Fig 1. (a) 2-dimensional photonic crystal (b) reciprocal space representation (c) photonic crystal band diagram with photonic bandgap in grey
Fig 2. Heterostructure photonic crystal membrane laser
Progress in fabrication technologies has allowed for material manipulation on the order of the wavelength of light. Such small manipulations can be used to make photonic crystals. Photonic crystals are formed by periodic variations in material refractive index. These periodic variations lead to photonic band gaps, frequencies of light not allowed to propagate in the photonic crystal, and slow group velocity. These phenomena can then be exploited in order to make optical cavities and eventually lasers.
Much work has already been done in the area of photonic crystal membrane lasers. This research focuses mostly on creating a two-dimensional photonic crystal in a thin semiconductor membrane containing quantum wells. Either slow group velocity effects or photonic bandgap confinement can be used to create the optical resonator for the laser.
Our work focuses on the creation of original photonic crystal cavities and lasers in semiconductor membranes. The photonic crystal heterostructure cavity is shown above in Figure 2. Here the heterostructure is formed by the interfacing of two different types of photonic crystals, kagome and hexagonal, with the same nearest neighbor hole spacing. The design is such that the bandgap of the hexagonal region provides feedback to the slow light modes of the kagome region. This allows for good optical properties with a medium sized cavity. The device has a slow turn on and exhibits single mode lasing as shown in Figures 3 and 4, respectively. The below threshold emission spectrum indicates a quality factor of about 2400.
Fig 3. Collected output power vs. instantaneous pump power. (note: the input pump power is not calibrated to pump spot size).
Fig 4. Lasing spectrum of photonic crystal heterostructure cavity laser. The inset shows the measured spectrum below threshold.