pump–probe measurements (original) (raw)

Definition: techniques for investigating ultrafast phenomena, where a pump pulse excites a sample and a probe pulse is used for probing the sample after an adjustable delay time

Categories: optical metrology, methods

Related: time-resolved spectroscopyultrafast opticssaturable absorberssemiconductor saturable absorber mirrorsoptical samplingsynchronization of laserslaser spectroscopy

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DOI: 10.61835/p9f Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Pump–probe Measurements?

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 probe 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).

Figure 1: Reflectance change in a semiconductor saturable absorber, hit by a short pulse at ($t = 0$).

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 vary 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.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is the principle of pump–probe measurements?

A strong 'pump' pulse excites a sample, and a weaker 'probe' pulse, arriving after a variable time delay, measures changes in the sample's transmittance or reflectance. Varying the delay allows one to track the evolution of the excitation on ultrafast timescales.

What limits the time resolution of pump–probe measurements?

The temporal resolution is fundamentally limited by the pulse duration of the pump and probe pulses. A fast photodetector is not required for achieving high temporal resolution.

What are two-color pump–probe measurements?

These are measurements where the pump and probe beams have different wavelengths. This requires two synchronized sources of short pulses and provides additional capabilities for laser spectroscopy, such as probing different energy transitions than were excited.

What are pump–probe measurements used for?

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains nine suppliers for pump–probe measurements. Among them:

⚙ hardware

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.

⚙ hardware

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.

⚙ hardware

The K2-ASOPS is a 60 MHz femtosecond laser optimized for pump–probe spectroscopy, enabling precise ultrafast measurements without the need for mechanical delay lines. Utilizing a patented single-cavity dual-comb architecture, it generates two highly stable pulse trains with a freely tunable repetition rate difference, allowing for asynchronous optical sampling (ASOPS) with exceptional timing precision.

Key advantages of pump–probe spectroscopy:

• Freely tunable repetition rate difference for optimal ultrafast signal sampling.
• Eliminates mechanical delay stages — improves stability, speed, and precision.
• Ultra-low relative timing noise — even when free-running due to shared cavity architecture.
• Compact, quasi-monolithic design — robust and reliable for both research and industrial applications.

Ideal for semiconductor wafer inspection, ultrafast material science, and molecular dynamics, the K2-ASOPS delivers the next-generation solution for high-resolution pump-probe spectroscopy.

⚙ hardware

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 complement our family of ultrafast optics and laser systems, including low-GDD beamsplitters, to serve various pump-probe applications.

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