Exploration of novel optical process: designable spatially coherent wideband radiation
Frequency conversion is always the research theme in optics, and beams with spatial coherence are of great significance. Here a novel designable radiation process by the synergistic effect of broadened phonons and absorption modes opens a new chapter in optical-frequency conversion technology.
In condensed matter systems, the interaction modes between particles and quasiparticles are rich, among which the most well-known mode is phonon-photon interaction. Mediated by the radiation process of the excitation center in the lattice, the phonon modes corresponding to the normal vibrational modals of the lattice matrix take participation in the radiation process and directly affect the wavelength and momentum of the radiated photons. This is so-called phonon-assisted radiation process. Common raman scattering can directly reflect the vibrational mode of the solid matrix. Later discovery of more complex phonon-assisted radiation processes, e.g. resonance raman, phonon side band emission, further proves the importance of phonon-assisted radiation (1,2).
Then several critical question arise: if the state density of various phonon modes in the lattice matrix is similar, are all these phonon modes likely to participate in the de-excitation process? What characteristics does the radiation process possess under such circumstances? Suppose mode competitions are present, what rules should be obeyed? With these questions in mind, our team members led by Prof. Linde Zhang have explored the answers for almost ten years. From a theoretical view, the answer to the first critical question is definitely “yes”，but answers to other questions are not known yet. From an experimental view, how to confirm the “yes” answer and disclose other answers is really a challenge.
It is indeed a tough process to explore the answers, during which trying and repeating different experiments is the main theme. We investigated various lattice matrices in the early stage. It was found that the phonon modes mainly involved in the radiation process are optical branches or other high-frequency modes. The corresponding radiation processes differ little from conventional ones like fluorescence, phosphorescence and raman. To be honest, the progress in the exploring research is so slow that we have experienced upset, self-doubt or even depression during the trial and error process.
Then we cannot help asking ourselves: whether are the original ideas wrong? How can we design materials and experiments to support our proposals? Shall we give up? Luckily, in 2019, a ray of light shot on us, who were still suffering the exploration in the dark. We noticed uncommon strong infrared absorption in the glass system doped with phosphorus, sulfur, silicon and other components. In the case of some specific concentration ratios, the optical path for infrared test couldn’t receive any signals at all. In contrast, common glass samples with comparable thickness were almost transparent to infrared lights. When the formation was adjusted, i.e., the concentration of one component was highly raised, the infrared absorption characteristic of the multi-component system would be similar with common glass samples. We realize that the phonon modes in this multi-component material are likely to exhibit anomalous broadening, which may lead to interesting radiation processes. Combined with the research on high-entropy alloys, we tend to believe that the desired phonon broadening may result from the high entropy of the multi-component glass system.
Finally, we designed the glass rods based on the close packing of oxides. From a mesoscopic view, many tetrahedral and octahedral voids were formed through close packing of O2-. Then we applied the strategy of high entropy alloys to fill the tetrahedral and octahedral voids with different ions. After trying different formulas and filling ions, we obtained the desired glass material, named as high entropy glass system (HEGS). The absorption spectrum of the HEGS indicated the presence of high-frequency optical phonons or allowable multi-phonon processes in the system, which made it exhibit a much stronger and wider infrared absorption than conventional glass system.
We doped neodymium ions as the radiation center in this system and studied its de-excitation behaviors in detail. The HEGS showed that pumped by with laser of 520 nm, blue-shifted and red-shifted emission came out. The preliminary observation really excited us! We knew that we have broken the research bottleneck.
Figure 1. Excitation of 520 nm laser generating blue-shifted and red-shifted emission
Based on the optimized HEGS, we identify a radiative de-excitation process consisting of a broadened-phonon-assisted wideband radiation (BPAWR) process and a subsequent self-absorption coherence modulation (SACM) process, i.e., BPAWR-SACM. The BPAWR results from the high-frequency phonon modes or allowable multi-phonon process in the HEGS. The wideband radiation is permitted to propagate only in the non-absorption band of the medium, which consists of the blue-shifted band produced by the phonons' self-absorption and the red-shifted band produced by the phonons’ emission. This process leads to SACM. For more details, please refer to the article “Design of coherent wideband radiation process in a Nd3+-doped high entropy glass system” published in Light: Science & Applications. Link: https://www.nature.com/articles/s41377-022-00848-y%C2%A0.
Figure 2. Mechanism of BPAWR-SACM
Overturning the laser forming paradigm, the synergetic effect of the broadened phonons and absorption modes contributes to the desired emission spectra, which can be tuned by changing excitation wavelengths, sample size, and doping concentrations of rare earth ions. Further experiments prove that through combining the line shape of the absorption spectrum and the wavelengths of pump lasers, obtaining spatially coherent light of any desired wavelengths is possible. This means a new chapter in optical-frequency conversion technology has been opened.
The potential applications of BPAWR-SACM endow the discovery with more significance, which are our research focus for next steps. With a continuous-wave (CW) laser as a pump source, a modulated emission with wide-spectrum and spatial coherence can be obtained by designing the absorption spectrum of the medium. Compared with traditional supercontinuum lasers, utilizing the BPAWR-SACM process is a more direct way to obtain CW white light lasers. The repetition frequency and pulse width of white light lasers can be tuned by a Q-switched or mode-locked way, converting the white light lasers to short pulse light for more application requirements. Furthermore, the energy transfer and amplification of the ultrashort pulse can also be carried out by BPAWR-SACM. The energy of single pulse can be boosted by combining chirped pulse amplification (CPA) technology.
In all, combining with optical parametric oscillators/amplifiers (OPO/OPA) technology, the amplification and combination of BPAWR-SACM has the potential to generate mid-/far- infrared beams with high intensity and spatial coherence. In addition, frequency modulation in the visible light range is possible, thereby realizing the wavelength division multiplexing communication technology in this range. This means the light source is designable and tunable from the perspectives of frequency, power density, repetition frequency and pulse width.
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- Chernikov, A. et al. Phonon-assisted luminescence of polar semiconductors: fröhlich coupling versus deformation-potential scattering. Physical Review B 85, 035201 (2012).