APPARATUS AND METHOD FOR SUPER-RESOLUTION BRILLOUIN MICROSCOPY

An apparatus for super-resolution Brillouin microscopy includes a probe laser that emits a first laser beam and a first objective lens that focuses the first laser beam onto a sample. The apparatus further includes a pump laser that emits a second laser beam and a second quarter-wave plate that receives the second laser beam. The apparatus further includes a depletion laser that emits a third laser beam that passes through a phase plate to modify its wavefront phase such that the third laser beam has a donut shape, and a second objective lens that focuses the second laser beam and the third laser beam onto the sample. Characteristically, the beam spot from the depletion laser is overlaid with the Gaussian-shape beam spots of the first laser beam and the second laser beam at the same focal plane. A detector is configured to detect a stimulated Brillouin gain signal and a stimulated Brillouin loss signal.

TECHNICAL FIELD

The present disclosure relates to the improvements to Brillouin microscopy techniques. Specifically, the present disclosure relates to the development of the apparatus and method of super-resolution Brillouin microscopy. The invented apparatus and method break the optical diffraction limit and can achieve a spatial resolution that is 5 times better than existing Brillouin techniques, which allows to quantify the mechanical properties of biomedical material at ˜100 nm scale.

BACKGROUND

Cells experience physical changes of microenvironments during many physiological and pathological activities including cancer metastasis, immune response, and development. To adapt to such changes, cells use cytoskeleton and nucleus to sense and respond to the external stimuli via subcellular structures including focal adhesion and microfilament proteins1,2. For example, the stress fibers, bundles of actomyosin filaments, is a crucial cytoskeletal component to regulate cell's functions including adhesion, contraction, migration, differentiation, and maintaining its shape3. To understand the complex mechanical interactions, it is fundamentally necessary to quantify the mechanical elasticity of these subcellular cytoskeletal components in situ. However, this is highly challenging due to the inherent limitation of existing techniques.

Cytoskeletal structure such as stress fibers has size of hundreds of nanometers and mechanically connected with other components within the cell body. Existing methods for measuring mechanical property of cell can be classified into two categories: contact-based and noncontact-based. Contact-based techniques such as atomic force microscopy (AFM) or micropipette stretching requires physical contact and applies force to cell body4,5, making it an indirect probe and impossible to assess the intracellular structure exclusively due to the mechanical interaction of cytoskeletal components6. On the other hand, noncontact-based technique such as optical elastography7and confocal Brillouin microscopy8do not have enough spatial resolution to differentiate the microfilament structures. Therefore, A technique that can directly quantify the mechanical property of microfilament structures inside the cell with high resolution in 2D/3D is highly desired but an unmet need.

In recent years, confocal Brillouin microscopy has been emerged as a promising complementary tool to conventional technologies for quantifying the mechanical properties of biomedical materials, as it is an all-optical technique and can conduct the measurement in a noncontact, non-perturbative, and label-free manner9-24. However, current Brillouin microscopy is based on the spontaneous Brillouin scattering and optical confocal configuration. As such, its spatial resolution is limited to approximately half of the wavelength (˜500 nm), which is not enough to identify cytoskeletal structures. Therefore, there exists a general need for developing new apparatus and method of Brillouin microscopy that can significantly improve the spatial resolution of Brillouin technology so that the mechanical properties of the intracellular structure that has nanometer size can be measured.

SUMMARY

In at least one aspect, an apparatus and method of super-resolution Brillouin microscopy (SBM) that overcomes the aforementioned limitations is provided. The innovation of the proposed SBM is based on the physical principle of stimulated Brillouin scattering25,26and the idea of stimulated emission depletion27. Different from the spontaneous scattering used in confocal Brillouin, stimulated Brillouin scattering is a highly controllable scattering process: Brillouin signal (gain or loss) is only excited when the frequency difference between a pump laser and a probe laser matches the Brillouin frequency shift of the material26. To achieve super-resolution, we first build up the stimulated Brillouin scattering with a pump laser and a probe laser, both beams have Gaussian-profile intensity shape. We then introduce a third laser (depletion laser) that has a donut shape and make it overlap with the Gaussian beam. By tuning the frequencies of three lasers, we can deplete Brillouin gain within the overlapping region, thus the detected Brillouin signal will be only from the subtracted region, whose size is much smaller than the original diffraction-limited Gaussian beam.

In another aspect, an apparatus for super-resolution Brillouin microscopy is provided. The apparatus includes a probe laser that emits a first laser beam, a polarizer through which the first laser beam passes, a first quarter-wave plate through which the first laser beam passes after the polarizer, and a first objective lens that focuses the first laser beam onto a sample. Characteristically, the first laser beam has a beam spot with a diffraction-limited Gaussian intensity profile. The apparatus further includes a pump laser that emits a second laser beam, a second quarter-wave plate that receives the second laser beam. The apparatus further includes a depletion laser that emits a third laser beam, a phase plate through which the third laser beam passes to modify its wavefront phase such that the third laser beam has a donut shape, and a second objective lens that focuses the second laser beam and the third laser beam onto the sample. Characteristically, the beam spot from the depletion laser is overlaid with the Gaussian-shape beam spots of the first laser beam and the second laser beam. The apparatus also includes a detector configured to detect a stimulated Brillouin gain (SBG) signal and a stimulated Brillouin loss (SBL) signal. Advantageously, the apparatus is configured to adjust the frequencies of the probe laser, the pump laser, and the depletion laser to establish stimulated emission depletion such that the Brillouin signal created from the subtracted region of the Gaussian beam and donut beam is collected by the second objective lens and redirected into the detector.

In another aspect, a method for establishing stimulated emission depletion with the apparatus described herein is provided. The method includes steps of:moving the phase plate out of the beam path so that the beam spot of the depletion laser has a Gaussian intensity shape and is overlapped with the beam spots of the pump laser as well as the probe laser at the focal plane of the objective lens;locking the frequency of the pump laser to the absorption line of the Rubidium (Rb) gas. scanning the frequency of the probe laser until the detector detects the stimulated Brillouin gain (SBG) signal;blocking the pump laser beam, and scanning the frequency of the depletion laser until the detector detects the stimulated Brillouin loss (SBL) signal;re-switching on the pump laser beam, and adjusting the power of the depletion laser until the SBG signal and SBL signal cancel out with each other; andinserting the phase plate into the beam path to reshape the beam spot of the depletion laser into a donut shape at the focal plane of the objective lens.

DETAILED DESCRIPTION

When referring to a numerical quantity, in a refinement, the term “less than” includes a lower non-included limit that is 5 percent of the number indicated after “less than.” A lower non-includes limit means that the numerical quantity being described is greater than the value indicated as a lower non-included limited. For example, “less than 20” includes a lower non-included limit of 1 in a refinement. Therefore, this refinement of “less than 20” includes a range between 1 and 20. In another refinement, the term “less than” includes a lower non-included limit that is, in increasing order of preference, 20 percent, 10 percent, 5 percent, 1 percent, or 0 percent of the number indicated after “less than.”

With respect to electrical devices, the term “connected to” means that the electrical components referred to as connected to are in electrical communication. In a refinement, “connected to” means that the electrical components referred to as connected to are directly wired to each other. In another refinement, “connected to” means that the electrical components communicate wirelessly or by a combination of wired and wirelessly connected components. In another refinement, “connected to” means that one or more additional electrical components are interposed between the electrical components referred to as connected to with an electrical signal from an originating component being processed (e.g., filtered, amplified, modulated, rectified, attenuated, summed, subtracted, etc.) before being received to the component connected thereto.

The term “electrical communication” means that an electrical signal is either directly or indirectly sent from an originating electronic device to a receiving electrical device. Indirect electrical communication can involve processing of the electrical signal, including but not limited to, filtering of the signal, amplification of the signal, rectification of the signal, modulation of the signal, attenuation of the signal, adding of the signal with another signal, subtracting the signal from another signal, subtracting another signal from the signal, and the like. Electrical communication can be accomplished with wired components, wirelessly connected components, or a combination thereof.

The term “one or more” means “at least one” and the term “at least one” means “one or more.” The terms “one or more” and “at least one” include “plurality” as a subset.

The term “substantially,” “generally,” or “about” may be used herein to describe disclosed or claimed embodiments. The term “substantially” may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such instances, “substantially” may signify that the value or relative characteristic it modifies is within ±0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or 10% of the value or relative characteristic.

The term “electrical signal” refers to the electrical output from an electronic device or the electrical input to an electronic device. The electrical signal is characterized by voltage and/or current. The electrical signal can be stationary with respect to time (e.g., a DC signal) or it can vary with respect to time. The term “computing device” refers generally to any device that can perform at least one function, including communicating with another computing device. In a refinement, a computing device includes a central processing unit that can execute program steps and memory for storing data and a program code.

When a computing device is described as performing an action or method step, it is understood that the one or more computing devices are operable to perform the action or method step typically by executing one or more lines of source code. The actions or method steps can be encoded onto non-transitory memory (e.g., hard drives, optical drive, flash drives, and the like).

This invention is based on the principle of stimulated Brillouin scattering and the idea of stimulated emission depletion. The stimulated Brillouin scattering is a nonlinear optical process where the acoustic phonons are driven by the resonant interaction of counter-propagating pump and probe lasers. This is a controllable process as the Brillouin signal (stimulated Brillouin gain or stimulated Brillouin loss) will be excited only when the frequency difference of the pump laser and the probe laser matches the Brillouin shift of the material. The idea of stimulated emission depletion has enabled the breakthrough of fluorescence super-resolution microscopy, where the resolution below diffraction limit is achieved by selectively deactivating fluorophores. In this invention, we adapt the idea of stimulated emission to the scenario of stimulated Brillouin scattering and achieve super-resolution in biomechanical imaging.

FIG.1shows the first exemplary embodiment of an apparatus for super-resolution Brillouin microscopy. Apparatus10includes probe laser12that emits a first laser beam. A first optical system14focuses the first laser beam into a first Gaussian-shaped beam spot on a sample16at a first focal plane that has a first Gaussian intensity profile. Pump laser18emits a second laser beam. A second optical system20focuses the second laser beam into a second Gaussian-shaped beam spot on the sample at a second focal plane that has a second Gaussian intensity profile. Characteristically, the second focal plane overlaps the first focal plane. The first Gaussian-shaped beam spot overlaps the second Gaussian-shaped beam spot. A depletion laser22emits a third laser beam. A third optical system24focuses the third laser beam into a third beam spot having a donut shape with zero intensity in its center. The third beam spot is overlaid with the first Gaussian-shaped beam spot and the second Gaussian-shaped beam spot. A detector28is configured to detect a stimulated Brillouin gain (SBG) signal and a stimulated Brillouin loss (SBL) signal. Advantageously, the apparatus is configured to adjust frequencies of the probe laser, the pump laser, and the depletion laser to establish stimulated emission depletion such that a Brillouin signal created from a subtracted region of the first Gaussian-shaped beam spot and the second Gaussian-shaped beam spot and donut beam is collected and redirected into the detector.

In another aspect the first optical system14includes a polarizer32through which the first laser beam passes and a first quarter-wave plate34through which the first laser beam passes after the polarizer. First optical system14also includes a first objective lens36that focuses the first laser beam onto the sample, wherein the first laser beam has a beam spot with a diffraction-limited Gaussian intensity profile. In a refinement, the first optical system14also includes a mirror38. The first laser beam is reflected by mirror38from the polarizer32to the first quarter-wave plate34.

In another aspect, the second optical system20includes a second quarter-wave plate40that receives the second laser beam. Second optical system20also includes second objective lens42that focuses the second laser beam onto the sample. In a refinement, second optical system20also includes beam splitter45, a polarized beam splitter46, and a mirror48. The second laser beam emitted from the pump laser is redirected into the second quarter-wave plate40by a beam splitter45, a polarized beam splitter46, and mirror48.

In another aspect, the third optical system24includes a phase plate44through which the third laser beam passes to modify its wavefront phase such that the third laser beam has the donut shape. Third optical system24also includes the second objective lens42that also focuses the third laser beam onto the sample16. Third optical system24also includes mirror50, second quarter-wave plate40, beam splitter45, a polarized beam splitter46, and a mirror48. The third laser beam is guided from the phase plate44into the second objective lens and focused into the sample after passing through a mirror50, a beam splitter45, a polarized beam splitter46, the mirror48, and the second quarter-wave plate40.

In another aspect, the first objective lens36and the second objective lens42have the same configuration.

Referring more specifically toFIG.1, the first laser beam emitted by probe laser12first passes through a polarizer32. After reflected by a mirror38, the beam passes through a quarter-wave plate34and is focused into the sample16by an objective lens36. At the focal plane, the beam spot has a diffraction-limited Gaussian intensity profile. The second laser beam emitted from the pump laser18is redirected into a quarter-wave plate40by a beam splitter45, a polarized beam splitter46, and a mirror48. After that, the second laser beam is focused into the sample16by an objective lens42that is identical to the objective lens36. The objective lenses36and42are adjusted such that their focal planes are overlapped within the sample16. Therefore, the beam spots of the first laser beam and the second laser beam are perfectly overlapped within the sample16and have Gaussian intensity profile. The third laser beam emitted from a depletion laser22first passes through a phase plate44to modify its wavefront phase. The laser beam is then guided into the objective lens42and focused into the sample16after passing through the mirror50, the beam splitter45, the polarized beam splitter46, the mirror48and the quarter-wave plate40. The beam spot from the depletion laser22is overlaid with the Gaussian-shape beam spots of the probe laser12and the pump laser18. Because of the phase plate44, the beam spot from the depletion laser22has donut shape featuring a zero intensity in the center. By adjusting the frequencies of the lasers12,18, and22, the stimulated emission depletion is established such that the Brillouin signal created from the subtracted region of the Gaussian beam and donut beam is collected by the objective lens42and redirected into the detector28by the quarter-wave plate40, the mirror48, and the polarized beam splitter46. Since the subtracted region is much smaller than the diffraction-limited Gaussian beam, super-resolution Brillouin detection is achieved.

FIG.2provides a flowchart of exemplary steps to establish the stimulated emission depletion with the setup ofFIG.1. First, the phase plate44is moved out of the beam path so that the beam spot of the depletion laser has a Gaussian intensity shape and is overlapped with the beam spots of the pump laser as well as the probe laser at the focal plane of the objective lens42. Then, the frequency of the pump laser is locked to the absorption line of the Rubidium (Rb) gas. Next, the frequency of the probe laser is scanned until the detector detects the stimulated Brillouin gain (SBG) signal. Next, the pump laser beam is blocked, and the frequency of the depletion laser is scanned until the detector detects the stimulated Brillouin loss (SBL) signal. Next, the pump laser beam is re-switched on, and the power of the depletion laser is adjusted until the SBG signal and SBL signal cancel out with each other. In the end, the phase plate44is inserted into the beam path to reshape the beam spot of the depletion laser into a donut shape at the focal plane of the objective lens42.

FIG.3shows the examples on simulation results of the beam shape manipulation for achieving super-resolution. The probe and pump lasers have a Gaussian intensity profile (as shown in the lower panel) at the focal plane of the objective lenses36and42, respectively. With the phase plate44, the depletion laser has a donut intensity profile and is overlapped with the diffraction-limited Gaussian beam at the same focal plane within the sample. Within the overlapping region, the SBG signal is depleted by the depletion laser. Therefore, the detected SBG signal will be only from the subtracted region. Since the spot size of the subtracted region is much smaller than the Gaussian beam, super-resolution that breaks the diffraction limit is achieved.

FIG.4shows an exemplary setup developed based on the content of this invention for super-resolution Brillouin measurement. The setup is built using an inverted microscope stand. To establish stimulated Brillouin scattering, the pump laser beam and probe laser beam are counterpropagating and coincided within the sample by using two identical objective lenses. This creates a diffraction-limited Gaussian beam spot at the focal plane. To achieve super-resolution, a third laser (the depletion laser with a donut beam shape) is coupled into the setup and overlapped with the Gaussian beam spot within the sample. By tuning the frequency of the depletion laser, Brillouin signal of the overlapped region of the Gaussian and donut beams will be selected depleted. As a result, the detector will only receive Brillouin signal from the subtracted region, whose size is much smaller than the initial Gaussian beam thus breaks the diffraction limit. A lock-in amplifier is used for high-sensitivity detection of the Brillouin signal, and a computer is used for signal collection.

FIG.5shows an exemplary setup developed based on the content of this invention for establishing stimulated Brillouin scattering. The pump laser (780.24 nm, TA pro, Toptica) is locked to the absorption line of the Rb gas cell (COSY, Toptica). The probe laser (780.24 nm, DL pro, Toptica) is freely running, and its frequency difference against the pump laser is monitored by the frequency counter (FC) in real time. Two identical objective lenses (60×, NA=1.2, Olympus) are used to focus the laser beams into a Gaussian shape and overlapping each other within the sample. The pump laser is further modulated by a AOM with a frequency of ˜1 MHz for high-sensitivity signal detection. To reject the stray light noise, a Rb gas cell (300-mm length) is placed in front of the photodiode detector. By scanning the frequency of the probe laser, the SBG signal is detected by the lock-in amplifier. A software interface based on LabVIEW platform is developed to control the lasers, monitor the status of the components, and detect the signal. We first quantify the dependence of the signal-to-noise ratio (SNR) on the power delivered to the sample by the pump and probe lasers. From there, we determine the optimized parameters that allows us to conduct experiments with lowest light dose.

FIG.6shows an exemplary setup developed based on the content of this invention for achieving stimulated emission depletion within the overlapping region of the Gaussian beam and donut beam. Briefly, it includes three steps. Step 1 (FIG.6a): lock the pump laser (ω1) to Rb gas cell, scan the probe laser (ω2) at 1 kHz until obtaining SBG signal. At the end of this step, we will record the Brillouin shift of the sample: ωB=ω1−ω2. Step 2 (FIG.6b): switch off the pump beam, and switch on the depletion beam; set the initial frequency of the depletion laser ω3=ω1−2ωB, then slowly scan (1 Hz) its frequency within a bandwidth of ±1 GHz until acquiring SBL signal; stop scanning the depletion laser. Step 3 (FIG.6c): switch on the pump beam, adjust the power of the depletion laser until the SBG signal and SBL signal cancel each other out. At the end of this step, the depletion condition will be successfully achieved. Motorized optical shutter is used to control on/off status of each laser beam. The frequency scan of the lasers is driven by the function generator and coordinated with each other.

REFERENCES