Open Access
Add to BibSonomy
Abstract
We propose a method for opto-thermophoretic trapping with a 2 µm Tm-doped fiber laser. The infrared continuous-wave laser beam is directly and strongly absorbed by water solution, and some local temperature gradient is generated around a focus. The particles are migrated along the temperature gradient, and form a hexagonal close-packed structure at a bottom-glass solution interface. On the other hand, the particles are not trapped in heavy water which does not absorb 2 µm light. The fact indicates that the local temperature elevation is the origin of this phenomenon. We have investigated the dependence of the phenomenon on the material, particle size, and laser power. To the best of our knowledge, 2 µm is the longest wavelength used for the opto-thermophoretic trapping.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A technique for manipulating micro and nanoparticles with light is indispensable for the development in nanoscience and nanotechnologies. Optical tweezers invented by A. Ashkin et al. in 1986 [1] are one of the important tools for the manipulation, which have been widely used in various research fields especially in biophysics [2–5]. Upon an irradiation of a tightly focused laser beam, the particles diffusing in a solution are trapped at the focus due to an optical gradient force which is originated from a spatial gradient of electric fields. Thus far, many efforts of fundamental studies of the optical force have been investigated by modifying either laser fields (such as locally enhanced plasmonic fields [6], vortex beam [7]) or target material properties (such as dye-doped colloidal particles [8], a single protein [9], molecular clusters [10]).
As an alternative method of particle manipulation, a temperature gradient induced by a laser beam can be also used for manipulating the particles. It is known as opto-thermophoresis and optothermal manipulation [11–13]. A thermophoretic force is sensitive to a variety of parameters such as surface charge of particles, solvent thickness and environmental temperature. In particular, the particle is driven from cold to hot regions along the temperature gradient when the sign of Soret coefficient is negative. To gain the temperature gradient, the laser energy needs to be converted to heat through certain light-absorbing materials which transfer the heat to the surrounding water solution. So far, various materials for light-heat conversion have been utilized for the demonstration of opto-thermophoretic trapping [14–28].
In recent years, a metallic thin film coated on the substrate is one of the common platforms using a plasmon-enhanced photothermal effect, where the steep temperature gradient is produced around the laser focus. The steep gradient is achieved by the strong coupling between light and matter based on surface plasmon resonance. For example, a thin Cr layer coated on the glass substrate was used as a 1064 nm light absorber, and 100 nm fluorescent polystyrene particles were trapped in a polymer solution [14]. Furthermore, a microbubble was produced by local heating of a gold nanoisland substrate with a visible laser, and the particles migrated around that bubble due to Marangoni flow [21,27]. Similarly, gold nanoparticles, gold coated silica nanoparticles and gold nanorods were also used as the light-heat conversion materials [20,23,25]. Apart from the surface plasmon resonance, other absorbers such as photalocyanine dyes contained in light-absorbing layer [15] or Hemin (iron-containing porphyrin) added to the solvent [16] that absorb 775 nm and 1064 nm laser light, respectively, were also used for the optothermal trapping experiments. All of these experiments require intermediate materials which indirectly transfer the laser energy to the solvent.
Herein, we use a 2 µm Tm-doped fiber laser (exact wavelength is 1956 nm). The laser beam is directly and strongly absorbed in water solvent without the intermediate materials. Although there were some reports that laser beams from 1400 to 1500 nm wavelength were used for heating water [25,29–32], the present laser at 1956 nm is the longest wavelength used for opto-thermophoresis, to the best of our knowledge. In particular, the imaginary part of refractive index, attenuation length, of water at 1956 nm is approximately six times higher than that at 1480 nm [33], which becomes more efficient to convert the laser energy to the water solvent directly.
2. Optical setup, samples and image analysis
We used a 2 µm continuous-wave Tm-doped fiber laser (Advalue Photonics, CW-1950-M200-LP) for opto-thermophoretic trapping. A maximum output power and the exact wavelength are 200 mW and 1956 nm, respectively (see Fig. 1(b)). The beam diameter is expanded to be about 5 mm by using a collimator lens to fulfill a back aperture of reflective type objective lens (Thorlabs-LMM40X-P01, NA 0.5), which results in focal diameter approximately 6 µm (Full width at half maximum, see in Fig. 1(c)). An infrared (IR) camera (Spiricon, Pyrocam IIIHR) is used for detecting the 2 µm laser beam. The 2 µm laser is reflected up by a dichroic mirror (Thorlabs-MDSP1800R) and focused at the bottom-glass/solution interface as illustrated in the inset of Fig. 1(a). The maximum laser power is about 50 mW after the objective lens and the laser power is controlled by using proper ND filters for each experiment.
Fig. 1. (a) Setup for opto-thermophoretic trapping. Inset shows an illustration of a focused laser beam and sample chamber configuration. (b) A spectrum of 2 µm Tm-doped fiber laser. (c) Inset shows a spatial profile of the focused laser spot measured with an IR camera. The length of the white bar is 50 µm.
For the samples, we used polystyrene particles (200 nm, 500 nm, 1 µm and 2 µm in diameter), poly (methyl methacrylate) (PMMA) particles (450 nm) and silica particles (500 nm). The particles are diluted by a deionized water (Milli-Q water) from a mother solution to optimal particle concentration (exact concentrations are denoted in each section). We chose relatively lower particle concentration to observe each particle dynamics one by one. A sodium chloride (NaCl) is slightly added (50 µg/ml) to the solvent in order to weaken the surface charge around the particles to form a relatively stable assembly structure. The sample solution is sandwiched by two glass substrates (Matsunami) with a spacer of 120 µm thickness (Electron Microscopy Sciences, 70327-20S). The substrates are soaked in a deionized water containing 2% detergent (Sigma-Aldrich, Hellmanex III, Z805939) overnight. After sonication, the substrates are carefully flushed with the deionized water. We also use heavy water (D2O, Sigma-Aldrich, 435767-100G) having less absorption to the 2 µm laser. The transmission images are simply observed with Kohler illumination of a halogen lamp and recorded by CMOS camera (JAI, APL-1600T-USB, 30 frames per second). In a data analysis of single particle tracking, the particle coordinates at each frame are extracted by analyzing each image. In particular, we binarize the images to separate the particle and background after adjusting the contrast, and find the center of the circle with the Image Processing Toolbox in MATLAB. From the spatial distribution of the trapped particle, a histogram (probability density, P(x, y)), a trapping potential (U(x, y)) and a trapping stiffness in x- and y-directions (kx, ky) are experimentally estimated. The potential is calculated by using the formula of P(x, y) = 1/Z exp(-U(x, y)/kBT), where Z, kB and T are normalized constant, Boltzmann constant and absolute temperature, respectively. Furthermore, the trapping stiffness is calculated by using the equation of kx= kBT/<x2> and ky = kBT/<y2>, where < x2> and < y2> are position variance. The absolute temperature, T, is set to 298 K. This enables us to quantitatively discuss the distribution of the particles around the focus and the velocity of each accumulating particle.
3. Results and discussion
3.1 Time evolution of particle assembly
To show a temporal evolution of the assembling, we first show the optothermal trapping of 1 µm polystyrene particles (see Fig. 2 and also Visualization 1). The 2 µm laser of 10 mW is directly absorbed into water solvent and creates the temperature gradient around the focus. Before turning on the laser, the polystyrene particles are homogeneously dispersed inside the solution, as shown in Fig. 2(a). When the laser is turned on, the particles are gathered one by one toward the focal spot. Within 30 s, the particles are trapped at the laser focus, forming a monolayer of hexagonal close-packed structure, and its assembly size grows gradually over time (see Fig. 2(c-f)). At least the particles are gathered 125 µm far away from the focus (for the detail see Supplement 1, Fig. S1). Typically, in optical tweezers, the particles very close to the focus are only trapped there because the optical force is, of course, only loaded at the place where the laser is irradiated. However, in the present opto-thermophoretic trapping, the particles are accumulated far away from the focus. When the laser is turned off, the assembly disperses gradually to the solution (see Fig. 2(g-j)). Namely, the assembly is reversible and the particles are not damaged.
Fig. 2. The transmission images of 1 µm polystyrene particles trapped by 2 µm Tm-doped fiber laser. (a) Before turning on the laser. (b-f) Temporal changes after turning on the laser. The laser is turned on at 0 s. (g-j) The assembly releasing process after turning off the laser. (k) Temporal growth of the assembly in 10 min. (m-p) Temporal changes when water solvent is replaced with D2O. The length of the black bars is 5 µm.
On closer inspection, not all the particles are actually gathered to the focus but some defocused particles located slightly above the interface (appearance in transmission images is white bright spots) are pushed along the laser propagation direction (see Visualization 1). We consider that this is due to a thermal convection flow. Interestingly, only the particles located near to the interface (appearance in transmission images is black spots) are collected toward the focus (see also Visualization 1) and other particles approaching to the focus from the optical axis direction (light propagation direction) are eventually pushed up. We assume that the balance between the thermophoretic force and thermal convection flow is important to achieve the stable trapping. Additionally, we carried out the trapping without adding the NaCl to the water solution. In fact, all the particles were pushed along the laser propagation direction and assembly formation was not observed. (see Supplement 1, Fig. S2). The purpose of adding NaCl is to neutralize the surface charge of the particles, and it indicates that the electrostatic interaction between the particle and glass substrate becomes moderate which enables the particle to gently slide toward the focal spot. Besides, it is worth mentioning that the irradiation of 2 µm laser to water produces a dark shade in the transmission image as shown in the Fig. 2(b). It seems that the refractive index of water is slightly changed due to the temperature elevation. We also conduct the longer irradiation time experiment to observe the steady state of assembly. Figure 2(k) shows the growth of assembly during the laser irradiation. In 2 min of irradiation, the assembly size almost approaches steady state. In 10 min of irradiation, the assembly is formed as a multi-layer circular shape. Among a five-time experiment, the average diameter is 41.8 µm, with standard deviation of 1.29 µm. Apparently, the assembly size reaches steady state much faster when the initial concentration is higher and/or the laser power is higher.
3.2 Experiment with heavy water
In order to clearly show that these phenomena are due to the local temperature elevation of water solution, we changed water solvent to a heavy water (D2O) solvent. The absorbance of D2O at 2 µm wavelength is less than H2O because the vibrational mode of D2O is shifted to the longer wavelength. Figure 2(m-p) are the results of polystyrene particles in the D2O solution with adding NaCl to the solution. Before the irradiation of the laser, the particles are distributed evenly inside heavy water in a similar manner as water. As a result, no particles are trapped at the focus (see Fig. 2(n-p)) and the particle motion is not affected by the laser. It is worth mentioning that when the laser power is increased to 40 mW, the particles start to be trapped but the speed of accumulation is much slower compared to the trapping in water solvent with laser power of 10 mW (see Supplement 1, Fig. S3). We also measure the laser power before and after the water and heavy water sample chambers to estimate how much of the laser is absorbed to the solvent (see Supplement 1, Fig. S4). This simple estimation shows that the laser is absorbed to the water about 2.2 times larger than heavy water. These experimental facts clearly indicate that the temperature elevation is indeed the origin of the accumulation of the particle at the focus.
3.3 Laser power dependence
The laser power dependence of the 1 µm polystyrene particle trapping is summarized in Fig. 3. Each transmission image is recorded after 20 s laser irradiation under different laser power. With the increase of the laser power, the particles are trapped more intensively as shown in Fig. 3(a-e). In the particle amount-power diagram shown in Fig. 3(g), the number of the trapped particles is linearly increased as the irradiation power increases (the particle number is manually counted one by one with watching the movie). The trapping threshold is approximately 1.5 mW, which is the laser power that can be able to distribute several particles around the focus. When the power increases to 45 mW, all the particles are rapidly collected but the heat sometimes causes damage to the sample which may due to the absorption to polystyrene particle itself. Therefore, around 30 to 40 mW are the optimum condition for gathering the 1 µm polystyrene particles faster.
Fig. 3. (a-e) Transmission images of assembly formed from 1 µm polystyrene particles under different power at 20 s irradiation time. (f) Damage caused by 45mW of power. The length of all the black bars is 5 $\mathrm{\mu}$m. (g) The number of collected particles at 20 s for different laser power.
3.4 Single particle tracking analysis
To quantitatively discuss the trapping dynamics such as particle distribution and velocity, the single particle tracking analysis is examined. Practically, we carried out the experiment of single particle trapping by reducing the particle concentration, and x- and y- coordinates of the particle in the focal plane are extracted by image analysis. Figure 4(a-c) shows the trajectories of each particle (500 nm, 1 µm and 2 µm) until the particles are trapped at the focus. First it can be observed that the trajectory of 500 nm polystyrene particles is fluctuating more than that of 1 µm and 2 µm ones, because the motion of smaller particles is much more dynamic due to Brownian motion. The net traveling distances within 14.85 s for 500 nm, 1 and 2 µm particles are approximately 60, 60 and 40 µm, respectively. In addition, the maximum velocity of 500 nm and 1 µm particles is both around 8 µm/s, and that of 2 µm is slower (around 5 µm/s). In other words, the 2 µm particles take much longer time to be gathered, while 500 nm and 1 µm particles are collected in a similar manner. To clearly compare the tracking trajectories for different sizes of the particle, we here only showed the particles that are trapped from a right-up side in field of view, but the particle is also accelerated to the focal center in the similar manner even when the initial particle position is at the different side.
Fig. 4. Single particle tracking analysis. Trajectory (a-c) and velocity (d-f) for 500 nm, 1 µm and 2 µm polystyrene particles, respectively. The time period of the start and end of trajectories are set to be 14.85 s for the comparison. The laser power after the objective lens is 10 mW. (g-i) Distribution map of trapped particles at the focus for 500 nm, 1 µm and 2 µm polystyrene particles, respectively. The filled pink circle indicates the center of the motion. (j-o) The histogram, trapping potential and trapping stiffness in x- and y-directions for 500 nm, 1 µm and 2 µm polystyrene particles, respectively.
Regarding the particle confinement at the focus, the larger particles tend to be much more confined than the smaller one, as shown in Fig. 4(g-i), which may be due to the less fluctuation of Brownian particles. In detail, the 1 µm and 2 µm polystyrene particles are confined in a circle with a diameter of approximately 2 µm. The trapping potential depth for 500 nm, 1 µm and 2 µm polystyrene particles are all approximately 3 kBT. The width of trapping potential well becomes narrower for larger particles, and correspondingly, the trapping stiffness for 500 nm, 1 µm and 2 µm polystyrene particles is about 7 fN/µm, 0.04 pN/µm and 0.08 pN/µm, respectively. The maximum stiffness achieved in the present study is about 1 pN/µm for 2 µm polystyrene particles with 40 mW laser power (see Supplement 1, Fig. S5). It is worth mentioning that the limit of maximum velocity which we can achieve in the current optical setup is about 36.4 µm/s for 1 µm particle at 40 mW. This value is nearly four times or even larger compared to that at 10 mW, which suggests that the velocity linearly scales up with the laser power. As a relevant experiment [31], a 6 µm polystyrene particle was drifted 180 µm far away from the focus with 1550 nm laser of 7 mW, yet the particles were not trapped at the focus and they were pushed and levitated along the light propagation direction. The maximum velocity was estimated to be around 20 µm/s near the focus. Although the experimental parameters are different and direct comparison is quite difficult, our system can trap the particles at the focus by directly heating the water solvent.
3.5 Particle size dependence
Different sizes of polystyrene particles are examined here. Figure 5 shows the selected transmission images recorded under the same irradiation power at the moment of 20 s laser exposure time. As a smallest particle we examined, the concentration of 200 nm polystyrene nanoparticles indeed increases at the focus after turning on the laser as shown in Fig. 5(a) and Visualization 2. The central part of the images is darker compared to the outside, which means that the particles are distributed around the focus. Typically, the laser power of several hundred milliwatts or even higher power was used for 200 nm polystyrene trapping with optical tweezers system [34,35], our opto-thermophoresis experiment only requires 10 mW.
Fig. 5. Transmission images of different sizes of polystyrene particles, (a) 200 nm, (b) 500 nm, (d) 1 µm and (e) 2 µm polystyrene particles under the laser power of 10 mW, respectively. (c) Enlarged view of the assembly in (b). The length of all the black bars is 5 $\mathrm{\mu}$m.
As similar to the formation of hexagonal structure of 1 µm polystyrene particles abovementioned, the hexagonal structure is also formed for 500 nm polystyrene particles as shown in Fig. 5(b). Moreover, a beautifully structured flower pattern is prepared in the 2 µm size of polystyrene particles (see Fig. 5(e)). Actually, we also tried the trapping for 50 nm polystyrene nanoparticles. However, due to the resolution of the transmission type bright field microscope, it was not obvious to observe that the particles are collected or not (we plan to observe the fluorescence from dye-doped 50 nm polystyrene nanoparticles in near future). Although the absolute concentration for each particle in this study is not the same, it can remark that the hexagonal structure can be prepared when the particle diameter is larger than 500 nm, The assemblies are formed by just directly focusing the 2 m laser beam to the bottom-glass solution interface without any substrate fabrication.
3.6 Material dependence
To show the present method can be used for other materials, we demonstrate the experiment with PMMA and silica particles which are also typically used as the standard targets for trapping (see Fig. 6). The size of all the examined particles is approximately 500 nm in diameter, and the particle concentrations are about 3.64×108, 1.8×108, and 8.04×108 particles/ml, respectively. They are collected around the focus (also see Visualization 3, Visualization 4, Visualization 5). The hexagonal structure is formed for the polystyrene and PMMA particles but the stable hexagonal structure is not clearly formed for silica particles. The formed assembly size is similar for polystyrene and PMMA, but silica particle assembly size is smaller even though the particle concentration is higher. Specifically, the silica particles are collected to the focus during the laser irradiation, however some of them after being trapped are pushed along the direction of laser propagation. Concretely, we can observe that some of the particles are defocused in the transmission images which means the particles are pushed up along the laser propagation. Although we cannot conclude the exact mechanism at the current stage, the environmentally sensitive thermophoretic force and optical absorption force will be the possible reasons. Especially, the imaginary part of the refractive index of silicon dioxide at 1956 nm [36] is about two times higher than that of polystyrene [37].
Fig. 6. Transmission images of 500 nm particles trapped by opto-thermophoretic trapping. (a-f) Polystyrene particles (g-l) PMMA particles and (m-r) silica particles are trapped and assembled at the bottom glass-solution interface. Insets in (f, l, r) are enlarged view of each assembly. The length of black bar is 5 µm. The laser is turned on at 0 s.
3.7 Possible mechanism and discussion
Based on all the experimental facts, here we summarize and explain the possible assembling dynamics in terms of thermophoresis, convection flow and optical force. The focused 2 µm laser is efficiently absorbed into the water, which produces a temperature gradient around the laser focus. This induces the convection flow which accumulates the particle toward the focus. Without adding the salt, the particles are just drifting along the convection flow and they are not trapped at the focus. As a sample space becomes thinner, the convection flow becomes weaker (see Visualization 6 and Visualization 7). This indicates the convection flow contributes to the gathering process. As described earlier, with adding the salt, the particles are trapped and confined at the focus at the bottom-glass/solution interface. We consider that the Soret coefficient becomes negative and/or the electrostatic interaction between the particle and interface becomes moderate, which enables the particle trapping.
Apart from the thermally induced forces, an optical scattering force is larger than the optical gradient force because the laser is loosely focused by low NA (0.5) objective lens (see the simple optical force calculation in Supplement 1, Fig. S6). Thereby, the present trapping behavior cannot be explained by the optical force. Based on all of these facts, we suggest that the opto-thermophoretic force is the key to explain the present results.
It should be emphasized that the 2 µm laser can be absorbed by the water six times larger compared to the 1400-1500 nm laser used in the past reports [25,29–32]. Numerically, the attenuation length of water at 1956 nm is approximately six times higher than that at 1480 nm [33]. It was reported that, by using the 1480 nm laser of 25 mW, an aggregate of 500 nm polystyrene particles diffusing in polyethylene glycol (PEG) solution were three-dimensionally trapped by directly heating the water [14]. Although, it is necessary to estimate the trapping potential and stiffness in the same experimental system for the direct comparison between 1480 nm and 1956 nm lasers, we remark that the present method can be used to assemble and order the 500 nm polystyrene particles without adding the PEG to solution with similar or even lower laser power of 1.5 mW.
Regarding the plasmonic-based optothermal trapping, it was reported that 100 nm metallic nanoparticles were trapped and manipulated by opto-thermoelectric nanotweezers [24]. Concretely, a visible laser with 0.05-0.4 mW/µm2 was used for exciting the surface plasmon resonance, and the trapping stiffness was about the range from 0.08 to 2.6 pN/µm. In the present study, the maximum stiffness we achieved is about 1 pN/µm for 2 µm polystyrene particles when the laser power is 40 mW (0.48 mW/µm2). As a fact, the particle size in the present study is much larger, and the plasmonic-based opto-thermal trapping is efficient. We believe that the opto-thermophoretic manipulation based on direct heating becomes more efficient by using a 3 µm laser, because an absorption coefficient at 3 µm is several hundred times larger than that at 2 µm. In near future, we plan to use a rare earth-doped fluoride-glass fiber laser [38,39] which can produce the 3 µm laser to achieve the opto-thermophoretic manipulation with lower laser power. Meanwhile, an absolute temperature measurement with thermo-sensitive fluorescent dyes and a finite element simulation for flow calculation will be conducted as future topics.
4. Conclusion
We have experimentally demonstrated opto-thermophoretic trapping and assembling of micro and nanoparticles with a 2 µm Tm doped fiber laser. To the best of our knowledge, the wavelength used in the present study is the longest wavelength which is directly and strongly absorbed in the water solvent. The study includes the comparison of trapping in water and heavy water, assembly formation under different laser power, analysis of single particle tracking, and particle trapping of different sizes and different materials, respectively. The micro and nanoparticles at the bottom glass-solution substrate of the sample are driven by the opto-thermal gradient force to move forwards and trapped at the focal center. It is possible to pull the particles to the center even with a milliwatt-level of power. Although most of the opto-thermophoretic trapping experiments in recent years utilize the intermediate targets to convert light energy to heat such as using gold nanoparticles and/or thin-film, our method can directly and strongly heat up the water solution due to the large imaginary part of refractive index, attenuation length, of water at 1956 nm. The present system with 2 µm direct optothermal trapping will be extended in various fields, such as bio sensing (DNA detection and sorting) [17,28] without fabricating light absorbers on substrates. This fabrication-free method only requires to focus the 2 µm laser into the water solvent.
Funding
Core Research for Evolutional Science and Technology (JPMJCR17N5); Japan Society for the Promotion of Science (JP 21K14555, KAKENHI); Kambayashi research grant.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11(5), 288–290 (1986). [CrossRef]
2. K. Dholakia, P. Reece, and M. Gu, “Optical micromanipulation,” Chem. Soc. Rev. 37(1), 42–55 (2008). [CrossRef]
3. D. Gao, W. Ding, M. Nieto-Vesperinas, X. Ding, M. Rahman, T. Zhang, C. Lim, and C.-W. Qiu, “Optical manipulation from the microscale to the nanoscale: fundamentals, advances and prospects,” Light: Sci. Appl. 6(9), e17039 (2017). [CrossRef]
4. H. Ishihara, “Optical manipulation of nanoscale materials by linear and nonlinear resonant optical responses,” Adv. Phys.: X 6(1), 1885991 (2021). [CrossRef]
5. C. J. Bustamante, Y. R. Chemla, S. Liu, and M. D. Wang, “Optical tweezers in single-molecule biophysics,” Nat. Rev. Methods Primers 1(1), 25 (2021). [CrossRef]
6. M. Righini, A. S. Zelenina, C. Girard, and R. Quidant, “Parallel and selective trapping in a patterned plasmonic landscape,” Nat. Phys. 3(7), 477–480 (2007). [CrossRef]
7. J. E. Curtis and D. G. Grier, “Structure of optical vortices,” Phys. Rev. Lett. 90(13), 133901 (2003). [CrossRef]
8. R. Bresolí-Obach, T. Kudo, B. Louis, Y.-C. Chang, I. G. Scheblykin, H. Masuhara, and J. Hofkens, “Resonantly Enhanced Optical Trapping of Single Dye-Doped Particles at an Interface,” ACS Photonics 8(6), 1832–1839 (2021). [CrossRef]
9. Y. Pang and R. Gordon, “Optical trapping of a single protein,” Nano Lett. 12(1), 402–406 (2012). [CrossRef]
10. T. Sugiyama, T. Adachi, and H. Masuhara, “Crystallization of Glycine by Photon Pressure of a Focused CW Laser Beam,” Chem. Lett. 36(12), 1480–1481 (2007). [CrossRef]
11. L. Lin, E. H. Hill, X. Peng, and Y. Zheng, “Optothermal Manipulations of Colloidal Particles and Living cells,” Acc. Chem. Res. 51(6), 1465–1474 (2018). [CrossRef]
12. P. Zemánek, G. Volpe, A. Jonáš, and O. Brzobohatý, “Perspective on light-induced transport of particles: from optical forces to phoretic motion,” Adv. Opt. Photonics 11(3), 577–678 (2019). [CrossRef]
13. S. Liu, L. Lin, and H.-B. Sun, “Opto-Thermophoretic Manipulation,” ACS Nano 15(4), 5925–5943 (2021). [CrossRef]
14. H. R. Jiang, H. Wada, N. Yoshinaga, and M. Sano, “Manipulation of colloids by a nonequilibrium depletion force in a temperature gradient,” Phys. Rev. Lett. 102(20), 208301 (2009). [CrossRef]
15. L. H. Thamdrup, N. B. Larsen, and A. Kristensen, “Light-induced local heating for thermophoretic manipulation of DNA in polymer Micro- and nanochannels,” Nano Lett. 10(3), 826–832 (2010). [CrossRef]
16. P. Kumari, J. A. Dhormadhikari, A. K. Dhormadhikari, H. Basu, S. Sharma, and D. Mathur, “Optical trapping in an absorbing medium: from optical tweezing to thermal tweezeing,” Opt. Express 20(4), 4645–4652 (2012). [CrossRef]
17. L.-H. Yu and Y.-F. Chen, “Concentration-Dependent Thermophoretic Accumulation for the Detection of DNA Using DNA-Functionalized Nanoparticles,” Anal. Chem. 87(5), 2845–2851 (2015). [CrossRef]
18. K. Namura, K. Nakajima, K. Kimura, and M. Suzuki, “Photothermally controlled Marangoni flow around a micro bubble,” Appl. Phys. Lett. 106(4), 043101 (2015). [CrossRef]
19. J. Chen, H. Cong, F. Loo, Z. Kang, M. Tang, H. Zhang, S. Wu, S. Kong, and H. Ho, “Thermal gradient induced tweezers for the manipulation of particles and cells,” Sci. Rep. 6(1), 35814 (2016). [CrossRef]
20. H. Chen, E. Gratton, and M. A. Digman, “Self-assisted optothermal trapping of gold nanorods under two-photon excitation,” Methods Appl. Fluoresc. 4(3), 035003 (2016). [CrossRef]
21. L. Lin, X. Peng, Z. Mao, W. Li, M. N. Yogeesh, B. B. Rajeeva, E. P. Perillo, A. K. Dunn, D. Akinwande, and Y. Zheng, “Bubble-pen lithography,” Nano Lett. 16(1), 701–708 (2016). [CrossRef]
22. L. Lin, X. Peng, X. Wei, Z. Mao, C. Xie, and Y. Zheng, “Thermophoretic tweezers for low-power and versatile manipulation of biological cells,” ACS Nano 11(3), 3147–3154 (2017). [CrossRef]
23. I. Aibara, J. Chikazawa, T. Uwada, and S. Hashimoto, “Localized phase separation of thermoresponsive polymers induced by plasmonic heating,” J. Phys. Chem. C 121(40), 22496–22507 (2017). [CrossRef]
24. L. Lin, M. Wang, X. Peng, E. N. Lissek, Z. Mao, L. Scarabelli, E. Adkins, S. Coskun, H. E. Unalan, B. A. Korgel, L. M. Liz-Marzán, E.-L. Florin, and Y. Zheng, “Opto-thermoelectric nanotweezers,” Nat. Photonics 12(4), 195–201 (2018). [CrossRef]
25. K. Setoura, T. Tsuji, S. Ito, S. Kawano, and H. Miyasaka, “Opto-thermophoretic separation and trapping of plasmonic nanoparticles,” Nanoscale 11(44), 21093–21102 (2019). [CrossRef]
26. M. Zhong, A. Liu, and F. Ji, “Opto-thermal oscillation and trapping of light absorbing particles,” Opt. Express 27(21), 29730–29737 (2019). [CrossRef]
27. A. Kotnala, P. S. Kollipara, J. Li, and Y. Zheng, “Overcoming diffusion-limited trapping in nanoaperture tweezers using opto-thermal-induced flow,” Nano Lett. 20(1), 768–779 (2020). [CrossRef]
28. T. Shoji, K. Itoh, J. Saitoh, N. Kitamura, T. Yoshii, K. Murakoshi, Y. Yamada, T. Yokoyama, H. Ishihara, and Y. Tsuboi, “Plasmonic manipulation of DNA using a combination of optical and thermophoretic forces: separation of different-sized DNA from mixture solution,” Sci. Rep. 10(1), 3349 (2020). [CrossRef]
29. S. Duhr and D. Braun, “Why molecules move along a temperature gradient,” Proc. Natl. Acad. Sci. U. S. A. 103(52), 19678–19682 (2006). [CrossRef]
30. P. Reineck, C. J. Wienken, and D. Braun, “Thermophoresis of single stranded DNA,” Electrophoresis 31(2), 279–286 (2010). [CrossRef]
31. Y. Liu and A. W. Poon, “Flow-assisted single-beam optothermal manipulation of microparticles,” Opt. Express 18(17), 18483–18491 (2010). [CrossRef]
32. H. Xin, X. Li, and B. Li, “Massive photothermal trapping and migration of particles by a tapered optical fiber,” Opt. Express 19(18), 17065–17074 (2011). [CrossRef]
33. L. Kou, D. Labrie, and P. Chylek, “Refractive indices of water and ice in the 0.65- to 2.5-µm spectral range,” Appl. Opt. 32(19), 3531–3540 (1993). [CrossRef]
34. C. Hosokawa, H. Yoshikawa, and H. Masuhara, “Cluster formation of nanoparticles in an optical trap studied by fluorescence correlation spectroscopy,” Phys. Rev. E 72(2), 021408 (2005). [CrossRef]
35. S.-F. Wang, K. Yuyama, T. Sugiyama, and H. Masuhara, “Reflection microspectroscopic study of laser trapping assembling of polystyrene nanoparticles at air/solution interface,” J. Phys. Chem. C 120(29), 15578–15585 (2016). [CrossRef]
36. J. Kischkat, S. Peters, B. Gruska, M. Semtsiv, M. Chashnikova, M. Klinkmüller, O. Fedosenko, S. Machulik, A. Aleksandrova, G. Monastyrskyi, Y. Flores, and W. T. Masselink, “Mid-infrared optical properties of thin films of aluminum oxide, titanium dioxide, silicon dioxide, aluminum nitride, and silicon nitride,” Appl. Opt. 51(28), 6789–6798 (2012). [CrossRef]
37. X. Zhang, J. Qiu, X. Li, J. Zhao, and L. Liu, “Complex refractive indices measurements of polymers in visible and near-infrared bands,” Appl. Opt. 59(8), 2337–2344 (2020). [CrossRef]
38. S. Tokita, M. Hirokane, M. Murakami, S. Shimizu, M. Hashida, and S. Sakabe, “Stable 10 W Er:ZBLAN fiber laser operating at 2.71–2.88 µm,” Opt. Lett. 35(23), 3943–3945 (2010). [CrossRef]
39. J. Liu, M. Wu, B. Huang, P. Tang, C. Zhao, D. Shen, D. Fan, and S. K. Turitsyn, “Widely Wavelength-Tunable Mid-Infrared Fluoride Fiber Lasers,” IEEE J. Sel. Top. Quantum Electron. 24(3), 0900507 (2018). [CrossRef]
