Ultrafast Lab

Since 2015, we have started up an ultrafast laser lab as part of our group. The aim is to study ultrafast optical processes in nanoscale materials for applications in photonics. 

Figure 1: Laser lab at our department

The core of the laser chain is a Mai Tai Ti:S oscillator which gives 110 femtosecond pulses at 800 nm (full range 690 - 1040 nm) with a repetition rate of 80 MHz. The average output power is over 2.5W at 800 nm resulting in a few tens of nanojoule energy per pulse. The output can be coupled to an amplifier (see below) or directly into a microscope setup where we can double the pulse energy to 400 nm an reduce the repetition rate using a pulse picker.

Although this energy is suffiicient for many applications where light is focused (e.g. a confocal microscope, time-resolved linear spectroscopy, ...), we couple the relatively low energy pulses into a regenerative amplifier, the Spitfire ACE (Spectra Physics). The latter uses a stretcher, amplification in a Ti:S crystal, pumped by a green Empower seed laser (Spectra) and finally pulse re-compression to reach 110 fs short pulses at 1 kHz with up to 4 mJ per pulse at 800 nm.

Five percent of this beam is used to generate white light in the transient absorption setup, the remaining 95% can be used directly or can be doubled to 400 nm to have mJ pulses at both 800 and 400 nm. This is often used as a pump beam for the transient absorption setup. The 95% can also be coupled into a TOPAS (Traveling Wave Optical Parametric Amplifier) (Light Conversion) which can generate microjoule pulses from 285 nm to 2400 nm.

Figure 2: Typical output energies of the TOPAS-Prime.

 

Transient Absorption Spectroscopy

The output of the TOPAS, the 800 nm (4mJ)  and the frequency doubled 800 nm (400 nm, <1 mJ) are used as the pump beam in a transient absorption spectrometer (see figure 3). In this experiment, a sample (e.g. a dipsersion of quantum dots) is pumped by a 'pump' beam at time zero. This photo-excitation changes the absorbance A of the sample by creating charge carriers. We monitor the change of this absorbance (dA) with a temporally short probe beam (110 fs) which is delayed in time (up to 6 ns with a 30 fs resolution) using a 2-folded delay stage (see figure 4). The probe is broadband - from 350 to 800 nm (CaF2 or sapphire) or 850 to 1600 nm (YAG) - which gives us information of dA both as function of time delay and energy (wavelength). Combining both allows us to study changes due to photo-excitation on sub picosecond timescales, a range relevant for almost every process relevant for light generation, amplification and detection.

Figure 3: Overview of transient absorption setup (TAS).

Figure 4: Pump and probe paths in the TAS

Figure 5: The 6.4 nanosecond delay stage in the TAS. 

Figure 6: The white-light generation stage, creating the broadband probe beam.

Single Photon Microscopy

Our microPL setup allows us to study the luminescence of single nanoparticles. The nanoparticles are excited by focussing a blue light beam - second harmonic generation of a Ti:S oscillator - on them. Using an objective lens, excitation-spot diameters of a few hundred of nanometres are achieved. The luminescence is collected by the same objective lens and separated from the excitation beam using a dichroic mirror.  An imaging spectrometer allows us to either image our field of view or to measure the spectrum of individual nanoparticles. To “see” individual nanoparticles a very sensitive EMCCD camera (350-1000 nm) is used. The luminescence can also be directed towards a pair of photon counters. These are even more sensitive than the camera and can detect single photons with an efficiency of 70% and a time resolution of 300 ps. We use the photons counters to measure the luminescence decay of single nanoparticles and  analyse the statistics of individual photon-emission events. We can study luminescence intermittency and perform time-correlation photon-counting spectroscopy.

Figure 7: Single Photon Emission of InP/ZnSe quantum dots. Taken from Chandrasekaran et al. (link)

 

Integrated Photonics

 

Figure 8 : Overview of the integrated photonics setup.

 

Contact:

Dr. Ir. Pieter Geiregat - pieter.geiregat [at] ugent.be

Prof. Dr. Ir. Zeger Hens - zeger.hens [at] ugent.be

Selected publications