Electron beam bleaching

Fig. 2.9 A microscope image of a microring resonators fabricated with the electron beam bleaching method. The lighter areas in the image are bleached by electron beam. Reprinted from Ref. 15 with permission. 2008 Institute of Electrical and Electronics Engineers...

The method of beam modulation used involved the attenuation of the beam by reflection off glass flats. The split bleach and monitoring beams were carefully recombined at the sample. Exposure of the sample to bleach beam was achieved using a computer controlled electronic shutter (UniBlitz SD 1000). The 1/e2 diameter of the Gaussian profile spot and the sample was 2.85 pm. The intensity of the attenuated laser beam was adjusted such that it was below the bleaching threshold of the fluorescent probe. The duration of the bleach pulse was kept below 10% of the recovery time of the sample under test. These measures ensured that the recovery time was not artificially prolonged by overbleaching the sample. The... 

The bleaching at the photon energies of the exciting beam, main 1 S3/2(h)— 1 S(e) transition, and at intermediate energies may be explained by state filling (saturation effect). For the case presented in Fig. 2a the estimated density of the excited electrons in the lP(e) state per QD (> 10) is enough to saturate this 6-fold degenerate state. 

In a typical pump-probe experiment, a sample is excited with a pulse with frequency < i and wavevector ki, and is probed by a second pulse with frequency C02 and wavevector 2- The optical path of (Mie of the pulses is varied to change the delay between the two pulses. The measured signal is the difference between the intensities of the transmitted probe pulses in the presence and absence of the excitation pulses, and usually is averaged over many pulses (Fig. 1.9). In a system with only two electronic states, the difference can reflect either stimulated emission from the excited state or bleaching of the absorption band of the ground state. The probe frequency often is selected by dispersing a spectrally broad probe beam after... 

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