Meng Xun

GaN nanorod light emitting diodes with suspended graphene transparent electrodes grown by rapid chemical vapor deposition

Ordered and dense GaN light emitting nanorods are studied with polycrystalline graphene grown by rapid chemical vapor deposition as suspended transparent electrodes. As the substitute of indium tin oxide, the graphene avoids complex processing to fill up the gaps between nanorods and subsequent surface flattening and offers high conductivity to improve the carrier injection. The as-fabricated devices have 32% improvement in light output power compared to conventional planar GaN-graphene diodes. The suspended graphene remains electrically stable up to 300 °C in air. The graphene can be obtained at low cost and high efficiency, indicating its high potential in future applications.

High Beam Quality of In-Phase Coherent Coupling 2-D VCSEL Arrays Based on Proton-Implantation

Two-dimensional coherently coupled arrays of vertical cavity surface emitting lasers were fabricated using proton implantation. A 2 × 2 array with the far field beamwidth of 3.6° ( 1.18×diffraction limited), the output power of 0.45 mW, and the spectral line width of 0.2 nm was achieved for continuous wave operation at room temperature. It showed an excellent beam quality due to the in-phase coupling among the elements in the array. The array performance dependence on injection current level and thermal effects was discussed. The processing for the arrays is considerably simple and can be used as an alternative to other array implementations.

Implant-Defined 3

Two-dimensional in-phase coherently coupled vertical cavity surface emitting laser (VCSEL) array with high beam quality was fabricated by proton implantation. Indium-tin-oxide (ITO) current spreading layer was employed to improve the uniformity of the injection current. This makes a coherent array to operate in in-phase mode even in a relatively larger scale. The appropriate thickness of ITO current spreading layer was obtained by experiment (80 nm). The procedure of proton implantation defined array is considerably simple and low cost. A 3 × 3 in-phase coherently coupled array with 2.4° (1.28 × D.L.) of lobe beamwidth was achieved. It sufficiently demonstrates the advantages of the high beam quality of the in-phase coherently coupled arrays. The output power of the array was up to 4.5 mw under pulse-wave condition.

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AbstractThe two-dimensional (2D) triangle lattice air hole photonic crystal (PC) GaN-based light-emitting diodes (LED) with double-layer graphene transparent electrodes (DGTE) have been produced. The current spreading effect of the double-layer graphene (GR) on the surface of the PC structure of the LED has been researched. Specially, we found that the part of the graphene suspending over the air hole of the PC structure was of much higher conductivity, which reduced the average sheet resistance of the graphene transparent conducting electrode and improved the current spreading of the PC LED. Therefore, the work voltage of the DGTE-PC LED was obviously decreased, and the output power was greatly enhanced. The COMSOL software was used to simulate the current density distribution of the samples. The results show that the etching of PC structure results in the degradation of the current spreading and that the graphene transparent conducting electrode can offer an uniform current spreading in the DGTE-PC LED. PACS: 85.60.Jb; 68.65.Pq; 42.70.Qs

3 In-Phase Coherently Coupled Vertical Cavity Surface Emitting Lasers Array

In-phase coherently coupled vertical cavity surface emitting laser (VCSEL) hexagonal arrays were fabricated using proton implantation. The near-field profiles, far-field profiles, and emission spectra under different injection currents were tested and analyzed. The arrays can maintain in-phase single mode in a considerably wide current range from 10 mA (I(th)) to 35 mA (3.5×I(th)), exhibiting excellent beam quality. The far-field divergence angle of the in-phase coupled array is 2.5 degrees. Approximately 29% of total power is localized in the central lobe. Compared with square structure arrays, hexagonal arrays can maintain a more stable in-phase mode because of stronger coupling among the elements. The maximum output power of 4.9 mW was obtained under pulse wave condition. The simulation of far-field was carried out to match the in-phase operation test results. The performance enhancement of the array is attainable if the condition of heat dissipation is better. The process procedure of proton implantation is relatively simple and of low cost. It can be used as an alternative to coherently coupled array implementations.

Enhanced performance of photonic crystal GaN light-emitting diodes with graphene transparent electrodes

The characteristics of polarization stable microstructure vertical-cavity surface-emitting lasers (PS-MS-VCSELs) with low threshold current and single fundamental mode (SFM) operation were theoretically and experimentally investigated. Elliptical air hole photonic crystal (EPC) structure was incorporated in the top mirror of the MS-VCSELs to realize single fundamental mode operation. By controlling the mode loss difference among the two orthogonal modes, the fundamental mode and other high order modes of the MS-VCSELs, through suitable oxide aperture shape and EPC parameters, a high performance PS-MS-VCSEL is achieved with output power of 1.6 mW, low threshold current of 0.8 mA as well as more than 20 dB orthogonal polarization suppression ratio (OPSR).

Wide operation range in-phase coherently coupled vertical cavity surface emitting laser array based on proton implantation.

Electronically controlled beam steering was achieved via highly coherent in-phase implant defined vertical cavity surface emitting laser arrays. The total power in the central lobe is above 36% in 1 × 2 array when steering.

Polarization stable low threshold current single fundamental mode VCSELs

Far-field properties dependent on array scale, separation, element width and emitted wavelength are systematically analyzed theoretically and experimentally. An array model based on the finite-difference method is established to simulate the far-field profile of the coherent arrays. Some important conclusions are obtained. To achieve a higher quality beam, it is necessary to decrease separation between elements, or to increase the element width. Higher brightness can be achieved in the array with larger scale. Emitted wavelength also has an influence on the far-field profile. These analyses can be extended to the future design of coherent vertical cavity surface emitting laser arrays.

Beam steering in highly coherent implant-defined vertical cavity surface emitting laser array

Implant-defined vertical-cavity surface-emitting laser (VCSEL) arrays can be designed to operate in in-phase mode. However, the nonuniformities in fabrication process impact the resonance selection and the devices do not follow expected trends. Coherent coupling was demonstrated in three-element VCSEL arrays via phase tuning of elements. In-phase mode and out-of-phase mode were both achieved in most of the arrays. Moreover, coherent coupling can decrease the threshold current of elements in the array. Improved output power was also clearly observed when the array operated in the in-phase mode. Arbitrary phase combination of the array elements can be obtained via the phase tuning. This technology is able to improve the reproducibility and practicability of the implant-defined coherently coupled VCSEL array.

Simulation of Far-Field Properties of Coherent Vertical Cavity Surface Emitting Laser Array*

We have investigated the dependence of modal behavior on the structures of 2-D implant-defined coherently coupled vertical cavity surface-emitting laser (VCSEL) arrays experimentally and theoretically. 2-D VCSEL arrays with different interelement spacing have been designed and fabricated. Through the experimental analysis of near-field and far-field distribution, in-phase and out-of-phase mode are obtained from the arrays, depending on the interelement spacing. Considering the effect of thermal and carrier injection, the refractive index distribution of the arrays under operation is calculated. Then, FDTD Solutions software is employed to calculate the near-field distributions of the arrays through establishing a simplified refractive index model. The simulation results agree well with the experiments. Meanwhile, according to the phase characteristics of in-phase and out-of-phase mode, as observed from the near-field profiles, the far-field distributions are calculated, in an excellent agreement with the experiments.