Far-infrared Transmission Research Articles

Purification and investigation of tellurium rich Te-As-Se chalcogenide glass for extended far-infrared transmission

In the present study, tellurium-rich (45–55 mol%) As-Se-Te chalcogenide glasses were synthesized and purified further to eliminate the impurities for achieving extended far infrared transmission. The glasses were characterized and studied for physical, thermal, optical, structural and mechanical properties. All the glasses have shown transparency in the wavelength range of ∼1.6–19 µm, with >45 % transmission with impurities. The purification process through vacuum distillation has eliminated the oxide impurities in the glass and the glass displayed ∼52 %T in the wide infrared region. The refractive index for purified glass decreased from 3.487 at 2 µm to 3.362 at 10 µm. The purified glass was heat treated for four hours, and the most dominant crystal phase, As2Te3 was observed in XRD analysis. Presence of sharp Raman peak at ∼120 cm−1 belonging to As2Te3 pyramids in heat-treated sample well agreed with the XRD results.

Decay processes of long-lived phonons in 6H-SiC

Temperature evolution of the high-resolution far-infrared transmission spectra of 6H-SiC single crystal slab is experimentally examined in a temperature range of 5–320 K. Temperature dependences of the phonon lifetimes for long-lived phonons, namely the IR active folded transverse acoustic (FTA) phonon doublet, are determined. These dependences are interpreted in the framework of a theoretical model developed using first principle anharmonic lattice dynamics. It is shown that three-phonon anharmonic processes dominate in the FTA phonon decay at temperatures above 160 K resulting in a rapid decrease of the phonon lifetimes with increasing temperature. Contribution of four-phonon anharmonic processes is negligibly small at temperatures below 320 K.

Influence of Er3+ concentration in Er:GGAG crystal on spectroscopic and laser properties

Detail spectroscopy study of erbium-doped active media is crucial for understanding of the behavior of erbium-based lasers. In this paper, for the first time, detailed study of the concentration dependence of Er3+ in garnet-based active media is presented. Wide range of novel samples Er:GGAG (26 in total) with various Er3+ concentration (0.5–38 at%), were investigated for the first time. The complete spectroscopy (absorption and emission cross-section, Raman, mid- and far-infrared transmission spectra, decay times at 4I11∕2 and 4I13∕2) together with Judd-Ofelt analysis and laser characteristics were measured at room temperature. Results shows strong dependence of the first excited state (4I13∕2) on concentration, on the other hand, the 4I11∕2 level is completely independent of the erbium doping level. The Judd-Ofelt analysis show that erbium doping has minimal effect on the structural properties of Er:GGAG. Also it is shown that Er:GGAG laser has a slope efficiency of 15.3 % (almost half of the theoretical limit) and generated maximal output power of 350 mW.

Preparation and imaging properties of coherent chalcogenide glass fiber bundles with large planar array for far-infrared transmission (invited)

Preparation and imaging properties of coherent chalcogenide glass fiber bundles with large planar array for far-infrared transmission (invited)

Complex Er-doped selenium-based chalcogenides in the far-infrared region: a structural bonding arrangement study

Selenium-based chalcogenide glasses show tremendous infrared transmission in the 2–15 μm region, and these amorphous glasses could be easily formed into optical devices i.e. optical fibers and lenses, owing to their good thermo-mechanical properties. Even though the phonon energy for tellurium-based glasses is on the lower side, still, selenium-based glasses are worthwhile for mid to long-wavelength infrared emissions. Here, we have developed Er-doped selenium-based, Ge 17 Sb 8 Se 75−x Er x where x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0, chalcogenides by conventional melting and quenching technique for possible mid to far-infrared applications. Far-infrared transmission spectra of the synthesized chalcogenides are obtained at room temperature in the spectral range of 35–450 cm−1. The bonding arrangements in the synthesized chalcogenides are investigated as a function of composition. It has been found that with the addition of Er content, the far-infrared transmission spectra shift toward the lower wavenumber side. The experimental results are correlated with the theoretically calculated parameters such as relative probability, bond energy, wave number, force constant etc. The obtained results provide insight into understanding the synthesized chalcogenides’ optical behavior, which is dynamic for designing the optical components operated in mid-infrared to far-infrared regions.

Assessing thermodynamical properties of Al1−xGaxSb alloys and optical modes for Al1−xGaxSb/GaAs epifilms and (AlSb)m/(GaSb)n superlattices

A generalized Green's function (GF) theory is adopted in the framework of a realistic rigid-ion-model (RIM) to assess the composition, x-dependent lattice dynamics, and thermodynamical characteristics of ideal random Al1−xGaxSb alloys. For simulating phonons, the alloy parameters are achieved by interpolating the values of the RIM force constants between AlSb and GaSb without requiring any additional interactions. The outcomes of phonon dispersions ωj(q→), Debye temperature ΘD(T), and specific heat Cv(T) compare favorably well with the existing experimental and theoretical data. An established methodology of multilayer optics is also employed for modeling the far-infrared reflectance and transmission spectra of ultrathin GaSb/GaAs, AlSb/GaAs, Al1−xGaxSb/GaAs epilayers, and (AlSb)m/(GaSb)n/GaAs superlattices at near normal (θi = 0) incidence and oblique (θi ≠ 0) incidence. An accurate appraisal of the x-dependent longitudinal-optical [ωLO(Γ)] and transverse-optical [ωTO(Γ)] phonon splitting by Berreman's effect, along with the calculated GF results of localized vibrational mode (GaSb:Al) and gap mode (AlSb:Ga), is carefully integrated into the modified-random-iso-displacement model to validate the two-phonon mode behavior in Al1−xGaxSb ternary alloys.

Optical access to folded transverse acoustic phonon doublet in 6H-SiC

We report on the study of far-infrared transmission in silicon carbide of 6H polytype in the region of IR active folded transverse acoustic (FTA) phonon doublet. Joint analysis of the transmittance spectra obtained at low and high resolutions is performed to determine spectral dependences of the refractive index and absorption coefficient. A dual Lorentz oscillator model is used to simulate the FTA phonon contributions to the dielectric function. The oscillator strengths and phonon lifetimes for both the components of the FTA phonon doublet in 6H-SiC are determined. At temperature decrease from 300 to 5 K, the FTA phonon lifetimes increase more than six times, while the oscillator strengths remain the same.

Structural correlation of GeTeSeGa system by XRD and far-infrared spectroscopy

The structural properties of quaternary Ge10Te80Se10-xGax (x = 0 to 10) glassy alloy have been studied with XRD (X-ray diffraction) and FTIR (Fourier transform infrared) spectroscopy. The position of FSDP (first sharp diffraction peak) (2θ) and its FWHM (full width half maxima) have been utilized to estimate the local structure parameters of FSDP like repeating distance y and structural correlation length (L). The far-infrared (IR) transmission spectra of Ge10Te80Se10-xGax (x = 0, 2, 4, 6, 8, 10) have been analysed in the range 30–300 cm−1 to study the formation of bonds using chain crossing model, random covalent network model and chemical bond approach. The theoretical calculations have been executed for bond energies, force constants, wave number, etc., for probable bonds, and the results justify the experimental values. The present study contributes to the understanding of the composition-dependent structural property relationship of chalcogenide glasses.

Far-infrared spectroscopy of folded transverse acoustic phonons in 4H–SiC

The weakest mode of the infrared active folded phonons in 4H–SiC, folded transverse acoustic (FTA) mode, has been experimentally studied. The lifetime of the FTA phonons has been determined. The lifetime is 58 ± 4 ps at room temperature and increases to 140 ± 7 ps when the temperature is decreased to 5 K. These results were obtained by the experimental study of far-infrared transmission of the SiC crystal and its theoretical simulation using an oscillator model for the dielectric function around the phonon resonance. The FTA phonon lifetime was a fitting parameter of the model when analyzing the irregular interference pattern in the transmission spectrum measured with a high resolution (0.2 cm−1). Oscillator strength of the FTA phonon resonance has also been determined. It is equal to (6.0 ± 0.5)×10−4 and does not depend on temperature.

Far-infrared absorption of undoped and Br-doped carbon nanofiber powder in stacked-cup cone configuration

We performed room-temperature far-infrared ($40--650\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) transmission measurements on undoped and bromine-doped powder samples of carbon nanofibers in stacked-cup cone geometry. The transmission spectra show enhanced transmittance after bromine doping and all spectra were fit to a Drude-Lorentz (DL) model. A decreased metallic conductivity along with a redshift of the lowest semiconducting gap was found with doped samples. A significant decrease in the scattering rate upon heavy doping has been qualitatively explained as partial ordering of intercalated dopant ions. Absorption spectra were derived from the transmission spectra under the assumption of nondispersive reflectance and subsequently compared with DL-model-derived spectra. The free-carrier density of the $n$-type powder and the electronic mean free path were estimated and compared with reported values for single-walled nanotubes and pyrolytic graphite.

Emission properties of Er3+-doped Ge20Ga5Sb10Se65 glasses in near- and mid-infrared

In this work, we reported the fabrications and characterization of Er3+-doped Ge20Ga5Sb10Se65 glasses and glass-ceramics and measured their transmission and fluorescence spectra. The results showed that, the fluorecence intensity of the glasses increased until Er3+ concentration was up to ∼1.1 wt% Er, and then decreased with further increasing Er3+ concentration that was due to concentration quenching effect. While it was found that the mid- and far-infrared transmission did not decrease significantly in the glasses annealed at 310 °C for a duration up to 50 h, seven-folded enhancement in the intensity of mid-infrared fluorescence at 2.78 μm was observed. This demonstrated the potentials of the materials used for Er-doped amplifier and fiber laser.

Effect of graphene on far-infrared transmission and absorption of FeF2 photonic crystals

The influence of graphene (Gr) on the far-infrared transmission and absorption of FeF2 photonic crystals (PCs) is investigated by the forth-order transfer matrix since Gr is anisotropic when the external field is perpendicular to the surface of PCs. The numerical results show that the transmission and absorption spectra largely depend on the structural symmetry of Gr/FeF2 PCs and the position of Gr layer. The optimal structure and number of dielectric bi-layers (N) are discussed. In addition, the introduction of Gr leads to the disappearance of the defect modes in the band gap. Meanwhile, the line width of absorption around of the resonant frequencies of FeF2 has been extremely broadened, which is compared with the one of FeF2 PCs. Once N is beyond a critical value, the absorber will become the reflector. The effect of Fermi energy and external field on the absorption is also investigated.

Telluride glasses with far-infrared transmission up to 35 μm

Telluride glasses are very attractive due to their unique infrared transparency window compared to other chalcogenide glasses. The extension of their infrared transmission by changing the composition appears to be very challenging. Glasses in the (GeTe4)100-x(AgI)x system, with 5 ≤ x ≤ 30 have been synthesized using the melt-quenching method. The effect of the addition of silver iodide is a widening of the infrared transparency range up to 35 μm that is much larger than for any other chalcogenide glass families. Moreover, the moulding of these telluride glasses is also achievable without affecting the optical properties.

Influence of Optical Nonlinearities on Plasma Waves in Graphene

A theory of the nonlinear plasma waves in graphene is developed in the nonperturbative regime. The influence of strong electric fields on the position and linewidth of plasma resonances in the far-infrared transmission experiments, as well as on the wavelength and the propagation length in the scanning near-field optical microscopy experiments is studied. The theory shows that the fields of order of a few to a few tens of kV/cm should lead to a red shift and broadening of plasma resonances in the first type and to a reduction of the wavelength and the propagation length in the second type of experiments.

Effect of uniaxial strain on the optical Drude scattering in graphene

Graphene is a mechanically robust 2D material promising for flexible optoelectronic applications. However, its electromagnetic properties under strain are experimentally poorly understood. Here we present the far-infrared transmission spectra of large-area chemical-vapor deposited monolayer graphene on a polyethylene terephthalate substrate subjected to uniaxial strain. The effective strain value is calibrated using the Raman spectroscopy and corrected for a relaxation of wrinkles and folds seen directly by atomic-force microscopy. We find that while the Drude weight and the Fermi level remain constant, the scattering rate increases by more than 10% per 1% of applied strain, showing a high level of reproducibility during strain cycling. As a result, the electronic mobility and optical absorption of graphene at terahertz and lower frequencies appear to also be sensitive to strain, which opens pathways to control these key parameters mechanically. We suggest that such a functionality can be potentially used in flexible optoelectronic and microelectromechanical systems based on graphene. By combining our findings with existing theoretical models, we discuss the possible mechanisms of strain-controlled Drude scattering.

Infrared spectroscopic study of carrier scattering in gated CVD graphene

We measured Drude absorption of gated CVD graphene using far-infrared transmission spectroscopy, and determined carrier scattering rate (g) as function of the varied carrier density (n). The n-dependent g(n) was obtained for a series of conditions systematically changed as (10 K, vacuum) -> (300 K, vacuum) -> (300 K, ambient pressure), which reveals that (1) at low-T, charged impurity (=A/sqrt(n)) and short-range defect (=B*sqrt(n)) are the major scattering sources which constitute the total scattering g=A/sqrt(n)+B*sqrt(n), (2) among various kinds of phonons populated at room-T, surface polar phonon of the SiO2 substrate is the dominantly scattering source, (3) in air, the gas molecules adsorbed on graphene play a dual role in carrier scattering as charged impurity center and resonant scattering center. We present the absolute scattering strengths of those individual scattering sources, which provides the complete map of scattering mechanism of CVD graphene for the first time. This scattering map allows us find out practical measures to suppress the individual scatterings, the mobility gains accompanied by them, and finally the ultimate attainable carrier mobility for CVD graphene.

Ultraviolet to far-infrared transmission properties of thin film multi-walled carbon nanotube random networks

Thin films of multi-walled carbon nanotubes forming random networks were produced by vacuum filtration method, and their broadband electromagnetic radiation transmittance spectra are presented. Thickness of the nanotube films was between 100 nm and 1 μm, and the transmission properties are demonstrated for the wavelength range from 300 nm to 400 μm. It is observed that transmittance is an increasing function of a radiation wavelength, and for the thickest films it almost saturates above 1 μm wavelength. To explain the experimental results in the ultraviolet–near infrared range, we employed effective medium theory (in the form of symmetric Bruggeman model) correlating properties of multi-walled carbon nanotubes with the effective dielectric function of a nanotube network. The optical properties of a single multi-walled carbon nanotube that were used for calculations were based on ordinary and extraordinary dielectric functions of bulk graphite. The proposed theoretical model has been successfully fitted to the experimental results. It has been also found that despite the fact that radiation undergoes multiple internal reflections at the film interfaces, the transmittance–thickness relation can be still described by exponential decay.

Modulation of far-infrared light transmission by graphene-silicon Schottky junction

Tunable conductive properties of graphene in terahertz and far-infrared regimes provide a prominent way to control electromagnetic waves. In this paper, we explore the photon-electric properties of the graphene-silicon heterostructure and its application in modulating the transmission of far-infrared light. Experimentally, we show this structure will give rise to different transmission modulation ratios with amplitudes varying from tens to few percentages, dependent on the operation wavelength. The modulation effect gradually decreases and saturates within wavenumbers 1000-2000 cm−1 influenced by the pump light power. The diode transmission behavior is explicitly interpreted in terms of the Schottky junction formed between the graphene-silicon interface. The results give a further deep understanding of the electromagnetic behavior of graphene in the far-infrared region that may be integrated for potential applications.

BETTER ALTERNATIVES TO “ASTRONOMICAL SILICATE”: LABORATORY-BASED OPTICAL FUNCTIONS OF CHONDRITIC/SOLAR ABUNDANCE GLASS WITH APPLICATION TO HD 161796

"Astronomical" or "circumstellar" silicate optical functions (real and imaginary indices of refraction n and k have been previously derived from compositionally and structurally disparate samples; past values were compiled from different sources in the literature, and are essentially kluges of observational, laboratory, and extrapolated or interpolated values. These synthetic optical functions were created because astronomers lack the quantitative data on amorphous silicates at all wavelengths needed for radiative transfer modeling. This paper provides optical functions that (1) are created with a consistent methodology, (2) use the same sample across all wavelengths, and (3) minimize interpolation and extrapolation wherever possible. We present electronic data tables of optical functions derived from mid-ultraviolet to far-infrared laboratory transmission spectra for two materials: iron-free glass with chondritic/solar atmospheric abundances, and metallic iron. We compare these optical functions to other popular n, k data used to model amorphous silicates (e.g., "astronomical" or "circumstellar" silicate), both directly and in application to a simple system: the dust shell of the post-AGB star HD161796. Using the new optical functions, we find that the far-IR profile of model SEDs are significantly affected by the ratio of glass to iron. Our case study on HD161796 shows that modeling with our new optical functions, the mineralogy is markedly different from that derived using synthetic optical functions and suggests a new scenario of crystalline silicate formation.

Measurements of phosphorus and boron concentrations in Czochralski silicon wafer-thick samples by p-polarized Brewster incidence far-infrared transmission

Determination of phosphorus and boron concentrations in Czochralski-grown silicon wafer-thick samples has been examined by means of low-temperature high-resolution far-infrared transmission measurements with p-polarized light incident at Brewster angle. The measurements were taken at about 8.7 K with a spectral resolution of 0.5 cm−1 under halogen light illumination. Linear correlations are clearly obtained between the absorption coefficients of several phosphorus and boron transitions and their concentrations, which are approximately in the range of 7.7 × 1013–1.6 × 1015 atoms cm−3 for phosphorus and of 2.5 × 1014–7.6 × 1014 atoms cm−3 for boron. The correlations are obtained regardless of the surface processing conditions of the samples. These are because interference fringes due to internal multiple reflections in wafer-thick silicon samples are almost suppressed using this incidence. The detection limit of phosphorus is about 1.2 × 1013 atoms cm−3 and that of boron is about 4.2 × 1013 atoms cm−3. P-polarized Brewster angle incidence has made it possible to determine phosphorus and boron concentrations in wafer-thick samples.