Biomolecules are the basic components of life. A further understanding of them could, in the long run, lead to the therapy of diseases such as Alzheimer's, Parkinson or Creutzfeld Jacob, or other neurodegenerative diseases related to the accumulation of incorrectly folded proteins.

One procedure for the investigation of biomolecular dynamics is Ultrashort Laser Pulse Spectroscopy. The principle is simple: A first laser impulse strikes the molecule, and a second, weaker and time shifted laser impulse measures the reaction.  Through repeated measurements with different time shifts, one can map a temporal image of the biomolecule's behavior.



In the laboratory of Professor Dr. Eberhard Riedle, chair for Bio Molecular Optics (BMO) department of the Ludwig Maximilians University in Munich, researchers work with impulses of less than 20 femtoseconds.  A femtosecond is 1x10^ -15 Seconds - an order of magnitude that is difficult to imagine.  A comparison may help here: In one second light travels 300,000 km, nearly the distance between Earth and moon. In a femtosecond, light travels just 0.3 µm.  That's about 1/100th the thickness of a strand of human hair.

Titanium sapphire lasers (Ti: SA) are used as impulse sources, and they supply impulses with wavelength of approx. 800 nm (red light), duration of 150fs and a 1 kHz repetition rate.  Spatially viewed, few cycles of red light can fit into a 20-fs impulse. Applying the well-known Fourier relationship (The shorter the impulse, the broader its frequency spectrum), the researchers shorten the laser pulses by widening the frequency spectrum and selecting the shorter wavelengths.

This transformation reaches the BMO team via a Noncollinear Optical Parametric Amplifier (NOPA). Through a sapphire crystal, feedback and mixing, and prism-based spectral and pulse shaping, the impulse is reduced to just a few femtoseconds.

 
The original pulse frequency of 1 kHz is preserved through the impulse transformation. However, the amplitude and phase of the laser can be distorted through this process.  So, the impulse intensity must be constantly monitored and regulated, because otherwise each test specimen behaves differently.

The BMO examines impulses using a digital storage oscilloscope.  The DL9000 measures and analyzes the laser impulses above the clock rate of 1kHz. The center shows a zoomed in shot of a single impulse.

 
The shape of the curve doesn't indicate the actual pulse envelope, but the output of the photo detector, which, although orders of magnitude slower, still corresponds to the impulse intensity. The list in the lower right indicates maximum values, indicated in mV, as well as the respective integration over the zoom shot range, indicated in millivolt-seconds (mVsec). These values correspond to the impulse intensity.  Also shown is the frequency distribution of the impulse intensity as histogram.  In the right hand column, the instrument displays the average values and standard deviations.  Since all values are computed on-line, it makes the alignment and optimization of the experimental setup possible.

 

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