Polarization selective reflection geometry trapping of nanoparticles
View Patent ↗Optical tweezer methods and apparatus that operate in reflection mode use polarization selection to isolate an input laser signal from optical signals associated with trapping. A trapping aperture or apertures can be defined in a conductive film such as a gold film have a polarization-dependent reflectance due to asymmetry of the aperture, and a reflected beam is polarized differently than an incident beam. Polarization of scattered light from the aperture can thus be distinguished from polarization of the incident beam. In some cases, an aperture is not used but instead polarizing nanoparticles, such as nanorods, are detected based on either transmitted or reflected optical radiation in a different state of polarization than that of the incident beam.
1 . A method, comprising:
situating at least one particle at a double nanohole defined in a conductive structure;
trapping the at least one particle by directing an input optical beam from a laser in a first linear state of polarization to the double nanohole and the at least one particle;
receiving a reflected portion of the input optical beam in a first linear state of polarization from the double nanohole and directing the received portion toward the laser;
directing a portion of an optical beam associated with the at least one particle responsive to the input optical beam from the conductive structure to a detector in a second linear state of polarization; and
based on the reflected portion of the input optical beam, determining the trapping of the at least one particle at the double nanohole.
2 . The method of claim 1 , wherein the first linear state of polarization and the second linear state of polarization are orthogonal linear states of polarization.
3 . The method of claim 1 , further comprising selectively directing the portion of the optical beam associated with the at least one particle to a detection system.
4 . The method of claim 1 , wherein the conductive structure comprises a conductive layer.
5 . The method of claim 1 , further comprising situating a polarizing beam splitter to direct a portion of the reflected portion of the input optical beam in the first linear state of polarization towards the laser and to direct a portion of the reflected portion of the input optical beam in the second linear state of polarization to a detector, wherein alignment of the at least one particle at the double nanohole is determined based on a signal produced by the detector in response to the portion of the optical beam associated with the at least one particle.
6 . The method of claim 1 , wherein the at least one particle is a polarizing nanoparticle.
7 . The method of claim 1 , wherein variation in an intensity of the portion of the portion of the optical beam associated with the at least one particle is indicative of trapping of objects in a size range of 0.5 nm-100 nm.
8 . The method of claim 1 , further comprising detecting Brownian motion of one or more particles based on fluctuations in the reflected optical beam associated with the at least one particle.
9 . The method of claim 8 , further comprising estimating a particle size based on the Brownian motion.
10 . The method of claim 1 , further comprising determining a particle shape or size based on the portion of the optical beam associated with the at least one particle or determining a shape or size of an object trapped at the double nanohole.
11 . The method of claim 1 , wherein the at least one particle is a protein, peptide, DNA, a biomolecule, a lipoprotein, a hormone, an inorganic nanoparticle, or a nanorod.
12 . The method of claim 1 , further comprising:
based on fluctuations in intensity of the portion of the optical beam associated with the at least one particle, transduce Brownian motion of the at least one particle into a detectable signal to determine size, or geometry of the at least one particle; and/or
with a Raman filter, detecting inelastically scattered light from the at least one particle to determine material properties of the at least one particle; and/or
detecting fluorescence or nonlinear optical signals from the at least one particle; and/or
detecting optical radiation associated with the trapping with a photon counting system.
13 . The method of claim 1 , wherein the input optical beam in a first linear state of polarization is directed to the at least one particle situated in a sample volume from a first side of the sample volume, and further comprising supplying a sample fluid to the sample volume from a second side of the sample volume, opposite the first side.
14 . The method of claim 1 , wherein the at least one particle is situated in a highly absorbing or scattering medium or in a medium that contains obstructing objects.
15 . The method of claim 1 , wherein:
(1) based on fluctuations in intensity of the optical beam associated with the at least one particle, transducing Brownian motion of the at least one particle into a detector signal; and/or
(2) the optical beam associated with the at least one particle comprises inelastically scattered light, and the detector signal is associated with a material property of the at least one particle; and/or
(3) the optical beam associated with the at least one particle comprises fluorescence or nonlinear optical signals from the at least one particle.