A high frequency sensor for optical beam deflection atomic force microscopy

Abstract

We demonstrate a novel electronic readout for quadrant photodiode based optical beam deflection setups. In our readout, the signals used to calculate the deflections remain as currents, instead of undergoing an immediate conversion to voltages. Bipolar current mirrors are used to perform all mathematical operations at the transistor level, including the signal normalizing division. This method has numerous advantages, leading to significantly simpler designs that avoid large voltage swings and parasitic capacitances. The bandwidth of our readout is solely limited by the capacitance of the quadrant photodiode junctions, making the effective bandwidth a function of the intensity of photocurrents and thus the applied power of the beam deflection laser. Using commercially available components and laser intensities of 1-4 mW we achieved a 3 dB bandwidth of 20 MHz with deflection sensitivities of up to 0.5-1 V/nm and deflection noise levels below 4.5 fm/Hz. Atomic resolution imaging of muscovite mica using FM-AFM in water demonstrates the sensitivity of this novel readout.

High-speed imaging upgrade for a standard sample scanning atomic force microscope using small cantilevers

“…Although our system is not optimized for low noise performance, we have measured baseline noise levels of our readout below 100 fm/ √ Hz. While this level does not reach the excellent noise performance of OBD systems optimized for low noise performance, [29][30][31][32] it is comparable to the noise levels we have measured for our commercial standard and small cantilever AFM systems (MultiMode and Dimension FastScan, Bruker Nano Surfaces).…”

“…Our high-bandwidth readout architecture uses a novel translinear photodiode readout circuit which is capable of high bandwidth and low noise performance. 29 The high bandwidth of this readout approach was tested using a small cantilever (BL-AC10DS-A2, Olympus, Japan) with dimensions 2 × 9 μm and fundamental resonance frequency 1.5 MHz. The power spectrum of the cantilever thermal deflections was captured on an oscilloscope and calibrated by comparison of the first thermal peak captured on the oscilloscope with the same calibrated thermal peak measured with the MultiMode AFM system.…”

Atomic-resolution imaging in liquid by frequency modulation atomic force microscopy using small cantilevers with megahertz-order resonance frequencies

“…Cantilever excitation methods To achieve the theoretically-limited performance given by equation (10), we should overcome two technical difficulties: the detection and excitation of the cantilever oscillation. So far, a number of detection techniques for a small cantilever have been reported [19,20,3,21,22,23]. In contrast, the excitation method for a small cantilever has yet to be established.…”

“…Several groups have presented practical ways to fabricate small cantilevers with a high resonance frequency [16,17,18]. In addition, several sophisticated designs for low noise and wideband deflection sensors for small cantilevers have been proposed [19,20,3,21,22,23].…”

Improving image contrast and material discrimination with nonlinear response in bimodal atomic force microscopy

“…Thus new cantilever designs could be used to enhance cantilever response27 and low noise detectors would help improve the SNR off resonance282930. The enhanced contrast clearly seen at several mixing frequencies points towards further possible development of the methods used here.…”