Controlling the Spectral Persistence of a Random Laser

Research Background

Since their theoretical proposal by Letokhov in the 1960s, Random Lasers (hereinafter referred to as RLs) have gradually become a widely researched field. A significant feature of RLs is that they do not require precisely manufactured optical cavities, which gives them considerable advantages in terms of fabrication and scalability. Due to their inherent multi-mode characteristics and low spatial coherence, these lasers show unique advantages in applications such as full-field, non-interferometric imaging. For instance, RLs, which generate coherent light through stimulated emission in light-scattering media, exhibit nonlinear responses and unique spectral fluctuation behaviors that promise potential applications in sensing and imaging. Furthermore, RLs have demonstrated their potential as nonlinear components in complex networks, making them ideal components for optical neural networks.

However, due to the disordered nature of their structure, RLs face challenges such as spectral fluctuations and poor repeatability in practical applications. These challenges are particularly notable in applications requiring high repeatability, such as synchronization in neural networks, where spectral fluctuations significantly affect performance. Hence, controlling the modal stability of RLs and reducing spectral fluctuations has become a current research focus.

Origin and Authors

This paper was published in the 2024 July issue of “Optica” (Volume 11, Issue 7), titled “Controlling the Spectral Persistence of a Random Laser”. The research was carried out by Pedro Moronta, Pedro Tartaj, Antonio Consoli, Pedro David García, Luis Martín Moreno, and Cefe López. The first and corresponding authors are affiliated with the Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), and Universidad Rey Juan Carlos, among others.

Research Process and Experimental Methods

Sample Preparation

The sample preparation involved three main chemicals: sodium salt of salmon DNA, CTMA chloride, and the dye DCM (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran). The sample was designed with a V-shape to facilitate adjustment of the cavity length. The specific process included mixing the sodium salt of salmon DNA with CTMA chloride to obtain the DNA-CTMA complex, then adding the dye DCM to the DNA-CTMA solution, and finally forming a V-shaped structure with rough edges by scraping off TiO2.

Optical Experimental Setup

The experiments used two laser systems: a nanosecond pulse laser and a picosecond pulse laser, emitting 15 ns and 30 ps pulses respectively, both operating at a wavelength of 532 nm. The energy output was controlled by rotating a half-wave plate and a fixed polarizer, and a cylindrical lens was used to generate a strip-shaped beam. Emitted light was collected by a microscope objective from the backside and analyzed by a spectrometer. The experiments measured single-shot lasing spectra at different cavity lengths to evaluate their spectral fluctuation.

Theoretical Model

The Coupled-Mode Theory (CMT) was adopted, with the electromagnetic field of the modes described by the following equation:

[ \frac{d a_k}{d t} = i \delta_k a_k - \alpha_k ak + \sum{j \neq k} c_{k,j} a_j + g(t, \delta_k) \frac{a_k}{1 + \gamma_k |a_k|^2} ]

Specific parameters include the complex amplitude of the mode, central frequency detuning, decay constant, mode coupling coefficient, gain, and saturation coefficient, among others. This model was used to simulate the relationship between different pump pulse widths and mode interaction times, aimed at explaining the experimental results.

Experimental Results and Discussion

Relationship Between Spectral Fluctuations and Cavity Length

In the experiments, researchers changed the cavity length while keeping the pump energy constant and found that the shorter the cavity length, the smaller the single-shot lasing spectral fluctuations. As shown in the figure, for a cavity length of 330±50 µm, the single-shot lasing spectrum was almost fixed. In contrast, for cavity lengths of 1300±50 µm and 1910±50 µm, spectral fluctuations were significant, manifested by changes in the positions of different peaks in the continuous lasing spectrum.

Correlation Coefficient Analysis

By calculating the Pearson correlation coefficients for different cavity lengths, the study found that the shorter the cavity length, the higher the correlation between spectra. When the pump pulse width was longer (15 ns), the spectra for different cavity lengths were almost identical (Pearson correlation coefficient close to 1). For shorter pulse widths (30 ps), only the spectra for shorter cavity lengths exhibited high correlation (correlation coefficient over 0.9).

Simulation Results

Numerical simulations using the CMT model further validated the experimental results. With an increase in the number of modes (equivalent to an increase in cavity length), single-shot spectral fluctuations intensified, and the accumulated spectrum gradually became smoother with a larger baseline signal. This result indicates the importance of the mode interaction time relative to the pump pulse width.

Discussion and Conclusion

The study demonstrated that by controlling the pump pulse width and cavity length, one can effectively regulate the spectral persistence of RLs. When the pump pulse width is sufficiently long, allowing photons to complete enough round trips in the cavity, the competition between modes fully develops, ultimately forming a stable spectral structure.

This finding is significant for fundamental laser physics research and provides a simple and reliable method for mode stability control in practical RL applications. Utilizing this control mechanism, the potential of RLs in synchronization, signal processing, and neural networks will be further explored.

Research Significance and Outlook

This research unveils the transition mechanism of RLs from instability to stability, highlighting the key roles of cavity length and pump pulse width in controlling spectral stability. This not only provides a practical control method for applications but also opens new research directions for future RLs and their applications in complex networks.

The study was funded by the Spanish Ministry of Science and Innovation and several research programs. Research data can be obtained from the corresponding author upon reasonable request, and the Python code used for the simulations is also available.

The success of this research showcases the team’s breakthroughs at the forefront of the random laser field, providing new insights for laser science and application technologies.