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Scanning Confocal Microscopy

Images from scanning confocal set-up. Spatial scanning image of (a) silver nanoparticles dispersed on self-organized microstructures (b) TEM grid with pitch = 5 µm, taken with our custom made confocal set-up.

Confocal microscopy is a well-established technique for high resolution imaging of small length scale objects in the far field. The basic principle lies in using a spatial pinhole aperture to collect the emission from the object being imaged, which effectively allows preferential imaging of a chosen focal plane. This has two primary advantages: it increases the resolution of an image by rejecting unfocussed light in samples that are thicker than the depth of focus; it also allows three dimensional reconstruction of samples by varying the focal plane in the collection set-up.

Our confocal microscopes substitutes the aperture of a collection fiber as the required pinhole, which allows us to easily change the size of the pinhole on demand as well as couple different detectors to the microscope without need for intensive re-alignment. Additional modifications to this custom-made set-up impart more flexibility. We use a high NA objective with a long working distance, so this microscope can not only measure samples at room temperature, but can be coupled to cryostats for low temperature measurements. Our spatial resolution in this set-up is diffraction limited to ~ 500 nm.

We are interested in investigating phenomena over different spatial length scales. For such scanning PL measurements, our confocal microscopes are equipped with linear translation stages and fast steering mirrors that allow us to dynamically scan our microscope collection with respect to the sample surface.

The spectral and spatial distribution of PL from samples imparts a lot of information. However, the state of the emission polarization in response to variation in the excitation polarization is often an important aspect. We utilize a liquid crystal variable waveplate in conjunction with other polarization optics to vary the excitation polarization. This is especially important for us because we tend to experiment with samples which have been shown to react drastically to different polarizations of incident light, particularly our work on the nanoscale self-assembled materials.

Ultrafast Spectroscopy

Electronic states in the Valence-Band (VB) and the Conduction-Band (CB) after an optical excitation play a key role in deciding the optical properties of a material system.

Using ultrashort laser pulses with pulse widths in regime of picoseconds (ps) and femtoseconds (fs), one can specifically alter the density of filled energy states at ultrafast rates, thereby giving one numerous possibilities to study and take time-resolved snapshots of the charge carrier dynamics, spin dynamics (spin-flip mechanisms) and altered (novel) physical or electronic states, optical characteristics of the system under investigation–at these short time intervals. This way, we have an opportunity to study exotic and transient phenomena, which could not be other wise explored. Furthermore, tuning the energy of the excitation photons directly to the bandgap of the investigated spectra, one can realize and study resonance energy transfer mechanisms with great detail.  

All ultrafast measurements use an solid-state, mode-locked Titanium-doped Sapphire (Ti:sapphire) laser (Coherent Mira 900) as the excitation source. It produces a train of 150 fs optical pulses at a nominal repetition frequency of 76 MHz, with a gap of 13 ns between subsequent pulses. The laser wavelength range is tunable from 700 nm to 1000 nm. A nonlinear β-BBO crystal, placed after the Ti:Sapphire system, is used to double the laser frequency, via second-harmonic generation, to give laser pulses with wavelengths between 350 nm and 480 nm range.  

For our experimental purposes, the time-resolved measurements chiefly used to study include: 

Time-resolved Photoluminescence methods temporally resolve the spectrally discrete PL in order to observe the lifetimes of excited excitonic and electronic states. The recombination times, along with their variations with external parameters such as electric and magnetic fields, excitation pump power and pulse energy, etc. shed considerable light on the details of the photoexcitation process.

Our primary technique for TRPL measurement is a Time-correlated single photon counting (TCSPC) set-up (PicoHarp 300) from PicoQuant GmbH. A detailed introduction to the concept of time-resolved measurements can be found at their website (http://www.picoquant.com).

Spin-resolved dynamical measurements provide very sensitive probes of spin coherence times in bulk and low-dimensional semiconductor structures, with the resolutions approaching single spin detection levels. We follow two methods for studying spin dynamics:

• Hanle effect, which tracks the degree of circular polarization in the emission PL as a function of an applied in-plane magnetic field.
• Time-resolved Faraday rotation (TRFR), the fundamental idea behind which can be simply stated as follows: when a linearly polarized beam of light traverses a material system – which has either a permanent or induced magnetic moment - the polarization of the beam rotates in proportion to the magnetization of the sample. This phenomenon is caused by the circular birefringence induced by this magnetization, which rotates the two opposite circularly polarized components of the initial linearly polarized beam by different magnitudes, thereby rotating the linear-polarization axis with time. This rotation provides us with an ability to probe the magnetization dynamics in any magnetizable-system, on broad time-, and spatial-scales. Time-Resolved FR is a pump-probe technique, where the pump beam (for injecting spin-polarized electrons) is circularly polarized and the probe beam is linearly polarized (for probing the dynamics of spin evolution).