Pump-probe sampling

Rapid and precise thin-film inspection.

Introduction

Pump-probe sampling is a powerful technique used to observe ultrafast processes (fs to ns) in materials and biological systems. It involves shining a brief, intense pulse of laser light onto a sample (the “pump” pulse), which excites the sample and initiates a physical process or reaction. A time-delayed second pulse of laser light (the “probe” pulse) is sent through the sample to measure changes in its optical properties that occurred because of the initial excitation. By varying the delay between the pump and probe pulses, a detailed temporal record of the sample’s response to the pump pulse with high temporal resolution is obtained.

Pump-probe sampling is particularly useful in materials science and chemistry, where it can help understand the fundamental mechanisms underlying energy transfer, photochemistry, and other important processes. There are multiple ways to implement a high-performance pump-probe measurement system. The figure below conceptually compares the elements needed to obtain a state-of-the-art performance pump-probe setup. K2 Photonics laser solution allows to obtain a simple implementation for the pump-probe measurements with the highest performance, making such pump-probe methods easy to deploy in practice.

The key challenge: optical delay scan

Long pump-probe delays are often necessary to resolve surface acoustic waves and thermal dynamics, and in applications such as picosecond ultrasonics. The long scan range enables the study of complex thin-film stacks with several tens of micrometer total thickness, such as those encountered in modern semiconductor microchips. Unfortunately, scanning over such long distances with a mechanical delay line is slow, susceptible to systematic errors due to beam deflection or divergence, and requires a complex optomechanical system. Moreover, the slow optical delay scan speed necessitates lock-in detection of the signal to obtain high-sensitivity, adding further complexity to the system.

Rapid optical delay sweeps without moving parts: two laser approach

ASOPS is an alternative method to obtain long optical delay scans in pump-probe measurements. It uses two different optical pulse rates, one for the pump and one for the probe, which allows for precise and fast scanning of the optical delay between them. This technique is commonly used in ultrafast photoacoustic and other transient absorption studies. The range of the scan is determined by the pump repetition rate, and the scan speed is determined by the difference between the pump and probe repetition rates.

The key ASOPS parameters are summarized in the table below:

Parameter	Variable
Pump repetition rate	ƒ_rep,pump
Probe repetition rate	ƒ_rep,probe
Repetition rate difference	Δƒ_rep = \| ƒ_rep,pump — ƒ_rep,probe \|
Delay scan range	1 / ƒ_rep,pump
Delay sweep time	1 / Δƒ_rep
Delay scan step	τ ≈ Δƒ_rep/ ƒ_rep²
Measurement bandwidth	BW, typically, up to ƒ_rep / 2
Time step resolution	τ ≈ Δƒ_rep / (ƒ_repBW)

ASOPS is often implemented using two separate ultrafast lasers that are synchronized via high-frequency phase-locked loops and high-bandwidth feedback electronics. Achieving precise timing control using ASOPS requires high measurement and feedback bandwidth to obtain femtosecond-level precision on the time axis.

Rapid optical delay sweeps without moving parts: single laser approach

K2 Photonics has developed a unique solution for ASOPS that uses a single laser to achieve optical delay scans without the need for two separate ultrafast lasers. This is achieved by generating two pulse trains within a single laser cavity, each of which can be used as the pump and probe source, respectively.

This single-cavity dual-comb laser solution provides several advantages over traditional two-laser ASOPS systems. Firstly, it greatly simplifies the experimental setup, reducing the number of components required and leading to a more compact and stable system. Secondly, it enables improved time axis stability as both pump and probe sources are generated from the same laser cavity and thus have correlated pulse noise characteristics. This suppresses the need for electronic feedback loops between two separate lasers and greatly improves the overall stability of the system.

Shot-noise limited signal detection capability

K2 Photonics has opted for solid-state laser technology to create their single-cavity dual-comb laser system. This technology enables laser light with an ultra-low intensity noise at high frequencies. Typically, the relative intensity noise (RIN) is below -160 dBc/Hz for frequencies above 1 MHz. This low noise floor is especially advantageous for ASOPS, as most of the signal of interest lies at high frequencies that are not affected by the laser noise. In fact, the noise on the signal is mainly originating from the shot noise of the probe detecting photodiode, which is determined only by the probe power and responsivity of the photodiode. As a result, the use of solid-state laser technology in the K2 Photonics system results in a higher signal-to-noise ratio ASOPS measurement with a perfectly linear time delay axis, making it a more sensitive tool for ultrafast spectroscopy and other applications.

K2 Photonics value proposition for pump-probe applications

Various pump-probe study examples with our technology are showcased in the Resources section.

Resource 1: Ultrafast photoacoustics

This publication showcases the use of a mode-locked solid-state dual-comb laser for ultrafast pump-probe measurements, with a specific focus on its application in ultrafast photoacoustics studies. The authors demonstrate the effectiveness of this technique by measuring the thickness of tungsten layers on a silicon wafer, achieving sub-nanometer precision. This approach offers a fast, sensitive, and precise method for pump-probe measurements, making it a valuable tool for a wide range of applications, especially those involving ultrasonic echoes, Brillouin oscillations, surface acoustic waves, and thermal dynamics.

Resource 2: Arbitrary on-demand optical delay generation

This publication describes a new technique for generating arbitrarily timed pairs of wavelength-tunable pulses using time filtering of femtosecond frequency combs by pulse gating in a laser amplifier. The technique allows for on-demand optical delays without the need for optomechanical lines or asynchronous scans. Using a dual-channel millijoule amplifier, the programmable generation of both extremely short and long interpulse delays is demonstrated. The versatility of the method is confirmed by measuring the cross-correlation and multicomponent population recovery kinetics. The technique has potential applications in pump-probe spectroscopy and other ultrafast optical studies.

Pump-probe sampling

Rapid and precise thin-film inspection.

Introduction

The key challenge: optical delay scan

Rapid optical delay sweeps without moving parts: two laser approach

Rapid optical delay sweeps without moving parts: single laser approach

Shot-noise limited signal detection capability

K2 Photonics value proposition for pump-probe applications

Let's explore the most innovative possibilities of laser technology.

Contacts

Useful links

Pump-probe sampling

Rapid and precise thin-film inspection.

Introduction

The key challenge: optical delay scan

Rapid optical delay sweeps without moving parts: two laser approach

Rapid optical delay sweeps without moving parts: single laser approach

Shot-noise limited signal detection capability

K2 Photonics value proposition for pump-probe applications

Speed

Precision

Compactness

Sensitivity

Let's explore the most innovative possibilities of laser technology.

Contacts

Useful links