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 at many wavelengths simultaneously by a white light supercontinuum probe as sensed by the combination of a spectrograph and camera or photodiode array.  To reduce the effect of laser fluctuations, the sample’s absorption is measured first without the pump, with the optical chopper blocking the beam, and then with the pump unblocked using the subsequent pulse of the laser.  The time between pump and probe is adjusted using a delay stage and the experiment is repeated until the kinetics of the sample is fully known.

The signal in TAS is reported as the change in absorption (OD) ΔA = log10(blocked/unblocked).  When the signal increases upon optical excitation, this often corresponds to ground state (S0) bleach, or a depletion of the population of the ground state, indicating that there are fewer photoreceptors in the ground state available to absorb the probe light.  When the signal is positive, there is less light detected when the sample is excited, which is indicative of excited state absorption (S1 or T1).

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.

TAS benefits from the newly released DL series Delay Line Stages, which are ideal for ultrafast spectroscopy due to the fast speed, acceleration, and repeatable positioning.  The standard maximum optical delay between pump and probe in TAS is 4.3 ns.  This is limited by the length of the stage (325 mm) and the number of optical passes that the probe beam makes to and from the high quality retroreflector (<1 arc sec deviation of return beam) used on the stage.  In standard configurations, there are four passes (to and from the retroreflector twice).  As an option, an additional retroreflector and associated optics can be added to double the maximum pump-probe delay to 8.6 ns.

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 ±1.0 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 1 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 ±2 fs.  In a fast experiment, delay stage settling time is also a consideration, and the positioning may suffer if the settling time is low.

Since the time resolution is set by the convolution of the pump and probe duration --- with some small consideration for crossing angle --- the motion stage is more than sufficient for a pump-probe experiment when pulse widths are greater than 20 fs.  In the case that your experiment requires higher time resolution, the ILS300LM stage and XPS controller can be used instead

TAS comes with easy-to-use LabVIEW based software designed to speed up and simplify data collection with free updates for the lifetime of the product.  The software seamlessly integrates the many devices used in TAS, such as the motion stage(s), camera(s), optical chopper, and spectrograph.   The software can be used to take a cursory UV-Vis-NIR transmission/absorption scan of the sample.  The software features a Setup tab, where you can optimize the probe signal level, processed sample signal, delay line zero, chopper phase, and more on a real-time basis.  After this is complete, data acquisition can be started on the Acquire and Plot tab.  There are options for setting up the scan range and step size, as well as number of averages and selection of random or linear scan.  The estimated time remaining for the scan is estimated initially and updates as the data is acquired.  The data updates live in any of several different plot types, including intensity vs time and / or wavelength cross sections, or two- or three-dimensional colormap.  Previous datasets can be loaded for live comparison of the results.  This can also be accomplished in the Analysis view.  Every data point is saved to disk so that in case of a stopped scan, data is not lost.  Saved data can also be reloaded for later viewing and a copy of the software can easily be run in data viewing mode on an office or home computer.  The standard output file type is .csv with headers, however, fully customizable output file formats are also possible, so loading the data into another software package is made very easy.  Lastly, calibrations and advanced functions are mostly handled on the Diagnostics tab.  Two access levels, Basic and Administrator, are incorporated, so that critical settings are not accidentally changed or saved.

A wide variety of samples can be tested using TAS.  The majority of the samples tested by Newport’s Technology and Application Center are transparent and tested in transmission, with the pump and probe passing through the sample.  It is also possible to test samples in reflection by reconfiguring the probe light collection optics to accept the reflected light.  The most difficult samples to test are those that scatter significantly, since refocusing the light into a fiber, or directly to the slit of the spectrograph, is difficult.  Adding additional lenses, a fiber with a larger core diameter, or even free-space coupling to the spectrograph can help with, but not solve the problem. 

Additionally, another concern is light induced damage or photobleaching from the pump beam.  If the sample has a very long lifetime, it can be repumped to a higher excited state and the dynamics may be different than what you expect.  In some cases, even the probe beam can cause damage, as the total energy can exceed 1 µJ.  Thus, it is usually necessary to move the sample between laser pulses.  If the sample is a non-viscous fluid, it can be stirred or recirculated to replenish the pumped sample volume between laser pulses.  For solid or viscous liquid samples, the sample can be moved to similar effect.  The standard TAS is shipped with a 2 mm path length cuvette and magnetic stirring assembly.  As an option, TAS can be upgraded to include a cell with less path length (down to 100 µm) and a peristaltic pump for recirculating the sample, or add motion stages to move the sample. 

If you are considering the purchase of a TAS system, we will test a sample for you and provide you with the data so that you can evaluate the quality of the system and applicability to your samples.  In general, a good sample will have greater than 10% transmission (or reflection) at every probe wavelength where data is desired.  Additionally, the sample should not be overly scattering since the probe light must be collected and refocused into a fiber for collection.

Below is an image of some of the sample data that we’ve collected with TAS.  It shows a sample of gold nanorods.  In our lab, TAS has been used to help researchers publish papers on the matter.  We have also tested nanoparticle, photovoltaic, photosynthetic, and photoinitiator samples, amongst others.  Please inquire for a list of papers published using our systems.