The manipulation of microscale objects using midinfrared light has recently opened a new direction in optical manipulation, enabling the separation of materials according to their chemical species. Here, we report an enhanced midinfrared optical force exerted on silica micro- and nanoparticles, which can be attributed to light reflection when the laser wavelength is slightly blue-detuned from the siloxane absorption peak. In other words, the wavelength is tuned into the Reststrahlen band between the transverse optical (TO) and longitudinal optical (LO) phonon modes of the siloxane bonds. Translational motion of various microparticles in an evanescent field generated by two midinfrared lasers (8.9 and 9.3 $\mu \mathrm{m}$ lasers) was compared: the 9.3 $\mu \mathrm{m}$ laser excites at the siloxane-bond absorption peak (TO phonon mode), whereas the 8.9 $\mu \mathrm{m}$ laser is blue-detuned from it, falling within the Reststrahlen band. The velocity of silica particles at 8.9 $\mu \mathrm{m}$, especially for nanoparticles, was found to be higher than that at 9.3 $\mu \mathrm{m}$, despite the lower absorbance at 8.9 $\mu \mathrm{m}$. The analytical evaluation of the optical force using the dipole approximation for nanoparticles shows a consistent enhancement of the optical force under blue-detuned conditions, where the phase difference between the incident electric field and the induced polarization is $\pi /2$. From the above evaluation, we conclude that enhanced optical force originates from localized surface phonon polariton excitation within the Reststrahlen band.

Close

PrevNext

View figure in article

• Chemical separation
• Light-matter interaction
• Optical forces
• Optics & lasers
• Vibrational states