The GaSb quantum dots (QDs) with type II band alignment have attracted great attention recently. They are predicted to be optimizing active region materials for achieving high efficient intermediate-band solar cells and for obtaining ultra-long storage time for memory cells. In this research, GaSb QDs sandwiched inside InAlAs matrix lattice-matched to InP (001) substrate have been obtained via droplet epitaxy. The droplet epitaxy enable us to achieve low density (~2.6 x 10^9/cm^2) and large size (average height ~6.5nm) for the QDs while the lattice mismatch between the GaSb and InAlAs matrix is only ~4%. PL measurements reveal a type-II band alignment for these GaSb/InAlAs/InP QDs. The PL peak energy of QDs shows a blue-shift of >100 meV when the laser intensity increases by six orders of magnitude. Time-resolved PL measurements further confirm the type-II band alignment for the QDs by showing a maximum carrier lifetime of ~4.5 ns. The abnormal dependence of peak energy of QD PL band on the temperature in together with the special PL decay curve indicate that these GaSb/InAlAs QDs likely have different physics mechanism from common GaSb/GaAs type-II QDs. This study provide useful information for understanding the band structure and carrier dynamics of the GaSb/InAlAs QDs grown on InP surface.
The InGaAs surface quantum dots grown on GaAs surface without a capping layer (surface quantum dots, SQDs) are expected to play an important role for sensor applications due to their special surface sensitive properties. In this research, we investigated the photoluminescence (PL) characteristics of such In0.35Ga0.65As/GaAs SQDs with a layer of buried InGaAs QDs (BQDs) as reference. The uncapped InGaAs SQDs are integrated into a hybrid nanostructure with SQDs and buried quantum dots (BQDs) spaced by a 70 nm GaAs layer. Due to this thick GaAs spacer, we assert there is no quantum coupling between the SQDs and BQDs so that each layer of QDs has independent emission. The PL spectra show that the SQD PL intensity is far less than BQDs at low temperature but exceeds BQDs at high temperature, indicating a possible carrier transfer between the SQDs and surface states. With increasing excitation intensity, the PL spectra show clearly broaden on the high energy side and a blueshift for both the SQDs and BQDs. Therefore, there is lateral carrier transfer among each layer of QDs due to their high areal density. The intra-layer carrier transfer among SQDs as well as the inter-layer carrier transfer between SQDs and surface states attribute to carriers dynamics that make the SQDs having optical performance very different from the BQDs.
Atomic coherence and interference play an important role in the study of the atom-photon interactions. Electromagnetically induced transparency (EIT) is an extensively studied two-photon coherence phenomenon theoretically as well as experimentally. EIT is mainly observed in three-level atomic systems which causes transparency by quenching absorption of the medium. In this paper, based on the lambda type three-level system including energy level |1>, |2> and |3>, a microwave driving field is introduced between the excited-state energy level |3> and another excited-state energy level |4> to form an inverted Y-type four-level system. We theoretically study the two- and three-photon coherence in this system. The results show that the coupling field makes the probing absorption intensity at the resonant frequency have a very narrow line-width depression, i.e., EIT. The microwave field causes a dynamic Stark splitting of the energy level |3> and induces the Aulter-Townes double peaks. Their frequency interval is exactly equal to the Rabi frequency of the microwave field. The presence of all three fields induces wide window of EIT at the line center owing to the enhanced depression results. The transient evolution is also discussed to understand the optical switching process in the system. Our theoretical study will be helpful to get a deeper insight into the three-photon effects in multilevel systems.
Multilayer graphene (MLG) produced by micro-mechanical exfoliation can usually be stacked layer by layer in a Bernal way through van der Waals coupling. During the exfoliation, a partial bilayer graphene (BLG) is folded onto the BLG flake itself to form the exfoliated twisted (2+2)LG. In this paper, we measured Raman spectra of a few pieces of twisted (2+2)LGs with different twisted angles in back-scattering at room temperature with a HR Evolution micro-Raman system. The modes on both sides of G mode were measured to be a signature to distinguish the twisted angle and determine the layer number in twisted (2+2)LGs. The further research was extended to a twisted (3+3)LG and some results obtained in the twisted (2+2)LGs were confirmed. These results provide an applicable approach to probe the interlayer coupling in twisted graphenes and thus benefit the future research studies on their fundamental physics and potential applications.
Different doped silicon substrates have different device applications and have been used to fabricate solar panels and large scale integrated circuits. The thermal transport in silicon substrates are dominated by lattice vibrations, doping type, and doping concentration. In this paper, a variable-temperature Raman spectroscopic system is applied to record the frequency and linewidth changes of the silicon peak at 520 cm-1 in five chips of silicon substrate with different doping concentration of phosphorus and boron at the 83K to 1473K temperature range. The doping has better heat sensitive to temperature on the frequency shift over the low temperature range from 83K to 300K but on FWHM in high temperature range from 300K to 1473K. The results will be helpful for fundamental study and practical applications of silicon substrates.
Tremendous interest has recently focused on the layered TMDs. Layer number is one of the fundamental parameters in TMDs. In this paper, layer-number dependent reflectivity of WS2 and WSe2 flakes on SiO2/Si substrate were measured by a simple and fast reflection spectrum probing technique. Characteristic excitonic peaks, A and B, and some higher energy density of states excitonic peaks were observed and their properties as a function of layer number were studied. Our results are in agreement with the previous reports.
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