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 whole system is compact and supported by the anti-vibration system of the microscope. At present, a femtosecond laser of Amplitude Systems and a nanosecond laser of Litron are used. The microscope is also equipped with an electron energy-loss 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 10 – 100 milliseconds. UTEM uses pulsed electron beams to take ‘snapshots’ where the exposure time is given by the pulse length, like flashlight photography. To achieve ultrashort and intense electron pulses, pulsed lasers are used that generate electron emission from a photocathode. The electron bunches are accelerated and focused 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 of variable wavelength is used that excites the object. After an adjustable time, the electron bunch, serving as the probe pulse, traverses the object, forms the image, and is recorded by a CCD camera. Such pump-probe experiments are applied since a long time in short-timescale physics with two pulsed laser beams where short timescales are achieved but not high spatial resolution. Now, UTEM allows us to work in both high spatial and high temporal resolution. At picosecond time resolution, the sub-nanometer scale is reached which is much beyond the spatial resolution of light optics.
Operation modes of UTEM
The UTEM in Strasbourg allows the operation in both stroboscopic and single-pulse mode.
1. Stroboscopic mode: reversible processes
If the specimen recovers after a laser pulse, the microscope can be operated in the stroboscopic mode. Here, a train of pulses is sent onto both specimen and gun so that the same experiment is repeated many times while the shutter of the camera remains open. This allows working with a low number of electrons in one pulse; in the extreme case, single-electron pulses can be achieved. A femtosecond laser is used to create both ultraviolet and infrared pulses at the same time. This is achieved by using an infrared laser, splitting the beam, and a nonlinear element for frequency conversion to send an ultraviolet beam onto the photocathode. The delay between pump and probe pulses is adjusted by an optical delay line. The stroboscopic mode, that is technically less demanding than the single-shot mode, is operational for experiments.
2. Single-pulse mode: irreversible processes
The single-pulse mode uses just one very intense electron pulse to take an exposure. This mode is applied when the specimen does not recover within a short time after the laser-induced excitation. Two lasers are used and synchronized by an electronic delay. The exposure of the image needs a large number of electrons that have to be compressed into a small bunch. Mutual repulsion of the electrons leads to pulse broadening in space and time. Therefore, a nanosecond laser is used for creating photoelectron pulses with a certain temporal length so that the pulses are also spatially extended in the direction of the electron beam. This reduces the mutual repulsion of the electrons. The single-pulse setup has been finished in 2016.