Measurements Of Optical Spectra Research Articles

Stability Investigation on NiFeOx Electrocatalysts during On and Off Cycles in Harsh Alkaline Conditions

Water electrolysis is the most practical and mature technique to produce green hydrogen. In addition to its efficiency, its durability also influences the production cost of hydrogen. The major concern is the durability of anodes for oxygen evolution reaction (OER), which are exposed to highly oxidative and acidic/alkaline environments at elevated temperatures. Additionally, intermittent operation is expected when coupled with renewable electricity, which leads to severe potential shifts on the electrodes during turning on and off. In this study, the stability of benchmark electrocatalysts, NiFeOx, was evaluated during on/off cycles in various electrolytes including commercially used highly alkaline solution (7 M KOH) at 80 °C. The NiFeOx showed a significant degradation particularly during on/off operation in 7 M KOH for 80 h.A protocol for on/off test was constructed based on the reported protocol,[1] which is intended to reproduce power-on and power-off conditions repeated in a short period of time. At the beginning of a unit cycle, chronopotentiometry (CP) was conducted at 600 mA cm−2 for 30 s to perform OER followed by a potential sweep to 0.3 V vs. reversible hydrogen electrode (RHE) to reproduce the sudden voltage drop. The power-off condition is reproduced by chronoamperometry (CA) at 0.3 VRHE for 30 s. The degradation of the electrocatalyst was evaluated by steady state CP and cyclic voltammetry (CV) every 100 cycles. These operations are defined as one set, and continued till the catalytic performance obviously changes. The electrolyte was refreshed every 24 or 48 h before the electrolyte concentration changed due to evaporation. A benchmark NiFeOx electrocatalyst was deposited on a Ni foam by reported hydrothermal method.[2] Using the three-electrode system, NiFeOx durability tests were conducted in 1 M KOH at 298 K and 7 M KOH at 353 K. In 1 M KOH at 298 K, the potential to reach 600 mA cm−2 remained relatively constant even after 3000 on/off cycling (Figure 1). Although the initial potential in 7 M KOH was superior to that in 1 M KOH, the potential in 7 M KOH continuously increased with cycles and reached the OER potential using Ni foam substrate. We note that, during constant current operation at 600 mA cm− 2, the potential remained stable at 1.47 VRHE for 80 h indicating that a redox reaction of material itself during the potential sweep causes the degradation of NiFeOx. The volume change between Ni(OH)2 and NiOOH may cause a mechanical stress and promote the loss of catalyst into the electrolyte. Indeed, the Ni redox peak in CV and double layer capacitance sharply dropped within 5 h. A severe increase of the potential was observed when the electrolyte was replaced after 24 h. Fe in NiFeOx is considered to be the active site for OER.[3] The degradation in 7 M KOH may come from the formation of soluble Fe complex ions between the hydroxide and iron,[4] which is favored in the highly alkaline solution leading to the dissolution in the electrolyte. Inductively coupled plasma optical emission spectra (ICP-OES) measurements performed on the spent electrolytes showed that the Fe continued to dissolve in the electrolyte (purple diamond symbol in Figure 1). At the end of the test, 1.2 mg of the Fe was lost, which corresponds to 80 % of that in the original catalyst (1.4 mg).To further reveal the degradation behavior, the electrolyte was saturated with Fe(NO3)3, and durability tests were conducted at 80 °C. It was found that the potential was maintained at around 1.53 VRHE in the solution saturated with Fe(NO3)3, even after replacing the electrolyte (closed symbol), indicating that severe deactivation was due to the loss of Fe species. However, the initial degradation to 1.53 VRHE still occurred even in the presence of the extra ferric ions, which may come from the mechanical stress due to the potential sweeping. The present results demonstrate the importance to investigate the stability at industrially relevant harsh conditions rather than the standard laboratory condition. Step-by-step clarification of the degradation mechanism would help to reveal the limitation and to develop highly durable electrocatalysts. Figure 1

Thermal and photochemical degradation of CsGeI3 and CsGeBr3: XPS and optical studies

The toxicity of complex lead halides significantly limits the commercialization and practical application of perovskite solar cells. In recent days, lead-free complex lead halides with a perovskite structure have become increasingly popular, among which germanium systems are the least studied. Here we present for the first time a systematic study of the thermal and photochemical stability of perovskite thin films of CsGeI3 and CsGeBr3. The measurements of optical absorbance and X-ray photoelectron spectra (XPS) of core levels and valence bands of Ge-based halide perovskites after light soaking and heat stress were thoroughly performed. It has been found the effect of thermal and photochemical degradation of Ge perovskites which is accompanied by reduction of I:Ge and Br:Ge ratios and appearance of tetravalent metal species fixed in XPS Ge 2p and Ge 3d spectra. The XPS O 1s measurements of irradiated and annealed Ge-based perovskites and their comparison with spectra of GeO2 have shown that formation of Ge4+-species is not due to metal oxidation but owing to defect-mediated hole-doping. While the Ge2+→Ge4+ process lowers the thermal and photochemical stability of Ge-based perovskites and limits their use in photovoltaics, the temperature induced hole doping is favorable for thermoelectric applications.

Influence of Congored on the Optical, Dielectric and Thermal Behavior of Polymethyl Methacrylate and Polyvinyl Chloride

Polymethyl methacrylate (PMMA)/Congo Red (CR) and polyvinyl chloride (PVC)/CR composite films were prepared on optical glass by the drop method for optical measurements and also prepared as tablets under pressure for dielectrical measurements. The optical transmission spectra measurements for PMMA and PVC film doped with CR showed a highly absorption in the ultraviolet-visible (UV-Vis) optical range (at 353 nm and 518 nm) and considerably blocking in broad visible range (190 nm–640 nm). PMMA/CR and PVC/CR composite films containing congo red at a concentration of 1 wt% showed maximum absorption bands at 290 nm and 416 nm, but their transmittance percentages were reasonably reduced compared to those of pure PMMA and PVC. Both of composites indicated high transmittances between 640 nm and 800 nm. The optical constants of the PMMA/CR and PVC/CR composite films such as optical band energy gaps (Eg) estimated using Tauc’s model, refractive index and reflectance (%) were calculated. UV–Vis, Fourier transform infrared spectroscopy (FTIR) and Thermogravimetry Analysis (TGA) measurements were used to confirm the breakdown of the -N = N- azo group bonds attached to the aromatic ring as a result of thermal decomposition of CR at 500 °C, and it was suggested that N2 gas was eliminated from the CR above 360 °C. The dielectric plots showed that the dielectric constant increased when the CR is doped up to 70 wt% in the polymer matrices. A further increase in the doping concentration of CR resulted in an increase of the dielectric constant. The dielectric constant decreased with an increase in the frequency for all of the samples. The ac (alternating current) conductivity values of PMMA and PVC showed an increase with increasing CR dopant in the polymer composites, while the ac conductivity values were found to be values between those of the pure polymers and CR.

Role of La2O3 in enhancement the properties of the BaO–B2O3 glass system: optical and radiation shielding study

This work aims to prepare modified glassy barriers of the composition (70 − x) mol% B2O3 + 20 mol% Na2O + 10 mol% BaO + x mol% La2O3 for use as radiation shielding materials. The cooling method has been considered for the sample’s preparation process. The structural properties were investigated using density and Infrared Spectroscopy. The hardness of the prepared glasses was studied. The optical transmittance and absorbance of the prepared glasses was measured using optical spectra measurements. The prepared samples were exposed to a parallel beam of gamma rays emitted from different radioactive sources. Moreover, the shielding properties were evaluated experimentally and theoretically. The resulting value of the proposed programs is largely in line with the experimental values. The results prove that the studied glass has a largely shielding behavior for gamma rays by increasing lanthanum and vice versa for neutrons. In addition, it has been observed that the absorbance of glass samples and hardness values increase with increasing lanthanum content.

Electron spin resonance study of an annealed Cr,Mg:YAG epitaxial saturable absorber

We have demonstrated the use of electron spin resonance (ESR) spectroscopy to observe the influence of high-temperature annealing in an oxidizing atmosphere on Cr3+ ion concentration changes in an epitaxial Cr,Mg:YAG saturable absorber. For this purpose, a set of samples of epitaxial Cr,Mg:YAG/YAG structures differing in both the concentration of Cr and Mg ions and the concentrations ratio of these ions have been synthesized using liquid phase epitaxy. The X-ray diffraction analysis and optical transmission spectra measurements were performed to characterize the obtained layers. The usefulness of the ESR technique for monitoring certain high-temperature induced changes in epitaxial Cr,Mg:YAG absorber has been verified.

Near-infrared (λ ∼ 1.2 μm) intersubband electroluminescence in Si/CaF2 quantum cascade structures

A clear electroluminescence (EL) from a Si/CaF2 quantum cascade structure has been successfully observed. The structure was equipped with 15 periods of an active region comprising of Si/CaF2 multi-quantum wells and a waveguide grown on the silicon-on-insulator substrate by MBE-based technique. As a result of an optical spectrum measurement by Fourier transform IR spectroscopy, a clear EL spectrum with a peak at λ ∼ 1.2 μm was observed. The EL spectrum is reasonably explained by fitting it with a Lorentzian model that considers the thickness fluctuation of a single monoatomic layer of a Si quantum well, the intra- and inter-subband scattering times, and the carrier escape time. These results indicate that the EL was generated by intersubband transitions in the Si quantum well.

Nanodiamond-promoted white light emission in Eu, Dy, and Cu-containing phosphate glass: quantitative analysis and colorimetric study.

A thorough analysis of glass containing Eu2 O3 and Dy2 O3 , or Eu2 O3 , Dy2 O3 , and CuO melted together with nanodiamond powder was pursued based on measurements of optical absorption, photoluminescence (PL) emission and excitation spectra, and colorimetry. Nanodiamond facilitated the stabilization of Cu+ and Eu2+ ions with blue-emitting characteristics that, along with yellow-emitting Dy3+ and red-emitting Eu3+ led to the white light-emitting glass. Novel intensity notations implemented in intensity-based spectral ratios, and difference intensity correlation analysis were proposed for the assessment of PL properties. The chromaticity and correlated colour temperature of the emission were ultimately investigated as a two-parametric problem based on: (1) the different ionic components; and (2) the various excitation wavelengths employed. The optical analysis approach adds to the characterization methods to further fundamental understanding and provide helpful analytical tools for designing materials for tunable white light-emitting devices.

Enhancement in Photoluminescence of Nematic Liquid Crystals Doped with CsPbBr3 Quantum Dots

We report the enhancement in photoluminescence (PL) of nematic liquid crystals (NLCs) doped with CsPbBr3 (perovskite) quantum dots (QDs). QDs play a vital role to enhance the PL intensity, which has been attributed to the increase in the planar alignment of NLC molecules by reducing light leakage centres and intrinsic defects. QDs were synthesized by the hot injection chemical method. The structural properties of QDs were examined by X-ray Diffraction (XRD) and Transmission Electron Microscopic (TEM) measurements. Orthorhombic crystallite structure of CsPbBr3 QDs without secondary phase formation was confirmed from XRD pattern. The particle size of QDs was calculated by TEM and found to be 7.3 (± 1.1) nm. The optical properties of QDs were studied by recording UV-Visible absorption and PL spectra at room temperature. The absorption spectrum reveals the excitonic peak at 509 nm and PL emission is observed at 514 nm. The chemically prepared QDs were further used to enhance the emission of NLC. Firstly, liquid crystal sample cells were fabricated by conventional polyimide technique. NLC doped with CsPbBr3 QDs were inserted into the sample cell via the capillary action. The filled sample cells were characterized by polarising optical microscope (POM) images and PL spectra measurements. The dark and bright state images of POM demonstrate the uniform alignment and homogeneous texture of liquid crystal molecules in both cells. The molecular alignment is further improved while the light leakage centres and defects were reduced after doping QDs in the host NLC material. This leads to a significant enhancement in absorption and hence the emission of NLC. We found the enhancement in PL intensity by 11% compared to pure NLC. The improved emission of NLC is attributed to modified molecular alignment in presence of QDs. Results based on present work will help to fabricate QDs based display devices with improved the optical contrast and lower the operating voltage

Preparation and ultraviolet detection performance of Cu doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> thin films

Solar-blind UV photodetectors (SBPs) have attracted great attention because they are widely used in missile tracking, fire detection, biochemical analysis, astronomical observations, space-to-space communications, etc. At present, it is found that wide bandgap semiconductor materials such as Al<sub><i>x</i></sub>Ga<sub>1-<i>x</i></sub>N, Mg<sub>1</sub>Zn<sub>1-<i>x</i></sub>O, diamond and <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> are ideal semiconductor materials for developing high-performance SBPs. The ultra-wide band gap semiconductor material, <i>β</i>-Ga<sub>2</sub>O<sub>3</sub>, has a large band gap width of 4.9 eV, strong breakdown electric field, absorption edge located in the solar blind ultraviolet band (200–280 nm), and it also has high transmittance in the near ultraviolet and the whole visible band. Therefore, <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> is a very suitable material for making solar blind UV photodetectors. However, the p-type <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> is difficult to dope, which limits the further development of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> devices. In this work, the <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> thin films with different Cu doping content are grown on sapphire substrates by chemical vapor deposition method, and the morphology, crystal structure and optical properties of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> films are measured. The test results show that the surfaces of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> films with different Cu content are relatively smooth, and the (<inline-formula><tex-math id="M2">\begin{document}$ \bar 201 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230971_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20230971_M2.png"/></alternatives></inline-formula>) diffraction peak positions shift toward the lower degree side with the increase of Cu content, which indicates that Cu<sup>2+</sup> replaces Ga<sup>3+</sup> and enters into the <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> lattice. The optical absorption spectrum measurement indicates that the energy gaps of samples are evidently narrowed with the increase of Cu doping concentration. Hall measurements indicate that the Cu doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> thin films have a p-type conductivity with a hole concentration of 7.36 × 10<sup>14</sup>, 4.83 × 10<sup>15</sup> and 1.69 × 10<sup>16 </sup>cm<sup>–3</sup>, respectively. In addition, a photoconductive UV detector with metal-semiconductor-metal structure is prepared by evaporating Au on a Cu-doped <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> thin film, and its UV detection performance is studied. The results show that the photocurrent value of the device increases with Cu content increasing. The photo-to-dark current ratio (<i>I</i><sub>l</sub>/<i>I</i><sub>d</sub>) is about 3.8×10<sup>2</sup> of 2.4% Cu content device under 254 nm-wavelength light at 10 V. The rise time and decay time are 0.11 s and 0.13 s, respectively. Furthermore, the responsivity and external quantum efficiency can reach 1.72 A/W and 841% under 254 nm-wavelength light with a light intensity of 64 μW/cm<sup>2</sup>.

Time-Resolved Optical Spectrum Measurement of Multi-Mode Fabry-Perot Laser Diodes

Fabry-Perot (FP) laser diodes usually have multiple longitudinal modes. An optical spectrum analyzer (OSA) is able to measure the average power distribution of the mode structure but cannot detect the dynamics of mode competition which is the origin of the mode partition noise. We demonstrate a time-resolved optical spectrum measurement technique based on frequency-to-time conversion through time gating, chromatic dispersion, and real-time waveform sampling. This allows to investigate the dynamics of mode competition and correlation of relative intensity noises among different FP modes of the diode laser.

Appreciably optimization of PVA/PVP nanocomposite blend for enhanced optoelectronics properties and multifunctional applications

The optical and electrical properties of PVA/PVP were manipulated by optimizing the blend ratio between those polymers. The (40/60) wt.% of PVA/PVP achieved the lowest energy gap and the highest electrical conductivity. After that, the optical and electrical properties of blend films were engineered using Ni(OAc)2 for enhanced optoelectronic properties. The doping impact on the semicrystalline and molecular structures of the prepared blend was investigated. The optical properties of the doped blend films were examined using optical transmission and absorption spectra measurements. The nonlinear optical parameters such as nonlinear refractive index and third-order optical susceptibility were estimated. The prepared films showed a pronounced laser cut-off performance. The electrical properties of the fabricated films showed a tunable behavior with the variation of doping level. The engineered doped polymer blend films exhibited good sensitivity to visible light with promising values of responsivity and specific detectivity ∼5.89 mA/W, and 4.31 × 109 Jones, respectively.

The Stability of Hybrid Perovskites with UiO-66 Metal-Organic Framework Additives with Heat, Light, and Humidity.

This study is devoted to investigating the stability of metal-organic framework (MOF)-hybrid perovskites consisting of CH3NH3PbI3 (MAPbI3) and UiO-66 without a functional group and UiO-66 with different COOH, NH2,and F functional groups under external influences including heat, light, and humidity. By conducting crystallinity, optical, and X-ray photoelectron spectra (XPS) measurements after long-term aging, all of the prepared MAPbI3@UiO-66 nanocomposites (with pristine UiO-66 or UiO-66 with additional functional groups) were stable to light soaking and a relative humidity (RH) of 50%. Moreover, the UiO-66 and UiO-66-(F)4 hybrid perovskite films possessed a higher heat tolerance than the other two UiO-66 with the additional functional groups of NH2 and COOH. Tthe MAPbI3@UiO-66-(F)4 delivered the highest stability and improved optical properties after aging. This study provides a deeper understanding of the impact of the structure of hybrid MOFs on the stability of the composite films.

Optical spectroscopy combined in situ with instrumented indentation

Modern trends in the development of experimental research methods imply not only an increase in the accuracy of a specific technique but also the possibility of combining diverse measurements in the course of one experiment. While optical spectroscopy remains one of the most powerful tools used in the chemical and physical sciences to study the structure of a wide range of materials, it is impossible to imagine a single study of local mechanical properties without instrumental indentation. A powerful investigation technique is the in situ combination of these two methods within one experiment. This can be made by focusing the laser either through the transparent sample or through the transparent indenter tip of the special geometry preventing the total internal reflection in diamond. This Tutorial discusses the preparation and characterization of such a transparent diamond indenter. The obtained experimental results and promising application areas of simultaneous measurement of optical spectra during indentation are considered.

From ACQ to AIE: The CN(π)-(π)Ar interaction driven structural and photophysical properties of aromatic ring conjugated novel diaminomaleonitrile derivatives

Studying solid and solution state photoluminescence properties are exclusive to understanding Aggregation Caused Quenching (ACQ) and Aggregation Induced Emission (AIE) behaviour of organic molecules to explore the mechanism of molecular luminescence in greater detail. A unique hydrogen-bonded dimer with imperative interaction (CN)π---π(Ar) was designed and synthesized by selecting diaminomaleonitrile (DMN) as stator and decorated with different aromatic rotors (phenyl to pyrene). Depending on the intermolecular interaction present in the molecular system, (AIE) or (ACQ) has been observed and found to be tunable. The single-crystal x-ray diffraction analysis of these derivatives shows that AIE characteristics could be seen where to (CN)π---π(Ar) intermolecular interactions between the aromatic rotors and stator, coupled with NH–-–NC intermolecular hydrogen bonding is present, while the absence of these interaction reflected in ACQ observation. The significant impacts of steric, conjugation and electronic effects on the ACQ / AIE properties are presented based on crystallography analysis, optical spectra measurements and theoretical calculations.

Characterization and electrochromic properties of NiO thin films prepared using a green aqueous solution by pulsed spray pyrolysis technique

Nickel acetylacetonate of 0.2 M aqueous solution is used to synthesize (NiO) thin films by pulsed spray pyrolysis method onto glass substrates. Structural and morphological properties of the deposited films were examined using (XRD) and (FESEM) techniques. The results revealed that at low substrate temperatures and short spray time, the obtained films were amorphous, whereas the films deposited at higher temperatures and long spray time has a cubic phase. Cross-sectional FESEM is used to properly estimate films thicknesses. The energy band gap is calculated to be 3.52–3.89 eV using optical spectrum measurements. Electrochromic properties of NiO films deposited on FTO working electrode substrates are measured in a three-electrode cell containing KOH with Ag/AgCl reference electrode at a scan rate of 20 mV/s. The charge density increases with the thickness, and it is 98.0 mC/cm2 at the thickness of 360 nm after 30-min deposition time at 400 °C and for most of the samples the film stability increases after cycling.Graphical abstract

Noncontact evaluation of the conductivity of CuPc thin films by near-field microwave microprobe method

AbstractThe conductivity of copper II phthalocyanine (CuPc) thin films with different crystal structures andmorphologies was evaluated by using a near-field microwave microprobe (NFMM) technique in thefrequency range of 4-4.5 GHz. CuPc thin films annealed at 200 °C and 350 °C showed the a- and yS-phasecrystal structures, respectively. Crystal phase structure and morphology of CuPc thin films werecharacterized by optical absorption spectra measurement, X-ray diffraction, and scanning electronmicroscopy methods. Conductivity measurements of CuPc thin films were done in the temperatures rangeof25-160 °C.

Crystallization effects on the photoluminescence efficiency of SrO-B2O3 glass activated by W6+, Eu3+, Dy3+ or Pr3+ ions

Binary strontium borate host glass together with samples doped with 0.25 %WO3 and one of the rare – earth ions (0.125% Eu3+, Dy3+ or Pr3+) were prepared by conventional melting – annealing technique. DTA analysis of the host base glass was carried out to derive the proper temperatures which are necessary to convert glasses into their corresponding glass –ceramic through controlled thermal heat –treatment regime. Measurements of optical, FTIR, and PL spectra were carried out with special analysis of the spectral properties. The identity of glass – ceramic derivatives were investigated through x-ray diffraction, and TEM & SEM tools to identify their crystalline morphological features. The absorption spectrum of WO3 – doped glass refers to the existence of dominant hexavalent tungsten ions. The RE- doped glass reveal characteristic visible – NIR absorption peaks due to the respective rare earth ions. FTIR spectra show absorption bands due to both triangular and tetravalent borate groups (BO3, BO4).PL spectra reveal excitation and emission characteristic peaks due to each specific rare earth ions. XRD indicate the preference of the host glass to form strontium pyroborate (SrB4O7) crystalline phase. This facilitates the clear knowledge of the crystallization behavior and hence can justify the effect of the crystallization on the studied properties.

Microwave Photonic Interrogation of a High-Speed and High-Resolution Multipoint Refractive Index Sensor

Microwave photonic interrogation of a high-speed and high-resolution multipoint refractive index (RI) sensor based on an Au-coated tilted fiber grating (Au-TFBG) array is proposed and experimentally demonstrated. Due to the surface plasmon resonance (SPR), the optical spectrum of an Au-TFBG has a dip in the spectral envelope. When an Au-TFBG is immersed in a solution and the RI of the solution changes, the location of the dip will shift, reflecting the change of the RI. For multipoint sensing, an Au-TFBG array with the array elements located at different locations is employed. However, due to the overlap of the spectra of the Au-TFBGs in the array, the dips cannot be precisely located by optical spectrum measurement. A solution is to convert the spectra to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping with the mapped temporal waveforms separated by different time delays. By using a digital signal processor (DSP), the sensing information can be extracted at a high speed and high resolution. The proposed approach is evaluated experimentally. Experimental results show that the sensor has high RI accuracies of 3.1×10^−5, 2.5×10^−5 and 2.6×10^−5 refractive index unit (RIU) and a fast sensing speed of 11.75 kHz.

Nonlinear Dynamics of a Single-Mode Semiconductor Laser with Long Delayed Optical Feedback: A Modern Experimental Characterization Approach

Semiconductor lasers can exhibit complex dynamical behavior in the presence of external perturbations. Delayed optical feedback, re-injecting part of the emitted light back into the laser cavity, in particular, can destabilize the laser’s emission. We focus on the emission properties of a semiconductor laser subject to such optical feedback, where the delay of the light re-injection is large compared to the relaxation oscillations period. We present an overview of the main dynamical features that emerge in semiconductor lasers subject to delayed optical feedback, emphasizing how to experimentally characterize these features using intensity and high-resolution optical spectra measurements. The characterization of the system requires the experimentalist to be able to simultaneously measure multiple time scales that can be up to six orders of magnitude apart, from the picosecond to the microsecond range. We highlight some experimental observations that are particularly interesting from the fundamental point of view and, moreover, provide opportunities for future photonic applications.

Uniaxial hybridization in the quasi-one-dimensional Kondo lattice CeCo2Ga8

We report the electron dynamics of the recently discovered quasi-one-dimensional Kondo lattice compound ${\mathrm{CeCo}}_{2}{\mathrm{Ga}}_{8}$ by optical spectra measurements using polarized light. For an electric field along the $c$ axis, the high-temperature Drude-like peak is found to split into two distinct components below the coherence temperature ${T}^{*}$, including a narrower Drude-like peak at low frequencies characterizing the emergent heavy electron state with a large mass enhancement, and a midinfrared Lorentz-like peak originating from interband transitions across the hybridization gap. The splitting is, however, absent for an electric field along the perpendicular directions, suggesting that the hybridization only occurs along the $c$ axis, although the high-temperature optical spectra from the background conduction electrons are isotropic. Our observations are in agreement with the resistivity data that show a broad maximum at around 20 K only along the $c$ axis but which keep increasing with a lowering temperature below 50 K along the $ab$ plane. Our work confirms the quasi-one-dimensional nature of the heavy electron physics in ${\mathrm{CeCo}}_{2}{\mathrm{Ga}}_{8}$ and establishes a direct correspondence between the bulk transport properties and microscopic electronic structures. It also pushes the previous observation of anisotropic coherence temperature in many heavy fermion compounds to an extreme situation and reveals the importance of intersite magnetic correlations beyond the local Kondo picture.