Optical Micro-traps

We will now present some examples for often used micro-traps. 

The first example is a micro-trap within the evanescent field of a laser wave closely above the horizontal surface of a transparent solid at z = 0, illustrated in Fig. 9.22 [1161]. 

A laser beam is sent through a prism and is totally reflected at the prism basis if the inclination angle 6 0t exceeds the limiting angel 0t for total reflection. The electric field of the totally reflected wave does not suddenly drops to zero at the reflecting boundary between the medium with refractive index i and air ( 2 i) but penetrates into the air with an exponentially decreasing amplitude E z) = Eo exp(-z/ze) (evanescent wave). After a pathlength 

The amplitude has decreased to 1 /e of its value at the boundary surface. Just above the surface therefore a large gradient of the electric field strength 

Because of the field gradient dV jdz in the z-direction the atoms with the electric dipole moment /t experience a force in the z-direction 

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A clever way to minimize the unwanted background from the cover plate or the environment is combining laser tweezers and confocal Raman spectroscopy (LTRS) [73, 74], While the single cells are levitated well off the surface and held in the focus of the laser beam the Raman spectral patterns of these cells are recorded with high sensitivity. Another appealing fact is the usage of one laser for both Raman excitation and optical trapping to keep the instrumental efforts as low as possible. Additionally the trapped cells can also be micro-manipulated and moved from one place to another, e.g., from the native matrix to a clean collection chamber. 

Ramser K et al (2007) Micro-resonance Raman study of optically trapped Escherichia coli cells overexpressing human neuroglobln. J Biomed Opt 12(4) 044009... 

As for lo, the surfaces of Ganymede and Callisto are intimately connected to both radiation and atmospheric processes. Optical spectra have identified molecular oxygen in a condensed state, trapped in the ices on both satellites, as well as Europa. The characteristics of this oxygen component have been attributed to interaction with the plasma environment (Calvin and Spencer, 1997 Spencer and Calvin, 2002 Spencer et al., 1995) and may be related to the processes controlling the formation and distribution of carbon dioxide discussed above. In addition, ozone has been detected on Ganymede and attributed to the presence of micro-atmospheres of O2 and O3 trapped in the ice (Noll et al., 1996). 

In this Section, we will describe briefly the most recent projects of atomic clocks involving/based on ion traps as described above. The first part concerns micro-wave clocks, while the one following will be dedicated to optical frequency clocks. Performances of atomic standards can be evaluated only by comparison (frequency beatings) with another devices. When a new atomic standard can be presumed to out-perform the norm, it can be evaluated only from the comparison with a second system, which must be build in a similar way. It is worth noting that performances of each scheme depend on the local oscillator a quartz (eventually, cryogenic) oscillator for the microwave range, and a laser for the optical one. 

For the future work, elastomer tunable optofluidic devices are expected to extend into the nano-optics or nanofluidic fields. Several tunable nano-optical antenna devices fabricated on a stretchable PDMS substrate have been demonstrated recently. Combining elastomer-based micro/nano-devices with nanoplasmonic elements can be interesting for molecule-level imaging and spectroscopy. A tunable elastic nanofluidic channel was demonstrated recently on a PDMS chip for nanoparticle separation and molecule trapping [12]. One of the challenges of PDMS-based tunable nano-devices is to realize the high accuracy in control. High-precision control of PDMS-based tunable structures could be realized by very fine pneumatic actuation or connection to a piezo-actuator. 

C. G. Xie and Y. Q. Li, "Cohfocal micro-Raman spectroscopy of single biological cells using optical trapping and shifted excitation difference techniques," Journal of Applied Ply sics, vol. 93, pp. 2982-2986, 2003. 

D. Morrish, X. S. Gan, and M. Gu, "Morphology-dependent resonance induced by two-photon excitation in a micro-sphere trapped by a femtosecond pulsed laser," Optics Express, vol. 12, pp. 4198-4202, 2004. 

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