optical tweezers and their contributions to biophysics. Specialized knowledge of biology is not assumed.

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Efficient extension of the trapping lifetime of single atoms in an optical tweezer by laser cooling

Jun He, Baodong Yang, Tiancai Zhang and Junmin Wang
Optical tweezers have become powerful tools for the confinement and manipulation of neutral atoms, molecules, mesoscopic biological molecules and living cells. In our experiment, a single caesium atom was prepared in a large-magnetic-gradient magneto-optical trap (MOT). It was then efficiently transferred back and forth between the MOT and a 1064 nm microscopic optical tweezer. The atomic transfer between the MOT and the tweezer can be employed to measure the trapping lifetime and the energy distribution of the single atom in the tweezer. In order to extend the trapping lifetime, laser cooling is used to decrease the atom's kinetic energy. The trapping lifetime was extended from ~75 to ~130 s by applying a 10 ms laser cooling phase just after the single atom is transferred into the tweezer.

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Trapping of a microsphere pendulum through cavity-enhanced optical forces

Yuqiang Wu, Jonathan M Ward, Vladimir G Minogin and Síle Nic Chormaic

Optical forces resulting from evanescently coupled microcavities can produce remarkable mechanical effects on micro- and nanoscale systems. Excitation of the symmetric and antisymmetric modes of the interacting whispering gallery modes (WGM) leads to significant attractive and repulsive forces. Here, we propose a method to spatially trap a microspherical resonator pendulum via the optical forces produced by two simultaneously excited WGMs of a photonic molecule, comprising two microspherical cavities. We discuss how the cavity-enhanced optical force generated in the photonic molecule can create an optomechanical potential of about 5 eV deep and 10 pm wide, which can be used to trap the pendulum at any given equilibrium position by a simple choice of laser frequencies. This result presents opportunities for very precise all-optical self-alignment of microsystems. Frequency splitting of a co-resonant mode from two similar-sized microspheres was observed experimentally and the mechanical characteristics of a microsphere pendulum were also studied.

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Calculating optical forces using the boundary integral method

Per Jakobsen

In this paper, we show that the boundary integral method is highly efficient for the calculation of optical forces on small dielectric and metallic objects. The boundary integral formulation for the Maxwell equations is stated, and an implementation of the equations is described, tested and used to derive new bistability results for two dielectric spheres in counterpropagating incoherent laser beams.

DOI