Longer Wavelength Side Research Articles

Design Concept of Red and Yellow Emitting Novel Oxide Phosphors

Recent years, Eu2+ or Ce3+-activated nitride and oxynitride phosphors have been usually used in the white LEDs as commercial phosphors due to their high chemical and thermal stabilities. For example, in the Eu2+-activated CaAlSiN3 phosphor, which is commonly used in white LEDs as commercial red-emitting phosphor, the Ca2+ ions located in the tunnels surrounding with 6 corner-sharing tetrahedra of SiN4 and AlN4 and are directly coordinated by 5 nitrogen ions having short bonding length about 0.2493 nm.1) The excitation and emission band of Eu2+-activated phosphors is due to the energy transitions between the 4f ground state and 5d excited state of Eu2+. Since 5d orbital electrons are not shielded by outer orbital electrons, it is strongly affected by the crystal field and the excitation and emission wavelengths of Eu2+-activated phosphors are also strongly depended on the crystal field.2) The short bonding length between activator and anions is causative of the increase of the crystal field splitting of the 5d band, which leads to decrease the energy gap between the 4f ground state and the lowest level of the 5d excited state. Therefore, the CaAlSiN3:Eu2+ phosphor shows the excitation and emission spectra locating in longer wavelength side.Meanwhile, this theory indicates that Eu2+ or Ce3+-activated oxide phosphors are also expected to be obtained the excitation and emission band in the longer wavelength region, if it select the host materials having compactible site for substitution of activator ions. Therefore, we focused on oxide materials including the 6 or 7 coordination site with short bonding length in the lattice as a host materials to develop novel red emission LED phosphors. As a result, we could successfully synthesize the novel red emission phosphors,3,4) and in this study, we presents the luminescence properties of the novel oxide phosphors developed.Figure 1 shows the excitation and emission spectra of olivine-type NaMgPO4:Eu2+ phosphor. This phosphor exhibits excellent optical absorption in the blue light region and shows strong broad red emission band peaking 628 nm under excitation at 450 nm.3) In the crystal structure of NaMgPO4, Na+ ions, which is substitution site for Eu2+, locate in three-dimensional framework with PO4 tetrahedra and MgO6 octahedra connection and are coordinated by 6 oxide anions. The average bonding length between Na+ and O2− in NaO6 octahedra was 0.2360 nm, which is smaller than that of N3−−Ca2+ (0.2493 nm) in CaAlSiN3:Eu2+ phosphor. This indicates that NaMgPO4:Eu2+ phosphor has stronger or similar crystal field strength of O2− around Eu2+ in the lattice than/to that of CaAlSiN3:Eu2+ phosphor. Accordingly, NaMgPO4:Eu2+ phosphor can shows the red emission with high luminescence efficiency.Figure 2 shows the excitation and emission spectra of M3Sc4O9:Ce3+ (M = Ba and Sr) phosphors. In the M3Sc4O9:Ce3+ (M = Ba and Sr) phosphors, from viewpoint of ionic radius and electrovalence, Ce3+ ions substituted into the Sc3+ site in the M3Sc4O9 lattice. The Sc3+ ions are coordinated by 6 oxide anions in the crystal structure and the bonding length between Sc3+ and nearest O2− is 0.1739 nm and it is shorter than that of Y3+−O2− in Y3Al5O12:Ce3+ phosphor (0.2397 nm), which indicates that the crystal field strength of O2− around Ce3+ in the M3Sc4O9:Ce3+ (M = Ba and Sr) phosphors is stronger than that of Y3Al5O12:Ce3+ phosphor. Therefore, the emission band of M3Sc4O9:Ce3+ (M = Ba (583 nm) and Sr (610 nm)) located in longer wavelength side than that of Y3Al5O12:Ce3+ phosphor (555 nm).This work was supported by a project from NEDO, New Energy and Industrial Technology Development Organization (Rare Metal Substitute Materials Development Project Development of Technology for Reducing Tb and Eu Usage in Phosphors for Fluorescent Lamp by High-speed Material Synthesis and Evaluation).ReferencesR. J. Xie, N. Hirosaki, Y. Li, and T. Takeda, Materials, 3, 3777 (2010).P. Dorenbos, in Phosphor Handbook, 2nd ed., W. M. Yen, S. Shionoya, H. Yamamoto, p. 139, CRC Press, Boca Raton, FL (2007).S. W. Kim, T. Hasegawa, T. Ishigaki, K. Uematsu, K. Toda, and M. Sato, ECS Solid State Lett., 2, R49 (2013).T. Hasegawa, S. W. Kim, T. Ishigaki, K. Uematsu, H. Takaba, K. Toda, and M. Sato, Chem. Lett., (2014) in press.

High efficiency structured EUV multilayer mirror for spectral filtering of long wavelengths.

High spectral purity at longer wavelength side is demanded in many extreme ultraviolet (EUV) and soft X-ray (together also referred to as XUV) optical systems. It is usually obtained at the expense of a high loss of XUV efficiency. We proposed and developed a new method based on a periodic, tapered structure integrated with an EUV multilayer. The longer wavelength radiation is scattered/diffracted away by the tapered multilayer structure while the EUV light is reflected. The first proof-of-principle showed a broadband suppression from λ = 100-400 nm with an average factor of 14. Moreover, a high EUV reflectance of 64.7% was achieved, which corresponds to 94% of the efficiency of a regular EUV multilayer mirror.

Glass Structures and Linea/Nonlinear Optical Properties of Ag<sub>2</sub>O-Doped TeO<sub>2</sub> Glasses

A family of TeO2-based glasses is known to have high third-order nonlinear properties and expected as nonlinear optical devices. In the present study, we fabricated Ag2O-TeO2 and Ag2O-Nb2O5-TeO2 glasses, and estimated their structures and linear/nonlinear optical properties. The main results showed that the absorption-edge of the glasses was shifted to longer wavelength side with increasing Ag2O content. By Raman spectroscopy, it was found that an increase in Ag2O content promoted to change TeO4 to TeO3/TeO3+1 units. X-ray photoelectron spectroscopy (XPS) revealed the presence of an electronic state of Ag2+ ions, which was supposed to have a large influence on the absorption-edge wavelength. In Ag2O-Nb2O5-TeO2 glass system, the third-order nonlinear susceptibility χ(3) increased with increasing Ag2O content at the expense of Nb2O5 content.

Study of a mode-locked erbium-doped frequency-shifted-feedback fiber laser incorporating a broad bandpass filter: Numerical results

We present a comprehensive numerical study on the spectral and temporal behaviors of a mode-locked erbium-doped frequency-shifted-feedback fiber laser as a function of the frequency shift and optical bandwidth of the laser cavity. For this we develop a numerical model which is based on the rate equation and nonlinear Schrödinger equation for the fiber-based active cavity. We numerically verify that if the ratio of the filter bandwidth to the frequency-shift value is higher than ~400 times, the spectral broadening of the laser output tends to break up and form a secondary spectral band (SSB) on the wavelength side of the spectrum where the spectral components of the cavity modes are constantly shifted by the intracavity frequency shifter. We also verify that the SSB forms a satellite pulse in the time domain, traveling on either the trailing edge or leading edge of the main pulse, depending on whether the SSB is formed on the shorter or longer wavelength side of the pulse spectrum. We emphasize that these numerical results are also in good agreement with the experimental results discussed in our previous report [25].

Laser from the dimer state of a conjugated polymer (PFO) in solution

The spectral and laser properties of the conjugated polymer poly (9,9-dioctylfluorenyl-2, 7-diyl) (PFO) in benzene have been studied and presented in this paper. The absorption spectra of PFO in benzene under low concentration showed that there was only one peak at 390 nm. When the concentration was increased, a new peak at 437 nm appeared additionally, indicating dimer formation at higher concentration of PFO in benzene. The fluorescence spectra for low concentration showed two peaks at 422 nm and 444 nm, which could be attributed to the monomer and the excimer. At the longer wavelength side of these spectra, there was a hump at 465 nm. This hump became dominant for higher concentrations and in low temperatures, while the other peaks became weaker. So, this new peak at 465 nm may be due to the dimer. For still higher concentrations, the peaks at 422 nm and 444 nm vanished and there was a dominant peak only at 465 nm. Under suitable concentration and pump power excitation of Nd: YAG laser (355 nm) the amplified spontaneous emission (ASE) spectra were obtained. It could be seen that there were three ASE peaks at 422, 444 and 470 nm. These peaks could be attributed to the monomer, excimer and dimer respectively. This is perhaps the first report on ASE from dimers of the conjugated polymer, PFO in liquid solution.

Short-wavelength multiline erbium-doped fiber ring laser by a broadband long-period fiber grating inscribed in a taper transition

A stable multiwavelength all-fiber erbium-doped fiber ring laser (EDFRL) based on a broadband long-period fiber grating (LPFG) inscribed in a fiber taper transition is presented. The LPFG’s characteristics were engineered to provide a higher loss at the natural lasing wavelength of the laser cavity. The LPFG inscribed on a taper transition provided a depth greater than 25 dB, and posterior chemical etching provided a broad notch band to inhibit laser generation on the long-wavelength side of the EDF gain. Up to four simultaneous laser wavelengths are generated in the range of 1530–1535 nm.

On the surface morphology and transport properties of chemical bath deposited CoxCd1−xS thin films: A correlation

Abstract Co x Cd 1− x S (0 ≤ x ≤ 0.5) thin composite films were grown onto glass micro slides from an aqueous alkaline medium using a chemical bath deposition process. A complex liquid phase was formed by the addition of equimolar (1 M) volumes of cadmium sulphate, thiourea and cobaltous sulphate. For deposition, various preparative parameters, viz., speed, temperature, pH, and time were selected as optimized earlier. The chemical composition and surface morphology of these films were determined by an Energy Dispersive Spectroscopy (EDS) and the Scanning Electron Microscopy (SEM), respectively. EDS revealed the film composites to be Cd-rich. SEM revealed polycrystalline randomly oriented crystallites of elongated rhomboids with sharp and clear edges having leaf like appearance (up to x = 0.1). Optical studies revealed that the absorption increased with the increase of Co content in CdS lattice. The strong absorption edge also shifted towards the longer wavelength side and hence the band gap of the materials decreased with the increase of Co content. The temperature and composition dependence of electrical conductivity and thermoelectric power were studied and analyzed to determine the characteristic film properties and to understand the mechanism of an electrical conduction. A continuous decrease in electrical conductivity has been observed when x was varied from 0 to 0.5. The thermo power measurements showed n -type conduction. The observed results are interpreted and supported by the microscopic observations on these films.

Quantum confinement effects in Gd-doped CdS nanoparticles prepared by chemical precipitation technique

CdS and Gd-doped CdS nanoparticles have been synthesized by chemical precipitation technique. The X-ray diffraction patterns show that the CdS and Gd-doped CdS nanoparticles exhibit hexagonal structure. The high resolution transmission electron microscope image shows that CdS and Gd-doped CdS nanoparticles have particle size lying in the range of 3.5 to 4.0 nm. Raman spectra show that 1LO, 2LO and 3LO peaks of the Gd-doped CdS nanoparticles are slightly shifted to lower wavenumber side when compared to that of CdS. Optical absorption spectra of Gd-doped CdS nanoparticles shows that absorption edge is slightly shifted towards longer wavelength side (red shift) when compared to that of CdS and this shift is due to the quantum confinement effect present in the samples.

Effect of annealing temperature on nanocrystalline TiO2 thin films prepared by sol–gel dip coating method

Nanocrystalline TiO2 thin films have been prepared by sol–gel dip coating method. X-ray diffraction results showed the formation of anatase phase TiO2 with grain size in the range of 18–26nm. The HRTEM studies show that the average particle size of TiO2 is 24nm. The EDX analysis confirmed that Ti and O elements are present in the samples. The optical absorption spectra reveal that the absorption edge shifts towards longer wavelength side with increase in annealing temperature.

Charge-Transfer Electronic Absorption Spectra of 1-Ethylpyridinium Cation and Halogen Anion Pairs in Dichloromethane and as Neat Ionic Liquids

The charge-transfer (CT) absorption bands of ion pairs composed of 1-ethylpyridinium (Epy(+)) and halogen anions (X(-): Cl, Br, or I) were measured in dichloromethane solutions of EpyX. The CT band of the Epy(+)I(-) ion pair shows clear splitting because of spin-orbit interaction in the excited state. The CT transition energy of an Epy(+)X(-) ion pair in a dichloromethane solution is related to electron affinity of X, which is in accordance with the Mulliken theory for CT bands. Extinction coefficients for the CT bands of the Epy(+)X(-) ion pairs in dichloromethane were determined using the measured absorbance, and the ion-pair concentration was estimated on the basis of electroconductivity. Structures of Epy(+)X(-) ion pairs were also evaluated on the basis of both quantum-chemical calculations and NMR spectroscopy. In addition, in the absorption spectrum measured for neat EpyI liquid, a broad band appeared at a longer wavelength side of the S1(ππ*) band. This new band has been assigned to the CT band of the Epy(+)I(-) ion pair formed in neat EpyI liquid.

Tuning Properties of External Cavity Violet Semiconductor Laser

A tunable grating-coupled external cavity (EC) laser is realized by employing a GaN-based laser diode as the gain device. A tuning range of 4.47 nm from 403.82 to 408.29 nm is achieved. Detailed investigations reveal that the injection current strongly influences the performance of the EC laser. Below the free-running lasing threshold, EC laser works stably. While above the free-running lasing threshold, a Fabry—Pérot (F-P) resonance peak in the emission spectrum and a smooth kink in the output power-injection current characteristic curve are observed, suggesting the competition between the inner F-P cavity resonance and EC resonance. Furthermore, the tuning range is found to be asymmetric and occurs predominantly on the longer wavelength side. This is interpreted in terms of the asymmetric gain distribution of GaN-based quantum well material.

Proposal and Analysis of Distributed Reflector-Laser Diode Integrated with an Electroabsorption Modulator

A novel integrated laser, that is, a distributed reflector laser diode integrated with an electroabsorption modulator, is proposed to improve the output efficiency, single-mode stability, and chirp. The proposed laser can be realized using the selective metalorganic vapor phase epitaxy technique (that is, control of the width of the insulating mask), and its fabrication process is almost the same as the conventional electroabsorption modulated laser (EML) process except for the asymmetric coupling coefficient structure along the cavity. For our analysis, an accurate time-domain transfer-matrix-based laser model is developed. Based on this model, we perform steady-state and large-signal analyses. The performances of the proposed laser, such as the output power, extinction ratio, and chirp, are compared with those of the EML. Under 10-Gbps NRZ modulation, we can obtain a 30% higher output power and about 50% lower chirp than the conventional EML. In particular, the simulation results show that the chirp provided by the proposed laser can appear to have a longer wavelength side at the leading edge of the pulse and a shorter wavelength side at the falling edge.

Study of a mode-locked erbium-doped frequency-shifted-feedback fiber laser incorporating a broad bandpass filter: Experimental results

We present rigorous experimental studies on the spectral and temporal behaviors of an erbium-doped frequency-shifted-feedback fiber laser (FSFL), with respect to various parameters of the laser cavity, including the direction of the frequency-shift mechanism, the quantity of frequency-shift, and the output coupling ratio (OCR) of the cavity. We show that if the filter bandwidth is much broader than the laser linewidth, the laser spectrum tends to split and form a secondary spectral band (SSB) on the shorter or longer wavelength side of the primary spectrum, depending on whether the direction of the frequency-shift mechanism is upward or downward, respectively. We found that the SSB forms a parasitic pulse with much lower peak power traveling on the leading or trailing edge of the primary pulse, which leads to a significant asymmetry in the whole pulse formation in the time domain.

Excited States Energies and Dynamics of Peridinin Analogues and the Nature of the Intramolecular Charge Transfer State in Carbonyl-Containing Carotenoids

The lifetime of the lowest excited singlet state of carbonyl-containing carotenoids typically depends on the polarity of the solvent, an effect that has been attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this ICT state has yet to be clarified. In the present work, steady-state and ultrafast time-resolved optical spectroscopic experiments have been performed on peridinin and three synthetic analogues, C33-peridinin, C35-peridinin, and C39-peridinin, which have different extents of π-electron conjugation. Steady-state absorption at cryogenic temperatures revealed new absorption bands on the long-wavelength side of the strongly allowed S0 (1(1)Ag(-)) → S2 (1(1)Bu(+)) transition that can be assigned to S0 (1(1)Ag(-)) → S1 (2(1)Ag(-)) absorption. Analysis of the time-resolved absorption and fluorescence data sets revealed that the influence of polarity of the solvent on the excited state lifetime is unique for each molecule, leading to subtle differences in the values in highly polar solvents. Measurements in the most polar solvent, acetonitrile, demonstrated that the ICT state lifetime is shortest at 6.4 ps for C39-peridinin and gradually increases as the extent of π-electron conjugation decreases, becoming 10.6 ps for C33-peridinin. This suggests that the energy of the ICT state is dependent on the number of conjugated carbon-carbon double bonds.

Triple amplified spontaneous emissions from a conjugated copolymer BEHP-co-MEH-PPV in solution

The spectral and laser properties of conjugated polymer {[2-[2′,5′-bis(2″-ethylhexyloxy)phenyl]-1,4-phenylenevinylene]-co-[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]} (BEHP-co-MEH-PPV) in benzene had been studied and presented in this paper. BEHP-co-MEH-PPV was dissolved in benzene to form solutions of concentrations ranging from 1mM to 10mM. The absorption spectra had shown no new band when concentration increased; this indicates no dimer formation in these solutions for all concentrations mentioned above. The fluorescence spectra for the concentration 2.5mM have shown two peaks at 560nm and 600nm, which could be attributed to monomer and excimer of MEH-PPV alone. At higher concentration (5mM) the band at 560nm almost disappeared and the band at 600nm became dominant. At still higher concentration, the longer wavelength side of these spectra, there was a hump at 650nm. So, this new band around 650nm could be due to double excimer of MEH-PPV alone. When the laser pump power at 355nm and concentration of above solution were suitably chosen, we observed amplified spontaneous emission (ASE) at 570nm, 605nm and 650nm. These ASE peaks could arise from the monomer, excimer and double excimer states of MEH-PPV as a segment. To the best of our knowledge this is perhaps the first report on ASE from double excimer of the conjugated copolymer, BEHP-co-MEH-PPV, in liquid solution.

Quantum confinement effect in multilayer structure of alternate CdSe and SiOx insulator matrix thinfilms

Multilayer (ML) structure of layer-by-layer deposited CdSe/SiOx thin films and their monolayers were prepared using sequential thermal evaporation technique. X-ray diffraction study confirmed the (002) plane of CdSe with wurtzite structure. It is noticed that the microstrain, developed in ML thin films, increased with decreasing particle size. Experimentally measured band gap energies confirmed the splitting of valence band energy levels which rise due to hole confinement in CdSe. Crystallite sizes (5–7nm) were calculated using the effective mass approximation model (i.e., Brus model) which shows that the diameter of crystallites was smaller than the Bohr exciton diameter (11.2nm) of CdSe. The main band in the emission spectra of ML samples gradually shifted to longer wavelength side when particle size was increased from 5 to 7nm. This is characteristic of quantum size effect. It is inferred that disorderliness in CdSe/SiOx ML thin films would increase when the thickness of CdSe sublayer is greater than that of SiOx matrix layer.

CdS quantum dot sensitized nanocrystalline Gd-doped TiO2 thin films for photoelectrochemical solar cells

CdS quantum dot sensitized Gd-doped TiO2 nanocrystalline thin films have been prepared by chemical method. X-ray diffraction analysis reveals that TiO2 and Gd-doped TiO2 nanocrystalline thin films are of anatase phase. The absorption spectra revealed that the absorption edge of CdS quantum dot sensitized Gd-doped TiO2 thin films shifted towards longer wavelength side (red shift) when compared to that of CdS quantum dot sensitized TiO2 films. CdS quantum dots with a size of 5 nm have been deposited onto Gd-doped TiO2 film surface by successive ionic layer adsorption and reaction method and the assembly of CdS quantum dot with Gd-doped TiO2 has been used as photo-electrode in quantum dot sensitized solar cells. CdS quantum dot sensitized Gd-doped TiO2 based solar cell exhibited a power conversion efficiency of 1.18 %, which is higher than that of CdS quantum dot sensitized TiO2 (0.91 %).

Evidence for amplified spontaneous emission from double excimer of conjugated polymer (PDHF) in a liquid solution

The spectral and laser properties of conjugated polymer Poly [9, 9-di-(2′-ethylhexyl) fluorenyl-2, 7-diyl] (PDHF) in benzene had been studied and presented in this paper. PDHF was dissolved in benzene to form solutions of concentrations ranging from 12 μM to 6 mM. The absorption spectra had shown no new band when concentration increased; this indicates no dimer formation in these solutions for all concentrations mentioned above. The fluorescence spectra for the concentration 12 μM have shown two peaks at 415 nm and 435 nm, which could be attributed to monomer and excimer. At the longer wavelength side of these spectra, there was a hump at 465 nm. This hump became dominant at concentration 6.25 mM and the peak at 415 nm almost disappeared. So, this new band around 465 nm could be due to double excimer. When the laser pump power at 355 nm and concentration of above solution were suitably chosen, we observed amplified spontaneous emission (ASE) at 415 nm, 435 nm and 465 nm. These ASE peaks could arise from the monomer, excimer and double excimer states of the macromolecule. To the best of our knowledge this is perhaps the first report on ASE from double excimer of the conjugated polymer, PDHF in liquid solution.

Effects of average index variation in apodized long-period fiber gratings

We analyze the effects of average index variation on the transmission characteristics of an index-apodized long-period fiber grating (LPFG) by the transfer matrix method and study how these effects depend on the grating length, the grating profile, the modal dispersion factor, and the duty cycle of the index modulation. Apart from shifting the resonance wavelength and modifying the rejection band, average index variation can give rise to significant side lobes that may appear on the short-wavelength or long-wavelength side of the rejection band, depending on the signs of the average index change and the modal dispersion factor. Our results provide general guidance for the writing of LPFGs for the minimization of side lobes. Our analysis compares well with published experimental results and should be useful for the design and fabrication of LPFGs.

Optical properties of InAsSbN single quantum wells grown on InP substrates for 2-μm-wavelength region

Growth condition dependences of optical properties of InAsSbN single quantum wells (SQWs) grown on InP substrates by molecular beam epitaxy (MBE) for 2-μm-wavelength region were studied. The results for Sb composition dependences indicate that the optimum Sb composition at the growth temperature of 480°C is 2% in the case that the nitrogen composition is 1%. Also, it was found that the PL peak shifts to longer wavelength side with decreasing the growth temperature from 480°C to 450°C, suggesting that the longer wavelength laser operation is possible with the growth temperature of 450°C. In fact, laser operation was achieved for the laser diode structure grown at 450°C. The lasing wavelength is 2.36μm at 220K, which is the longest wavelength for InAsSbN quantum well lasers grown on InP substrates.