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Time and Frequency as New Frontiers in Microscopy

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Electron energy loss spectroscopy in the 1eV to 2keV range with spatial resolution on the near-atomic scale is now well established as a vital ancillary technique in high resolution electron microscopy. Taking a wider view of the whole energy loss or frequency spectrum however, it has been noted that the situation is rather less impressive. Over many decades of energy loss below 1eV or frequencies below 1015 Hz, scanned probe microscopy, near-field microscopy or scanning optical microscopy techniques appear to be more effective. On the evidence these provide, there is no reason to reject the view that these spectral regions are heavily populated with interesting phenomena. Using RF and other pulsing techniques, the frequency range below about 1MHz has been explored in a few isolated TEM or SEM investigations.

Within the limits of linear behaviour, a complete knowledge of the complex response as a function of frequency is sufficient to specify the time response of a specimen to any applied signal including a sharp impulse. Indeed the standard EELS method, being based on the delivery of a series of such impulses from individual electrons in the beam, operates that principle in reverse. When the response to a periodic input signal of frequency w is mapped out by phase-sensitive detection methods, as for example in scanning electron acoustic microscopy (SEAM), it is even possible to study non-linear features of behaviour by measuring the response at say 2w.

Although it is not so far possible to scan whole images by scanning tunneling microscopy in less than about 1ms, the basic non- linearities of the tunneling process facilitate the probing of high frequency dynamic response at selected positions. Methods include magnetostrictive oscillation of the tunneling distance and the use of laser pulses in sum-difference frequency methods and in overlapping pulse techniques. With split laser pulses in the traditional pump- probe procedure, the probe pulse can also be used to collect the signal at a controllable time delay by using it to trigger a photo- activated gate in the tunneling circuit. Pump-probe optical pulse techniques can also be employed in apertureless near-field microscopy where the tip field enhancement effect provides the necessary localization of the pump pulse. Scanned probe microscopy has led the field in exploring these new possibilities, but it is probably true to say that we still await really exciting applications.

TEM imaging in pump-probe operation has recently been demonstrated by splitting a train of laser pulses to pump the sample and to generate a probe pulse of electrons by exciting a photocathode electron source with a controllable delay time. Images at x100,000 were obtained in seconds using sub-100fs pulses with as few as one electron per pulse and a repetition rate of 80MHz. This must be the best approach for dynamic imaging of non-damaging phenomena which can be repeated many times under identical starting conditions. More general, ultra-fast imaging requires single shot operation with millions of electrons in one pulse and for high spatial resolution raises truly formidable problems of electron optics.

This talk is part of the Electron Microscopy Group Seminars series.

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