This paper is going to review the state-of-the-art of the high-speed 850/940-nm vertical cavity surface emitting laser (VCSEL), discussing the structural design, mode control and the related data transmission performance. InGaAs/AlGaAs multiple quantum well (MQW) was used to increase the differential gain and photon density in VCSEL. The multiple oxide layers and oxide-confined aperture were well designed in VCSEL to decrease the parasitic capacitance and generate single mode (SM) VCSEL. The maximal modulation bandwidth of 30 GHz was achieved with well-designed VCSEL structure. At the end of the paper, other applications of the near-infrared VCSELs are discussed.

Enhanced nonlinear refractive indices and absorption coefficients of nonstoichiometric Si_xC_1−x with varying C/Si composition ratios from 0.51 to 1.83 grown by low-temperature plasma-enhanced chemical vapor deposition are demonstrated. When increasing the [CH₄]/[CH₄ + SiH₄] fluence ratio from 70% to 92%, two Raman scattering signals at 795 and 970 cm⁻¹ contributed by transverse and longitudinal optical modes of Si–C bonds are simultaneously enhanced in stoichiometric and C-rich Si_xC_1−x samples, whereas the Si-associated Raman peak at 520 cm⁻¹ decreases with increasing C/Si composition ratio. The C-rich Si_xC_1−x film shows additional Raman scattering peaks at 1330 and 1580 cm⁻¹, which are individually contributed by the diamond-like C–C bonds and the G peak of the graphite-like C–C bonds, respectively. The nonlinear refractive index increases to 1 ± 0.1 × 10⁻¹¹ cm² W⁻¹ upon increasing the C/Si composition ratio up to 1.83, which is four orders of magnitude higher than that of bulk SiC. The increased C/Si composition ratio reduces the lattice constant and the effective mass of the Si_xC_1−x film and enhances its nonlinear refractive index effectively. In comparison with the sp³-orbital C–C bond, the sp²-orbital C–C bond in Si_xC_1−x films dominates the enhancement of nonlinear refractive index. By contrast, the nonlinear absorption coefficient changes sign and decreases magnitude when transferring the Si_xC_1−x film from Si-rich to C-rich conditions. This sign reversal is primarily attributed to the conversion in the absorption mechanism from two-photon absorption to nonlinear saturable absorption. Unlike Si–Si bonds which favor two-photon absorption, the sp²-orbital C–C bonds with delocalized π-electrons essentially enhance the saturable absorbance. Hence, a higher nonlinear refractive index and a higher saturable absorption coefficient can be obtained by enriching the sp²-orbital C–C bonds in C-rich Si_xC_1−x films.

A green 2 × 2 micro light-emitting diode ( μ - LED ) array with nanostructured grating patterns grown on a semipolar (20-21)-oriented gallium nitride (GaN) buffered layer on (22-43)-oriented sapphire substrate is specially transistor-outline can (TO-can) packaged with a sub-miniature-A (SMA) connector for high-speed data communication beyond 5 Gbit/s. Through a specific design for suppressing the quantum-confined Stark effect (QCSE) in the green 2 × 2 μ - LED array with a low polarization-related electric field and flat quantum well band diagram, the green 2 × 2 μ - LED array exhibits a turn-on voltage of 2.5 V and output power of 0.3 mW at 1 A / cm 2 . The green 2 × 2 μ - LED array also reveals a wavelength shift from 543 nm to 537 nm smaller than that of conventional devices grown on c -plane buffered GaN substrate due to the inhibited QCSE. The 50 µm emission aperture of the green 2 × 2 μ - LED array ensures a lower capacitance for a larger − 3 dB modulation bandwidth, which exhibits − 1 dB power compression at a larger bias under high-speed operation, as it is less affected by the high resistance of the single μ - LED element. With a specific TO-can+SMA package, the green 2 × 2 μ - LED array exhibits maximal data rates exceeding 1.5 Gbit/s for the non-return-to-zero on–off keying format and beyond 5.02 Gbit/s for the bit-loaded discrete multitone (BL-DMT) format, which is very promising for optical wireless communication. As the sampling rate increases from 4 GSa/s to 16 GSa/s, the μ - LED array’s received signal-to-noise ratio (SNR) improves dramatically from 15.4 dB to 12.2 dB. The SNR remains about 15.4 dB, with a matching bit-error ratio (BER) of 2.7 × 10 − 3 , whereas the 10-fold oversampling of the eight-ray quadrature amplitude modulation orthogonal frequency-division multiplexing (8-QAM OFDM) data stream with 16 GSa/s appears to reduce the SNR by − 3 dB , resulting in a decoded BER of 3.3 × 10 − 3 . The green 2 × 2 μ - LED array has demonstrated greater potential in data transmission beyond 5 Gbit/s using the BL-DMT algorithm for future applications in domains of visible light communication or optical wireless communication when packaged with handed mobile devices.

The nonstoichiometric ITO/n-SiC/i-SiC/p-Si/Al light-emitting diodes (LEDs) with dense Si quantum dots (Si-QDs) embedded in the Si-rich Si_xC_1-x -based i-SiC layer are demonstrated. The Si-rich Si_xC_1-x films with buried Si-QDs are grown by the plasma-enhanced chemical vapor deposition with varying substrate temperatures. After the annealing process, the average Si-QD size in the Si-rich Si_0.52C_0.48 film is 2.7 ± 0.4 nm with a corresponding volume density of 1.43 × 10¹⁸ cm^-3. By increasing the deposition temperatures from 300°C to 650°C, the turn-on voltage and turn-on current of the ITO/n-SiC/i-SiC/p-Si/Al LEDs are found to decrease from 13 to 4.2 V and from 0.63 to 0.34 mA, respectively. In addition, these Si-rich Si_xC_1-x LEDs provide the maximal electroluminescent (EL) power intensity increasing from 1.1 to 4.5 μW/cm². The yellow (at 570 nm) EL emission power of the ITO/n-SiC/i-SiC/p-Si/Al LEDs reveals a saturated phenomenon due to the Auger effect. The dissipated energy by the lattice thermal vibration contributes to a decayed EL emission power at higher biased currents. The corresponding power-current slope is observed to enhance from 0.45 to 0.61 μW/A with the substrate temperature increasing to 650°C.