Laser tweezer system for measuring acoustic vibrations of nanoparticles

Optically trapped specimens (typically nanoparticles having diameters between 1 and 50 nm) are excited with an optical beam so as to induce vibrations in the specimens. A trapping optical beam and the excitation optical beam can produce vibrations based on a difference frequency based on the trapping optical beam and the excitation optical beam. Scattered optical radiation as a function of modulation frequency can be recorded and used to identify or characterize the specimen.

BACKGROUND

Optical tweezers have been used to trap nanoscale dielectric particles using forces applied with focused laser beams. An electric field gradient at the beam waist draws particles to the center of the beam. Particles also experience a force in the direction of beam propagation, and thus tend to be situated slightly displaced from the beam waist along the direction of propagation. While conventional optical tweezers are useful in many applications, improvements are needed to expand the range of available applications, particularly for trapping particles <50 nm in size.

SUMMARY

In the examples discussed below, high frequency optical excitation is applied so that trapped samples are heated. Motion from heated samples is detected based on elastic scattering of light that can be detected at much lower frequencies than actual specimen vibration frequencies. Some methods comprise optically trapping a sample and applying an excitation optical signal to the optically trapped sample so as to excite a vibration of the optically trapped sample. A portion of the excitation optical signal received from the sample (or a portion of another optical signal) is detected so as to determine a vibrational frequency associated with the sample. In alternative examples, the sample is optically trapped in response to the applied excitation signal, in response to a trapping optical signal, or in response to a combination of the excitation optical signal and the trapping optical signal. In typical examples, the sample is optically trapped at or near an aperture, wherein the aperture is a single nanohole or a double nanohole having an effective trapping region diameter between 0.1 nm and 300 nm. According to some embodiments, the aperture can have one or more cusps. In some cases, the aperture is defined in a metallic film. In a particular example, the excitation optical signal includes first and second optical signals, and the vibration of the sample is excited in response to a frequency difference between the first and second optical signals. Typically, the frequency difference is a heterodyne frequency between the first and second optical signals. The excitation optical signal can be produced by one or two semiconductor lasers (external cavity, distributed feedback, VCSEL or distributed Bragg reflector lasers), solid state lasers, a dye laser, or a mode-locked laser. In most cases, two lasers of any kind can used to produce a heterodyne frequency. A single mode-locked laser can also be used to induce specimen vibrations. With a pulsed laser, a pump-probe setup can be used, which either has a probe beam from a CW laser or from the pulsed laser itself as a beam portion that is split from the main beam.

Apparatus comprise an excitation optical radiation source that delivers an excitation optical beam to an optically trapped specimen. A control system is coupled to the optical radiation source so as to select a modulation frequency associated with the excitation optical beam. A detector is situated to receive at least a portion of the excitation optical beam responsive to vibrations induced in the trapped specimen by the excitation optical beam. Typically, a detection signal is produced that is associated with the magnitude of the induced vibrations. Modulation frequencies that are at or near specimen resonance frequencies tend to produce larger detection signals due to the increase of vibrational amplitudes at such modulation frequencies. In some examples, the control system selects a plurality of modulation frequencies and the detector provides a detection signal corresponding to the plurality of modulation frequencies. Typically, the control system selects the plurality of modulation frequencies by sweeping the modulation frequency in a frequency range. According to representative examples, the optical excitation source comprises a first optical source and a second optical source, and the control system adjusts a frequency of at least the first optical source and the second optical source to establish a heterodyne frequency, wherein the modulation frequency corresponds to the heterodyne frequency. The first optical source and the second optical source can be semiconductor lasers such as distributed feedback lasers, vertical cavity surface emitting lasers (VCSELs), solid state lasers, gas lasers, or other lasers that can provide pulsed or continuous wave outputs, including mode-locked pulses. In addition, external cavity semiconductor lasers (including Littman and Littrow configurations) can be used.

In some alternatives, a trapping optical radiation source applies a trapping optical beam to the sample, wherein the trapping and excitation optical radiation sources includes respective lasers and the control system is coupled so as to select the modulation frequency as a heterodyne frequency between the trapping optical beam and the excitation optical beam. In other examples, at least one nanohole, is situated so that the trapping optical beam is directed toward the nanohole so as to trap the specimen at the nanohole. The at least one nanohole can be a double nanohole. In further examples, the detector is situated to receive at least a portion of the excitation optical beam inelastically or elastically scattered by the trapped specimen.

These and other features and aspects of the disclosed technology are set forth below with reference to the accompanying drawings.

DETAILED DESCRIPTION

The interaction of light with mechanical vibrations has had broad impact ranging from cavity optomechanics of micron structures (<10 GHz) to infrared and Raman spectroscopy of molecular vibrations (>3 THz). Between these frequency extremes is the so-called “terahertz gap” in which new approaches are needed to efficiently probe the vibrations of nanoparticles such as colloidal particles, quantum dots, proteins, DNA, and virions. Disclosed herein are methods and apparatus than can be applied to probing vibrations of nanoparticles using laser tweezers. The disclosed approaches can be used with single molecules, and thus can be applied to ultra-sensitive spectroscopy and detection applications. One or more optical beams are used to induce vibrations in a trapped specimen, and one or more beams (the same or different beams than the stimulus beams or a trapping beam) are directed to the specimen to detect vibrations. High frequencies are needed to induce high amplitude vibrations due to the high resonance frequencies of typical specimens. These induced high frequency variations can be detected at low frequencies based on scattered portions of optical beams. High frequency detection is not required. Thus, scattered portions of optical beams measured at low frequencies can be used to detect trapped specimen resonance frequencies of many GHz or THz.

A representative system100is shown inFIG. 1. A source102produces optical radiation that is directed to a specimen106via beam forming optics103so as to trap the specimen. A second source104directs a stimulus beam to the specimen106. A detector110receives optical radiation from the source as collected by receiver optics105and produces a signal that varies based on the specimen's response to the optical radiation from the stimulus source. A controller112is coupled to the trap source102and the stimulus source104so as to control trapping and the frequency associated with one or both of the trapping and stimulus optical radiation. In some cases, the controller112produces a variable frequency, and the detector signal is recorded as a function of the variable frequency. For example, the controller112can adjust the frequencies of the stimulus optical radiation so as to produce a variable heterodyne frequency based on interference. The detector110can be coupled to a display to show detector signal or signal variations as a function of frequency, or the detector signal can be recorded for analysis or transmission.

Another example system200is shown inFIG. 2. First and second distributed feedback lasers (DFB lasers)202,204produce first and second optical beams that are coupled into fibers206,207(typically single mode fiber) connected to a polarization maintaining 50/50 coupler208. Other types of beam combiners and coupling ratios other than 50/50 can be used. The optical beams of the lasers202,204are at optical frequencies that can be tuned by adjusting laser temperature, drive current, or other parameters. As shown inFIG. 2, the fiber207delivers a portion of the combined beams to an optical spectrum analyzer (OSA)212to measure the frequency difference between the lasers202,204. A temperature controller210(or other frequency controller) is coupled to one or both of the lasers202,204to adjust laser temperature so as to tune emission frequency of one or both of the lasers202,204to establish a selected heterodyne frequency. In typical examples, the frequency difference between the lasers202,204is varied so as to induce vibrations in a sample under investigation.

Lasers such as gas or solid state lasers can be used, but semiconductor lasers are convenient. DFB lasers are generally selected due to their relatively narrow spectral bandwidths. Laser output power and frequency can be severely affected by back reflections, and as shown inFIG. 2, an isolator216is placed between fiber launch (FL) optics218and beam forming/delivery optics220, shown in the inverted microscope arrangement. A fiber polarization controller (FPC)224is used to align the polarization of the optical beams from the lasers202,204before passing through the isolator216to minimize, reduce, or otherwise control back reflections. In the example ofFIG. 2, the beam forming/delivery optics includes an attenuating optical filter230, a half-wave plate232, a beam expander234that includes lenses236,237, a turning mirror238, and an objective lens240(shown as a 100× oil-immersion microscope objective) that focuses the combined beams into a sample secured to a specimen holder244. A stage246retains the sample, and permits translation of the specimen holder244with the beam in transverse and longitudinal directions.

The combined beams or scattered optical radiation associated with one or more of the beams are directed through the specimen holder244to a receiver optical system249that includes first a condenser lens248, a mirror250, and a second condenser lens252. The receiver optical system249directs the combined beams to a detector260such as an avalanche photodiode.

The specimen holder244includes a double-nanohole (DNH) aperture as a trapping site. DNHs are described in detail in Y. Pang and R. Gordon, “Optical trapping of a single protein,” Nano Letters, 12(1), 402-406 (2012), which is incorporated herein by reference. Changes in transmission through the DNH are measured as voltage changes at the detector260. These changes are actuated by a nanoparticle trapped in the DNH, as the traps are sensitive to dielectric loading. Interference between the optical beams of the first and second lasers202,204leads to modulation of a local intensity at the DNH. This modulation occurs at a difference frequency (a heterodyne or beat frequency) between the two lasers. By tuning the frequency difference, a wide range of beat frequencies in the ˜10 GHz-10 THz range can be obtained. This modulated intensity also modulates the electrostriction force (elongation of a particle under an applied field), and this vibrates the molecule. There is an increase in detector signal fluctuations when the beat frequency matches or approaches a vibrational resonance of the nanoparticle, corresponding to increases in particle motion so as to heat the particle by applying a modulation that matches the vibrational resonance.

FIG. 3is a plot of root mean squared (RMS) variation in detector signal for a trapped 20 nm diameter polystyrene particle as a function of beat frequency. Standard theory for the vibrational resonances of a sphere predicts a peak at ˜44.0 GHz, as seen here. The other peak has been attributed to the lowest order acoustic mode in Raman experiments on smaller nanoparticles. Expected l=2 resonance of 20 nm diameter polystyrene spheres in vacuum (higher frequency) and in water (lower frequency) are noted with vertical lines. Note, those Raman measurements have lower resolution, can only probe higher frequencies, do not probe single particles and require complex and expensive spectroscopy systems—such as triple monochromators. Other methods to measure such peaks include Brillouin scattering, but Brillouin scattering based approaches are unable to probe single particles having sizes that are less than about 200 nm. Still other approaches are based on the Optical Kerr Effect, but such approaches have only been able to probe vibrations of strongly scattering (i.e., plasmonic) single nanoparticles, or of many particles in solution.

Titania spheres of 20.5 nm diameter show a similar peak, but at a much higher wavelength detuning (beat frequency) as shown inFIG. 4. The two peaks are attributed to one of two possible factors: the slightly ellipsoid nature of titania nanoparticles, or the crystal structure of titania particles leading to elastic anisotropy (the latter being the most likely explanation since it agrees well with the elastic anisotropy values in the literature). The value of the detuning is in the range expected for that size of titania particle. Based on these measurements, it is clear that a type of particle can be identified, and combined with other factors, such as the step height at trapping or the RMS roll off frequency, particle size can be assessed or estimated. Expected resonances of 21 nm diameter titania particles are at about 157 GHz and 161 GHz.FIGS. 5A-Eare graphs similar to those ofFIGS. 3-4obtained based on trapping of carbonic anhydrase, conalbumin, aprotinin, cyclooxygenase-2, and streptavidin, respectively. Referring toFIG. 5D, the two peaks are attributed to the ellipsoidal nature of cyclooxygenase.

Similar approaches can be used for the investigation, manipulation, and identification of DNA (e.g. for sequencing), viruses (e.g., for detection, noting heterogeneity, etc.), colloids (e.g. for measuring polydispersity), protein complexes (e.g., for measuring binding affinities), antibodies (e.g. for controlling antibody synthesis) and other applications.FIG. 6is a graph of root mean squared (RMS) variation in photodetector signal obtained from the bacteriophage (bacteria virus) MS2.

Referring toFIGS. 7A-7B, a representative fiber-based system700includes an optical excitation and/or trapping source702that delivers an excitation or trapping optical beam to a beam input end706of an optical fiber704or other optical waveguide. The beam (or beams) are guided to a beam output surface710at a beam output end708. The output surface710includes a nano-aperture716defined in a metallic layer714situated on the output surface710. The nano-aperture716is situated at the core region718of the optical fiber704, and is shown inFIG. 7Bas centered on the fiber core, but other locations can be used. A detection system722receives optical radiation responsive to the optical excitation beam and collected by receiver optics720from a specimen situated at the nano-aperture716. Typically, the received optical radiation is associated with increased specimen vibration induced by the excitation optical beam.

REPRESENTATIVE EXAMPLES

Examples of the disclosed technology include a laser tweezer system that traps nanoparticles (e.g., colloidal particles, quantum dots, DNA, proteins, viruses, etc.) and uses a modulated laser source to excite the vibrations of the trapped nanoparticles and probe these vibrations. In some cases, the tweezer laser is also the modulated laser or is separate from the modulated laser. The modulation can be obtained by interfering two laser beams of different frequencies. The lasers can be distributed feedback lasers operating at wavelengths from 500 nm to 2 μm. In other examples, the two lasers are VCSEL. One or more of the lasers can be an external cavity laser, a Ti:Sapphire CW laser, a dye laser, or a mode locked laser (e.g., Ti:Sapphire). In some embodiments, the laser tweezer system uses an inverted microscope setup. In particular examples, the laser tweezer system uses an aperture in a metal film for trapping, and in some examples, the aperture is a double nanohole, or similar shape with cusps to enhance trapping of nanoparticles in the 0.1 nm to 50 nm range. Fibers can be used to deliver beams to and from the specimen, and the aperture can be integrated on the end of a fiber coated with a metal. In typical examples, the vibrational resonances of the nanoparticles lead to increased motion of the particle when the laser is at or near a resonance frequency and this can be detected in the optical radiation scattered by the particle, as detected with a photodiode (or avalanche photodiode or other optical detector). In some cases, light scattering shows the vibrational resonance by heating the particle, resulting in increased Brownian motion. Alternatively, light scattering can be associated with a vibrational resonance related to particle polarizability. In some examples, a spectrometer (or other optical spectrum analyzer) is used to monitor the wavelengths of the lasers and/or the wavelength of the inelastically scattered photons (such as a Raman signal) that shows the vibrational resonance. In typical applications, nanoparticles are identified based on, for example, size or composition, or on dynamic changes in their state (e.g., protein binding). Other applications include DNA sequencing or the evaluation of DNA-protein interactions, viruses (virions), virus interactions, protein-small molecule interactions, protein-peptide interactions, protein-protein interactions (including with antibodies), macromolecules and macromolecular interactions, colloidal particle analysis, so as to measure material properties and polydispersity.

Having described and illustrated the principles of the disclosed technology with reference to the illustrated embodiments, it will be recognized that the illustrated embodiments can be modified in arrangement and detail without departing from such principles. The particular arrangements above are provided for convenient illustration, and other arrangements can be used, and we claim all that is encompassed by the appended claims.