Theoretical investigation of metal-cavity nanolasers and nanoLEDs and comparison with experiments
Using metal to confine light in a small cavity and to make a light-emitting device has been intensively investigated recently since it provides a new approach to reduce the size of light-emitting devices. In the presence of metal, the optical mode volume of either a dielectric mode or a plasmonic mode can be suppressed to be smaller than its counterpart in conventional dielectric cavities. Furthermore, the mode volume of the plasmonic mode can be smaller than the diffraction limit, which is the volume with half a wavelength in three dimensions, due to the nature of the surface wave. Many groups have shown experimental work on metal cavity lasers. To understand the metal effect on metal-insulator-semiconductor-insulator-metal (MISIM) slab and circular waveguides, we derive their guiding conditions. The analysis gives us the intuition on metal cavity laser design. For the non-circular cross section of a nanodisk such as ZnO, which has a hexagonal cross section, the resonant modes in different metal cavities will be calculated with the finite-difference time-domain method (FDTD). Our investigation shows the enhancement of metal on the optical field confinement and the reduction of the radiation loss from metal. The threshold material gain can be improved by one third compared with that of the same cavity without metal encapsulation. On the other hand, due to the dispersive characteristics of metal, we use a rigorous formula for electromagnetic energy in dispersive material and the positive energy is always obtained whatever the operating frequency. We then apply Poynting’s theorem to calculate the quality factor (Q) of a nanobowtie antenna and further analyze its radiation pattern and material loss. The calculated Q agrees well with the experimental data and, thus, the validity of the formula is verified. The third study is to analyze the metal-cavity surface-emitting lasers, fabricated by our group. The metal-cavity microlasers show the highest power among current metalcavity lasers and they operate at continuous-wave (CW) electrical injection at room temperature. We study this structure from calculating its gain profile of coupled multiple quantum wells (MQWs) and fit the experimental light output power versus the current (L-I) curve using the rigorous rate equations with temperature dependence. Our study shows that the nonradiative recombinations, including the surface recombination and Auger recombination, dominate the threshold current. The high-speed modulation response of metal-cavity light-emitting devices is then investigated. Since the spontaneous emission plays an important role in a small cavity, especially when it works in the LED region, we derive the complete representation for the spontaneous emission in a metal cavity and show that the Purcell effect appears naturally in the spontaneous emission formula, instead of being artificially placed in the spontaneous emission rate in free space. We show the dependence of the maximum bandwidth on the quality factor Q and the normalized effective optical modal volume Vn, for bulk, multiple quantum wells, and quantum dots, using our rigorous rate equations. The effects of the optical mode volume, the quality factor, and the active materials are thoroughly discussed. Finally, to realize a small metal-cavity laser, the potential design rules are presented.
Nanolasers, nanoLEDS, high-speed modulation, Thermal effect, Light-emitting diode (LED)
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign