Nanodiamonds with color centers are to an increasing degree investigated as intracellular biosensors for magnetic fields, electrical fields, or temperature changes, as well as for abundance of free radicals or pH inside live cells. A common color-center is the nitrogen-vacancy (NV) center, where a substitutional nitrogen atom is positioned next to a vacancy within the diamond host crystal. The nanodiamond with negatively charged NV− center is particularly versatile due to its biocompatibility and its purely optical addressability. Our work aims to use NV-center nanodiamonds both as intracellular biosensors and as probe particles within an optical trap to determine the viscoelastic properties of the intracellular environment in single-cell studies. For this to be successful, several prior steps are needed: 1) The uptake of nanodiamonds within the cells should be characterized, including studies of subcellular localization, and a controllable protocol developed; 2) Any effect of the trapping laser on the NV-center sensing should be characterized and understood, and a protocol for stable trapping along with accurate biosensing should be developed. In this work we summarize the preliminary findings of our ongoing investigations to address these points. We show results of T1-relaxometry with and without CW NIR laser irradiation in a suspension cell model, analyze optical trapping of nanodiamonds with CW light in an adhesion cell model, and investigate implications of the presence of the optical trapping laser on T1-relaxometry measurements.
Intracavity optical tweezers are a powerful tool to trap microparticles in water using the nonlinear feedback effect produced by the particle motion when it is trapped inside the laser cavity. In such systems two configurations are possible: a single-beam configuration and counterpropagating one. A removable isolator allows to switch between these configurations by suppressing one of the beams. Trapping a particle in the counterpropagating configuration, the measure of the optical power shows a feedback effect for each beam, that is present also when the two beams are misaligned and the trapped particle periodically jumps between them.
We demonstrate a novel intracavity optical trapping approach based on a self-adjusted counter propagation beam. In this configuration, the trapping part is placed within a backward pumping fiber ring cavity. The particle in this setup is trapped by two counter propagation beams including pump, from top to bottom and signal from bottom to top. This trapping scheme benefits from the cavity design in which the individual beams are adjusted depending on the displacement of the trapped particle. When the particle is in the center of the trap and turns the laser signal off, the pump then automatically turns on and this keeps the particle in the trap position. We believe this approach can find its applications in the trapping of submicron particles where the previous intracavity OTs were not able to trap, especially biological samples that are subjected to photo-damage due to the intensity of laser light.
In standard optical tweezers optical forces arise from the interaction of a tightly focused laser beam with a microscopic particle. The particle is always outside the laser cavity and the incoming beam is not affected by the particle position. Here we describe an optical trapping scheme inside the cavity of a fiber laser where the laser operation is nonlinearly influenced by the displacement of trapped particle and there is a coupling between laser operation to the motion of the trapped particle and this can dramatically enhances optical tweezers action and gives rise to nonlinear feedback forces. This scheme operates using an aspheric lens at low numerical aperture (NA=0.125), NIR wavelength (λ = 1030 nm), and very low average power which results in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. Ultra-low intensity at our wavelength can grant a safe, temperature-controlled environment, away from surfaces for microfuidics manipulation of biosamples that are sensitive to light intensity. As the main advantage of our approach and highly relevant application, we observed that we can trap single yeast cells at a very low power, corresponding to an intensity of 0.036 mW μm-2, that is more than a tenfold less intensity than standard techniques reported in the literature.
We propose a novel approach for trapping micron-sized particles and living cells based on optical feedback. This approach can be implemented at low numerical aperture (NA=0.5, 20X) and long working distance. In this configuration, an optical tweezers is constructed inside a ring cavity fiber laser and the optical feedback in the ring cavity is controlled by the light scattered from a trapped particle. In particular, once the particle is trapped, the laser operation, optical feedback and intracavity power are affected by the particle motion. We demonstrate that using this configuration is possible to stably hold micron-sized particles and single living cells in the focal spot of the laser beam. The calibration of the optical forces is achieved by tracking the Brownian motion of a trapped particle or cell and analysing its position distribution.
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