Figure 2.

Detecting FIB-modified ultrashort cantilevers on a commercial AFM. (a) Schematic of the custom-built small-spot-size detection module integrated with a commercial AFM. A diode laser (λ = 853 nm) passes through an acousto-optic modulator (AOM) before being coupled into a single-mode polarization-maintaining fiber. Following a 50/50 fiber-based splitter, one output arm is used to measure the laser intensity using a photodiode (PD) and thereby stabilize the laser intensity using feedback to the AOM. The other fiber output is coupled into the AFM using a triplet collimator. A pre-existing polarizing beam splitter (PBS) and quarter-wave plate (λ/4) within the AFM act as an optical isolator to direct the reflected laser light onto the quadrant photodiode (QPD). Acronyms represent the following: optical isolator (OI) and neutral density filter (ND). (b, c) Optical images comparing the spot size generated by (b) our custom small spot-size module and (c) the commercial small spot-size module reflecting off a BioLever Mini for image clarity. (d) The positional power spectral density (PSD) plotted as a function of frequency for a modified BioLever Fast cantilever at 50 nm over the surface in liquid when using the commercial (red) and custom (green) detection module. The estimated noise floor (black) for detecting modified cantilevers with this custom-detection module is ∼20-fold lower than the commercial one. (e) Sensitivity for detecting modified ultrashort cantilever using the commercial (red) and custom (green) detection modules determined by pushing the cantilever into a hard surface. Note that an optical-interference artifact leads to a sinusoidal modulation in the deflection signal as a function of the cantilever’s height over the surface. The voltage amplitude of this artifact for both detection modules was similar, implying at least a three-fold reduction in this artifact when deducing force and displacement with our custom-built module due to its higher sensitivity.