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We show that the free-carrier-density dependence of internal optical loss gives rise, in general, to the existence of a second lasing threshold above the conventional threshold. Above the second threshold, the light-current characteristic is two-valued up to a maximum current at which the lasing is quenched.

We describe a mechanism of nonlinearity of the light–current characteristic common to heterostructure lasers with a reduced-dimensionality active region. It arises from (i) noninstantaneous carrier capture into the active region and (ii) nonlinear (in the carrier density) recombination rate outside the active region. Because of (i), the carrier density outside the active region rises with injection current above threshold, and because of (ii), the useful fraction of current (that ends up as output light) decreases. We derive a universal closed-form expression for the internal differential quantum efficiency that holds true for quantum well, quantum wire, and quantum dot lasers.

The multimode generation threshold in quantum-dot (QD) lasers is calculated as a function of the parameters of structure and temperature. Thermally excited escapes of carriers away from QDs are shown to control the multimode generation threshold. A decrease in the QD size dispersion is shown to increase considerably the relative multimode generation threshold. The maximum tolerable QD size dispersion and the minimum tolerable cavity length, at which the lasing is possible to attain, are shown to exist. Concurrent with the decrease of threshold current, the reduction of multimode generation threshold is shown to occur with decrease of temperature.

We show that the internal optical loss, which increases with free-carrier density in the waveguide region, considerably reduces the modulation bandwidth omega(-3 dB) of a quantum dot laser. At a certain optimum value j(0)(opt) of the dc component of the injection current density, the maximum bandwidth omega(max)(-3dB) is attained and the modulation response function becomes as flat as possible. With internal loss cross-section sigma(int) increasing and approaching its maximum tolerable value, omega(max)(-3dB) decreases and becomes zero. As with j(0)(opt), there also exists the optimum cavity length, at which omega(-3 dB) is highest; the larger is sigma(int), the longer is the optimum cavity. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697683] U.S. Army Research Office (Grant No. W911-NF-08-1-0462) China Scholarship Council Russian Foundation for Basic Research (Grant No. 11-02-00573) Program "Fundamental Research in Nanotechnology and Nanomaterials" of the Presidium of the Russian Academy of Sciences

We show that the carrier capture from the optical confinement layer into quantum dots (QDs) can strongly limit the modulation bandwidth omega(-3) dB of a QD laser. As a function of the cross-section sigma(n) of carrier capture into a QD, omega(-3) dB asymptotically approaches its highest value when sigma(n) -> infinity the case of instantaneous capture). With reducing sigma(n), omega(-3) dB decreases and becomes zero at a certain nonvanishing sigma(n)(min). The use of multiple-layers with QDs significantly improves the laser modulation response-omega(-3) dB is considerably higher in a multilayer structure as compared to a single-layer structure at the same dc current. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3571295] U.S. Army Research Office (Grant No. W911-NF-08-1-0462) China Scholarship Council Russian Foundation for Basic Research (Grant No. 08-02-01337) Program "Fundamental Research in Nanotechnology and Nanomaterials" of the Presidium of the Russian Academy of Sciences

We derive a closed-form expression for the upper limit for the modulation bandwidth of a semiconductor quantum dot (QD) laser. The highest possible bandwidth increases directly with overlap integral of the electron and hole wave functions in a QD, number of QD-layers, and surface density of QDs in a layer, and is inversely proportional to the inhomogeneous line broadening caused by the QD-size dispersion. At 10% QD-size fluctuations and 100% overlap, the upper limit for the modulation bandwidth in a single QD-layer laser can be as high as 60 GHz. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3446968] U.S. Army Research Office (Grant No. W911-NF-08-1-0462) Russian Foundation for Basic Research (Grant No. 08-02-01337)

We study the optical output power of a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells. Even if there is out-tunneling leakage of carriers from QDs, the intensity of parasitic recombination outside QDs remains restricted with increasing injection current. As a result, the light-current characteristic becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents-a fascinating feature favoring the use of tunneling-injection of both electrons and holes into QDs for high-power lasing. (C) 2008 American Institute of Physics. U.S. Army Research Office under Grant No. W911-NF-05-1-0308

Internal loss, which increases with the carrier density outside the active region, causes a rollover of the light-current curve and strongly limits the output power of a diode laser with a single layer of quantum dots. The maximum power is calculated as a steeply decreasing function of internal loss cross section. The use of multiple layers of quantum dots is shown to significantly improve the laser output characteristics. U.S. Army Research Office under Grant No. W911-NF-05-1-0308