Pump–probe Measurements

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

pump–probe measurements can be used to obtain information on ultrafast phenomena, typically on picosecond or femtosecond time scales. The general principle is the following:

• The investigated object is hit by some pump pulse, which generates some kind of excitation (or other modification) in the sample.
• After an adjustable time delay, a typically weaker probe pulse hits the sample, and its transmittance or reflectance is measured. That reveals to which extent the sample is still affected by the pump pulse at that time.
• By monitoring the probe signal as a function of the time delay, it is possible to obtain information on the decay of the generated excitation, or on other processes initiated by the pump pulses.

The pump and probe pulses are in most cases derived from a common source, e.g. from a mode-locked laser and a beam splitter. The relative timing is controlled with an optical delay line. The change in time delay is directly inferred from the change in path length of the delay line, e.g. using an interferometric sensor.

Note that a fast photodetector is not required; the temporal resolution is fundamentally limited only by the pulse duration of pump and probe pulses.

In some cases, the wavelengths of pump and probe beam are not identical. A so-called two-color pump–probe measurement, based on two synchronized sources of short pulses (e.g. a laser and an optical parametric oscillator, or two parametric oscillators pumped with the same laser) has additional capabilities in ultrafast laser spectroscopy. One may then perform pump–probe measurements with different pump and signal wavelengths. It is vital in such cases to ensure tight synchronization of the different laser sources with a very low relative timing jitter, as the jitter could otherwise spoil the attainable temporal resolution.

The probe signal is often quite weak. It may be averaged over many pulses to obtain a better signal-to-noise ratio, assuming that the behavior of the tested object is fully reproducible. That is often the case if the applied pulse intensities are not too high.

Examples of Applications of pump–probe Measurements

Characterization of Saturable Absorbers

pump–probe measurements can be used, for example, to monitor the recovery of a saturable absorber (e.g. a SESAM) after its excitation. For SESAMs, one often observes a rather fast partial recovery, followed by a slower complete recovery of the saturable loss (see Figure 1).

Photodiodes

The speed of a photodiode can depend on various factors, one of them being the diffusion of photoexcited carriers. For investigating that, one may also very the exact position where the probe pulses are applied.

Laser Material Processing

In laser material processing, e.g. involving laser ablation, the occurring processes may be rather complicated. To some extent, they can be investigated with pump–probe measurements. For example, one may find out during which time interval after an intense pulse the material is found in liquid form.

Time-resolved Spectroscopy

pump–probe methods are also used in various methods of time-resolved spectroscopy. Obviously, the addition of the temporal dimension to the spectral one greatly expands the opportunities for investigating systems.

More to Learn

Encyclopedia articles:

• time-resolved spectroscopy
• ultrafast optics
• saturable absorbers
• semiconductor saturable absorber mirrors
• optical sampling

Suppliers

The RP Photonics Buyer's Guide contains nine suppliers for pump–probe measurement equipment. Among them:

TOPTICA Photonics

TOPTICA contributes to the field of pump–probe measurements with newest fiber technology which is outstanding in performance and versatility. A single laser system can be equipped with various outputs that are intrinsically optically synchronized down to the attosecond level. All of these can be configured individually to satisfy the experimental needs: Pulse durations as short as 15 fs or broad tuning ranges of 490 nm to 700 nm, 850 nm to 1000 nm or 980 nm to 2200 nm are available. Within the third generation of TOPTICA’s ultrafast fiber lasers a novel, versatile toolbox is available to realize even more advanced pump–probe spectroscopy schemes. The FemtoFiber dichro bioMP outputs two different wavelength simultaneously (1050/780 nm at 150 fs), while the system can control the relative time delay of both colors and the GDD as well.

Thorlabs

Ready-to-use optical delay lines manufactured by Thorlabs include the drive system necessary for computer-controlled variation of optical path lengths. With delays up to 4,000 ps and time resolution down to 0.67 fs, these systems compliment our family of ultrafast optics and laser systems, including low-GDD beamsplitters, to serve various pump-probe applications.

APE

The APE ScanDelay is a variable optical delay line, allowing the introduction of a well-defined time delay into an optical path – for example, for pump–probe measurements. The delay is periodically controlled by a fast scanning shaker at a frequency of up to 20 Hz.

The heart of APE’s optical delay line series is a special linear translation stage that is supplied together with appropriate control and drive electronics. The linear drive has been designed especially for optical applications. It combines low moving mass with compactness, reaching a high speed, as well as high precision and resolution. The delay drive moves frictionless, has a large and precise travel range, and allows for very small displacements without any stick-slip-effects.

The control electronic contains the motion driver and a quartz stabilized signal synthesizer. It can be synchronized with an external clock for a precise, phase-locked scanner movement.

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