  Micromechanical transducers such as cantilevers for AFM often rely on optical
readout methods that require illumination of a specific region of the
microstructure. Here we explore and exploit the diffraction effects that have
been previously neglected when modeling cantilever bending measurement
techniques. The illumination of a cantilever end causes an asymmetric
diffraction pattern at the photodetector that significantly affects the
calibration of the signal in the popular optical beam deflection technique
(OBDT). Conditions for optimized linear signals that avoid detection artifacts
conflict with small numerical aperture illumination and narrow cantilevers
which are softer and therefore more sensitive. Embracing diffraction patterns
as a physical measurable allows a richer detection technique that decouples
measurements of tilt and curvature and simultaneously relaxes the requirements
on the alignment of illumination and detector. We show analytical results,
numerical simulations and physiologically relevant experimental data
demonstrating the usefulness of these diffraction features. We offer
experimental design guidelines and identify and quantify possible sources of
systematic error of up to 10% in OBDT. We demonstrate a new nanometre
resolution detection method that can replace OBDT, where Frauenhofer and Bragg
diffraction effects from finite sized and patterned cantilevers are exploited.
Such effects are readily generalized to arrays, and allow transmission
detection of mechanical curvature, enabling in-line instruments. In particular,
a cantilever with a periodic array of slots produces Bragg peaks which can be
analyzed to deduce the cantilever curvature. We highlight the comparative
advantages over OBDT by detecting molecular activity of antibiotic Vancomycin,
with an RMS noise equivalent to less than $2.5 \mu M$ (1.5 nm), as example of
possible multi-maker bio-assays.
