Small photonic-crystal lasers have offered one promising method for generating single photon sources, but research has only demonstrated devices that are optically pumped. A research team at the Korea Advanced Institute of Science and Technology (KAIST; Taejon, Korea) recently demonstrated an electrically driven, single-mode, low-threshold current (~260 µA), photonic bandgap laser that operates at room temperature.
This SEM image of an intentionally broken sample shows the center post clearly.
Research team leader Yong Lee says the team's small step toward creating a single-photon source may be of special interest to researchers working with photonic crystals, cavity quantum electrodynamics, and quantum information. "One of the biggest problems we faced in trying to create a single-cell free-standing photonic-crystal laser was how to make electrical contact with the tiny sub-micron-sized photonic-crystal resonator structure," Lee says. "In the end, we sat the resonator on a sub-micron post that was large enough for good electrical activity and small enough not to disrupt resonator performance."
The group used an inverted heterojunction (n-i-p instead of p-i-n) structure that limits bimolecular recombination to the area around the sub-micron central post. This, in turn, utilizes the low mobility of the hole to generate photons. The team placed an indium phosphorous semiconductor post at the center of a single-cell photonic-crystal resonator. The post was 1.0 µm high and 0.64 a by 0.51 a, where a is the lattice constant of the photonic crystal.
"The post connected to a 50-µm diameter mesa, which consisted of a modified single-cell photonic-crystal cavity surrounded by five heterogeneous photonic-crystal sections with the same lattice constant but different sizes of air holes," Lee says. "This heterogeneous surrounding improved [our ability to] position and size the central post." The team found that the size of the air holes affected the speed of the post-etching process. By modifying the air-hole size, they could improve the position and size of the post. In addition, the chirped resonator structure improved the Q factor slightly without changing either resonant frequency or modal volume of relevant modes. "We confirmed that with 3-D finite-difference time-domain calculations," Lee says.
The team electrically pumped the single-cell photonic-crystal cavities at room temperature, using ~6-ns pulses for a period of 2.5 µs. Emitted photons were collected by a 50X microscope and fed to a spectrometer. "With this process," Lee says, "we observed single-mode lasing action at 1519.9 nm. Still, several issues remain to be addressed before this electrically driven, ultra-small cavity can become a practical on-demand single-photon source," Lee says. Issues to be solved include how to place well-defined quantum dots or impurity atoms at the antinode of the cavity and how to inject single electron-hole pairs efficiently. Lee estimates it will be several years before a realistic single-photon gun is achieved. "Nevertheless," Lee says, "we believe the demonstration of an electrically driven single-cell photonic-crystal laser represents a small but meaningful step toward the ultimate photon source."
Toshihiko Baba of Yokohama National University (Yokohama, Japan) said that Lee's electrically pumped photonic-crystal laser was an important step after researchers at the California Institute of Technology (Pasadena, CA) first demonstrated an optically pumped single-cell crystal laser. It is a very promising step toward creating a functional photonic chip, practical single-photon emitter, or sensing photon source, he adds.