Optical trapping and manipulation have been extensively utilized to organic programs, and their cutting-edge strategies are creating present traits in nanomaterial sciences. The resonant absorption of supplies induces not solely the vitality switch from photons to quantum mechanical movement of electrons but in addition the momentum switch between them, leading to dissipative optical forces that drive the macroscopic mechanical movement of the particles. Nonetheless, optical manipulation, in accordance with the quantum mechanical properties of particular person nanoparticles, continues to be difficult. Right here, we show selective transportation of nanodiamonds with and with out nitrogen-vacancy facilities by balancing resonant absorption and scattering forces induced by two different-colored lasers counterpropagating alongside a nanofiber. Moreover, we suggest a technique for exactly figuring out the absorption cross sections for single nanoparticles by monitoring the optically pushed movement, which known as as “optical drive spectroscopy.” This technique supplies a novel course in optical manipulation know-how towards growth of useful nanomaterials and quantum gadgets.
Nanoparticles and nanomaterials—reminiscent of quantum dots, nanocrystals, carbon nanomaterials, molecular aggregates, and steel nanoparticles—have attracted nice consideration owing to their distinctive mechanical and quantum mechanical properties and have been utilized in varied photonic, digital, mechanical, and biomedical gadgets, reminiscent of mild emitters, photo voltaic cells, photocatalysts, molecular electronics, structural supplies, drug supply, and bioimaging (1–6). As a result of these properties of nanoparticles/nanomaterials are strongly influenced by the encircling surroundings and are considerably totally different from the majority properties, reminiscent of quantum measurement impact, the characterization of particular person nanoparticles supplies vital data for advancing nanomaterial and quantum materials sciences. Moreover, the choice and sorting of single nanoparticles in accordance with their traits are important and desired for the exact design of useful nanostructures and growth of single-quantum sensors, single-photon sources, and quantum data gadgets (7, 8).
Optical trapping and manipulation based mostly on optical forces are promising instruments for positioning, transporting, and aligning wonderful particles with out mechanical contacts (9, 10). Optical tweezers proposed by Ashkin et al. have been utilized in varied analysis fields, reminiscent of biophysics, cell biology, microfluidics, complete analytical programs, and micromechanics (11, 12). Optical sorting of dielectric objects has been developed utilizing holographic optics, move cytometry, interference know-how, and close to subject photonics (13–15). Steel nanoparticles may also be separated by optical forces based mostly on the floor plasmon resonances (16). Nonetheless, these strategies are restricted to the particle choice by the scale and refractive index. The optical gradient and scattering forces exerted on small particles and their dependences on the diameter, wavelength, and relative refractive index are decided by the Mie principle. Moreover, the reported strategies are relevant solely to the sorting of submicrometer or larger-sized dielectric particles. Trapping and manipulation of smaller-sized particles stay difficult as a result of the optical drive turns into weaker in proportion to the particle quantity.
On this examine, we show the optical choice and sorting of nanoparticles in accordance with their quantum mechanical properties. Semiconductor quantum dots exhibit attribute optoelectronic properties because of the quantum confinement of the electron-hole pairs within the nanovolume (1, 2). Diamond nanoparticles exhibit quantum resonances of level defects (17, 18). The optical forces mirror these quantum mechanical properties of nanoparticles and their optical traits (19, 20). The interplay between mild and nanomaterials induces not solely an vitality switch from the photons to the quantum mechanical movement of the electrons but in addition a momentum switch between them. The change within the photon momentum give rise to optical forces, which drive the macroscopic mechanical movement of the nanoparticles. We notice that there are three sorts of optical forces: (i) gradient drive arising from the inhomogeneous depth distribution of the electrical subject, (ii) dissipative scattering drive brought on by the actual a part of the refractive index, and (iii) quantum resonant absorption drive exerted on nanomaterials. Subsequently, we are able to understand the characterization and selective manipulation of single nanoparticles having varied properties by monitoring and controlling the particle motions. This system supplies a brand new course in optical drive know-how towards advances in nanomaterial sciences.
RESULTS AND DISCUSSION
Optical trapping and sorting system
To understand the sorting of particular person nanoparticles, we use counterpropagating different-colored lasers that may extract the resonant absorption drive by cancelling out the scattering forces. The counterpropagating beam programs had been constructed utilizing a pair of lenses with massive numerical aperture positioned reverse to one another (21) and the inversely directed evanescent waves (16). Nonetheless, it’s tough to exclude the affect of the gradient drive that simply negates the small impact of the quantum resonance drive. Thus, we centered on tapered optical fibers, i.e., nanofibers (22, 23). We ready a nanofiber with a diameter of a number of hundred nanometers and size of a number of millimeters (24), which exhibited the traits of single-mode propagation, thereby forming an intense evanescent subject across the fiber and enabling long-distance propagation whereas sustaining a tightly centered beam of sunshine. Utilizing these traits, a uniform electrical subject distribution might be generated alongside the fiber by which the particle movement was restricted to 1 dimension. As well as, the optical gradient drive and thermophoretic drive, arising from the temperature gradient (e.g., Soret impact), had been exerted in a course perpendicular to the fiber axis such that the particle movement alongside the nanofiber was pushed solely by the resonant absorption and scattering forces. Moreover, as a result of the momentum of the photons in a waveguide is dependent upon the propagation constants of the person modes, the single-mode wave in our nanofiber had the fixed photon momentum; this supplies a perfect platform for analyzing the optical forces exerted on the nanoparticles. On the premise of the steadiness of the absorption and scattering forces induced by the different-colored lasers counterpropagating alongside the nanofiber, we succeeded in attaining the selective transportation of single nanoparticles in accordance with the quantum resonant absorption (Fig. 1A).
Along with the choice and sorting, the proposed system can exactly decide the resonant absorption cross sections of single nanoparticles. Fluorescence and photothermal spectroscopies have been extensively used for characterizing single nanoparticles and nanomaterials due to their excessive sensitivity on the degree of single-molecule detection (25, 26). Nonetheless, these strategies probe the comfort processes emitting a photon and thermal vitality, that are thought to be oblique absorption measurements. When the excited states of the supplies irreversibly transit to different states with out present process rest processes, reminiscent of photochemical reactions, these strategies can now not observe the resonant absorption. Absorption spectroscopy, which straight measures the excitation processes, is an indispensable instrument for analyzing the interplay strengths between mild and matter. Specifically, absolutely the values of the absorption cross sections of single nanoparticles/nanomaterials are important for experimental physics in materials science and are essential for designing nanostructured supplies at a single-quantum state degree (27). Nonetheless, it’s nonetheless difficult to detect extraordinarily small absorption alerts of single nanoparticles and nanomaterials. In our technique, correct measurement of quantum resonant absorption is realized by exactly observing the optical drive–pushed motions of the nanoparticles, referred to as as optical drive spectroscopy. This spectroscopy based mostly on the optical momentum change as a substitute of the vitality change is conceptually totally different from the traditional strategies.
Figure 1B illustrates the experimental setup. A nanofiber with a diameter of 400 nm was fabricated from a industrial single-mode optical fiber (24). The diameter is fixed within the waist a part of the fiber over a size of a number of hundred micrometers. The nanofiber was soaked in an aqueous answer of diamond nanoparticles, i.e., nanodiamonds (NDs). As a result of nitrogen-vacancy facilities (NVCs) in NDs have superior properties, reminiscent of no photobleaching, excessive sensitivity to the encircling surroundings, and sharp zero phonon line absorption, they’ve been gaining consideration as luminescent and magnetic-responsive nanomaterials that can be utilized for organic imaging, sensing, and single-photon supply (17, 18). Thus, choice and sorting of NDs with and with out NVCs are extremely fascinating. We ready two sorts of NDs; one contained NVCs (>300 per particle), i.e., quantum resonant ND (r-ND), and the opposite was nearly free from the NVCs, i.e., nonresonant ND (n-ND). The diameters of each r-NDs and n-NDs had been 50 ± 15 nm. Steady-wave inexperienced (GR; 532 nm) and near-infrared (NIR; 1064 nm) diode lasers had been launched from each ends of the nanofiber. The NVCs exhibit absorption on the GR area however not on the NIR area (28, 29). Moreover, we introduce a weak crimson laser within the fiber as a probe mild (690 nm, 0.1 mW) to watch the movement of the NDs, which was recorded by an optical microscope outfitted with a charge-coupled system (CCD) digital camera.
Selective transportation of single nanoparticles
Figure 2A depicts the trapping and transportation of a single r-ND, the place solely the GR laser (70 mW) is incident from the left finish of the fiber and the movement of the r-ND is noticed within the waist a part of the fiber. The outcome reveals that the r-ND is attracted by the gradient drive of the evanescent subject and strikes alongside the fiber due to the dissipative forces. The particle pace is fixed at 110 μm/s (see a trajectory in fig. S1). We consider the drive exerted on the r-ND as 89 fN by contemplating the steadiness between the optical drive and viscous drag utilizing the Faxen method for correcting the impact of the fiber floor [(23) and see the Supplementary Materials). When the NIR laser is simultaneously incident from the other end of the fiber (from the right), where the NIR laser power is fixed at 250 mW, and the GR laser power is varied from 70 to 0 mW, we achieve the motion control of a single r-ND (Fig. 2B). At the GR laser power of 70 mW, the r-ND moves toward the propagation direction of the GR laser (from left to right). As the GR laser power decreases, the motion decelerates and subsequently stops (~8 s). On further decreasing the GR laser power, the r-ND moves toward the opposite direction. The motion control experiment for an n-ND is illustrated in the Supplementary Materials (fig. S3).
The dissipative optical force exerted on an r-ND along a nanofiber is composed of two components, namely, absorption and scattering forces (Fabs, Fsca), which are represented by the absorption and scattering cross sections (σabs, σsca), as follows
(1)where I and c represent the intensity and velocity of light in a vacuum, respectively, and neff is the effective refractive index of the nanofiber (neff = 1.354 at 532 nm). The scattering cross section for Rayleigh particles is theoretically given by
(2)where n1 and n2 are the refractive indices of diamond and surrounding water, respectively, λ is the incident laser wavelength in vacuum, and V is the volume of the particle. In the case of r-NDs including NVCs, σabs is given by the transition dipole strength of an NVC and the number of NVCs in r-ND. The NIR laser induces only the scattering force, as NVCs exhibit no absorption at 1064 nm.
We perform a motion control experiment for an n-ND without NVCs to measure the balanced powers of the GR and NIR lasers for restricting the motion of the particle. The NIR laser power was fixed at 160 mW, corresponding to the intensity of 108 MW/cm2 estimated from the mode profile of the nanofiber, while the observed balanced power of the GR laser was 7.61 mW (intensity, 6.06 MW/cm2). As Fabs is not exerted on the n-ND, scattering forces (Fsca) by the GR and NIR lasers balance each other. Moreover, σsca strongly depends on the wavelength (Eq. 2), which is compensated by the large difference between the intensities of the GR and NIR lasers. As σsca is proportional to the square of the particle volume, the scattering force also changes significantly depending on the particle size. Fortunately, the ratio of the scattering forces at 532 and 1064 nm is constant for any particle size. This is because the volume dependence of σsca is the same (∝V2) for both wavelengths. Thus, it is noted that the balanced powers of the two counterpropagating lasers remain unchanged for n-NDs of any size.
Furthermore, we demonstrate the selective transportation of r-NDs and n-NDs (Fig. 3 and movie S1). The same experimental setup and nanofiber were used, and the NIR laser power was 160 mW. The GR laser power was adjusted to 7.40 mW to drive different motions of the r-NDs and n-NDs. This value is slightly lower than the balanced power of the n-ND such that the scattering force exerted by the NIR laser is stronger for n-NDs than that by the GR laser, whereas the resonant absorption force on the r-NDs by the GR laser reverses the force strength relation. By switching the probe laser on and off, we can measure the emission from the NVCs and thus distinguish between the r-NDs and n-NDs. The two particles at both ends are r-NDs (numbered 1 and 4) and the other two particles are n-NDs (numbered 2 and 3). Scattered light spots of four NDs have nearly the same intensities when the probe laser is off, while the spots of r-NDs are brighter than the spots of n-NDs in Fig. 3 because the NVC emission is added to the scattered light. The r-NDs slowly move to the right (along the propagation direction of the GR laser), whereas the n-NDs move in the opposite direction (see trajectories in fig. S2). This result clearly demonstrates the selective transportation of NDs according to the quantum resonant absorption of NVCs by using the optical forces.
Determination of the absorption cross section
Next, we analyze the absorption cross section (σabs) of a single r-ND. We prepared the same experimental conditions and used the same nanofiber that was used for the balanced power measurement of an n-ND. At the NIR laser power of 160 mW, the balanced power of the GR lasers for an r-ND was measured to be 6.75 mW (intensity, 5.37 MW/cm2). Then, by turning the NIR laser off, the motion of r-NDs driven by the GR laser was observed to determine the strength of the optical force. On the basis of the balance with the viscous drag, the optical force composed of Fabs and Fsca was calculated as 6.30 fN. As a reference, the data of the n-ND were used for the present absorption analysis. By comparing the balanced powers of the GR laser for the r-ND (Pr-ND = 6.75 mW) and n-ND (Pn-ND = 7.61 mW), we obtain the ratio of Fabs and Fsca exerted on the r-ND as (Pn-ND − Pr-ND):Pr-ND (Fig. 4) such that the measured optical force on the r-ND (F = 6.30 fN) can be decomposed into Fabs = 0.71 fN and Fsca = 5.59 fN. This result demonstrates that the absorption and scattering forces exerted on a single r-ND can be separately determined with subfemtonewton order accuracy. Thus, from Eq. 1, we evaluate the σabs to be 2.9 × 10−14 cm2. We repeated the measurements for 10 different r-NDs using the same nanofiber to perform the experiments under the same conditions. The average and SD of the evaluated σabs were 3.3 × 10−14 and 1.1 × 10−14 cm2, respectively. The deviations in σabs can be attributed to the variations in the number of NVCs contained in the r-NDs with different sizes and defect densities. The detailed distribution of σabs and estimated number of NVCs are shown in the Supplementary Materials (see fig. S4).
Here, we emphasize that the present method can detect the absorption cross section in the order of a square nanometer, which is close to those of single molecules (typically as large as 10−15 cm2). Under the diffraction-limited illumination condition, this absorption cross section corresponds to a transmittance of ~10−6. Recently, Kukura et al. (30) and Celebrano et al. (31) succeeded in measuring the extremely small absorption using highly sensitive detectors, and the accuracy of their method is comparable with that of our method. However, in their technique, the Rayleigh scattering caused by nanoparticles and nanomaterials attenuates the transmitted light intensity as well such that the absorption signals cannot be extracted separately from the scattering components. In contrast, our proposed optical force spectroscopy can separately determine the absorption and scattering cross sections of single nanoparticles from the momentum change. The sensitivity is not limited by the signal-to-noise ratio of light intensity detection but restricted by the accuracy of the motion detection. Although nanometer-level position sensing techniques are available, the random thermal motion is the main factor that determines the accuracy. If the experiment is performed using superfluid helium at the cryogenic temperature, then the detection accuracy will be ultimately improved.
We demonstrated the selective transportation of single nanoparticles based on the relation between the quantum mechanical properties of nanomaterials and their macroscopic motion driven by the quantum resonant optical forces. This selective transportation is applicable to the precise sorting of nanocrystals, quantum dots, and molecular nanoparticles according to their resonant absorption properties. Optical force spectroscopy directly and sensitively measures the interaction between light and nanoparticles separately from the scattering effects based on the photon momentum change and not the energy change. It is noted that even if the reference nanoparticle having the same parent material but without absorbers is unavailable, the proposed absorption detection can still be achieved (see Materials and Methods). Although we focus on NDs as the samples for the first demonstration, note that other kinds of nanoparticles can be equally interesting targets. Size-selective optical transport of semiconductor quantum dots has been successfully demonstrated (32). Furthermore, it was reported that organic dye-doped nanoparticles have unique optical trapping characteristics according to their quantum resonance properties (33, 34). Applying the present technique to these nanomaterials will be our future endeavor. In conclusion, we believe that our scheme can enable a new class of optical force methodologies to investigate the characteristics of advanced nanomaterials and quantum materials and develop state-of-the-art nanodevices.
MATERIALS AND METHODS
We used commercially available NDs having a mean diameter of 50 nm (r-NDs, FND Biotech Inc.; n-NDs, Microdiamant Japan. The absorption of NVCs appears at 532 nm after proton irradiation for fabricating r-NDs; contrarily, n-NDs exhibit no absorption at 532 nm. These were dispersed in pure water with 0.1 weight % surfactant. The concentration was adjusted such that a single ND is trapped by a nanofiber during the experiment.
Fabrication of a nanofiber
A commercially available single-mode optical fiber (780HP, Thorlabs) was used to fabricate a nanofiber. It was heated with a ceramic heater at ~1400°C and stretched at both ends. The waist diameter of the nanofiber used in this study was 400 nm, which remained constant (variation of <2%) over a length of several hundred micrometers. From the mode dispersion curve obtained by the fiber mode analysis, the single-mode propagation is valid when the wavelength of incident light is longer than 360 nm. The fiber was fixed on a glass slide using ultraviolet glue and soaked in a cell filled with an ND-dispersed aqueous solution.
Continuous-wave GR (532 nm) and NIR (1064 nm) diode lasers were introduced from both ends of the fabricated nanofiber. The laser powers were controlled using rotational neutral density filters. To record the motions of the NDs, we introduced a weak red laser (690 nm), and its light, scattered light from the particles, was monitored using a CCD camera. When nanoparticles other than the observed particles are trapped on the fiber, their scattering reduces the laser intensity irradiated on the particle. To avoid this disturbance, the experiments were performed after ensuring no change in the transmitted laser power.
Analysis without the reference particles
When the reference nanoparticle having the same parent material but containing no absorbers is unavailable, the proposed absorption detection can still be realized by the following method: The measurement of the balanced laser powers for the reference particle is replaced by the calculation of the ratio of the scattering cross sections at two different wavelengths (using Eq. 2). When the refractive index of the parent material is constant at two laser wavelengths, the ratio of the scattering cross sections can be obtained using the inverse fourth power law. Using this value, we can determine the balanced laser powers for the virtual nonabsorbing particle. We analyzed the same data for 10 r-NDs as the abovementioned experiments but without using the data for n-NDs; consequently, the absorption cross sections were determined as (3.8 ± 1.0) × 10−14 cm2. The variation from the above value [(3.3 ± 1.1) × 10−14 cm2] would have been brought on by a deviation from the Rayleigh scattering principle (Eq. 2) owing to the form, measurement, and refractive index of the particles, in addition to the random and systematic errors within the measurements.
Acknowledgments: Funding: The authors acknowledge the funding obtained from JSPS KAKENHI (grant numbers JP16H06504, JP16H06506, JP18H03882, JP18H05205, JP17K05016, and JP19H04529) and the Cooperative Analysis Program of “Community Joint Analysis Middle for Supplies and Units.” Creator contributions: H.I. and Okay.S. developed the idea and supervised the experiments. Okay.Y., H.F., and Okay.S. carried out the experiments. H.I. and T.W. theoretically elucidated the phenomena. H.F., Okay.Y., T.W., H.I., and Okay.S. participated in dialogue of the outcomes. H.F., H.I., and Okay.S. ready the manuscript. Competing pursuits: The authors declare that they don’t have any competing pursuits. Knowledge and supplies availability: All knowledge wanted to guage the conclusions within the paper are current within the paper and/or the Supplementary Supplies. Further knowledge associated to this paper could also be requested from the authors.
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