In this study a blue‐light‐emitting conjugated polymer, poly(9,9‐dioctylfluorene), is confined to the interlayer space of inorganic, layered metal dichalcogenide materials, metallic MoS2, and semiconducting SnS2. The nanocomposites are prepared through Li intercalation into the inorganic compound, exfoliation, and restacking in the presence of the polymer. X‐ray diffraction and optical absorption measurements indicate that a single conjugated polymer monolayer, with an overall extended planar morphology conformation, is isolated between the inorganic sheets, so that polymer aggregation or π–π interchain interactions are significantly reduced. Photoluminescence (PL) measurements show that the appearance of the undesirable green emission observed in pristine polymer films is suppressed by incorporating the polymer into the inorganic matrix. The blue emission of the intercalated polymer is stable for extended periods of time, over two years, under ambient conditions. Furthermore, the green emission is absent in the PL spectra of nanocomposite films heated at 100 °C for 7 h in air with direct excitation of the keto defect. Finally, no green emission was observed in the electroluminescence spectrum of light‐emitting devices fabricated with a polymer‐intercalated SnS2 nanocomposite film. These results support the proposed hypothesis that fluorenone defects alone are insufficient to generate the green emission and that interchain interactions are also required.

The maximum operating temperature reported so far for THz-QCLs is ∼200 K. With the well-known degradation mechanism of thermally activated LO-phonon scattering, one straightforward strategy to improve their temperature performances is the use of diagonal structures in which the upper-to-lower state scattering time is lengthened. However, the effectiveness of this method for achieving room temperature operation remains to be demonstrated. Here, we studied the temperature degradation of highly diagonal GaAs/Al0.15GaAs THz-QCLs. By analyzing their output power dependence on temperature, we identified the physical mechanism that limits their performance to be thermally activated leakage into the continuum, as evidenced by the large activation energy of ∼80 meV extracted from the Arrhenius plot. This observation is further supported by a careful analysis of current-voltage characteristics, especially in regions of high biases. In order to significantly improve the temperature performances of diagonal THz-QCLs, this leakage should be eliminated.

In this paper, we demonstrate a method to investigate the temperature degradation of THz quantum cascade lasers (QCLs) based on analyzing the dependence of lasing output power on temperature. The output power is suggested to decrease exponentially with some characteristic activation energy indicative of the degradation mechanism. As a proof of concept, Arrhenius plots of power versus temperature are used to extract the activation energy in vertical transition THz QCLs. The extracted energies are consistent with thermally activated longitudinal optical-phonon scattering being the dominant degradation mechanism, as is generally accepted. The extracted activation energy values are shown to be in good agreement with the values predicted from laser spectra. Semiconductor laser performance versus temperature is frequently characterized by examining the threshold current evolution according to a phenomenological relationship J th ¼ J 1 þ expðT=T 0 Þ, where T 0 is an experimentally determined parameter. Unfortunately, there is no straightforward way to relate T 0 to the underlying physics of the temperature degradation. This paper suggests an alternative characterization of semiconductor lasers based on the evolution of maximum lasing output power versus temperature. The method is validated through examining the temperature degradation of vertical transition terahertz quantum cascade lasers (THzQCLs), where thermally activated LO phonon scattering ( Fig. 1(a)) is widely believed to be the dominating mechanism.The output power of a semiconductor laser is given bywhere P out is the output power, ht is the photon energy, A ¼ L  W the contact area, e the electron charge, a m is the mirror loss, a w is the waveguide loss, J and J th are the injected current density and threshold current density, respectively, and g i is the internal quantum efficiency. The current density due to stimulated emission is related to the injected current and threshold current densities byJ À J th ð Þ, and can be approximated bywhere Dn th ¼ g th r g L mod is the 2D clamped population inversion at the lasing threshold g th determined by the lasing condition (balance of gain and loss). r g is the gain cross section, L mod is the QCL period (module) thickness, N mod is the number of modules, and 1 s st is the stimulated emission rate. Therefore,Assuming that mirror and waveguide losses are temperature independent, then the clamped population inversion, Dn th , is also temperature independent. Any temperature dependence of P out must, therefore, be due to the stimulated emission rate,, for simplicity we focus on the case of a three-subband THz QCL (see Figure 1(b)). Neglecting backfilling effects, the stimulated emission rate can be shown to bewhere s à 13 is the injector (1 0 ) to the upper lasing level (3) tunneling time, s 31 is the LO-phonon scattering time from the upper lasing level (3) to the injector level of the next module (1), s 21 is the LO-phonon scattering time from the lower lasing level (2) to the injector level of the next module (...

The mechanisms that limit the temperature performance of GaAs/Al0.15GaAs-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated LO-phonon scattering and leakage of charge carriers into the continuum. Consequently, the combination of highly diagonal optical transition and higher barriers should significantly reduce the adverse effects of both mechanisms and lead to improved temperature performance. Here, we study the temperature performance of highly diagonal THz-QCLs with high barriers. Our analysis uncovers an additional leakage channel which is the thermal excitation of carriers into bounded higher energy levels, rather than the escape into the continuum. Based on this understanding, we have designed a structure with an increased intersubband spacing between the upper lasing level and excited states in a highly diagonal THz-QCL, which exhibits negative differential resistance even at room temperature. This result is a strong evidence for the effective suppression of the aforementioned leakage channel.

ULL (level 3) to Injector (level 1) LO-phonon scattering time.

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We have investigated the composition and optical properties of GaInAsN/GaAs single quantum wells grown using metal organic chemical vapor epitaxy at 500 °C. Using time-of-flight secondary ion mass spectrometry and photoluminescence spectroscopy, we have shown the presence of a 1–2 nm thick nitrogen-rich interfacial layer at the first interface grown. The inhomogeneous asymmetric distribution of nitrogen atoms along the growth direction is attributed to the dominance of surface kinetics, nonlinear dependence of N incorporation on In content, and the strain gradient effect on the effective diffusion of N. We have utilized this finding to grow high quality quantum wells.

We present a so-called "split-well direct-phonon" active region design for terahertz quantum cascade lasers (THz-QCLs). Lasers based on this scheme profit from both elimination of high-lying parasitic bound states and resonant-depopulation of the lower laser level. Negative differential resistance is observed at room temperature, which indicates that each module behaves as a clean 3-level system. We further use this design to investigate the impact of temperature on the dephasing time of GaAs/AlGaAs THz-QCLs.