1 Advanced Control of Photochemical Reactions …
21
1.4 Mesoscopic Motion of Small Particles Induced
by Switching of Photon Force Through Photochromic
Reactions
In this section, we introduce the movement of the nano- and microparticles synchronized with the photochemical reaction under the laser trapping, which is one of the
photosynergetic responses integrated with the momentum change of photon and the
photochemical reaction of molecules.
A small particle under photo-irradiation experiences photon force (photon pressure) originating from the transfer of the momentum of photon to the particle [32].
This photon force is generally classified into three parts: absorption, scattering, and
gradient forces. The absorption and scattering forces are, respectively, due to the
momentum transfer of photons through absorption and scattering, which push a
photo-irradiated particle toward the light propagation. On the other hand, the gradient
force directs the spatial gradient of the light intensity. When the refractive index of
a particle is larger than that of the medium, the gradient force pushes the particle
to the direction along the increasing light intensity. Under the irradiation of the
tightly focused laser beam, the gradient force keeps a small particle at the focal
point. Because the contribution of the gradient force is generally dominant in the
condition with no absorption of the particle at the wavelength of the trapping laser
(off-resonant), the particle is trapped in the vicinity of the focal point. Although the
absorption force is negligible in the off-resonant condition, it largely contributes in
the case where the absorption of the particle is resonant to the trapping light. Hence,
the change of the color of the particle may induce the change of the trapping position
of the particle through the switching of the absorption force.
In the present study, we employed a photochromic diarylethene (DAE) derivative shown in Fig. 1.20a as a color switching material. In addition to the change
of the absorption wavelength, photochromic reactions of this DAE are accompanied with the fluorescence switching; the open-ring isomer is non-fluorescent while
the closed-ring isomer shows bright fluorescence. This fluorescence was used for
tracking the position of a trapped particle [33]. Figure 1.20b shows extinction spectra
of an aqueous colloid of 300-nm sized poly(methyl methacrylate) particles containing
the DAE derivative (DAE-PMMA particle) for different UV irradiation time. The
initial spectrum consists of the scattering of the particle and the absorption of the
open-ring isomer of DAE. With an increase in UV dose, absorption band of the
closed-ring isomer in the visible region increases and that of the open-ring isomer in
UV region decreases, indicating that the photochromic reaction actually takes place
in the PMMA particle suspended in water.
Figure 1.20c shows the time trace of the Z-position of the trapped DAE-PMMA
particle under periodic irradiation with UV light. Reciprocating motion by several
hundreds of nm, which are synchronized with the UV irradiation, was observed along
the optical axis (Z-axis) of the trapping beam. Figure 1.20d schematically illustrates
the mesoscopic motion of DAE-PMMA particle observed in Fig. 1.20c. The picture
in the left-hand side shows one DAE-PMMA particle trapped in water by the CW
21
1.4 Mesoscopic Motion of Small Particles Induced
by Switching of Photon Force Through Photochromic
Reactions
In this section, we introduce the movement of the nano- and microparticles synchronized with the photochemical reaction under the laser trapping, which is one of the
photosynergetic responses integrated with the momentum change of photon and the
photochemical reaction of molecules.
A small particle under photo-irradiation experiences photon force (photon pressure) originating from the transfer of the momentum of photon to the particle [32].
This photon force is generally classified into three parts: absorption, scattering, and
gradient forces. The absorption and scattering forces are, respectively, due to the
momentum transfer of photons through absorption and scattering, which push a
photo-irradiated particle toward the light propagation. On the other hand, the gradient
force directs the spatial gradient of the light intensity. When the refractive index of
a particle is larger than that of the medium, the gradient force pushes the particle
to the direction along the increasing light intensity. Under the irradiation of the
tightly focused laser beam, the gradient force keeps a small particle at the focal
point. Because the contribution of the gradient force is generally dominant in the
condition with no absorption of the particle at the wavelength of the trapping laser
(off-resonant), the particle is trapped in the vicinity of the focal point. Although the
absorption force is negligible in the off-resonant condition, it largely contributes in
the case where the absorption of the particle is resonant to the trapping light. Hence,
the change of the color of the particle may induce the change of the trapping position
of the particle through the switching of the absorption force.
In the present study, we employed a photochromic diarylethene (DAE) derivative shown in Fig. 1.20a as a color switching material. In addition to the change
of the absorption wavelength, photochromic reactions of this DAE are accompanied with the fluorescence switching; the open-ring isomer is non-fluorescent while
the closed-ring isomer shows bright fluorescence. This fluorescence was used for
tracking the position of a trapped particle [33]. Figure 1.20b shows extinction spectra
of an aqueous colloid of 300-nm sized poly(methyl methacrylate) particles containing
the DAE derivative (DAE-PMMA particle) for different UV irradiation time. The
initial spectrum consists of the scattering of the particle and the absorption of the
open-ring isomer of DAE. With an increase in UV dose, absorption band of the
closed-ring isomer in the visible region increases and that of the open-ring isomer in
UV region decreases, indicating that the photochromic reaction actually takes place
in the PMMA particle suspended in water.
Figure 1.20c shows the time trace of the Z-position of the trapped DAE-PMMA
particle under periodic irradiation with UV light. Reciprocating motion by several
hundreds of nm, which are synchronized with the UV irradiation, was observed along
the optical axis (Z-axis) of the trapping beam. Figure 1.20d schematically illustrates
the mesoscopic motion of DAE-PMMA particle observed in Fig. 1.20c. The picture
in the left-hand side shows one DAE-PMMA particle trapped in water by the CW