The overall goal of this experimental system is to generate and measure rotation and torque at the nano scale using circularly polarized optical tweezers. The rotating objects can be interrogated using additional instrumentation for brownian dynamics analysis and light scattering spectroscopy. This method can help us answer key questions in nano scale thermal and optic physics.
For instance related to nano particle brownian diffusion and transfer of optical angular momentum into a nano particle. The main advantage of this technique is that it allows stable nano particle rotation at high frequencies, and that the measurements provide detailed information about the nano particle and its immediate environments. The idea is to trap a metallic nano particle by a focused laser beam in optical tweezer setup.
If the trapping laser is circularly polarized the nano particle will rotate due to transfer of angular momentum from the light. Construct the setup around a suitable inverted microscope. The setup includes elements for both the optical tweezers and measurements on a trapped particle.
This is a schematic representation of the setup. For now, focus on the elements for the optical tweezers. Choose a 660 nano meter linearly polarized laser with a stable output power of up to 500 milliwatts.
Use lenses in a keplerian telescope configuration to expand the beam. Use mirrors in kinematic mounts to direct the laser beam to the microscope. Inside the microscope, use a 50 50 beam splitter to couple the laser light into objective.
For alignment and data collection include a camera in the path of the reflected light. In this case a pivoting mirror inside the microscope can direct the light to it. Adjust the keplerian lenses to expand the beam.
Make the beam slightly larger than the back aperture of the trapping objective to optimize the focus. Move along the beam path to check the collimation. The beam diameter should be constant as it propagates to the objective.
Focus the laser on a glass slide or a mirror. Perform fine tuning with the mirrors in the beam path. An improperly aligned laser has a non-radially symmetric intensity pattern.
If the laser is aligned, its intensity pattern is radially symmetric when changing the focus above and below the focal point. Now circularly polarize the beam by introducing a quarter wave plate. Orient it with its fast axis at 45 degrees to the linear polarization of the light.
For optimal rotary performance its critical to have a good circular polarization of the laser. In case problem arises at this stage insert the half wave plate to correct adverse birefringence of optical components in the beam path. Finally, set up a dark field system and collar illumination using oil immersed condenser.
Repair the elements for photon corelation spectroscopy. Install beamsplitter to redirect light from the microscope to a port. At the port, place an assembly of a linear polarizer, followed by an x y stage with a collection fiber.
Focus light on the collection fiber on the x y translation stage. The fiber will ultimately go to a fast single pixel silicon photo diode. Make sure the core size of the collection fiber is large enough to contain the image of the nano particle during its excursions due to translational brownian motion.
If this criterion is not met the collected data can be difficult to interpret. To align the collection fiber, couple visible light to it's unattached end. This will illuminate the substrate in the microscope and allow visitational of the collection region of the fiber Adjust the fiber position with the translation stage.
Stop when the collection region coincides with the position of the optical trap. Then move the fiber end from the light source to the detector. In this schematic, these elements represent the setup needed for photon corelation spectroscopy.
For the dark field spectroscopy setup, have the spectrometer in place. Direct light to the spectrometer with the mirror in the reflected beam's path. In this case, the mirror is located with the microscope body Use a notch filter before the mirror to remove the trapping laser light.
Adjust the guiding mirrors to have the optical tweezers coincide with the spectrometer slit. Begin by preparing the particles for the experiment. Dilute the particles in deionized water to an appropriate concentration.
Sonicate the diluted solution for two minutes to break up aggregates and create a uniform dispersion. Next prepare the sample cell. Use a microscope slide and cover glass.
Sonicate them for five minute each in acetone, followed by isopropanol. When done, create a 100 micrometer deep spacer tape well on the glass slide. Disperse two micro liters of the diluted nano particle solution within the well.
Also deposit two micro liters on the cover glass. Put the two parts of the cell together. There should not be any air bubbles inside the chamber.
Move the cell to the microscope stage. There place one drop index matched immersion oil on top of the sample. Place another drop on the condenser.
Start the experiment with the trapping laser blocked. Located a suitable particle through observation in the dark field imaging system. Unblock the laser and manipulate the stage and focus to push the particle forward toward the water glass interface.
For rotational dynamics measurements, collect the intensity oscillation with the silicon photo detector. For spectroscopic measurements, start with the substrate of densely dispersed uniformly scattering polystyrene beads. Record the white light spectrum from the substrate scattering response.
Next replace the bead sample in the microscope with the nano particle sample. Record a background spectrum with stray light from the trapping spot. Next block all light from the detector, record a dark spectrum under this condition.
Remove the block from the beam, finally trap a nano particle and record the raw spectrum associated with it. These are intensity fluctuations of back scattered laser light from a trapped, rotating, nano rod. From such untreated data, it is difficult to extract information on any time scale.
However, when the intensity data is auto correlated, it shows oscillations related the nano rod's rotation frequency. The oscillatory decay after a few periods is due to the rotational brownian motion. The red line is a fit to the theoretical auto correlation function from which one can extract detail information about the rotary motion.
Here the raw scattered intensity from a trapped nano rod is in dark blue. It is distorted by the presence of a notch filter. To isolate the nano rod's scattering spectrum, calibrate the raw data with the background spectrum, in red, and a white light excitation spectrum in orange.
This is the corrected scattering spectrum. The spectrum shows two distinct localized surfaced plasmon resonances, as expected. In this plot, the blue points are the scattering spectrum from a trapped nano rod with the distorted spectrum region disregarded.
The red curve is the fit of the bi-Lorentzian model function. The fits components are in light blue and orange. Once the setup is constructed and the technique mastered, optical trapping and rotation experiment can be done in a few hours if it's performed properly.
While attempting this procedure, it's important to remember to use appropriate nano rods who dominate resonance is on the blue wavelength side of the laser, it should also have clearly separated resonance peaks yet be physically large enough to be stably trapped. Optical tweezers for nano rod rotation experiments can be constructed using a range of different laser wavelengths microscope objectives and microscopes with only minor alterations. Moreover, instrumentation for translational brownian measurements can easily be added for complementary data collection.
This nano particle rotation platform has proved useful as oscilloscope gauge of viscosity and local temperature, for tracking morphological changes of nano rods and molecular coatings and as a transducer and probe of photo-thermal and thermodynamic processes. After watching this video, you should have a good understand of how to perform optical tweezing experiments on metallic nano-rods. How to rotate them using circularly polarized light and how to extract information through photon corelation spectroscopy and dark field spectroscopy.
Don't forget that working with lasers can be extremely hazardous to the eyes and precautions such as wearing laser safety glasses and proper laser filters and handling procedures should always be taken while performing this procedure.
