Self-frequency-conversion nanowire lasers

Nanowire (NW) lasers with the NW simultaneously acting as gain medium and optical cavity are attractive for miniaturized photonic integration systems, which have merits of ultracompact footprint, easy integration, low energy consumption, and potential large-scale production. After two decades of development, NW lasers have been demonstrated in a variety of materials, such as III-V and II-VI compound semiconductors, and perovskites. And the introduction of quantum wells and quantum dots in NWs can further reduce the lasing threshold and improve the temperature stability of NW lasers. In recent years, the development of selective area epitaxy enables NW array lasers with high output power. These achievements greatly promote the applications of NW lasers, including optical interconnects, sensing, displaying, and microscopy.

Unfortunately, limited by specific gain spectral range inherent in semiconductor, there are still some awkward wavelengths that cannot be achieved from NW lasers, thereby narrowing their application. Although the emission peaks of semiconductor NW could be theoretically shift over a broad range by controlling the alloy composition, there are many material-related challenges to the NW growth. For example, green InGaN NW lasers are still not realized due to the high density of dislocations in the high-indium content quantum well.

In the history of the development of lasers employed in laboratory and industry, the laser operation wavelength range is also limited by the gain medium. But now, the commercial lasers could provide coherent output ranging from ultraviolet to THz. One of the most critical technologies is to utilize an external nonlinear optical crystal to achieve nonlinear optical parametric conversions, including second-harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG), optical parametric amplifier (OPA), optical parametric oscillation (OPO), etc. For example, the high-power green laser at 532 nm is typically realized by frequency-doubling the Nd:YAG laser at 1064 nm via the SHG. In addition, based on OPO or OPA, lasers could be continuously tuned beyond 1000 nm in the near infrared range. These successes inspire the utilization of nonlinear frequency conversion to extend the output wavelength range of NW lasers.

In a new paper published in Light Science & Application, a research group, led by Professor Xuetao Gan and Professor Jianlin Zhao from Key Laboratory of Light Field Manipulation and Information Acquisition, Ministry of Industry and Information Technology, and Shaanxi Key Laboratory of Optical Information Technology, School of Physical Science and Technology, Northwestern Polytechnical University, China and their collaborators have proposed that the working wavelength of NW laser could be expanded by utilizing the second-order nonlinear effect of NW crystal. However, unlike traditional solutions using an external non-linear crystal, the proposed solution exploits the high non-linearities of III-V NW itself and the strongly localized optical field within the laser cavity to achieve frequency conversion (as shown in Figure 1). And the second-order nonlinear optical processes could be self-realized effectively. Note, it is impractical to employ NW lasers as the source to pump another nonlinear crystal to achieve frequency conversion due to the low output power of NW lasers.

In our experiment, high-quality GaAs/In_0.16Ga_0.84As core/shell NWs were grown by selective area epitaxy (SAE) method and transferred to a SiO₂/Si substrate. As shown in Figure 2a, the two flat end facets function as two parallel mirrors, and form a Fabry–Pérot cavity as the optical cavity for the laser. From a NW with a single lasing mode at 1016 nm, a strong visible lasing peak at 508 nm is also obtained (Figure 2b). By analyzing the far-field polarization dependence with numerical modelling (Figure 2c) and the intensity dependence (Figure 2d), it is confirmed that the visible laser mode at 508 nm is generated via doubling the frequency of the fundamental lasing mode (SHG), i.e., the self-frequency-conversion process.

Besides SHG, the second-order nonlinear effect of III-V NW also support other three-wave mixing processes, such as SFG, DFG, etc., which could also be utilized to extend the output wavelength of NW lasers. In the experiment, multimode lasing in near-infrared is demonstrated in a NW with larger diameter, and multiple lasing peaks are obtained in the visible wavelength range (shown in Figure 3). According to the calculation of the frequency conversion and corresponding experiment, the lasing modes at these wavelengths could be directly correlated to the SHG and SFG signals of the fundamental lasing modes. Note that the three-wave-mixing processes of these fundamental lasing modes also include their DFGs, which could generate signals at the mid-infrared wavelength range. For example, the corresponding DFG of λ₂ and λ₄ (ω₂-ω₄) is expected at the wavelength of 6678 nm. However, limited by the spectrum range of test instruments, the researchers did not observe the DFG signal. Further reducing the frequency difference of two fundamental lasing modes, the frequency of DFG could be redshifted even to THz. Finally, the authors believed that by exploiting the second nonlinear effect of the NWs, the proposed self-frequency-conversion NW lasers provide a new path for expanding the working wavelength of these lasers, with potentially applications in optical parametric generators, amplifiers, and oscillators operating over a large wavelength range from deep ultraviolet to THz.

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