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Tunable potential well for plasmonic trapping of metallic particles by bowtie nano-apertures

View Article: PubMed Central - PubMed

ABSTRACT

In this paper, the tunable optical trapping dependence on wavelength of incident beam is theoretically investigated based on numerical simulations. The Monte Carlo method is taken into account for exploring the trapping characteristics such as average deviation and number distribution histogram of nanoparticles. It is revealed that both the width and the depth of potential well for trapping particles can be flexibly adjusted by tuning the wavelength of the incident beam. In addition, incident wavelengths for the deepest potential well and for the strongest stiffness at bottom are separated. These phenomena are explained as the strong plasmon coupling between tweezers and metallic nanoparticles. In addition, required trapping fluence and particles’ distributions show distinctive properties through carefully modifying the incident wavelengths from 1280 nm to 1300 nm. Trapping with lowest laser fluence can be realized with1280 nm laser and trapping with highest precision can be realized with 1300 nm laser. This work will provide theoretical support for advancing the manipulation of metallic particles and related applications such as single-molecule fluorescence and surface enhanced Raman spectroscopy.

No MeSH data available.


Related in: MedlinePlus

Trapping forces and trapping potential well.(a) The trapping force spectrum as a function of laser wavelength with different locations of the gold nanoparticles along Z axis (Z coordinates). (b) The trapping forces depending on the location of the nanoparticle under different incident wavelength. (c) Trapping potential energy depending on the varying incident wavelength. The Z direction is vertical to the surface of the gold film and the original point is in the center of the tweezers. (d) Half width at half maximum (HWHM, ◾) and depth (⦁) of potential well depending on incident wavelength. The laser fluence applied in this figure is 4 × 108 W/m2.
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f2: Trapping forces and trapping potential well.(a) The trapping force spectrum as a function of laser wavelength with different locations of the gold nanoparticles along Z axis (Z coordinates). (b) The trapping forces depending on the location of the nanoparticle under different incident wavelength. (c) Trapping potential energy depending on the varying incident wavelength. The Z direction is vertical to the surface of the gold film and the original point is in the center of the tweezers. (d) Half width at half maximum (HWHM, ◾) and depth (⦁) of potential well depending on incident wavelength. The laser fluence applied in this figure is 4 × 108 W/m2.

Mentions: The trapping forces spectrum as a function of laser wavelength for different locations of gold nanoparticles is shown in Fig. 2(a). The wavelength with respect to the strongest trapping force shows an evident blue-shift with increasing distance between nanoparticles and the center of tweezers. As the distance increases from 10 nm to 60 nm, the wavelength with respect to the strongest trapping force decreases from 1300 nm to 1260 nm. This observed blue-shift could be attributed to the weakening of the coupling between the plasmon of the particles and the tweezers. The wavelength dependence of the relationship between the location of the nanoparticle and the trapping force is also revealed as shown in Fig. 2(b). The displacement of particle to the center of tweezers with respect to the strongest trapping force exhibits an evident increase with the decreasing of incident wavelengthfrom 1300 nm to 1260 nm. The strongest trapping force exists with a displacement of nearly 20 nm at the laser wavelength of 1300 nm. As the incident wavelength decreases to 1260 nm, however, the displacement with respect to the strongest trapping force extends to nearly 60 nm. When the particle is in vicinity of tweezers, the resonance wavelength of the particle-tweezers system approximates to 1300 nm. As a result, under the incident wavelength of 1300 nm, the strongest trapping force exists when the distance between the particle and the tweezers is 20 nm. When the particle escapes away from tweezers, the particle-tweezers system is off the resonance state under wavelength of 1300 nm due to the decrease of the coupling strength and the blue shift of the resonance wavelength. As a result, the trapping force drops quickly. Similarly, when the distancebetween the particle and the tweezers is small, the particle-tweezers system is in off-resonance state under the wavelength of 1260 nm because the resonance wavelength of the entire system is a much longer one. As the nanoparticle gradually escapes from the tweezers, the particle-tweezers system is approaching to the resonance state. As a result, the strongest force exists at the distance of 60 nm under the wavelength of 1260 nm. More detailed information about particle displacement for maximum trapping forces depending on incident wavelengths has been shown in supplementary figure, Fig. S1 on line. It can be seen from Fig. S1 that when decreasing incident wavelengths from 1330 nm to 1250 nm, the displacement for maximum trapping forces show so evident increase from10 nm to 70 nm. The modification of the trapping force under different incident wavelengths would definitely lead to the modification of the shape of the potential energy well, which is shown in Fig. 2(c). With increasing the incident wavelength, the area corresponds to considerable depth of the potential well (bright area) shows an evident decreasing, indicating the rapidly decreasing of potential well width. More detailed information can be seen from the wavelength dependent half width at half maximum (HWHM) potential well shown in Fig. 2(d). The HWHM potential well shows the same trend as the displacement for maximum trapping force depending on incident wavelength, which is not hard to understand when considering the quasi-Gaussian shape of potential well. With the incident wavelength ranging from 1250 nm to 1300 nm, the HWHM decreases from 69 nm to21 nm. For a longer wavelength, the trapping force, namely, the gradient of the trapping potential well, reaches maximum at a smaller distance between particles and tweezers and then drops quickly when the distance increases. As a result, the rapid dropping of the trapping force leads to the narrow trapping potential well. As the incident wavelength decreases, however, the distance between particles and tweezers with respect to the strongest trapping force increases. As a result, the width of the trapping potential shows an evident increase. Interestingly, the HWHM increases slightly when the incident wavelength is below 1250 nm and above 1300 nm, which could be attributed to the too short or too long incident wavelength which beyond the resonance wavelength of the particle-tweezers system all the time. The tunabilities of the depth of the potential well are also shown in Fig. 2(c,d). The deepest potential wellnear to minus 12 kT can be acquired with the incident wavelength of 1280 nm and the depth of potential can be controlled below minus 6 kT with the incident wavelength varying from 1240 nm to 1310 nm. Considering that stable trapping can be realized when the depth of potential well is below minus 10 kT1, it is possible to realize stable trapping with a laser fluence no higher than 8 × 108 W/m2 (two times as that used in Fig. 2) by tuning the incident wavelength within the range from 1240 nm to 1310 nm. When the incident wavelength reaches to 1200 nm or 1350 nm, the potential well becomes ignorable. Supplementary figure, Fig. S2 on line shows thedifferent groups of wavelength dependent potential well depth with difference bowtie nano-aperture sizes. Incident wavelength for deepest potential well evidently increases when enlarging the aperture size, which accords to the previous works20.


Tunable potential well for plasmonic trapping of metallic particles by bowtie nano-apertures
Trapping forces and trapping potential well.(a) The trapping force spectrum as a function of laser wavelength with different locations of the gold nanoparticles along Z axis (Z coordinates). (b) The trapping forces depending on the location of the nanoparticle under different incident wavelength. (c) Trapping potential energy depending on the varying incident wavelength. The Z direction is vertical to the surface of the gold film and the original point is in the center of the tweezers. (d) Half width at half maximum (HWHM, ◾) and depth (⦁) of potential well depending on incident wavelength. The laser fluence applied in this figure is 4 × 108 W/m2.
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Related In: Results  -  Collection

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f2: Trapping forces and trapping potential well.(a) The trapping force spectrum as a function of laser wavelength with different locations of the gold nanoparticles along Z axis (Z coordinates). (b) The trapping forces depending on the location of the nanoparticle under different incident wavelength. (c) Trapping potential energy depending on the varying incident wavelength. The Z direction is vertical to the surface of the gold film and the original point is in the center of the tweezers. (d) Half width at half maximum (HWHM, ◾) and depth (⦁) of potential well depending on incident wavelength. The laser fluence applied in this figure is 4 × 108 W/m2.
Mentions: The trapping forces spectrum as a function of laser wavelength for different locations of gold nanoparticles is shown in Fig. 2(a). The wavelength with respect to the strongest trapping force shows an evident blue-shift with increasing distance between nanoparticles and the center of tweezers. As the distance increases from 10 nm to 60 nm, the wavelength with respect to the strongest trapping force decreases from 1300 nm to 1260 nm. This observed blue-shift could be attributed to the weakening of the coupling between the plasmon of the particles and the tweezers. The wavelength dependence of the relationship between the location of the nanoparticle and the trapping force is also revealed as shown in Fig. 2(b). The displacement of particle to the center of tweezers with respect to the strongest trapping force exhibits an evident increase with the decreasing of incident wavelengthfrom 1300 nm to 1260 nm. The strongest trapping force exists with a displacement of nearly 20 nm at the laser wavelength of 1300 nm. As the incident wavelength decreases to 1260 nm, however, the displacement with respect to the strongest trapping force extends to nearly 60 nm. When the particle is in vicinity of tweezers, the resonance wavelength of the particle-tweezers system approximates to 1300 nm. As a result, under the incident wavelength of 1300 nm, the strongest trapping force exists when the distance between the particle and the tweezers is 20 nm. When the particle escapes away from tweezers, the particle-tweezers system is off the resonance state under wavelength of 1300 nm due to the decrease of the coupling strength and the blue shift of the resonance wavelength. As a result, the trapping force drops quickly. Similarly, when the distancebetween the particle and the tweezers is small, the particle-tweezers system is in off-resonance state under the wavelength of 1260 nm because the resonance wavelength of the entire system is a much longer one. As the nanoparticle gradually escapes from the tweezers, the particle-tweezers system is approaching to the resonance state. As a result, the strongest force exists at the distance of 60 nm under the wavelength of 1260 nm. More detailed information about particle displacement for maximum trapping forces depending on incident wavelengths has been shown in supplementary figure, Fig. S1 on line. It can be seen from Fig. S1 that when decreasing incident wavelengths from 1330 nm to 1250 nm, the displacement for maximum trapping forces show so evident increase from10 nm to 70 nm. The modification of the trapping force under different incident wavelengths would definitely lead to the modification of the shape of the potential energy well, which is shown in Fig. 2(c). With increasing the incident wavelength, the area corresponds to considerable depth of the potential well (bright area) shows an evident decreasing, indicating the rapidly decreasing of potential well width. More detailed information can be seen from the wavelength dependent half width at half maximum (HWHM) potential well shown in Fig. 2(d). The HWHM potential well shows the same trend as the displacement for maximum trapping force depending on incident wavelength, which is not hard to understand when considering the quasi-Gaussian shape of potential well. With the incident wavelength ranging from 1250 nm to 1300 nm, the HWHM decreases from 69 nm to21 nm. For a longer wavelength, the trapping force, namely, the gradient of the trapping potential well, reaches maximum at a smaller distance between particles and tweezers and then drops quickly when the distance increases. As a result, the rapid dropping of the trapping force leads to the narrow trapping potential well. As the incident wavelength decreases, however, the distance between particles and tweezers with respect to the strongest trapping force increases. As a result, the width of the trapping potential shows an evident increase. Interestingly, the HWHM increases slightly when the incident wavelength is below 1250 nm and above 1300 nm, which could be attributed to the too short or too long incident wavelength which beyond the resonance wavelength of the particle-tweezers system all the time. The tunabilities of the depth of the potential well are also shown in Fig. 2(c,d). The deepest potential wellnear to minus 12 kT can be acquired with the incident wavelength of 1280 nm and the depth of potential can be controlled below minus 6 kT with the incident wavelength varying from 1240 nm to 1310 nm. Considering that stable trapping can be realized when the depth of potential well is below minus 10 kT1, it is possible to realize stable trapping with a laser fluence no higher than 8 × 108 W/m2 (two times as that used in Fig. 2) by tuning the incident wavelength within the range from 1240 nm to 1310 nm. When the incident wavelength reaches to 1200 nm or 1350 nm, the potential well becomes ignorable. Supplementary figure, Fig. S2 on line shows thedifferent groups of wavelength dependent potential well depth with difference bowtie nano-aperture sizes. Incident wavelength for deepest potential well evidently increases when enlarging the aperture size, which accords to the previous works20.

View Article: PubMed Central - PubMed

ABSTRACT

In this paper, the tunable optical trapping dependence on wavelength of incident beam is theoretically investigated based on numerical simulations. The Monte Carlo method is taken into account for exploring the trapping characteristics such as average deviation and number distribution histogram of nanoparticles. It is revealed that both the width and the depth of potential well for trapping particles can be flexibly adjusted by tuning the wavelength of the incident beam. In addition, incident wavelengths for the deepest potential well and for the strongest stiffness at bottom are separated. These phenomena are explained as the strong plasmon coupling between tweezers and metallic nanoparticles. In addition, required trapping fluence and particles’ distributions show distinctive properties through carefully modifying the incident wavelengths from 1280 nm to 1300 nm. Trapping with lowest laser fluence can be realized with1280 nm laser and trapping with highest precision can be realized with 1300 nm laser. This work will provide theoretical support for advancing the manipulation of metallic particles and related applications such as single-molecule fluorescence and surface enhanced Raman spectroscopy.

No MeSH data available.


Related in: MedlinePlus