Laser tuning in AlN single crystals

In the fields of trace gas detection, electric field detection, visible optical communication, and beyond, laser tuning technology plays an increasingly important role, as shown in Fig. S1. Following the miniaturization of optoelectronic devices, it is desirable to integrate a large number of lasers with different wavelengths into one chip [1]. However, the dynamic wavelength allocation efficiency of fixedwavelength laser is quite low, which greatly limits the monolithic integration of optoelectronics systems. Therefore, the development of laser wavelength tuning technology is urgent and crucial to the efficiency of optoelectronics [2–7]. Raman scattering is a physical phenomenon that can be explained by mature normal form [8,9]. However, most studies only pay attention to the fact that Raman spectrum is a non-destructive material characterization method with fingerprint identification characteristics, while its essential feature of inelastic scattering is always ignored [10]. The natural frequency shift characteristics indicate the possibility of Raman scattering as a feasible method to realize laser wavelength tuning [11–13]. An important prerequisite for achieving laser wavelength tuning by the Raman effect is the scattering light with polarization characteristics dependent on laser medium [14]. For isotropic optical crystals, the Raman scattering signal is often accompanied by depolarization phenomenon. Thus, optical crystals with strong anisotropy may be an ideal carrier for laser wavelength tuning. Here, we adopted wurtzite aluminum nitride (AlN) as the laser medium. As a mature binary semiconductor, AlN has a relatively simple phonon dispersion relationship compared with other multicomponent systems, which directly determines a “clean” Raman spectrum [15–18]. In addition, high-quality and large-size AlN single crystals (SCs) with different crystal planes have been fabricated via physical vapor transport (PVT) technology [19,20], which avoids the interference of Raman signals from other crystal planes. In this letter, we discovered that when the polarization direction of incident light is along the c-axis of AlN SC, an “extinction phenomenon” will appear in E1(TO) and E2 2 modes, and only A1(TO) mode can be observed. In this situation, the scattering light corresponding to A1(TO) mode shows perfect linear polarization characteristics with a degree of polarization I I I I (( ) / ( + )) approaching 1. This phenomenon indicates that AlN can realize laser frequency shift with the help of Raman scattering. In addition, based on the theory that phonon frequency is limited by temperature, an ultra-high precision laser wavelength tuning (0.0006 nm K) has been realized by controlling the temperature [21,22]. Our experimental results indicate that wurtzite AlN with high thermal conductivity and high stability may be an ideal carrier for laser wavelength tuning [23]. Theoretically speaking, changing laser medium can achieve arbitrary wavelength tuning, which may help with the development of integrated sensing field. Here, we adopted backscattering geometry to collect the angle-resolved polarization Raman signal of m-plane AlN SC with Renishaw Raman spectrometer (inVia Reflex). The measurement geometry is shown in Fig. 1a [24–34], where a polarizer with fixed polarization direction was placed in the collecting optical path to ensure that the polarization direction of scattering light was strictly consistent with that of pump light. In order to avoid the influences of pump light absorption and resonance Raman scattering on the intensity of scattering light, 488-nm Ar+ gas laser was used as pump light and focused on the AlN surface through a 50× quartz lens [35–39]. Meanwhile, in order to smooth the Raman signal and prevent the sample from being damaged, exposure time and integration time were set to 1 s and 10 times, respectively. Previously, we have reported an angle-resolved polarization Raman spectra of AlN microwires with a diameter of 200 μm under the excitation of a 532-nm laser, in