MSc assignment: Building a Transient Absorption Microscope

Master assignment: Building a Transient Absorption Microscope

Chair: Prof. Dr. Guido Mul

Supervisor: Dr. Ir. Annemarie Huijser

Daily supervisor: Lisanne Einhaus (contact: l.m.einhaus@utwente.nl)

For who?

-          You are a master student in applied physics or nanotechnology

-          You are interested in optics and like to get some hands-on experience on this

-          You like to design, build and test a new optical setup, thereby physically creating new measurement equipment which will be used in state-of-the-art research.

Ultrafast Transient Absorption

Ultrafast transient absorption (TA) spectroscopy is a pump-probe optical measurement technique. It is used to measure the photogenerated excited state absorption energies and charge carrier processes. A measurement exists of two phases. First, a white light pulse (“probe”) is directed on a sample, and the transmitted light is measured. From this, the steady state absorption spectrum is obtained. Then, a stronger, narrow wavelength “pump” pulse is directed on the sample to bring it in an excited state, thereby creating excitons or free carriers (electrons/holes). Now again the white light is used to measure the absorption spectrum. This “excited state absorption spectrum” will be different from the “steady state absorption spectrum”. By subtracting both spectra, we obtain the differential absorption . This can be done for different “delay times”, arrival times between the pump and probe pulse, to obtain  over a certain time range after excitation. From this we can obtain information on the behaviour of excitons and charges within a sample. This is very important for materials used in opto-electronic applications, such as solar cells or LEDs.

Figure 1: Working principle of Transient Absorption

Figure 2: Impression of the current TA setup

The assignment

Right now, our setup measures at spot sizes of 250um/100um (pump/probe). However, we want to extend our setup to be a transient absorption microscope, creating smaller spot sizes to be able to measure physical charge transport in for example patterned samples. This is not a trivial thing, because smaller spot size means smaller measurement signals, so part of the setup would need to be rebuild to work on higher repetition rate. This means we should implement a new way of converting 800nm light in 400nm (second harmonic generation), design the focusing optics, scanning equipment, maybe use a lock-in amplifier etc.

The goal of the assignment is to design and build this extension on the TA setup, and eventually test it’s specifications