attoMICROSCOPY
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Atomic Force & Confocal Microscopy ( AFM / CFM ) fundamentals attoMICROSCOPY
After decades of evolution in magnetic imaging , combining the sensitivity needed to detect single electron or nuclear spins with a spatial resolution of a few nanometers may soon get within reach of current state-of-the-art instrumentation : optically detected magnetic resonance ( ODMR ) is commonly considered to be the most promising candidate to achieve this goal by using the unique properties of single nitrogen-vacancy ( NV ) centers in diamond . While there is huge scientific activity to reliably prepare appropriate nano-diamonds with tailored NV center characteristics and to attach them to atomic force microscope tips , attocube as a commercial supplier of scanning probe microscopes is complementing these efforts by providing an ideal platform for ODMR : the attoAFM / CFM combines a cantilever based atomic force microscope ( AFM ) with a high-numerical aperture confocal microscope ( CFM ) ( see figure ).
The attoAFM / CFM features an Akiyama probe to investigate any tip-sample interaction forces on the nanometer scale . These probes combine the convenience of conventional AFM cantilevers with electrical detection of tip-sample interactions via a tuning fork , and thus eliminate the need for a laser-based AFM detection mechanism . As a result , the AFM tip is so compact that it fits in between the sample and an objective with high numerical aperture and low working distance . The tip is typically operated in non-contact mode using a phase-locked loop ( PLL ) to excite the probe at resonance and track any frequency shifts due to tip-sample interactions . The PLL keeps this shift at a constant value while scanning over the surface and hence keeps the force on the tip constant . Simultaneously to the information provided by the AFM probe , the CFM reveals complementary optical information of the sample surface . To maintain a constant distance between the low temperature compatible lens and the sample , upon force changes , only the AFM tip is adjusted in height by a dedicated scanner .
Local magnetic fields are subsequently measured via the Zeeman shifts of the NV defect spin sublevels which are directly proportional to the local magnetic fields encountered by the tip . This condition can be detected by a decrease in photoluminescence intensity of the NV center under resonant microwave excitation , referred to as ODMR . Moreover , using a lock-in technique allows tracking of the resonance shift while rastering the sample , thus enabling to record a local iso-magnetic field map with nanometer resolution .