Double tunneling-injection quantum dot laser: Temperature-stable, high-power, and high-speed operation

Abstract

A laser exploiting tunneling-injection of both electrons and holes into quantum dots possesses the potential for temperature-stable and high-power operation and may also be a candidate for high modulation bandwidth.There has been much effort to use quantum dots (QDs) as an active region in diode lasers [1,2]. In the 'conventional' design of QD lasers, the carriers are first injected from the cladding layers into a bulk reservoir [which also serves as the optical confinement layer (OCL) and includes a wetting layer] and then captured into QDs. Due to bipolar (i.e., both electron and hole) populations in the reservoir, a certain fraction of the injection current goes into electron-hole recombination there. The parasitic recombination outside QDs is a major source of the temperature dependence of the threshold current [3]. In addition, the carrier capture from the reservoir into QDs is not instantaneous. For this reason, the carrier density in the reservoir, and hence the parasitic recombination rate, rise, even above the lasing threshold, with injection current. This leads to sublinearity of the light-current characteristic (LCC) and limits the output power, especially at high pump currents [4,5]. Suppression of the parasitic recombination would thus be expected to significantly enhance the temperature stability and the output optical power of a laser.In [6][7][8], to suppress the recombination outside QDs, tunneling injection of both electrons and holes into QDs was proposed from two separate quantum wells (QWs) (Fig. 1). In [9,10], the structures of [6-8] were referred to as double tunneling-injection (DTI) QD lasers. There have been experimental developments [11][12][13][14] related to the concept of DTI QD lasers. A DTI QD laser should be distinguished from the laser of [15][16][17] exploiting tunneling injection of only electrons into QDs. The latter laser serves a purpose different from suppressing the recombination outside QDs (namely, minimizing hot carrier effects) and was referred to as a single tunneling-injection (STI) QD laser in [9, 10].Ideally, out-tunneling of each type of carriers from QDs into the opposite-to-injection-side QW should be completely blocked and hence the electron-hole recombination outside QDs totally suppressed. As a result, the threshold current of such an ideal DTI QD laser would be virtually temperature-insensitive (the characteristic temperature T 0 above 1000 K -see [6][7][8]) and the LCC strictly linear (the dashed line in Fig. 3).To scrutinize the potential of a realistic DTI QD laser for temperature-stable and high-power operation, out-tunneling leakage of carriers from QDs (shown by the inclined arrows in Fig. 1) was considered in [9,10,18,19]. For such a DTI QD laser, Fig. 2 shows T 0 versus root mean square δ of QD-size fluctuations normalized to its maximum tolerable value δ max [18]. As seen from the figure, T 0 is very high throughout the entire range of δ and not significantly affected by the QD-size fluctuations, which is a clear manifestation of robustness of the D...