In comparison to traditional one-dimensional spectroscopy, where absorption or emission is measured as a function of energy (typically wavelength or frequency), two-dimensional spectroscopy allows analysis of the spectrum along two energy axes, exposing detail that is not present in one-dimension.  These details consist of connections between electronic transitions, exciton – exciton coherences, population transfer, separate measurements of the homogeneous and inhomogeneous linewidth, or structural change to the molecules of interest.  In a complex system, where there is an ensemble of different molecular configurations, this allows for tracking the evolution of subsets of the ensemble in time, whereas the one-dimensional spectrum is only the average of the ensemble.   In other words, one can track the changes to absorption or emission of a sample at a given probe frequency caused by excitation in a narrow bandwidth.  

Two-dimensional spectroscopy with ultraviolet, visible, or infrared pulses in the time domain is only possible with the use of femtosecond pulses.  The first pulse creates a coherent superposition between the ground and excited state of the sample, which oscillates and decays over the course of the coherence time, often denoted as τ.  The second pulse converts and stores the superposition as a population in the ground and excited state.  After a waiting time, denoted as T, another pulse, or series of pulses, recreates a superposition of states, which emits radiation that is typically measured in the spectral domain.  The time between the third pulse and the measurement of the signal is referred to as the detection time and is denoted as t.  Often, the data is converted to the frequency domain by Fourier transform with respect to τ and plotted versus the detection frequency. 

It is important that there is a known and controllable phase relation between electric fields of the first two pulses.  It is also important to have phase-resolved detection for the emitted signal, which also entails a high degree of phase stability.  With visible pulses, this corresponds to less than a femtosecond stability between the pulses.  It is then required to use a pulse shaper to form the first two pulses and in some cases to have active measurement or control of the relative phase of the pulses.  The same can be said for the last two pulses, however, as previously discussed, when recorded in the frequency domain by interference , the phase stability is passively ensured.  

2DSpec requires the use of Fastlite's Dazzler™ acousto-optic programmable dispersive filter to form the first two "pump" pulses with high precision in the control of relative phase and delay.  The pump pulses take the optical path; they are collinear.  The third pulse, or "probe," is crossed at a small angle with the pumps and the emitted signal is collinear and phase matched with the probe beam for subsequent heterodyne detection, with the probe serving as the local oscillator. Other 2D implementations use non-collinear phase matching of all pulses.   While there are some advantages to this technique, the experimental implementation and resulting data analysis are much more difficult.  In the "pump-probe" geometry the "absorptive," narrowed, line shape is directly extracted after Fourier transform.  It is also possible to implement phase-cycling to extract other signals of interest. 

2DSpec benefits from the newly released DL series stages, which are ideal for ultrafast spectroscopy due to the fast velocity, acceleration, and repeatable positioning.  The standard maximum optical delay between pump and probe in 2DSpec 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 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 2DSpec 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 2DSpec 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 2DSpec, 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.

The 2DSpec is designed with flexibility in mind.  The basic system comes with the essential components required for ultrafast spectroscopy; however, almost every system that is shipped is specially built and tailored to meet the customers’ needs.  Some things that are customized or upgraded are:

1. Pump-probe delay step size (down to 0.13 fs)
2. Addition of ultraviolet probe (CaF2 crystal) and motion system
3. Reference channels for monitoring and correcting probe fluctuations
4. Automated system alignment/realignment using patented GuideStar™ technology
5. Sample – moving stage or thin flow cell with pump
6. Additional gratings for higher spectral resolution / range
7. Transient Absorption (TAS)
8. Femtosecond Stimulated Raman Spectroscopy (FSRS)

Newport scientists and engineers are always working to support new additions and types of pump-probe spectroscopy, so, if you have an idea that you don’t see listed, feel free to inquire. Most features are offered as upgrades and can be added in the future as they are required, or as budget allows.