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In this study, we report the generation of stable even-order nonlinear polarization in the above gap states in strained multiple quantumwells. Using the strain field, the even-order polarization to emit terahertz waves is generated in a zincblende crystal. Our results can make the material and laser choice ranges wide because the exciton resonant excitation is unnecessary.

Magnetic fields can increase the intensity of terahertz (THz) waves due to changing the dipole moment direction using the Lorentz force. This study reports the increase in the THz-wave intensity generated by differential frequency mixing using commercial permanent magnets under exciton-excitation. While a weak magnetic field applied to a multiple quantum well increases the THz-wave intensity due to excitons, a strong field causes its decrease. According to the calculations, the increase is caused by the electron-hole separation due to the Lorentz force. Furthermore, the calculations suggest the importance of carrier acceleration to enhance the intensity. Importantly, the increase in the THz-wave intensity due to differential frequency mixing does not require a strong magnetic field and can be achieved with inexpensive commercially available magnets.

Intense terahertz-wave emission in the higher frequency region can result in various applications such as terahertz spectroscopy and ultrafast data communication. In this study, an increase in terahertz waves by the overlap of exciton states in different quantum wells and spectroscopic demonstration are reported. The excitation energy dependence of signal intensity shows the effect of the overlap. The signals measured under the condition of square dependence of intensity on the excitation power indicate interference with the periods corresponding to the laser energy difference. Furthermore, the absorption coefficient of the transparent sheet is obtained at specific frequency. These results indicate that the generation of intense terahertz waves at various frequencies using excitons is possible and that difference frequency mixing is a useful terahertz-wave source.

Al electroplating using ionic liquids (ILs) has been extensively studied for various potential applications, including corrosion-resistant coatings. When Al electroplating is to be performed commercially for many small objects, adopting the barrel plating method is reasonable for improving productivity and reducing manufacturing costs. However, little is known about the barrel-plating conditions for the formation of Al coatings. In this study, the barrel plating conditions for forming Al coatings on steel bolts using ILs composed of 1-ethyl-3-methylimidazolium chloride (EMIC) and aluminum chloride (AlCl3) were investigated. Hull cell tests showed that the IL with an AlCl3/EMIC molar ratio of 1.2 has a higher covering power than the IL with a higher AlCl3 ratio. Barrel plating using the IL with an AlCl3 ratio of 1.2 enabled complete coverage of the bolt with the Al coating. The uniformity of the Al coating on the bolts was further improved by adjusting the rotation speed of the barrel. Furthermore, a simple pretreatment procedure for steel bolts was proposed to obtain an electrodeposited Al coat that adheres well to steel bolts.

Photon upconversion (PU) is a process where an electron is excited from the valence band to the conduction band of a wide-gap semiconductor by the sequential absorption of two or more photons via real states. For example, two-step PU can generate additional photocurrent in the so-called intermediate-band solar cells. In this work, we consider two- and three-step processes; we study multi-step PU in a quantum dot (QD)-based single-junction solar cell with a double-heterointerface structure. The solar cell consists of three different absorber layers: Al0.7Ga0.3As, Al0.3Ga0.7As, and GaAs, which form two heterointerfaces. Just beneath each heterointerface, an InAs/GaAs QD layer was inserted. After band-to-band excitation, electrons accumulate at each heterointerface, and then, below-bandgap photons excite a certain fraction of these electrons above the barrier energy. The photoluminescence spectra of the InAs QDs reveal slightly different QD size distributions at the two heterointerfaces. We show that the external quantum efficiency is improved by additional irradiation with below-bandgap infrared photons, which suggests a multi-step PU process that involves the two heterointerfaces. The dependence of the photocurrent on the infrared excitation power density only shows a superlinear behavior when the GaAs layer is excited but the Al0.3Ga0.7As layer is not. These data demonstrate a multi-step PU process that consists of one intraband transition at each of the two heterointerfaces and one interband transition in GaAs.

Nanostructured intermediate band solar cells have been proposed as one of the promising candidates to surpass the limited efficiency of single-junction photovoltaic devices, while challenges still remain for realizing their practical implementation due to intricate carrier behaviors occurring in confined states. In this report, we have elucidated a detailed photo-carrier collection mechanisms in a quantum dot-in-well intermediate band solar cell, which is varied by effective electron intraband excitation process. Results from the photoluminescence response of quantum dots are discussed to examine the occupation state inside confined levels. Detailed analyses point out that diffusion of holes plays an important role in modulating the electrical generations.

Laser cooling in solids has been demonstrated, and cooling from room temperature to cryogenic temperatures has so far been the main focus of research. However, operation at high temperatures can be advantageous. We propose that laser cooling of Yb-doped yttrium aluminum garnet ceramic discs is enhanced above 300 K. Photoluminescence (PL) spectra is evidence that the phonon-assisted energy transfer from resonant states, which exhibit a narrow PL peak, to inhomogeneously distributed energy states becomes stronger at higher temperatures. This results in an enhanced non-resonant anti-Stokes PL at high temperatures. The ideal cooling efficiency determined from the PL spectrum at 470 K is 1.7 times higher than that at 300 K.

A photoelectrochemical cell utilizing porous carbon derived from Binchotan charcoal as the anode is demonstrated. Visible pulse laser irradiation of the Binchotan charcoal anode induces the ablation of carbon cations into NaNO3 aqueous solution. Electrons remaining in the anode reduce the potential of the anode with respect to a platinum cathode. As the electrodes are short-circuited, photogenerated electrons are transported from the anode to the cathode as a diffusion current. The photoinduced charge density at the anode exhibits superlinear and linear excitation laser power density dependences at low and high laser power densities, respectively. This difference in excitation power dependence is caused by the valences of the carbon cations generated by absorbing two-photon energy. The revealed reaction mechanism for electron generation at the Binchotan charcoal anode in the photoelectrochemical cell suggests that the sacrificial reagent for the anode can be not only Binchotan charcoal but also any materi...

Silicon (Si) and silicon carbide (SiC) nanoparticles embedded in an amorphous matrix of SiO xCy were grown using a very safe and economic precursor, tetra-ethyl orthosilicate (TEOS-(Si(OC2H5)4 )) by an organic catalytic chemical vapor deposition technique (O-Cat-CVD) at a relatively low filament temperature of 1700 °C and a substrate temperature of 300 °C. These crystalline nanoparticles were generated during the growth process in the thin film matrix, without the need for any post-annealing treatment. Morphology and crystalline phase identification of the nanoparticles were studied using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) respectively. X-ray Photoelectron Spectroscopy (XPS) and Fourier transform infrared (FTIR) revealed the presence of Si, O, C presence in the matrix and additionally different bonding peaks such as Si O Si rocking, Si C stretching, Si H bending, Si OH, Si O Si antisymmetric stretching, Si CH3 bending, C C and C H modes confirmed the formation of SiOxCy matrix. Optical studies were carried out using the photoluminescence spectroscopy , and broad, intense visible emission was observed from the sample. Further, investigation of the luminescence mechanism was carried out using the time-resolved photoluminescence (TRPL) and low-temperature photoluminescence (LTPL). The decay time was found in the order of few nanoseconds, which remarked that the emission is mainly from the radiative recombination inside the Si and SiC nanoparticles embedded in SiOxCy matrix and was attributed to the quantum confinement effect (Q.C.E). Additional, measurements were carried out by varying the laser pump power, and two extreme powers (low pump power, 2 mW, and high pump power, 30 mW) were used to find their impact on the emission. As in overall, the emission found in the present study can be used to develop potential applications such as LED, the third generation of solar cells among others.

We elucidate a photocarrier collection mechanism in intermediate band solar cells (IBSCs) with InAs-quantum dots (QDs)-in-an-Al0.3Ga0.7As/GaAs-quantum well structures. When the Al0.3Ga0.7As barrier is excited, the device electrical output can be varied by additional infrared light for the electron intraband optical transition in QDs. The photocurrent in IBSC with a single QDs-in-a-well structure shows a monotonic increase with the intraband-excitation density. Conversely, IBSC with a multilayered QDs-in-a-well structure exhibits a photocurrent reduction when electrons in QDs are optically pumped out. The simultaneously measured photoluminescence spectra proved that the polarity of QD states changes depending on the intraband-excitation density. We discuss the drift and diffusion current components and point out that the hole diffusion current is significantly influenced by carriers inside the confinement structure. Under strong intraband excitations, we consider an increased hole diffusion current occurs by blocking hole-capture in the quantum structures. This causes unexpected photocurrent reduction in the multilayered device.

Cooling via efficient emission of anti-Stokes photoluminescence can be utilized to realize novel cooling devices. It has been shown that rare earth-doped materials are good candidates for solid-state laser cooling because of their narrow emission spectra and discrete energy levels. We systematically study laser cooling efficiency of rare earth-doped oxide crystals. The photoluminescence spectrum evidences that the phonon-assisted energy transfer from resonant states to inhomogeneously distributed energy states sensitively depends on temperature. We discuss detailed properties of the cooling efficiency of Yb-doped yttrium aluminum oxide crystals and, furthermore, touch emission properties of their crystal thin films.

We aimed to clarify usefulness of the modified Frailty Index 11 (mFI-11) for assessing risk of postoperative complications (POCs) and effectiveness of perioperative management team (POMT) intervention for improving postoperative status of frail aged patients requiring colorectal cancer (CRC) surgery.We compared, retrospectively, surgical outcomes among 151 consecutive CRC surgery patients aged ≥80 years. Patients were grouped by mFI-11 scores and by POMT intervention (vs. no POMT intervention).POCs were more prevalent, postoperative stays were longer, and discharge status was poorer among high-risk (mFI-11 ≥ 3/11) patients without POMT intervention than among low-risk (mFI-11 ≤ 2/11) patients (p = 0.04, p = 0.02, p 0.01). Multiple POCs occurred less frequently and performance of activities of daily living was better for high-risk patients with (vs. those without) POMT intervention (p = 0.04, p = 0.03).POMT intervention appears beneficial for frail aged patients scheduled for CRC surgery.

Precise measurement of wafer flatness with high sensitivity and high spatial resolution is essential to realize high yields in nano-scale lithography because the depth of focus in this technology is relatively small. We report on a highly precise site flatness measurement system that employs a pair of heterodyne interferometers and achieves sub-nanometer precision for polished 300-mm silicon wafers. The determined overall mean standard deviation for the site flatness front-surface least-squares fit range is 0.21 nm. Furthermore, this system allows us to obtain images of the wafer flatness via scanning with a high spatial resolution of approximately 12 µm. These results suggest that the heterodyne interferometer-based system is a suited candidate for use at the next-generation 16-nm half-pitch technology node.

Intermediate-band solar cell is one of the promising concepts to jump out the dilemma of limited efficiency for traditional single junction photovoltaic devices. Albeit fruitful explorations have been done on revealing its working principle and promoting output performance, optical and electrical match of outer system can sometimes get it complicated because of extra below-bandgap photon absorptions. In this study, we reported a voltage backtracking behavior under intensive bi/uni-color irradiation circumstance within this kind of cell. Two different mechanisms are separately disclosed here for the sake of deep understanding how it operates and by which it can better work.

Two-step photon up-conversion solar cell is a newly-proposed concept for high-conversion efficiency photovoltaics, in which a heterointerface of different semiconductor materials is delicately merged inside common single-junction cell structure. Recently, we have reported an enhancement of its photocurrent under a reverse bias condition due to the boosted carrier collection efficiency. To push it further, in this study, modulation-doping method is employed in order for a better elucidation of two-step photon up-conversion at the heterointerface. It is observed that the difference of the external quantum efficiency with additional infrared light irradiation on the TPU-SCs can be greatly enlarged by changing doping concentration selectively reflecting the importance of the electric field at the heterointerface.

We demonstrated an enhancement of the ideal laser-cooling efficiency in Yb-doped yttrium aluminum garnet, (Yb:Y)AG, and Er-doped yttrium aluminum garnet, (Er:Y)AG, ceramic discs above 300 K. The temperature-dependent photoluminescence (PL) spectrum indicates that the phonon-assisted energy transfer from the narrowband, resonant state into the inhomogeneously distributed energy states is enhanced with increasing the temperature. The enhanced ideal cooling efficiency of the (Yb:Y)AG ceramic disc and the (Er:Y)AG ceramic disc at 470 K is 1.7 times higher than that at 300 K. The enhanced ideal cooling efficiency of (Er:Y)AG at 470 K marked 4.0% which is 1.8 times higher than that of (Yb:Y)AG.

Mid-infrared sensors detect infrared radiation emitted from objects, and are actually widely used for monitoring gases and moisture as well as for imaging objects at or above room temperature. Infrared photodetectors offer fast detection, but many devices cannot provide high responsivity at room temperature. Here we demonstrate infrared sensing with high responsivity at room temperature. The central part of our device is an Al0.3Ga0.7As/GaAs heterostructure containing InAs quantum-dot (QD) layer with a 10-nm-thick GaAs spacer. In this device, the electrons that have been accumulated at the heterointerface are transferred to the conduction band of the Al0.3Ga0.7As barrier by absorbing infrared photons and the following drift due to the electric field at the interface. These intraband transitions at the heterointerface are sensitized by the QDs, suggesting that the presence of the QDs increases the strength of the intraband transition near the heterointerface. The room-temperature responsivity spectrum exhibits several peaks in the mid-infrared wavelength region, corresponding to transitions from the InAs QD and wetting layer states as well as the transition from the quantized state of the triangular potential well at the two-dimensional heterointerface. We find that the responsivity is almost independent of the temperature and the maximum value at 295 K is 0.8 A/W at ~ 6.6 µm for a bias of 1 V, where the specific detectivity is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1.8\times {10}^{10}$$\end{document}1.8×1010 cmHz1/2/W.

The generation of two-photon photocurrent plays an essential role in realizing intermediate-band solar cells (IBSCs) with high conversion efficiencies. This current generation process strongly depends on the photocarrier dynamics in the intermediate levels, which can sometimes give rise to a deficient output current unexpectedly. In this work, we investigated the two-photon photocurrent generation process in InAs quantum dot (QD)-in-well intermediate-band solar cells. The two-photon photocurrent is generated by an interband transition in the structure (for example, in the well or the QD) and a subsequent intraband transition in the QD, and we used two different light sources to separately control these transitions. We found that, in the case of QD interband excitation in a sample with multiple wells, the carrier collection efficiency does not simply increase with the intraband excitation intensity; in the range from about 0.08 to 0.5 W/cm2, the collection efficiency decreases with increasing intraband excitation density. A comparison between samples with different numbers of wells revealed that the repetition of carrier trapping and detrapping during the transport in a multi-well structure can effectively modulate the recombination rate. This modulation induces a reduction of the current yield under certain illumination conditions. We propose a model to explain this phenomenon and verify it by investigating the bias dependence of the two-photon photocurrent from the QD.

A two-step photon upconversion (TPU) solar cell is a single-junction solar cell with a suitably designed heterointerface in the intrinsic layer. At this interface, upconversion can take place by absorbing infrared light, which excites carriers to energies above the heterojunction barrier, and then an electric field extracts these upconverted carriers to the barrier material. In this work, we study this photoabsorption process in a TPU solar cell with a modulation-doped structure. The modulation doping technique enables us to control the electric field at the heterointerface, and we demonstrate that the efficiency of the TPU process strongly depends on the electric field at the heterointerface. While we show that the TPU process induced by infrared light leads to a significant increase in the external quantum efficiency, we also note that this TPU process is an adiabatic process. It is shown that the adiabatic intraband photoexcitation at the heterointerface is able to increase the open-circuit voltage even above the open-circuit voltage that we predict from the diode equation and the actual current increase. We demonstrate obvious voltage boost effects and elucidate the relationship between the current and voltage.

Interactions between excitons with different energies are significant in device application, particularly in the emission of intense terahertz (THz) waves caused by difference frequency mixing. In this paper, the interactions of excitons generated by two resonant continuous-wave laser beams are assessed via the change in reflectivity. While calculations suggest that the reflectivity change depends on the exciton number, experimental results measured for various excitation powers, probe powers, and excitation energies (heavy-hole (HH) and light-hole (LH) excitons) show different profiles. Furthermore, the signals probed at a HH exciton differ from those at a LH exciton. These results indicate the importance of careful control of excitation levels for THz wave generation.

Using X-band Ferromagnetic Resonance (FMR) Spectroscopy, we demonstrate the microscopic ferromagnetic resonance features of degenerated GdN semiconductor. The FMR spectrum suggests a single resonance mode below 10 K; interestingly, this particular structure is found to exhibit a peculiar magnetic resonance (PMR) on the top of the uniform FMR while temperature increases from 12–36 K. The low-field PMR mode attributed to the differently magnetized part of the film with an easy in-plane axis. The narrow-field gap between PMR and uniform FMR suggests the strong coupling owning to the differently magnetized part with easy in-plane axis and the magnetized part with an out-of-plane axis. The saturation magnetization, cubic magnetocrystalline anisotropy, and uniaxial anisotropy of GdN epitaxial film have been evaluated by the angular-dependence FMR.

We have recently proposed a solar cell (SC) structure called two-step photon up-conversion SC (TPU-SC). In this SC, we have observed an obvious enhancement in the open- circuit voltage as well as the short-circuit current. In this study, we investigated the excitation intensity dependence of TPU. We find that the extra-photocurrent caused by TPU shows a sublinear relationship with the interband excitation intensity. In addition, the enhancement of the open-circuit voltage by TPU is larger than the value estimated from the short-circuit current, suggesting a voltage-boost at the hetero-interface.

Two-step photon up-conversion solar cell (TPU-SC) we have recently proposed is a single-junction solar cell containing a hetero-interface of different semiconductor materials. Although efficient two-step photon up-conversion is achieved in the TPU-SC, the detailed mechanism of the significant intraband photoexcitation occurring at the hetero-interface is still unclear. In this study, we performed simultaneous measurements of photoluminescence and photocurrent as a function of the applying bias voltage in the TPU-SC. We experimentally demonstrate the reciprocity relationship between the radiative recombination and the photocurrent of the TPU-SC.

We demonstrated hot-carrier (HC) extraction in GaAs solar cells containing InAs/GaAs quantum dot superlattices (QDSLs) functioning as a light absorber at 15 K. The short-circuit current density and the open-circuit voltage in the QDSL solar cells show step-wise changes as a function of the excitation photon density because of state filling under below-bandgap excitation. Furthermore, the short-circuit current density and the open-circuit voltage originated from the HC extraction were enhanced by increasing the period of the QDSL due to the improved absorptivity.

We report the change in the terahertz wave properties originating from the electron dynamics in the GaAs layer excited by the ultrashort pulse, which is caused by growth of InAs quantum dots and covering the quantum dots by the nitrogen atoms with or without the GaAs spacer layer. Due to the growth of InAs quantum dots, the excitation energy to generate the terahertz wave extends to the lower energy side than the bandgap of GaAs. This enables the use of the various laser system to generate the terahertz wave except for the Ti:sapphire pulse lasers.

The solar cells covered in the previous chapters are idealized solar cells, which allowed us to evaluate the efficiency limits under various conditions. Of course, besides the conditions employed so far (like the transmission of photons with Ephoton < Eg), there exist plenty of other factors that lead to losses in the amount of generated electricity and thus are important for the actual development of solar cells. Due to these losses, the conversion efficiency of an actually fabricated solar cell is smaller than the theoretical efficiency limit. For example, if we use silicon (Si) for the solar cell absorber material, the detailed balance limit of the conversion efficiency is about 30%. This value can be obtained from Fig. 5.10 in Chap. 5 by looking up the efficiency for the curve (b) at 1.1 eV, which is the band gap of Si. On the other hand, when we look up the present values for actual Si p–n junction solar cells, we find that the single crystal type has reached a maximum of 26.3% and the multi-crystalline type has reached a maximum of 22.3%, both being well below the theoretical efficiency limit[1]. In Chap. 3, we discussed the trade-off relation between the transmission and thermalization losses in the single-junction solar cell and derived the conversion efficiency limit that is imposed by these two types of losses. In considerations on the solar cell performance, such trade-off relations exist for various parameters. As another example for a trade-off relation, in the following sections, we introduce the semiconductor light absorption characteristics, which differ for each material. With this new concept, we discuss the solar cell conversion efficiency from a viewpoint that is different from the detailed balance model elaborated in the previous chapters. The trade-off presented in the following sections is a trade-off between the thickness of the semiconductor that is required for sufficient light absorption and the distance that can be travelled by the generated electrons and holes. The latter property is essential for the efficient extraction of carriers at the electrodes. It is straightforward that a thicker semiconductor can absorb more light. On the other hand, if the semiconductor is too thick, the distance that has to be travelled by the photoexcited electrons and holes also becomes longer. If the path to be travelled is long, there is a high probability for the electron to collide with an obstacle before its electrical energy can be used. Then the energy is unfortunately lost in spite of the efforts to generate it. A thicker semiconductor is not necessarily better; we have to consider the optimum thickness for efficient extraction of the electrical energy. After discussing the light absorption characteristics of semiconductors in Sect. 7.1, we explain the transport of the generated electrons and holes inside the semiconductor in Sect. 7.2. In the last section, we consider two specific semiconductor materials, Si and GaAs, as examples and discuss the optimum semiconductor thickness in a solar cell.