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Lasers & Sources

A photonic crystal nanocavity laser with ultralow threshold

Strong photon confinement in a semiconductor quantum dot gain material enables lasing with less than 1 microwatt pumping at room temperature.
11 December 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0915

Confining photons in a small space is a difficult task. Recent nanotechnology advances now enable their confinement in a cubic wavelength volume for as long as 1 nanosecond in semiconductors when using 2D photonic crystal (PhC) structures. As shown in Figure 1, a PhC is a periodic optical structure whose periodicity is on the order of the wavelength of light. PhCs are designed to affect the motion of photons1 because they have the optical equivalent of the energy gap of conventional semiconductors. The possibility to manipulate photons in solid materials offers many attractive applications such as nanocavity lasers, tunable slow light devices, and optical integrated circuits.

Figure 1. Scanning electron micrographs of a photonic crystal nanocavity structure. Top (a) and cross sectional (b) views. QD: quantum dot.

The PhC nanocavity laser is considered one of the best candidates to achieve ultra-low threshold lasing due to its small mode volume and high quality factor (Q, which expresses the ability of an oscillating system to keep oscillating before running out of energy). The first reported PhC laser (1999) used multiple quantum wells (QWs) as the gain material at low temperature in pulsed operation.2 Recently, continuous wave (CW) laser operation at low temperature was also reported. However, CW lasing is inherently difficult to achieve at room temperature due to fast non-radiative losses that translate into very high Q nanocavity requirements. Our group was the first to demonstrate room temperature operation of a high-Q PhC nanocavity CW laser.3 Further improvement of the cavity Q reduced the optical pumping threshold down to the microwatt level.4

3D photon confinement can be achieved by fabricating the PhC slab structure with a nanocavity (see Figure 1). The 2D triangular lattice confines photons in the plane of the slab. The excitation beam generates excitons in quantum dots (QDs) embedded into the slab layer and the excitons recombine with photon emission. Our PhC nanocavity was designed to have its resonant wavelength at the excitonic photoluminescence (PL) peak, allowing CW lasing at 1.33μm at room temperature with a Q of 87,000. Figure 2 shows PL spectra recorded below and above the laser threshold.

Figure 2. Photoluminescence (PL) spectra of the quantum-dot-based photonic crystal nanocavity laser below (a) and above (b) the laser threshold.

Figure 3 shows the light-in versus light-out (L-L) plot of this laser. We estimated a laser threshold of 2.5μW in irradiated pump power corresponding to 375nW in effective power absorbed by the slab layer. To the best of our knowledge, this laser has the smallest threshold pump power ever reported at room temperature.5,6 Its low threshold allows stable operation far above threshold without thermal damage. The L-L plot shown in Figure 3 depicts soft turn-on behavior close to the laser threshold, in marked contrast with conventional lasers that display plots with pronounced kinks. The soft turn-on observed in PhC lasers results from efficient coupling of the spontaneous emission to the lasing mode.

Figure 3. Light-in versus light-out plot of the photonic crystal nanocavity laser. The laser can operate with microwatt optical pumping. Pth: pump threshold.

One of the unique characteristics of our nanolaser becomes apparent when the L-L curve is plotted on a log-log scale, as shown in Figure 4, which compares the experimental L-L plot to simulation curves with different β values. The spontaneous emission coupling factor β is the rate of spontaneous emission at the lasing wavelength divided by the total spontaneous emission rate. A large value of β results in efficient lasing and reduces the laser threshold. The best fit is found for β = 0.90, which means that 90% of the photons emitted from the QDs couple to the cavity mode via the Purcell effect. This plot effectively describes nearly thresholdless behavior, a unique characteristic of the PhC nanocavity laser when compared to conventional lasers.

Figure 4. Simulated and experimental light-in versus light-out plots on a log-log scale. Nearly thresholdless behavior is a unique characteristic of the nanocavity laser.

To summarize, we demonstrated PhC nanocavity CW laser operation at room temperature with less than 1 microwatt of optical pumping. This laser has a uniquely soft turn-on lasing behavior due to highly efficient coupling of its spontaneous emission. To further lower the threshold, the use of QDs in InP-based PhC structures may represent the best approach, because the surface recombination velocity in InP is much lower than that of GaAs.

This work was supported by the Special Coordination Funds for Promoting Science and Technology.

Masahiro Nomura
Institute for Nano Quantum Information Electronics
University of Tokyo
Tokyo, Japan 

Masahiro Nomura is a research associate at the Institute for Nano Quantum Information Electronics of the University of Tokyo.

Satoshi Iwamoto, Yasuhiko Arakawa
Institute for Nano Quantum Information Electronics
University of Tokyo
Tokyo, Japan
Research Center for Advanced Science and Technology
University of Tokyo
Tokyo, Japan 

Satoshi Iwamoto is an associate professor at the Institute for Nano Quantum Information Electronics and at the Research Center for Advanced Science and Technology of the University of Tokyo.

Yasuhiko Arakawa is the director of the Institute for Nano Quantum Information Electronics and a professor at the Research Center for Advanced Science and Technology of the University of Tokyo.