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Full Information Acquisition


dynamic detection principle [ 35 , 36 ]. In this method, either the probe or the sample is excited mechanically, electrically, magneti- cally, or thermally using a sinusoidal wave with known frequency, amplitude, and phase. A lock-in amplifi er (LIA) isolates the probe response at the driving frequency or its harmonics that are used as the detected signal. Alternatively, phase-locked loops (PLL) are used to maintain the system at resonance by using the phase feedback between excitation and response signal. Over the last decade it has been recognized that these single-frequency detection methods do not adequately capture the complete information from the tip-surface interactions [ 25 ]. T is led to the development of multi-frequency SPMs in which the system is excited and measured at two or more frequencies. Passive [ 37 – 40 ], intermodulation [ 26 , 41 – 44 ], and feedback- based [ 29 ] multi-frequency SPMs have provided considerably deeper insight into the physics of tip-surface interactions in force-based SPMs and have enabled high-resolution imaging and reconstruction of force-distance curves. From passive multi-frequency SPMs, the band excitation (BE) [ 45 , 46 ] method allowed full characterization of linear tip-surface dynamics. Multi-frequency methods were recently reviewed by Garcia [ 25 ]. Information compression . Until recently all dynamic SPM methods universally used the lock-in method of processing to compress the information stream from 10 MHz as limited both by the bandwidth of the photodetector and fundamental physics of the cantilever, to the ~1–10 kHz rate of feedback operation and pixel acquisition. T ough these signal streams can be multimodal (for example, intermodulation signals or harmonics), the data fl ow is still extremely compressed. In the same vein, BE can be represented as a set of parallel lock-ins that compress the data fl ow to ~100 kHz. Even in this case, the data is compressed to about 1% of the initial data volume. Overall, these approaches restrict the analysis to a-priori postulated physical models (for example harmonic response) and ignore information on transients, single events, and incommensurate harmonics appearing in the


response. In general, we note there is a direct benefi t in increasing the amount of information that can be generated by an imaging tool, as illustrated in Figure 1 . General Mode SPM . In this section we provide an overview of the progress and opportunities for dynamic AFM imaging and analysis based on capturing and analyzing the full data stream from the detector, referred to as General Mode (G-Mode) SPM [ 47 ]. T e G-Mode SPM concept allows full exploration of complex tip-surface interactions, spatial mapping of multi-dimensional variability of material properties and their mutual interactions, and imaging at the full capacity of the information channel. T is approach circumvents limitations of heterodyne detection and, as a result, unlocks capabilities such as simultaneous multi-resolution imaging at multiple frequencies, smart data compression, noise analysis, and spectroscopic methods, which oſt en lead to multiple orders of magnitude improvements in speed compared to classical SPM techniques. Importantly, this approach facili- tates the application of a broad set of machine learning tools to the measured responses, thereby taking advantage of existing big data infrastructure and overcoming the limitations imposed by preselected physical model-based analysis [ 48 – 50 ]. Principles of the G-Mode SPM . Figure 2 illustrates the fundamental paradigm of G-Mode SPM, which is the information-theory-based analysis of the full information fl ow from the detector. T e cantilever is driven by a suitably chosen excitation signal corresponding to conventional single frequency, dual frequency, band excitation, or more complex excitation modes. However, unlike the heterodyne or parallel heterodyne processing in the classical and band excitation SPM (BE-SPM), G-Mode SPM captures the full time-dependent response of the cantilever and (temporarily) stores it for the whole image. T e raw data is subsequently analyzed and compressed for long-term storage. Multiple information channels such as vertical response, lateral response, and collected current can be captured simulta- neously. T e stored data are then de-correlated, simplifi ed, and


Figure 2 : Principles of G-Mode SPM. On the right side, G-Mode captures the complete raw signal from the photodetector thereby adding the time dimension for each spatial pixel. In contrast, the traditional lock-in paradigm (lower left) integrates the product of the excitation and response signals over the time constant and produces a single pair (amplitude and phase) of values at each spatial pixel.


36 www.microscopy-today.com • 2017 July


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