Fig. 1. Normalized device volume by the wavelength cubed versus the occurrence time of different types of lasers. EEL:Edge-Emitting Laser; VCSEL: Vertical Cavity Surface Emitting Laser; MD laser: Microdisk laser; PC laser: Photonic Crystal laser; NW laser: Nanowire laser; P-laser/SPASER: Plasmonic Laser/Surface Plasmon Amplification by the Stimulated Emission of Radiation^[21]

Fig. 2. Schematic of typical VCSEL device^[27]

Fig. 3. Schematic of VCSEL structure with high contrast grating^[29]

Fig. 4. Structure diagram of electrically pumped photonic crystal. (a) Schematic of photonic crystal laser; (b) image of a photonic crystal defect cavity laser by scanning electron microscopy^[36]

Fig. 5. Laser resonators composed of ZnO semiconductor nanowire arrays^[12]. (a)-(e) ZnO nanowire arrays imaged by SEM; (f) morphology of single nanowire imaged by SEM

Fig. 6. Deep subwavelength plasmonic laser^[44]. (a) Plasmonic laser consists of CdS semiconductor nanowires and a silver substrate, with nanoscale MgF₂ in the middle; (b) stimulated electric field distribution; (c) CCD imaging, spectrogram, input-output curve (logarithmic), and linewidth variation of the spectrum

Fig. 7. Structure diagram of nanorod laser^[45-46]. (a) Nanolaser composed of a GaN nanowire and silver substrate; (b) schematic of the spectroscopically tunable nanorod laser; (c) field distribution obtained by simulation; (d) all-color, single-mode lasing spectra obtained from single nanorods

Fig. 8. Principle of a cavity-free laser based on subwavelength light localization^[50]

Fig. 9. Metal-coated semiconductor cavity heterojunction nanolaser (the upper right images are SEM images of the corresponding device)^[51-52]. (a) Cross-section of a rectangular heterojunction nanolaser; (b) cross-section of a circular nanometer laser

Fig. 10. Various types of cladding lasers^[55-58]. (a)(e) InP nanodisk laser; (b)(f) silver wrapped InP/InAsP/InP quantum well gain layer laser; (c)(g) nanoscale square laser; (d)(h) coaxial metal-clad nanometer laser

Fig. 11. V-shaped structure diagram of gain media wrapped around the metal core^[61]

Fig. 12. The core shell structure of the SPASER and its lasing characteristics^[65]. (a) Schematic of the core shell structure; (b) electric field intensity distribution of the SPASER; (c) coupling between surface plasmon mode and gain mode and the corresponding stimulated emission spectrum

Fig. 13. Ag nano-pillar array and the modes it supports ^[70]. (a) Structure diagram; (b) distribution diagram of dark mode field; (c) distribution diagram of bright mode field

Fig. 14. One-dimensional photonic crystal laser composed of two-dimensional materials^[73]. (a) Crystal structure of MoTe₂; (b) schematic of the device; (c) SEM images of a photonic crystal nanocavity

Fig. 15. Input-output curves (in logarithmic form) of lasers with different β values ^[80]

Fig. 16. Different definition methods of threshold^[80,82-84]. (a) Input-output curve method; (b) mean photon number in lasing mode method; (c) Fano factor method; (d) second order correlation function method

Fig. 17. g⁽²⁾(τ) and input-output curves of resonators with different diameters. (a) g⁽²⁾(τ) and input-output curves of resonator with 1.5 μm diameter; (b) g⁽²⁾(τ) and input-output curves of resonator with 5 μm diameter

Fig. 18. g⁽²⁾(0) (red curve) and input-output curve of the laser^[93]

Fig. 19. Research on threshold behavior of nanolaser according to the full quantum laser model^[94]. (a) Cavity quantum dynamics model; (b) experimental schematic of second-order correlation function characterization; (c) g⁽²⁾(0) characterization results of different types of light sources

Fig. 20. Two coupled photonic crystal nanolasers. (a) Pump diagram^[108]; (b) corresponding obtained spectrogram^[108]; (c) the second order correlation function obtained by pumping the two resonators is characterized ^[112]

Fig. 21. Nanolaser with double heterojunction ridged waveguide structure^[121]. (a) Schematic of structure composition; (b) SEM of heterogeneous structure

Fig. 22. Etched cylindrical electrically pumped nanolaser^[122]. (a) Schematic of structure; (b) different designs of Bragg mirrors

Fig. 23. Different schemes for the integration of nanolasers with waveguides^[123-126]. (a) Metal-cavity nanolaser and silicon-based waveguide coupling scheme; (b) laser and waveguide coupling scheme realized by CMOS technology; (c) nanowire laser and V-channel plasmonic coupling scheme; (d) field distribution of nanowire laser and V-channel plasmonic coupling