Nanomanipulation: why optical methods are best

In everyday experience, movement in the world we perceive is most often a result of contact forces. The slightest puff of breeze stirs the treetops only through direct physical contact between the leaves and the moving substance of the air. Such is the familiarity of the forces that we most commonly observe and deploy, that any occurrences of objects being physically moved without material contact seem intrinsically more exotic, and often rather enigmatic. Gravity, whose effects provide the most obvious example of producing motion without contact, was long considered a mystery; it succumbed to Newton's analysis without yielding much mechanistic insight, and it is indeed still frustratingly difficult to weave into a satisfactory Grand Unified Theory. At least for the present, gravity also remains a force beyond our capacity to control.

Over many centuries—at least since the first use of a lodestone—forces of electrical and magnetic origin have held a particular fascination. They too manifest a clearly non-contact nature, yet of a kind that is indeed amenable to control and application. The eventual dawning of an understanding that light is itself electromagnetic in nature thus provided a logical basis for expecting it to have a capacity to exert a mechanical force. Indeed, such an expectation is consistent with what we have more recently discovered—that at the atomic level, even what we would normally call 'contact' forces are also fundamentally electrodynamic in origin, manifesting in bulk behaviour the effect of hidden interactions between the elementary charges that comprise every piece of matter.

To discover demonstrable manifestations of light-induced forces nonetheless proved much more difficult than for electric or magnetic effects. Maxwell and Bartoli [1, 2], amongst others, showed how and why light should have a capacity to convey and confer momentum, but the experimental confirmation awaited studies by Lebedev [3] and by Nichols and Hull [4] at the turn of the twentieth century. These works confirmed the presence of such effects, but also verified that conventional sources would produce radiation forces of so little magnitude as to be scarcely amenable to measurement—let alone of meaningful practical use—and often completely masked by thermal forces. Indeed, it was only with the arrival of intense light sources in the form of lasers, in the early 1960s, that the determination and exercising of control over optical force first became a practicable proposition.

The rapid pace of development of laser systems quickly provided experimental access to previously unattainable levels of intensity, substantially as a result of the high degree of angular confinement and controllable direction in their optical emission. Exploiting this feature, Ashkin's experimental studies first revealed the remarkable possibilities that laser beams could controllably manipulate small, neutral particles of matter [5]. In this inaugural work, he overcame any problems with thermal forces by using relatively transparent particles (i.e. latex spheres) suspended in a relatively transparent medium (water). These spheres were observed to be simultaneously drawn to the beam axis, and accelerated in the direction of the light: optical manipulation was demonstrated for the first time. In addition, a narrow beam-width also proved significant in another sense; it provided for sharp intensity gradients—and hence differential forces—across the beam. This force is the basis for the renowned optical tweezer technique [6].

The minimum physical width of an optical beam is generally prescribed by the Abbé diffraction limit [7], which is of the order of a wavelength—and therefore usually somewhere in the nanometre (sub-micron) regime for optical frequencies. (A variety of means have indeed been found to overcome this limit in super-resolution microscopy [8–11], notably the Nobel Prize winning technique of stimulated emission depletion, STED, developed by Hell et al [12, 13]). Consequently, the use of focused laser beams offers scope to exert controllable forces on micro- or nanometre-scale particles, including biological cells. Non-contact forces are especially useful in the nano regime because, on this scale, physical contact with manipulative equipment will often introduce problems due to adhesion, surface friction, stiction and contamination etc.

As we consider particles of progressively smaller size, gravity—whose effect essentially scales down with the cube of particle diameter—soon ceases to represent a significant obstacle. To completely offset its effect, micron sized particles can usually be stably suspended in a liquid—and this is indeed a common configuration for optical manipulation experiments. Here, viscous drag is the only major limitation to particle motion. (Intense light can in fact detach particles that are in physical contact, but the intensities required are much higher. Laser ablation operates on this principle [14]. However, such effects are not usually termed 'optical manipulation' and they lie outside the scope of this book.)

There are other problems to confront as the dimensional scale is further reduced to atomic and molecule-sized particles. Here, Brownian motion comes into play, serving as a reminder that at nanoscale levels all matter is in continual flux. Even at a temperature approaching absolute zero there is a residual energy of motion, reflecting the intrinsic quantum mechanical zero-point energy of all matter. The thermal energy that registers temperature is associated with the random kinetic motions of atoms and molecules in the air, and other gases at ambient temperatures. For this reason, the optical manipulation of atoms and very small molecules is usually studied at the extremely low temperatures of ultra-cold gases (rather than liquid or solid states, since deposition or condensation would frustrate the sought particle mobility).

Due to the difficulties in scaling up ultra-cold techniques, or scaling down microparticle trapping procedures, particles of intermediate scale (including quantum dots, nanowires and nanotubes) are optically manipulated using different approaches. So, as we shall see, both the methodologies and types of optical force used for nanomanipulation exist in numerous different forms. The size range of particles that can be optically trapped is shown in figure 1.1.

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