Optical properties of the rat head tissues (brain cortex, cranial bone and scalp skin) are assessed, aiming at transcranial light applications such as optical imaging and phototherapy. The spectral measurements are carried out over the wide spectral range of 350 to 2800 nm, involving visible, near-infrared (NIR) and short-wave infrared (SWIR) regions. Four tissue transparency windows are considered: ~700 to 1000 nm (NIR-I), ~1000 to 1350 nm (NIR-II), ~1550 to 1870 nm (NIR-III or SWIR) and ~2100 to 2300 nm (SWIR-II). The values of attenuation coefficient and total attenuation length are determined for all windows and tissue types. The spectra indicate transmittance peaks in NIR, NIR-II and SWIR-II, with maximum tissue permeability for SWIR light. The use of SWIR-II window for the transcranial light applications is substantiated. Furthermore, absorbance of the head tissues is investigated in details, by defining and describing the characteristic absorption peaks in NIR-SWIR.

heterojunctions have been constructed and demonstrated for controlling optical, [1] electrical, [2] mechanical [3] and magnetic characteristics. [4] In particular, semiconductor heterojunctions as the core of light-emitting diodes (LEDs) have played vital roles in electric-driven lighting and display devices. The electric potential of a semiconductor heterojunction has a strong positive effect on charge carrier transport at the interface and can tune/ control the behaviors of light emitting. [5] The advances in lighting technology have greatly promoted the development of artificial intelligence, biotechnology and flexible optoelectronics. [6] At present, almost all LEDs are driven by external power supply through wire connecting electrodes. However, the high-efficiency heterojunction material driven by Newton force to achieve the stress light-emitting devices is still limited in the present research. Thus, the exploration of such a new type of light-emitting device without wires and electrodes not only supplies advanced heterojunction systems for light-emitting but also provides a prospective reference for the future multiapproach energy conversion with extended applications.As a special type of light source, mechanoluminescence (ML) materials are capable of generating photon emissions in response to mechanical stimuli. In comparison with LEDs based on electroluminescence (EL), ML provides sustainable light output by excitation of mechanical energy ubiquitously available in nature. During the past decade, ML materials have attracted widespread interests due to their promising applications in stress sensing, display, artificial skin, bioimaging, anti-counterfeiting, structure fatigue diagnosis, night surveillance and flexible optoelectronics. [7][8][9][10] However, the recent developments of highperformance ML materials are not as fast as other luminescence systems such as photoluminescence (PL)/EL, which is attributed to the lack of rational design of ML material systems guided by the in-depth theoretical exploration in the mechanism. ML materials known to date are typically homogenous structures, which offer limited space for optimizing the ML performance. Therefore, further improving the ML performance by exploiting heterostructures remains a challenge for present research. [11,12] In this work, we fabricate a class of ZnS/CaZnOS heterostructures, which flexibly tune the efficient and reproducible Actively collecting the mechanical energy by efficient conversion to other forms of energy such as light opens a new possibility of energy-saving, which is of pivotal significance for supplying potential solutions for the present energy crisis. Such energy conversion has shown promising applications in modern sensors, actuators, and energy harvesting. However, the implementation of such technologies is being hindered because most luminescent materials show weak and non-recoverable emissions under mechanical excitation. Herein, a new class of heterojunctioned ZnS/CaZnOS piezophotonic systems is presented, which disp...

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Deep levels in metamorphic InAs/In x Ga 1−x As quantum dot (QD) structures are studied with deep level thermally stimulated conductivity (TSC), photoconductivity (PC) and photoluminescence (PL) spectroscopy and compared with data from pseudomorphic InGaAs/ GaAs QDs investigated previously by the same techniques. We have found that for a low content of indium (x=0.15) the trap density in the plane of self-assembled QDs is comparable or less than the one for InGaAs/GaAs QDs. However, the trap density increases with x, resulting in a rise of the defect photoresponse in PC and TSC spectra as well as a reduction of the QD PL intensity. The activation energies of the deep levels and some traps correspond to known defect complexes EL2, EL6, EL7, EL9, and EL10 inherent in GaAs, and three traps are attributed to the extended defects, located in InGaAs embedding layers. The rest of them have been found as concentrated mainly close to QDs, as their density in the deeper InGaAs buffers is much lower. This an important result for the development of light-emitting and light-sensitive devices based on metamorphic InAs QDs, as it is a strong indication that the defect density is not higher than in pseudomorphic InAs QDs.

Optical and photoelectric properties of metamorphic InAs/InGaAs and conventional pseudomorphic InAs/GaAs quantum dot (QD) structures were studied. We used two different electrical contact configurations that allowed us to have the current flow (i) only through QDs and embedding layers and (ii) through all the structure, including the GaAs substrate (wafer). Different optical transitions between states of QDs, wetting layers, GaAs or InGaAs buffers, and defect-related centers were studied by means of photovoltage (PV), photoconductivity (PC), photoluminescence (PL), and absorption spectroscopies. It was shown that the use of the InGaAs buffer spectrally shifted the maximum of the QD PL band to 1.3 μm (telecommunication range) without a decrease in the yield. Photosensitivity for the metamorphic QDs was found to be higher than that in GaAs buffer while the photoresponses for both metamorphic and pseudomorphic buffer layers were similar. The mechanisms of PV and PC were discussed for both structures. The dissimilarities in properties of the studied structures are explained in terms of the different design. A critical influence of the defects on the photoelectrical properties of both structures was observed in the spectral range from 0.68 to 1.0 eV for contact configuration (ii), i.e., in the case of electrically active GaAs wafer. No effect of such defects on the photoelectric spectra was found for configuration (i), when the structures were contacted to the top and bottom buffers; only a 0.83 eV feature was observed in the photocurrent spectrum of pseudomorphic structure and interpreted to be related to defects close to InAs/GaAs QDs.

Having used thermally stimulated conductivity (TSC) technique, we identified deep electron traps that produce strong effects on charge carrier transport and photoconductivity in InGaAs/GaAs quantum dot (QD) structures. The values of deep levels below the conduction band of GaAs at 0.16, 0.22, and 0.35 eV are obtained from the analysis of the shapes of TSC curves after the excitation with the quanta energy hv = 0.9, 1.2, and 1.6 eV. The level 0.16 eV in depth is an effective electron trap that provides crossing of lateral conductivity with a high-resistance mode and, therefore, causes a high photocurrent sensitivity of about 3 A/W at 77 K with excitation by interband transitions in QDs. We determined the charge density of electrons captured by the (Ec – 0.16 eV) level to be 2 × 10−6 C/cm2 at 77 K that induces electric field ∼ 105 V/cm in a vicinity of QDs. The state at Ec – 0.22 eV is shown to be related to the recombination center that can hold non-equilibrium holes over a long time under the condition that the non-equilibrium holes are localized by the quantum states of QDs. In the course of long-term electron storage in a vicinity of QDs, an electron trapped at the (Ec – 0.16) eV level can be recaptured by a deeper spatially remote (Ec – 0.22 eV) level that allows the TSC peak observation at 106 K.

The metal nanoparticle size and shape impact the plasmonic enhancement of Raman and photoluminescence (PL) spectra of monolayer and few-layer MoS2 decorated with them. The plasmonic enhancement is investigated for Ag nanotriangles (NTs or nanoprisms) of different sizes in comparison to Ag nanospheres (NSs) at room temperature. After the decoration with Ag NTs, the intensity of both Raman modes of MoS2 increases up to 6.8 times. The μ-PL spectra of bare MoS2 show the presence of the A and B exciton bands as well as of a weak trion component. After covering the flakes with 50 nm Ag NTs, the highest integrated PL enhancement factors are 2.9 and 2.1 under 532 and 405 nm excitations, respectively. The revealed shape effect is that Ag NTs provide much stronger Raman and exciton emission enhancement than Ag NSs, which is due to the generation of plasmonic hot spots near the sharp edges and tips of NTs. Another mechanism of enhancement is the plasmonic coupling between the neighboring Ag NTs that causes the generation of hot spots in the gap between NTs. The revealed size effect is a decrease of Raman and PL enhancement with an increase in size of Ag NTs or NSs, which is due to an increase in radiative damping of plasmon oscillation occurring with an increase in nanoparticle size. The important feature is a strong enhancement of the A– trion component after decorating MoS2 with Ag nanoparticles. The phenomenon may be explained by the surface-plasmon-mediated generation of hot electrons in Ag nanostructures, which then inject to MoS2 flakes. This work gives new fundamental insights into the physical mechanisms of light–matter coupling in hybrid two-dimensional (2D) semiconductor/plasmonic nanoparticle structures, which are highly promising for next-generation optoelectronic and nanophotonic devices.

Photoelectric properties of laterally correlated multilayer InGaAs/GaAs quantum dots (QDs) heterostructures are studied. The response of the photocurrent to increasing excitation intensity is found to be nonlinear and varying with excitation energy. The structures are photosensitive in a wide range of photon energies above 0.6 eV. The spectral dependence of the photoconductivity (PC) is caused by strong interaction between the bulk GaAs and the lower energy states of the wetting layer, the QDs, as well as the defect states in the GaAs band gap. In particular, a mechanism for the participation of deep electron trap levels in the photocurrent is clarified. These structures also demonstrate a high sensitivity of up to 10 A/W at low excitation intensities. However, at higher excitation intensities, the sensitivity reduces exhibiting a strong spectral dependence at the same time. The observed sublinear PC dependence on excitation power results from a direct electron-hole recombination both in the QDs and in GaAs host. The solution of rate equations included the contributions of QD ground and exited states, bulk GaAs states and the states of defects within the GaAs bandgap describes well the experimental data.