Xiangai Cheng

Temperature-dependent excitonic photoluminescence excited by two-photon absorption in perovskite CsPbBr 3 quantum dots.

Recently, lead halide perovskite quantum dots have been reported with potential for photovoltaic and optoelectronic applications due to their excellent luminescent properties. Herein excitonic photoluminescence (PL) excited by two-photon absorption in perovskite CsPbBr3 quantum dots (QDs) has been studied at a broad temperature range, from 80 to 380 K. Two-photon absorption has been investigated and the absorption coefficient is up to 0.085 cm/GW at room temperature. Moreover, the PL spectrum excited by two-photon absorption shows a linear blue-shift (0.32 meV/K) below the temperature of 220 K. However, for higher temperatures, the PL peak approaches a roughly constant value and shows temperature-independent chromaticity up to 380 K. This behavior is distinct from the general red-shift for semiconductors and can be attributed to the result of thermal expansion, electron-phonon interaction and structural phase transition around 360 K. The strong nonlinear absorption and temperature-independent chromaticity of CsPbBr3 QDs observed in temperature range from 220 to 380 K will offer new opportunities in nonlinear photonics, light-harvesting, and light-emitting devices.

Identification of the formation phases of filamentary damage induced by nanosecond laser pulses in bulk fused silica

Employing a pump-probe polarization-based two-frame shadowgraphy setup, the formation of filamentary damage induced in bulk fused silica by a nanosecond pulse at 1064 nm is investigated with a picosecond probe. Three different phases are exhibited in the damage experiments. The first phase is the formation of a micrometric plasma channel along the laser direction during the beginning of the pulse likely caused by multi-photon ionization. This channel exhibits growth during ∼400 ps, and the newly grown plasma is discrete. Then, during the end of the pulse, this channel evolves into a tadpole-like morphology showing an elliptical head upstream the laser flux followed by a thin tail. This observed asymmetry is attributed to shielding effects caused by both the plasma and hot modified silica. Once the damage shows its almost final morphology, a last phase consists in the launch of a pressure wave enlarging it after the laser pulse. The physical mechanisms that might be involved in the formation of plasma chan...

Giant photoluminescence enhancement in monolayer WS 2 by energy transfer from CsPbBr 3 quantum dots

Monolayer (ML) transition metal dichalcogenides (TMDCs) are thought to be highly promising materials for the optoelectronic and nanophotonic applications. However, the low absorption cross section and photoluminescence (PL) quantum yield in such atomically thin layers restrict their applications. Considering that the energy transfer in a heterostructure can modulate TMDCs’ optical properties, a type I heterostructure geometry comprising ML TMDCs and lead halide perovskite quantum dots (QDs) has the potential to overcome these shortcomings. In this work, spin-coating the CsPbBr3 QDs on ML WS2 results in ~12.7 times enhancement in the PL intensity of ML WS2 at 295K. This giant enhancement is attributed to the energy transfer process from CsPbBr3 QDs to WS2 with a ~40% energy transfer efficiency and ~2 × 108 s−1 energy transfer rate. Besides, we observed that the internal quantum efficiency of ML WS2 is increased from 6.35% to 29.01%. The result demonstrates the feasibility of using perovskite QDs and ML TMDCs to form a type I heterostructure and improve the performance of the TMDC-based optoelectronic devices.

The over-saturation phenomenon of a Hg0.46Cd0.54Te photovoltaic detector irradiated by a CW laser

A Hg0.46Cd0.54Te (~0.91 eV bandgap) photovoltaic detector is exposed to 10.6 ?m and 1319 nm intense laser radiations. The open-circle voltage (Voc) of the detector is measured and found to decrease with laser power density above the saturation threshold. The characteristics of over-saturation phenomena are quite sensitive to the temperature. For the Hg0.46Cd0.54Te detector, the photon energy of a 10.6 ?m laser (sub-bandgap laser) is less than the bandgap, and the photon energy of a 1319 nm laser (above-bandgap laser) is above the bandgap. Both lasers can induce an over-saturation phenomenon, but the dominant mechanism is different. The results indicate that photovoltage and thermoelectric voltage are the dominant mechanisms for above-bandgap illumination, and thermovoltage and thermoelectric voltage are the dominant mechanisms for sub-bandgap illumination.

Effects of thermally generated carrier and temperature dependence mobility in InSb photoconductive detector under CW 10.6 µm laser irradiation

The response mechanism of n-type indium antimonide photoconductive detectors under intense continuous wave (CW) 10.6 µm laser irradiation is investigated. The magnitude of the Voc signals and the exact shape of the signals vary greatly with laser power density and irradiation time. It is found that the signals begin to decrease at a critical time when laser power density is held in a constant value. If the irradiation time is fixed, the signals begin to decrease in a critical laser power density. The Voc signals for both radiation process and the end of laser irradiation have two different response time scales. A two-dimensional model of the detector for CW laser irradiation is presented. The calculated response curves agree well with the experimental results. The two separate time scales are found to be due to two thermally resistive bonding layers. Temperature-dependent mobility is the domain mechanism for the increasing tendency of Voc signals, and the decreasing tendency results from thermally generated carrier effect.

Dielectric properties of a CsPbBr_3 quantum dot solution in the terahertz region

In recent years, CsPbBr3 quantum dots (QDs) have attracted much attention due to their bright prospects in solar cell studies. Dielectric properties are important for the fabrication of optoelectronic devices. Here, the dielectric properties of a CsPbBr3 QD solution are investigated between 0.1 and 2.0 THz by terahertz time-domain spectroscopy. The measured frequency-dependent transmitted ratio is found to decrease from 0.96 to 0.80 in this range. By comparing different concentrations of the QD solution, the frequency-averaged absorption is linearly increased with the increase in QD concentration. After that, the frequency-dependent dielectric constant, including the complex refractive index, complex dielectric constant, and conductivity, is extracted by Fourier transform of the time-domain spectrum. An effective medium approach method is adopted to extract the complex dielectric constant of a CsPbBr3 QD inclusion, and a slight peak around 0.4 THz is found in the imaginary part of the dielectric constant. The result of Drude-Lorentz fitting shows that the phonon plays a dominant role in the dielectric properties of a CsPbBr3 QD solution. Moreover, the THz response of a CsPbBr3 QD is found to be unchanged when the test is conducted under illumination. We attribute this phenomenon to the discrete energy level of excitons in CsPbBr3 QDs due to quantum confinement, and design a comparative experiment to validate it. This study is significant for its deeper insight into the dielectric properties of CsPbBr3 QDs, and thus is helpful through its applications in optoelectronics.

The simulation of behaviors of photodetectors under pulsed laser irritation

Precise simulation of transient electrical behaviors of photodetectors under laser irradiation is becoming an increasingly concern. It not only can allow a detailed study and analysis of complex phenomena that cannot be carried out by experiments, but gives valuable information about the physical mechanisms which ultimately determine the response of the photodetectors. Finite difference numerical technique is adopted in the simulation to calculate the current response of photodetectors under pulsed laser irritation in this paper. To simulation the behaviors of photodetectors under pulsed laser irritation, the transport and trapping of carries and external circuit effects, including load resistance, junction capacitance, and parasitic capacitance, are considered. The basic equations governing the carrier behaviors are solved, including Poisson’s equation, the carrier motion equations, and the carrier continuity equations. The simulated transient carrier density and velocities are present, as well as corresponding transient electric field distributions. The behaviors of electrons and holes and its contributions to the external current response are analyzed. Then a general and brief image of the transient progress of photodetectors under pulsed laser irritation is established. How the carrier is induced, transported, and trapped and whether they make any significant contribution to the external current response are discussed. Besides, bias dependent response is also studied. Higher bias will improver the behaviors of photodetectors under pulsed laser irritation. The simulated results and theory analysis will show valuable clue for future research on the behaviors of photodetectors irradiated by pulsed laser.

The output of photovoltaic detector irradiated by spectral unrelated laser

The composition of open-circuit voltage of photovoltaic detector under irradiation of spectral unrelated laser is analyzed. The structure of detector, the energy band of PN junction, temperature gradient in the detector and the movement of carriers are investigated to explain the mechanism of thermovoltage. For illumination, the detector whose irradiation surface is n-type or p-type semiconductor is taken as an example to make an analysis of the composition of open-circuit voltage. Results show that, the open-circuit voltage of photovoltaic detector under irradiation of spectral unrelated laser is decided by thermovoltage and temperature difference-EMF (Electric Motive Force). Moreover, the voltage directions of them are opposite no matter what region (p or n) is taken as irradiation surface.

The different electrical responses of HgCdTe and InSb photovoltaic infrared detectors under pulsed laser irradiation

In the experiments of photovoltaic detectors illuminated by CW lasers, some new mechanism has been found, such as power saturation of photovoltage, hot carrier effect, as well as thermovoltage effect. To investigate whether there is similar mechanism with pulsed laser irradiating, an 808nm femtosecond pulsed laser is adopted. In the experiments, three photovoltaic infrared detectors are used, namely short wavelength HgCdTe detector, medium wavelength HgCdTe detector and medium wavelength InSb detector. Actually, the 808nm pulsed laser is spectral related laser for short wavelength HgCdTe detector while spectral unrelated laser for medium wavelength HgCdTe and InSb detector. Under various power densities, the detectors have a series of outputs. Power saturation of photovoltage is observed. However, the characteristics of the outputs of these three detectors are quite different, even between medium wavelength HgCdTe and InSb detector, which have the same packing method. There are three major contributions in the paper. Firstly, explain the mechanism of power saturation of photovoltage, mainly from hot carrier effect and the depressed ability of PN junction to separate electrons and holes with the higher temperature induced by the laser. Secondly, compare the differences between medium wavelength HgCdTe and InSb detector and give a qualitative analysis. Finally, the difference of the outputs between short and medium wavelength HgCdTe detector is compared and qualitatively analyzed, too, with the different mechanisms of interaction between infrared detectors and spectral related or spectral unrelated laser. The experimental results and theory analysis will show valuable clue for future research on photovoltaic detector irradiated by pulsed laser.