We use optical techniques to investigate function and structure of various natural proteins as well as of biomimetic devices. In particular, the group specializes in time resolved optical techniques that allow one to follow the sequence of events that occur in these systems with the time resolution of less that 100 fs. In fact, the time resolution of these experimental methods are so high that vibrational motion of a molecule could be observed in real time.
1. Time resolved pump-probe spectroscopy: technique description
In a pump-probe technique, a sample is first excited with a very short light pulse initiating changes in the material of interest. In the case of photosynthetic complex, for example, it could be a chlorophyll molecule that is promoted into its excited state after it absorbs a pump photon (left).
Once the molecule is in excited state, its optical properties change: (i) it will not absorb a photon anymore the way as it absorbed in its ground state (photobleaching, PB), (ii) it may amplify light as the next photon can promote transition from excited state to the ground state (stimulated emission, SE), and (iii) it may absorb another photon provided the second photon energy matches transition energy from excited state to higher excited states (excited state absorption, ESA)
These changes in optical properties can be monitored by the second probe pulse that can probe optical absorption of the system after excitation at controlled wavelength and at controlled time after excitation. The time resolution of this technique is determined by the pump and probe pulse duration.
2. Ultrafast optical pump-probe spectrometer: low repetition rate.
The main laser system that provides pump and probe pulses is based on a Kerr-lens mode-locked Ti:Sapphire laser that is capable of generating pulses shorter than 100 fs (1 fs = 10-15 s) at a repetition rate ~80 MHz and average power 300-600 mW. Thus, a single light pulse peak power is in the order of 100 kW. In order to provide broad spectral tunability, the pulse energy is further amplified using a sequence of Faraday isolator, optical pulse stretcher, Ti:sapphire light amplifier and pulse compressor. The resulting pulses have a peak power of up to 10 GW (1010 W) at repetition rate of 1 kHz. This very high pulse peak power is essential for further nonlinear frequency conversion of the laser pulse wavelength in optical parametric amplifier (OPA) and in white light continuum generator. The output of the first one is used as a source of pump pulses, while the femtosecond white light continuum is used to probe absorption change in a sample after excitation. This is a home-built system that offers extremely high sensitivity comparable with the intrinsic photon statistics noise and electronic shot noise allowing detection of absorbance differences DA ~ 10-5. This enables it to visualize kinetics of one excited molecule out of 100,000 molecules.
3. Ultrafast optical pump-probe spectrometer: high repetition rate.
The alternative femtosecond laser pump-probe spectrometer uses non-amplified Ti:sapphire laser analogous to the one used in previous system. Due to the high output frequency (80 MHz) both pump and probe beams can be modulated at very high frequency (6.5 MHz and 0.5 MHz) and the signal is detected at a sum 7 MHz frequency. Since laser noise at that that frequency is determined only by photon statistics, this system is capable of detecting signals at DA ~ 10-6 level using sub-picojoule pump and probe pulse energies. That means we can pick up changes in the sample if only one out of a 1000,000 molecules changes its optical state. Such an unsurpassed sensitivity makes us one of the few groups in the world who can study photosynthetic complexes under light excitations conditions that are comparable to natural sunlight. Most of the other pump-probe spectrometers need 100-10,000 times higher pulse powers to detect optical signals. Graph to the right shows a sample pump-probe profile measure with this system. The key to the high sensitivity of the system lays in the home-built ultrasensitive resonant photodetector that was developed by S. Savikhin (Rev. Sci. Instrum., 1995. 66: p. 4470-4474).
4. Nano-micro-milli-second pump-probe spectrometer
An alternative system bases on excimer laser pumped dye laser is used to monitor absorption change dynamics in nano-milli second time range. It uses ~10 ns laser pulses as a pump and employs ultrastable quasi-pulsed Xe lamp. This system runs at 0.5 Hz repetition rate and the home-built detectors achieve sensitivity in DA ~ 10-4 – 10-5. Data is accumulated in a fast digital oscilloscope.
5. Time correlated single photon counting system
This system is based on standard equipment and is well described here.
6. Terahertz spectrometer
This technique is realized using one of our fs Ti:sapphire lasers and a commercial THz spectrometr. See detailed description of the principle here. This system is in used in a joint project with Prof. S. Durbin’s group.
7. Standard equipment and facilities
· Steady state absorption spectrometer
· Closed cycle helium cryostat
· Deoxygenation chamber
· Chemical hood
· Deep freezer