Fast and accurate thin film detection

introduction

Pump probe sampling is a powerful and effective technique primarily used for observing ultrafast processes (fs to ns) in materials and biological systems. Irradiate ultra short laser pulses ("pump" pulses) onto the sample to excite physical processes or chemical reactions within the sample. Subsequently, a second laser pulse with time delay (the "detection" pulse) passes through the sample to measure the changes in the optical properties of the sample caused by the initial excitation. By changing the delay time between the pump pulse and the probe pulse, detailed time records of the sample's response to the pump pulse can be obtained with high time resolution.

Pump probe sampling is particularly useful in the fields of materials science and chemistry, as it helps to understand the fundamental mechanisms of energy transfer, photochemistry, and other important processes. There are multiple methods to achieve high-performance pump probe measurement systems. The following diagram conceptually compares the equipment required to achieve excellent performance of a pump probe device.The K2 Photonics laser solution enables high-performance pump probe measurements in a simple manner, making this pump probe method easy to deploy in practical applications.

Main challenge: Optical delay scanning

Longer pump probe delay is typically necessary for analyzing surface acoustic waves and thermodynamics, and is also applicable in applications such as picosecond ultrasound. The long scanning range can study complex thin film stacks with a total thickness of tens of micrometers, such as those encountered in modern semiconductor microchips. Unfortunately, using mechanical delay lines for such long-distance scanning is slow, and system errors are prone to occur due to beam deflection or divergence, requiring complex optomechanical systems. In addition, the slow optical delay scanning speed requires phase-locked detection of the signal to achieve high sensitivity, which further increases the complexity of the system.

Fast optical delay scanning without moving parts: dual laser method

Asynchronous Optical Sampling (ASOPS) is an alternative method for obtaining long optical delay scans in pump probe measurements. It uses two different light pulse rates, one for pumping light and the other for probing light, which enables precise and fast scanning of the optical delay between the two. This technique is commonly used for ultrafast photoacoustic and other transient absorption studies. The scanning range is determined by the repetition frequency of the pump light, while the scanning speed is determined by the difference in repetition frequency between the pump light and the probe light.

The following table summarizes the key parameters of ASOPS:

Asynchronous optical sampling (ASOPS) is typically achieved using two independent ultrafast lasers that are synchronized through high-frequency phase-locked loops and high bandwidth feedback electronics. The use of Asynchronous Optical Sampling (ASOPS) for precise timing control requires high measurement and feedback bandwidth to achieve femtosecond level accuracy on the time axis.

Fast optical delay scanning without moving parts: single laser method

K2 Photonics has developed a solution for Asynchronous Optical Sampling (ASOPS) technology, which uses a single laser to achieve optical delay scanning without the need fortwoA solitary oneA standing ultrafast laser. This is achieved by generating two pulse sequences within a single laser cavity, each pulse sequence can be used as a pump source and a detection source, respectively.

Compared with traditional dual laser asynchronous optical sampling systems, this single cavity dual comb laser solution has many advantages. Firstly, it greatly simplifies the experimental setup, reduces the required number of components, and makes the system more compact and stable. Secondly, due to the fact that both the pump source and the probe source come from the same laser cavity and have related pulse noise characteristics, the stability of the time axis is improved. This eliminatestwoA solitary onestandThe requirement for electronic feedback loops between vertical lasers greatly enhances the overall stability of the system.

Particle noise limits signal detection capability

K2 Photonics chose solid-state laser technology to create their single cavity dual comb laser system. This technology enables lasers to have ultra-low intensity noise at high frequencies. Typically, for frequencies above 1 MHz, the relative intensity noise (RIN) is below -160 dBc/Hz. This low background noise is particularly advantageous for ASOPS, as most of the signals of interest are located at high frequencies that are not affected by laser noise. In fact, the noise on the signal mainly comes from the scattered particle noise of the probe detecting the photodiode, which is determined only by the probe power and responsiveness of the photodiode. Therefore, using solid-state laser technology in the K2 Photonics system can achieve higher signal-to-noise ratio ASOPS measurements and havehaveThe excellent linear time delay axis makes it a more sensitive tool for ultrafast spectroscopy and other applications.

Advantages of K2 Photonics in Pump Detection Applications

• High precision

The single cavity structure and common noise suppression ensure femtosecond level accuracy on the time axis throughout the entire optical delay scanning process.

• Small size

Without the need for mechanical delay blocks and lock-in amplifiers, the implementation process of high-performance pump probe setup is greatly simplified.

• High sensitivity

A mode-locked solid-state laser produces lasers with ultra-low noise (RIN and timing) at high frequencies, enabling high-sensitivity measurements.