Piezoelectric Actuation Offers Light Microscopy New Capabilities
Scott Jordan PI (Physik Instrumente) L.P., 16 Albert St., Auburn, MA 01501
scottj@pi-usa.us
Introduction Te intersection of nanopositioning and light microscopy
is expanding and deepening rapidly. Evolving disciplines as diverse as single-molecule biophysics, super-resolution microscopy, and automated microassay scanning share several common themes: higher throughput, reduced resolution tolerances, and dynamic (on-the-fly) techniques. Tese combine to increase the prevalence of piezo-based positioning systems added to or integrated into microscope setups. Piezoelectric actuators have traditionally been layered
structures of lead zirconate titanate (PZT) ceramic interleaved with electrodes. Tis ceramic is ferroelectric, and application of a varying voltage produces a nearly proportional change in dimension [1]. Te dimensional change can be controlled to minute levels, which is why PZT actuation has been the foundation of atomic force microscopes and other applications requiring nanoscale-controlled motions. PZT actuation is also very quick, so throughput has been another reason for adoption. Tus, recently developed stage mechanisms present
new opportunities for microscopy applications. Tis article examines how piezo stages can remove previous constraints in ways that advance science.
Focusing Applications In microscopy, a common application is Z-stack data
acquisition, in which either the sample or the focusing lens is rapidly stepped or scanned along the optical axis. PZT mechanisms are the choice for this because of their combination of speed and precision. Integration of a frictionless lever amplification element allows design of compact and cost-effective mechanisms with many hundreds of microns of travel. Incorporation of a position sensor such as a strain gauge or capacitive sensor allows the PZT actuation to be made highly linear and repeatable, and the user can thereby acquire position information synchronously with the optical data without stopping, ensuring a reliable and highly deterministic position-domain datum set for their data (Figure 1). In these applications, a recent trend has been toward
longer and longer scan travels, even beyond the several- hundred-µm travels of which conventional lever-amplified PZT mechanisms are capable. Long travel is especially important for microscopy applications with large penetration depth, such as two-photon microscopy. Tis has necessitated development of novel mechanisms that use revolutionary modalities of piezo actuation. For example, in a novel class of walking-style actuators, the piezo ceramic is configured in a manner that confers longitudinal force on a ceramic actuation rod of arbitrary length. Te ceramic elements can be actuated down into the sub-nm realm for small distances, or they may be phased to alternately push and retract, allowing unlimited travel actuation with high axial stiffness and
30 Figure 1: PIFOC® Piezo Lens Positioner and Piezo-Z Stage (PI).
holding force—ideal for focusing mechanisms where stiffness is desirable for stability reasons and to accommodate heavy, high-NA objectives (Figure 2). Similarly, piezo actuation’s high speed and resolution
makes it the ideal technology for responsive autofocus implementations in microscopy. Te need for focusing mechanisms (either objective positioners or Z stages) to instantly acquire focus and keep it locked in despite structural driſts and specimen motions has outstripped the capabilities of previous- generation probe-based sensors that could compensate for only some driſt mechanisms. Te industry has responded with both image- and sensor-based autofocus approaches that meet these emerging needs and that leverage sophisticated new interfacing capabilities. In particular, a unique laser autofocus sensor coupled with fast nanopositioning controllers can now acquire and lock in perfect focus in milliseconds, even starting from many hundreds of micrometers out of focus. Tis approach offers many benefits over previous, probe-based platform stabilization, which merely compensated for structural deflections of the microscope platform or other gross elements. Responsiveness is on a millisecond time scale, and all structural and optical contributors to defocusing are
Figure 2: PiezoWalk principle and new 1 mm PIFOC Focusing Drive (PI). doi:10.1017/S1551929511000460
www.microscopy-today.com • 2011 July
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