A single source femtosecond-millisecond broadband spectrometer

A single source femtosecond-millisecond broadband spectrometer
E. C. Carroll, M. P. Hill, D. Madsen, K. R. Malley, and D. S. Larsena
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, USA

Time-resolved measurement of population dynamics extending over femtosecond to millisecond time scales typically requires a combination of transient absorption techniques involving different laser systems and detection schemes. The spectrometer design presented here facilitates transient absorption measurements over 12 decades with a single ultrafast laser system by picking pump and probe pulses independently from the laser oscillator pulse train. Unamplified pulses seed a photonic crystal fiber to a supercontinuum probe source for spectrally resolved measurements. The utility of the system is demonstrated by measuring triplet state dynamics following photoexcitation of vitamin B6 in aqueous solution.

Time-resolved transient absorption TA, or optical pump-probe, spectroscopy has become an indispensable technique for characterizing photoinduced population dynamics in biological, chemical, and solid-state chromophores.
Dynamics in such samples often occur over multiple time scales ranging from femtoseconds e.g. photoisomerization to milliseconds e.g. protein conformational changes. In a TA experiment, a pulsed light source excites a subpopulation of the sample into an electronic excited state and its relaxation dynamics is subsequently probed by monitoring changes in the TA spectrum with a time-gated light source. The time range and resolution depend on temporal characteristics of the pump and probe light sources and the method of detection.
Short-pulse spectrometers with femtosecond resolution tend to be limited to a range of 10 ns because pump-probe delay is generated by varying optical path lengths; electronically gated flash photolysis spectrometers can have ranges out to milliseconds, but tend to have resolution limited to several nanoseconds. Thus, the task of measuring dynamics extending from femtoseconds to milliseconds has traditionally required multiple TA spectrometers. This approach, however, can pose serious challenges to assessing the continuity of multidecade dynamics if, for instance, kinetics depend on pump wavelength, fluence, or intensity.
Many approaches to constructing high resolution multidecade spectrometers have synchronized a femtosecond laser system to a second independent femtosecond or picosecond laser system, picosecond pulsed laser diode, or highpower light-emitting diode. Spectrometers of these designs have the important advantage that identical excitation pulses are used for both ultrafast and microsecond experiments.
Nonetheless, multiple laser systems and detection equipment can be complex and expensive. Presented here is an experimental solution that bridges femtosecond, nanosecond, and microsecond domain measurements with two relatively inexpensive additions to a modern, commercial amplified Ti-:sapphire laser system: 1) an external pulse picker was introduced to slice unamplified 800 nm pulses from the 75 MHz laser oscillator independent of the amplified pulse train, and 2) oscillator pulses were used to generate supercontinuum SC in a photonic crystal fiber PCF to provide a broadband probe source. The high nonlinearity of PCFs enables SC generation at much lower energies than in slab media such as sapphire and CaF2. PCF supercontinua have been used as a coherent femtosecond probes in ultrafast TA experiments at megahertz laser repetition rates. In our spectrometer design, SC generated with oscillator pulses are synchronized to the 1 kHz amplified pulse train, which offers the advantage of using high intensity pulses for pump wavelength conversion. The resulting instrument is a flexible single-source, broadband transient spectrometer capable of measuring dynamics over 12 decades 10−1510−3 s, with spectrally resolved detection at 1 kHz using equipment now commonly found in a state-of-the-art ultrafast amplified laser laboratory.

To extend the temporal range of the spectrometer beyond 8 ns, probe pulses were sliced from the laser oscillator. These pulses were isolated by inserting a Faraday isolator, Pockels cell pulse picker Del Mar Photonics Pismo 8-1-1, and polarizing cube beam splitter in the optical path between the laser oscillator and amplifier.

Category:

Science & Technology

Tags:

• dynamics
• photoreceptors
• photo-voltaic
• photo-catalytic
• materials
• Vitamin
• B6
• triplet
• state
• photovoltaic
• exciton
• decay
• polaron
• photocell
• lifetime
• charge
• timescale
• chemistry
• pulse
• picker
• Pismo
• transient
• absorption
• spectrometer
• Hatteras
• Nobel
• Prize
• Chemistry
• Ahmed
• H.
• Zewail
• transition
• states
• chemical
• reactions
• femtosecond
• spectroscopy

[TheSolarmike]

The female in this video should be my friend and we could talk about science all the time.

[cdurack]

opps i spilled my coffee

[jeriellsworth]

Very cool.

[TheHouseBlog]

no offense intended but I find this video SUPER SUPER hot. >.>

[2222554]

2) oscillator pulses were used to generate supercontinuum SC in a photonic crystal fiber PCF to provide a broadband probe source.

[ibelieve1fly]

Wait, what does "table" mean?