Ultrafast Transmission Electron Microscopy
UTEM at the IPCMS
An ultrafast TEM is installed at the IPCMS since summer 2014. It has been funded in the second round of the national initiative Investissements d’Avenir as an EQUIPEX project.
The microscope is based on a Jeol 2100 with thermionic gun that has been modified by IDES Inc. Two mirror systems allow the illumination of both specimen and electron emitter with laser pulses of different wavelengths. An extra electron lens system above the condenser increases the brightness of the electron beam. The optical tables with the lasers and delay lines are directly coupled to the column so that the system is compact and supported by the spring system of the microscope. At present, a femtosecond laser of Amplitude Systems is used. The TEM is also equipped with an EEL spectrometer.
Principle of UTEM
Ultrafast Transmission Electron Microscopy (UTEM) has been developed to study microscopic objects with high temporal resolution. Conventional electron microscopy (TEM) uses continuous electron beams that do not allow taking images of objects with exposure times much below 0.1 seconds. UTEM uses pulsed electron beams to take ‘snapshots’ where the exposure time is given by the pulse length, much like flashlight photography. To achieve ultrashort and intense electron pulses, pulsed lasers are used that generate electron emission on a photocathode. The electron bunches are accelerated in the column of the electron microscope and traverse the specimen. The image or diffraction pattern is formed in the same way as in a conventional TEM.
To study the temporal evolution of nanoscale materials, a pump laser pulse is used that excites the object. After an adjustable time, the electron bunch, serving as the probe pulse, crosses the object, forms the image, and is recorded by a CCD camera. Pump-probe experiments are applied since a long time in short-timescale physics with two pulsed laser beams where high temporal resolution is achieved but not high spatial selectivity. Now, UTEM allows us to work on the scale of nanometers, much beyond the limit of optical microscopy.