Integrated Optical Sciences
Biomolecules and nanostructures
The Optical Sciences group studies the interaction of light and matter at the nanoscale.
We do this by exploring ways to shape light and its environment. It's what we call
active and passive control. Our current focus is on the interaction of light with
biomolecules and nanostructures. We are part of Twente
University's Department of Science and Technology and member of the
Pulse shaping setup for 20fs Ti:Sapphire pulse
In short: the spectrum of a short pulse is dispersed on a grating. The spectrum is collimated and sent through a spatial light modulator (LCD screen) on which individual lines can be addressed to add phase to the pathway of that particular bit of spectrum. The spectrum is then recombined and the spectral modulation reveals itself in the time domain. This is the (standard) approach that we also intend to follow. We will use a grating for the dispersion and a 4096 element beamsteerer as the shaper. However, a number of choices concerning resolution and bandwidth have to be made.
schematic of a grating based shaper
The ability to separate the effects of different transitions is determined by the resolution of the spectral shaping and is given by the number of pixels on the shaper that can be addressed individually within the bandwidth of the laser. More pixels imply a finer resolution, which translates into pulses with longer modulations in the time domain. A finer spectral resolution can also be achieved by having less total spectrum on the modulator. This again leads to modulations over longer timescales but with less total spectrum or less width in the energy. More total bandwidth means less modulation on the long timescales but a higher resolution in the time domain; modulations on shorter timescales. In other words: given a total number of pixels that is fixed by the modulator, the desired modulation depends on the timescales of the molecule that one wants to address. Roughly, the relevant molecular timescales can be divided as in the figure below. With an array of 4096 pixels used over a bandwidth of 100 nm around 800 nm, we can achieve pulse modulation with a resolution of 4 fs over a 15 ps time-window, a window that affects many of the relevant process in molecules. However, many transitions are not around the 800nm. To create shaped pulses at other center-frequencies we will mix the shaped 800nm pulses with a synchronized Nd:YAG modelocked oscillator and its harmonics in nonlinear crystals. In this way bands of wavelengths can be reached ranging from the blue (300nm) to the infrared (10micron). The narrow bandwidth of the Nd:YAG compared to the wide bandwidth of the Ti:Sapphire pulses ensures a pure transfer of the spectral shaping to the other wavelengths. Using these techniques (shaping and transfer) we can achieve shaping in many of the relevant wavelength ranges. We will create these shaped pulses with fairly low energies (low conversion efficiencies) but at high repetition rate (80MHz) to allow for rapid adjustments of the fields and high data acquisition rates. If higher energies are required we will parametrically amplify our pulses at a reduced repetition rate.