Measured Transmission Spectra Research Articles

High‐Performance and Stable Dopant‐Free Silicon Solar Cells with Magnesium Acetylacetonate Electron‐Selective Contacts

One of the challenges in fabricating high‐performance n‐type crystalline silicon (n‐type c‐Si) solar cells is the high‐quality n‐type c‐Si/metal contact. Schottky barriers are commonly found on the n‐type c‐Si/metal contact, which suppresses electron transportation. Herein, novel stacks of magnesium acetylacetonate (Mg(Acac)2)/magnesium (Mg)/silver (Ag) to form electron‐selective contacts for n‐type c‐Si solar cells are presented, which enables a dopant‐free process. An ohmic contact on n‐type c‐Si is formed using the Mg(Acac)2/Mg/Ag stacks. The transmission spectrum and ultraviolet photoelectron spectroscopy measurements show negligible conduction‐band offset and large valence‐band offset between Mg(Acac)2 and n‐type c‐Si, which indicates the electron‐transporting and hole‐blocking properties of Mg(Acac)2/n‐type c‐Si heterocontacts. Moreover, the contact resistivities (ρc) between the Mg(Acac)2/Mg/Ag electron‐selective heterocontacts and n‐type c‐Si substrates are lower than 10 mΩ cm2, which demonstrates the good electrode properties of the Mg(Acac)2/Mg/Ag stacks. The Mg(Acac)2/Mg/Ag electron‐selective stacks are applied on n‐type c‐Si solar cells with partial rear contact, and >20% efficiency is achieved, which is higher than that in a reference cell with only Ag contact. The stability of the n‐type c‐Si solar cell performance equipped with Mg(Acac)2/Mg/Ag contacts is verified under ambient conditions. This novel low‐temperature contact technique offers a reliable alternative for high‐performance n‐type c‐Si solar cells.

A Mie optimization model to determine optical properties of PCM based nanofluids for solar thermal applications of glazing window

Phase change material (PCM) filled in glazing windows keeps its temperature around the melting temperature during phase change, and stores energy in the day and releases energy in the night, which can reduce air-conditioning and heating energy consumption. The optical and thermal properties of phase change material can be improved by filling with nanoparticles to produce PCM based nanofluid, which is also fundamentally important in solar thermal applications of buildings. In the present work, two models to determine optical properties of PCM based nanofluids including Rayleigh and Mie optical scattering models were firstly investigated, in which PCM is paraffin wax and nanoparticle is TiO2. Then, the Mie optimization model was built and validated by transmittance spectrum measurement data of the paraffin nanofluid. And the influences of nanoparticle concentration and particle diameter on the optical properties of the paraffin nanofluid determined by the Mie optimization model were analyzed, including extinction and scattering characteristics. The results show that the calculation values of the Mie optimization model have a good match with the experimental transmission of the paraffin nanofluid. The larger the concentration of the nanoparticle concentration, the bigger the extinction coefficient and scattering coefficient of the paraffin nanofluid. When the nanoparticle concentration is constant, the larger the particle diameter is, the larger the scattering coefficient is doubled. When the diameter of the nanoparticles increases from 10 nm to 30 nm, the scattering coefficient of the paraffin nanofluid increases by 28.7 times.

High-performance plasmonic refractive index sensors via synergy between annealed nanoparticles and thin films

Plasmonic nanostructure-based refractive index (RI) sensors are the core component of biosensor systems and play an increasingly important role in the diagnosis of human disease. However, the costs of traditional plasmonic RI sensors are not acceptable to everyone due to their expensive fabrication process. Here, a novel low-cost and high-performance visible-light RI sensor with a particle-on-film configuration was experimentally demonstrated. The sensor was fabricated by transferring annealed Au nanoparticles (NPs) onto a thin gold film with polymethyl methacrylate (PMMA) as a support. RI sensitivities of approximately 209 nm/RIU and 369 nm/RIU were achieved by reflection and transmission spectrum measurements, respectively. The high sensitivity is due to the strong plasmon-mediated energy confinement within the interface between the particles and the film. The possibility of wafer-scale production and high working stability achieved by the transfer process, together with the high sensitivity to the environmental RI, provides an extensive impact on the realization of universal biosensors for biological applications.

Interactions between a magnon mode and a cavity photon mode mediated by traveling photons

We systematically study the indirect interaction between a magnon mode and a cavity photon mode mediated by travelling photons of a waveguide. From a general Hamiltonian, we derive the effective coupling strength between two separated modes, and obtain the theoretical expression of system's transmission. Accordingly, we design an experimental set-up consisting of a shield cavity photon mode, microstrip line and a magnon system to test our theoretical predictions. From measured transmission spectra, indirect interaction, as well as mode hybridization, between two modes can be observed. All experimental observations support our theoretical predictions. In this work, we clarify the mechanism of travelling photon mediated interactions between two separate modes. Even without spatial mode overlap, two separated modes can still couple with each other through their correlated dissipations into a mutual travelling photon bus. This conclusion may help us understand the recently discovered dissipative coupling effect in cavity magnonics systems. Additionally, the physics and technique developed in this work may benefit us in designing new hybrid systems based on the waveguide magnonics.

Multifractal and optical bandgap characterization of Ta2O5 thin films deposited by electron gun method

The micromorphology of tantalum pentoxide (Ta2O5) thin films, deposited on glass substrates by electron gun method, has been analyzed using atomic force microscopy (AFM), UV–Vis–NIR spectrophotometry and multifractal analyses. Two samples were grown at basic pressure of 7 × 10−6 mbar, work pressures of 1.3 × 10−4 and 2.0 × 10−4 mbar, and thicknesses of 0.38 μm and 0.39 μm, respectively. Subsequently, these samples were annealed at 300 °C for 2 h. The physical, structural and optical analyses were investigated by spectroscopic ellipsometry, spectrophotometry and AFM. The measured transmittance spectra were studied based on the Swanepoel method, whose results also yielded to the estimation of the film thickness and the refractive index. Finally, Ta2O5 thin films were characterized by AFM measurements and multifractal analyses for an accurate description of the 3-D surface microtexture features. The fractal examinations of the samples revealed that these microstructures exhibit multifractal characteristics. Essential parameters that characterized the thin films were compared and discussed thoroughly.

Fabrication of Cross-Sinusoidal Anti-Reflection Nanostructure on a Glass Substrate Using Imperfect Glass Imprinting with a Nano-Pin Array Vitreous Carbon Stamp.

Although polymer nanoimprinting on glass substrates has been widely employed for the fabrication of functional anti-reflective (AR) nanostructures, several drawbacks exist with respect to durability and delamination. The direct patterning of glass material is a potential solution for outdoor applications that require AR functional nanostructured glass plates. In this study, a glass imprinting technique was employed for the fabrication of an AR nanostructure on a soda-lime glass substrate using a vitreous carbon (VC) stamp. The VC stamp, which had a high aspect ratio nanopost array with a pitch of 325 nm, diameter of 110 nm, and height of ~220 nm, was fabricated by the carbonization of a replicated Furan precursor from an Si master. During the glass imprinting process using the nanopost array VC stamp, the softened glass material gradually protruded into the spaces between the nanopins owing to viscoelastic behavior, and one can achieve a cross-sinusoidal surface relief under specific imprinting condition, which can be used as an AR nanostructure with a gradually increasing refractive index. The effects of the processing temperature on the surface profile of the glass imprinted parts and the measured transmission spectra were analyzed, and a glass imprinting temperature of 700 °C and pressure of 1 MPa were found to be the optimum condition. The height of the fabricated cross-sinusoidal nanostructure was 80 nm, and the light transmission was increased by ~2% over the entire visible-light range. Furthermore, the measured transmission spectrum observed to be in good agreement with the simulation results.

Efficient Super Broadband NIR Ca2LuZr2Al3O12:Cr3+,Yb3+ Garnet Phosphor for pc‐LED Light Source toward NIR Spectroscopy Applications

AbstractSuper broadband near‐infrared (NIR) phosphor converted light‐emitting diodes (pc‐LEDs) are future light sources in NIR spectroscopy applications such as food testing. At present, a few blue LED excitable super broadband NIR phosphors (bandwidth > 300 nm) have been developed producing the NIR output powers below 26 mW at 100 mA input current after LED packaging. Here, an efficient super broadband NIR phosphor achieved by doping Yb3+ is reported in the NIR Ca2LuZr2Al3O12:Cr3+ (CLZA:Cr3+) garnet phosphor developed previously. Benefited from the superposition of Cr3+ emission and highly efficient Yb3+ emission excited by energy transfer from Cr3+, the codoped CLZA:Cr3+,Yb3+ phosphor shows a bandwidth of 320 nm and an internal quantum efficiency of 77.2% both higher than that (150 nm and 69.1%) of singly doped CLZA:Cr3+ phosphor. The codoped phosphor converts LED produced 41.8 mW NIR output at 100 mA input current. The pc‐LED as a light source is also well applied to the NIR transmission spectra measurement of water. The results indicate the great potential of CLZA:Cr3+,Yb3+ phosphor in super broadband NIR pc‐LED applications.

Counter-rotating effect on frequency shift of flux qubit in ultrastrongly coupled circuit-quantum-electrodynamics system

In recent years, quantum Rabi model has aroused considerable interest because of its fundamental importance and potential applications in quantum technologies. For a conventional cavity-quantum-electrodynamic (cavity-QED) system involving the interaction between an atom and photons in a cavity, the atom-photon coupling frequency is much smaller than the transition frequency of the atom and the frequency of the cavity mode. This cavity-QED system is usually described by the Jaynes-Cummings model in which the rotating-wave approximation can be adopted by neglecting the counter-rotating coupling terms in the Hamiltonian of the system. However, by designing the unique structure of the superconducting circuit, the ultrastrong-coupling regime can be achieved in a circuit-QED system in which the counter-rotating coupling terms become as important as the rotating terms. Thus, the rotating-wave approximation cannot be used in the ultrastrongly coupled circuit-QED system. Owing to the ultrastrong coupling, this circuit-QED system is described by the standard quantum Rabi model when a superconducting qubit is coupled only to a single resonator mode. In this work, we experimentally study an ultrastrongly coupled circuit-QED system consisting of a four-junction superconducting flux qubit and a muti-mode coplanar-waveguide resonator. The transmission-spectrum measurement and numerical simulations show that the system is in the ultrastrong-coupling regime. By changing the photon number in the resonator, we observe the frequency shift of the flux qubit via the spectroscopic measurement. This frequency shift contains the contributions from not only the rotating-coupling terms but also the counter-rotating terms, which is in good agreement with the theory. The result indicates that this ultrastrongly-coupled quantum system can be used as a good platform to investigate the quantum Rabi model and has potential applications in various aspects of quantum technology, such as quantum simulation, ultrafast quantum gates, entangled-state preparation and protected qubits.

Baseline-free quantitative absorption spectroscopy based on cepstral analysis.

The accuracy of quantitative absorption spectroscopy depends on correctly distinguishing molecular absorption signatures in a measured transmission spectrum from the varying intensity or 'baseline' of the light source. Baseline correction becomes particularly difficult when the measurement involves complex, broadly absorbing molecules or non-ideal transmission effects such as etalons. We demonstrate a technique that eliminates the need to account for the laser intensity in absorption spectroscopy by converting the measured transmission spectrum of a gas sample to a modified form of the time-domain molecular free induction decay (m-FID) using a cepstral analysis approach developed for audio signal processing. Much of the m-FID signal is temporally separated from and independent of the source intensity, and this portion can be fit directly with a model to determine sample gas properties without correcting for the light source intensity. We validate the new approach in several complex absorption spectroscopy scenarios and discuss its limitations. The technique is applicable to spectra obtained with any absorption spectrometer and provides a fast and accurate approach for analyzing complex spectra.

Symmetric Meandering Distributed Feedback Structures for Silicon Photonic Circuits

Fano lineshapes and electro-magnetically induced transparency-like peaks in the transmittance of a transverse electric polarization silicon-on-insulator symmetric meandering distributed feedback photonic structure are demonstrated. The coupling constants at the five identical directional couplers are varied to obtain the desired spectral responses. The numerically simulated and experimentally measured transmittance spectra are in good agreement with each other. The numerically calculated and experimentally measured insertion loss for the symmetric meandering distributed feedback structure with directional coupler coupling length Lc = 10 μm are respectively -5 dB and -17 dB, including the grating couplers. Fano lineshapes with mode splitting is observed at directional coupler coupling constant value C of 0.24. For coupling constant value of C ~ 0.78, electro-magnetically induced transparency-like peaks are observed, and spectrally adjusted by varying the directional coupler coupling length. Fano lineshapes show an extinction ratio of more than 26 dB and slope ratio of 368 dB/nm. Electro-magnetically induced transparency-like peaks show a quality-factor on the order of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . The symmetric meandering distributed feedback structure shows promise for possible applications as an optical switch, and an optical filter in wavelength division multiplexing and data networks, as well as optical sensors in optical diagnostics, using silicon photonics.

Observation of Collective Superstrong Coupling of Cold Atoms to a 30-m Long Optical Resonator.

We report on the observation of collective superstrong coupling of a small ensemble of atoms interacting with the field of a 30-m long fiber resonator containing a nanofiber section. The collective light-matter coupling strength exceeds the free-spectral range and the atoms couple to consecutive longitudinal resonator modes. The measured transmission spectra of the coupled atom-resonator system provide evidence of this regime, realized with a few hundred atoms with an intrinsic single-atom cooperativity of 0.13. These results are the starting point for studies in a new setting of light-matter interaction, with strong quantum nonlinearities and a new type of dynamics.

Comparative study of optical analysis methods for thin films

Three popular optical analysis methods (the transfer-matrix method, the Tinkham formula, and Beer's law) have been used for analyzing the optical spectra of thin films. While the transfer-matrix method is an accurate method, the Tinkham formula and Beer's law are approximate methods. Here we investigated the three methods using measured transmittance spectra of insulating transition-metal dichalcogenide (TMD) thin films on a quartz substrate. Three different semiconducting 2H-TMD systems (MoS2, MoSe2, and MoTe2) were measured and analyzed. The optical conductivities obtained from the measured transmittance spectra using the transfer-matrix method and Tinkham formula and the absorption coefficients obtained using the transfer-matrix method and Beer's law were compared. The comparisons show some discrepancies. The reasons for the discrepancies between the results obtained via the two different methods were examined and the application limitations of the Tinkham formula and Beer's law were discussed.

Optical properties of amorphous Ta2O5 thin films deposited by RF magnetron sputtering

Optical properties including refractive indices, extinction coefficient, thickness and both optical direct and indirect band gaps of the Ta2O5 thin films deposited by RF magnetron sputtering were determined in this work. The influence of sputtering power, which varies from 70 to 130 W, on microstructure and optical properties of the films were investigated as well. Phase structure, surface morphology, optical transmittance and optical constants of the samples were characterized by X-ray diffraction, atomic force microscopy, spectrophotometry and ellipsometry, respectively. The results show that deposition rate of the films increases with increasing sputtering power, and the maximum deposition rate, 2.1 nm/min, is achieved at 130 W, while the minimum one, 0.84 nm/min, is obtained at 90 W. The as-deposited thin films are amorphous according to the X-ray diffraction. Good surface quality featured by dense and compact microstructure is obtained at lower and higher sputtering powers while some micro-pores are found in the film deposited at medium sputtering powers. The optical constants, such as refractive index, extinction coefficient, and thickness of the Ta2O5 thin films were determined as a function of wavelength based on the ellipsometry measurements and fittings, and the dispersive refractive index was obtained for each sputtering power. The optical constants have been verified by comparing the measured transmittance spectra with the simulated ones, as they are consistent well with each other. Direct bandgap ranging from 4.05 to 4.18 eV and indirect bandgap ranging from 4.53 to 4.66 eV were acquired for the films deposited with the aforementioned sputtering powers.

High-order exceptional points of counterpropagating waves in weakly deformed microdisk cavities

In a recent publication [Phys. Rev. A 98, 023851 (2018)] the authors reported on a systematic approach based on perturbation theory to generate non-Hermitian degeneracies, so-called exceptional points, in weakly deformed microdisk cavities by coupling modes of different angular momentum. In the present paper we extent this approach to fully asymmetric boundary deformations. This allows us to increase the order of the exceptional point and create modes with interesting angular momentum patterns where a nonuniform clockwise and counterclockwise propagation is present. Furthermore, we combine the perturbation theory of weakly deformed microdisks with the coupled-mode theory to explain possible measurements of transmission and reflection spectra via a waveguide attached to the cavity.

Mid-infrared transmission by a tellurite hollow core optical fiber.

We experimentally demonstrate for the first time a successful fabrication of a tellurite hollow core optical fiber which has a mid-infrared transmission range. The wall thickness of each cladding air-hole is about 2.8 µm and the outer diameter of the full air-hole structure D is approximately 110 µm. The results show that the measured transmission spectrum can expand up to 3.9 µm. In addition, it is expected that the transmission can extend to around 6 µm. When the input light is linearly polarized, it can be maintained after propagating through a 17-cm-long fiber.

Spectral selectivity of multiple nanoparticles doped thin films.

Microscopic thin film doped with different species of nanoparticles displays a unique wavelength selectivity in the context of micro/nanoscale radiative heat transfer. We propose a methodology to shift, broaden, and suppress the thermal radiative selectivity in the desired wavelength ranges. Measured transmittance spectra of potassium bromide pellet doped with a single species of nanoparticles are compared with the theoretical prediction using refractive indices that are extracted by refitting transmittance spectra curve according to the Lorentz-Drude model. For a media doped with more than two species of nanoparticles, a successive effective dielectric function using the refitted complex refractive indices and Maxwell Garnett theory is used to evaluate the thermal radiative selectivity of the composites. It has been confirmed theoretically and experimentally that the wavelength selectivity in the transmittance spectra can be influenced by choosing proper species of materials and varying volume fractions of multiple nanoparticles. This work has shed light on the design and fabrication of novel composites doped with multiple particles for applications such as thermophotovoltaics, radiative cooling, and biosensing.

In situ Raman gain between hyperfine ground states in a potassium magneto-optical trap

We study optical gain in a gas of cold 39K atoms. The gain is observed during operation of a conventional magneto-optical trap without the need for additional fields. Measurements of transmission spectra from a weak probe show that the gain is due to stimulated Raman scattering between hyperfine ground states. The experimental results are reproduced by a simplified six-level model, which also helps explain why such gain is not observed in similar experiments with rubidium or cesium.

Manifestation of Сoncentration Quenching of Fluoroaluminate Glasses Doped with Erbium

Fluoroaluminate glasses of the composition 2Ва (РО3)2–98MgCaSrBaYAl2F14-xErF3, where x=0, 0.1, 0.5, 1.0 mol. % have been prepared by melt quenching technique and characterized by optical absorption, emission spectra and decay curve analysis. Measured transmission spectra indicate the high practical relevance of the composition of glasses under investigation for photonics and optoelectronics products. In the region of 500–700 nm, luminescence spectra with peaks at about 522, 550, and 665 nm were obtained. The positions of the luminescence bands have been described using an erbium ion energy scheme. The concentration dependences of the absolute quantum yield values for the series of Er3+-doped fluoroaluminate glasses were also established. The maximum value of absolute quantum yield was found for a sample with Er3+ concentration 0.21∙1020 сm-3. The main reason for reducing the values of absolute quantum yield is concentration quenching.

Efficient long period fiber gratings inscribed with femtosecond pulses and an amplitude mask.

We present efficient long period fiber gratings written with femtosecond laser pulses at 800nm and an amplitude mask, to the best of our knowledge, for the first time. The measured transmission spectra depict strong resonances, while the total grating length and polarization-dependent loss could be significantly reduced compared to previous results. Two gratings are exemplarily shown-one in a standard single mode, and one in a large-mode-area fiber revealing a predictable spectrum without intermediate peaks due to the suppression of coupling to asymmetric higher-order cladding modes.

Interaction of sub-terahertz radiation with low-doped grating-based AlGaN/GaN plasmonic structures. Time-domain spectroscopy measurements and electrodynamic modeling

We have presented the results of terahertz time-domain spectroscopy measurements and a rigorous electrodynamic modeling of the optical characteristics of grating-based AlGaN/GaN plasmonic structures with low-doped two-dimensional electrongas in the frequency range of 0.1...1.5 THz. Two samples with grating aspect ratios (strip width/period) of 2.4/3 and 1.2/1.5 μm have been investigated. The measured transmission spectra are reconstructed in the calculations with high accuracy. The transmission spectrafor p-polarized incident radiation exhibits Fabri–Perot oscillation behavior due to theoptically-thick substrate. The specific values of amplitude and spectral position of thetransmission maxima are associated with the coupling of terahertz radiation with 2Delectron gas due to plasmon excitations. Both calculations and transmission/reflectionmeasurements demonstrate that plasmonic structures with micro-scaled metallic gratinghave three-fold increase of non-resonant absorption of terahertz radiation in comparisonwith the bare heterostructure. The polarization measurements of the transmission spectra ofthe plasmonic structures well agree with calculations and indicate a well-pronounced filtering effect of the grating for the s-component of the incident electromagnetic wave. The obtained values of the transmission for p- and s-polarized incident radiation demonstrate the high quality of deposited metallic grating with the extinction ratio higher than 80:1 for sub- and few THz frequency range.