Quantum-dot nanolasers

The impact of semiconductor lasers on our daily lives is immense. They perform work every time someone gathers information for the internet, makes a telephone call, prints an article or pays for an item at a store. There are over 2.5 billion VCSELs † in use at our homes or offices, and roughly 100 million are produced each year to keep up with demand. VCSEL technology is now considered conventional, and research has progressed to smaller devices, modulated at higher speeds and producing output with greater spectral stability. This chapter focuses on two of the advances. One is the reduction of optical cavity volume by as much as two orders of magnitude, making a transition from microlasers (such as VCSELs) to nanolasers. Two is a change in active medium from quantum wells (QWs) to quantum dots (QDs), bringing about, e.g., devices operating with very few (tens) of emitters. These developments ushered in a new era for semiconductor device physics, one where quantum optical and many-body electron interaction effects dominate. The underlying motivation is the control of spontaneous emission [1]. The typical laser mitigates the randomness (noise) caused by spontaneous emission by overwhelming it with stimulated emission. Here, we are speaking of actually quieting the spontaneous emission noise, both spatially and temporally. In the former, we use nanocavities to inhibit spontaneous emission in undesirable directions. With the latter, we use very 628few QDs (ideally only one) to control the timing of photon emission, thereby improving photon statistics beyond the limit described by the Poisson distribution. Devices incorporating the two advances are being fabricated and experiments are being performed. The results are both promising and exciting from the device and physics aspects, respectively. They also lead to many new questions and renewed interest in some old ones: What is lasing and where is the threshold? Why is there not a phonon bottleneck? What is the homogeneous width of a QD transition [2-4]? What is the inhomogeneous broadening in my samples? Is there really thresholdless lasing [5-7]? Are QD lasers better than QW lasers [8]? Why do QD lasers not show modulation speeds as advertised? This chapter describes a theoretical framework capable of addressing all the abovementioned questions. The building blocks come from quantum electrodynamics (QED), which is the quantum theory of the interaction of light with matter, and carrier interactions described by many-body theory.