Electrically driven photonic crystal light emitters

Low threshold current 2-D slab photonic crystal lasers are reported. Two nondegenerate resonant modes with a central node are investigated. Highly-efficient photon out-coupling schemes will also be discussed. The 'practical' single photon source should have high Q/V value, high photon collection efficiency, single-modeness at the same time. The ability to localize photons into photonic bandgap semiconductor microcavities [1, 2] having wavelength-scale volumes and high quality factors enables us to study the cavity quantum electrodynamics [3] in semiconductor material systems. Various optically-pumped, ultra-small, photonic crystal lasers and electrically-driven light emitting structures based on the concept of photonic crystal have been recently reported. One of the practical difficulties of this very small, free-standing slab, photonic crystal laser [4, 5] is the electrical contact onto the sub-micron-size photonic crystal resonator. Locating the proper region inside the laser cavity to position an electrical contact requires an understanding of the resonant modes that are available in a single-cell triangular lattice photonic crystal cavity. In our resonator structures [6], the small central post functions simultaneously as an electrical wire, a mode selector and a heat sinker. A sub-micron-size semiconductor post is placed at the central region of the single-cell photonic crystal resonator where the photon density is almost zero. The thickness of the semiconductor slab is about a half the wavelength in the material. Electrons are supplied laterally from the top electrode while holes are injected directly through the bottom post. The carriers recombine in the InGaAsP quantum wells with electro-luminescence near communications wavelength 1,500 nm. A doping structure that is inverted from that of a typical semiconductor laser is used to exploit the high mobility of the electrons that has to travel a longer distance. The introduction of this heterojunction n-i-p structure [6] is expected to limit the occurrence of bi-molecular radiative recombination to the proximity of the central post. We observed lasing operations from the monopole mode with threshold of 260 μA. Electrically-driven hexapole mode lasing is also demonstrated. The existence of a central node of the hexapole mode makes it also suitable for electrical pumping. In Fig. 1(c), one can observe stepwise size tuning of the central post which is seen as a dim white shadow at the center of the resonator. The nondegeneracy of the mode is advantageous for the single-modeness. The single mode operation is experimentally confirmed. Threshold current and voltage are 100 μA and 0.9 V, respectively, at room temperature. Moreover, the parity-selective hexapole mode resonator shown in Fig. 2, six photonic crystal waveguides are introduced to spoil the quality factors of the other available modes within the gain spectrum [7]. The introduction of the waveguide puts additional parity matching constraints on the resonant modes. As a result, only the hexapole mode with odd symmetry with respect to the waveguides survives. Single mode operation over a wide spectral range has been experimentally confirmed. These waveguides serve also as current paths and reduce electrical resistance. In general, far-field radiations coming from wavelength-scale photonic crystal resonators are complicated and the efficient collection of output photons has been a long-standing issue. The distribution of the electric field in the hexapole mode is anti-symmetrically balanced. However, note that this fine balance can be perturbed by simple structural modification of the hexapole mode. In fact, we break the symmetry along the horizontal axis by moving two nearest air holes slightly outward. Through this simple structural perturbation, the DC component of Ex-field is newly created and escape into the free space. Thus mostly x-polarized far field radiation shows up. Over 70% of the photons generated in the resonator are expected to be contained within the angular span less than 30 degrees from the vertical direction. Moreover, the collection efficiency can be increased up to 84% with the help of bottom distributed feedback reflector. This electrically-operable vertically-beamed photon crystal laser will be a good candidate for a low power light source or a 'practical' single photon source.