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Transient Absorption Spectrometer (TAS)

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Our Transient Absorption Spectrometer (TAS) is a state-of-the-art instrument for femtosecond pump-probe spectroscopy. In an ultrafast time resolved experiment using TAS， a sample of interest is excited by an electronically resonant coherent femtosecond pulse referred to as the “pump.” This causes changes to the spectrum of the sample， which are monitored as a function of time by the white light supercontinuum probe， as sensed by the combination of a spectrograph and a camera or photodiode array.

• Flexibly designed femtosecond pump-probe spectrometer

• Supports a variety of ultrafast amplifiers & optical parametric amplifiers as input

• Broad pump-probe delay range and high pump-probe delay resolution

• Flexible imaging spectrograph supports automated switching of spectral range (detector) and resolution

• Robust， stable device is factory assembled and aligned for easy integration

• Worldwide installation and support is available

• Many samples require a pump with wavelength different from that of the fundamental wavelength of the ultrafast laser. For this reason， TAS is often sold with an external optical parametric amplifier or harmonics generator. TAS is also offered with an addition of an internal second harmonic generation (SHG) option for the pump beam. The energy of the pump beam can be controlled internally by using a variable reflective neutral density filter imprinted on a thin glass substrate. This reduces the GVD introduced to the beam while allowing power control over a range of 0-4 OD. In order to reduce the effect of laser fluctuations， a TAS measurement is most typically performed at half the repetition rate of the laser using a New Focus phase locked optical chopper to block every other pump pulse. The spectrum of the transmitted or reflected white light supercontinuum probe is compared to the spectrum when the pump is present at half the repetition rate of the laser， the fastest rate possible， in order to minimize the effect of any laser noise and fluctuations on the end measurement.

The standard maximum optical delay between pump and probe in TAS is 3.3 ns. This is limited by the length of the stage (250 mm) and the number of optical passes that the probe beam makes to and from the high quality retroreflector used on the stage. In standard configurations， there are four passes (to and from the retroreflector twice). The resolution changes depending on the mode of operation. In the best case， the resolution is set by the minimal incremental motion (MIM) of the delay stage. This is applicable when performing a single measurement where the stage is incremented linearly in only one direction. In the standard TAS configuration， this is 13.3 fs. If many scans are to be averaged， the repeatability of positioning must be considered. If performing the same measurement， the uni-directional repeatability must be accounted for， which is also 13.3 fs in the standard TAS configuration. Lastly， if the set of time points is acquired randomly， as is often the case to minimize the effect of laser and sample changes to the measured dynamics， the bi-directional repeatability is the most important factor. For the standard TAS， this has a value of 26.6 fs. While the standard specifications are more than sufficient for most experiments， upgrades are available to increase the maximum optical delay and to reduce the minimum step size. The simplest， and most cost effective， upgrade is achieved by simply adding an additional retroreflector to the delay stage. There are then eight passes (to and from both reflectors twice) and the optical delay is increased to 6.67 ns. The unfortunate disadvantage of this configuration is that the resolution is also reduced by a factor of 2. For example， the MIM becomes 26.6 fs and the bidirectional repeatability becomes 53.2 fs， which is insufficient for some laser systems. To combat the resolution limits， it is possible to replace the linear stage and motion controller from the standard TAS configuration. With the ILS250HA stage and 2 retroreflectors， the optical delay is 6.67 ns， the MIM is 8 fs and the bidirectional repeatability is 18.6 fs. Alternatively， for the very best performance， the ILS300LM stage can be used with Newport’s XPS motion controller. This allows for optical delay up to 8 ns， MIM of 0.26 fs， and bidirectional repeatability of 8 fs. Additionally， the stage can move at up to 5 times higher speed than the standard TAS model and therefore random scans do not take as much time to perform.