Mid-infrared Transition Research Articles

Experimental study on mid-infrared CO transition lines broadened by N2, He, and H2 at elevated temperatures

Tunable diode laser absorption spectroscopy (TDLAS) has been established as a strong technique for high-fidelity measurements at elevated temperatures. However, the accuracy of the measured data using TDLAS depends on the accuracy of the spectroscopic parameters. In this paper, the N2-, He-, and H2-perturbed collisional broadening coefficients of the CO P(0, 21), P(1, 21), and P(0, 37) lines were measured at 1040–2980 K. Ar-diluted mixtures were applied to measure these spectroscopic parameters at elevated temperatures up to 3000 K. Validation experiments highlighted that the measured temperatures and CO concentrations agreed well with the known values under thermal equilibrium and nonequilibrium conditions. Comparisons of residuals in Voigt fitting highlighted that the N2-perturbed Dicke narrowing effects are stronger than the He- and H2-perturbed phenomena. The differences among species-perturbed high-temperature collisional broadening coefficients for the three transition lines were found to be closely related to the intermolecular forces and effective collision cross-sections.

Comparing Pr3+ and Nd3+ for deactivating the Er3+: 4I13/2 level in lanthanum titanate glass

Erbium lanthanum titanate glasses were prepared by levitation melting for the spectroscopic study of ways to promote the mid-infrared fluorescence. Two series of heavily erbium doped glasses (15 wt%) were prepared with the addition of either Pr3+ or Nd3+ in amounts relative to Er3+ of 0.05, 0.1, and 0.2. Both ions quench the lower Er3+ laser level with the Pr3+ doing so more rapidly. Although high co-dopant concentrations result in higher energy transfer, as clearly evidenced in upconversion and downconversion fluorescence measurements, the mid-infrared lifetime also suffers a reduction and, therefore, a balance must be struck in the co-dopant concentration. Lifetime and spectral measurements indicate that, at a fixed relative co-dopant amount, Pr3+ is more effective than Nd3+ at removing the bottleneck of the Er3+ 4I13/2 level. Moreover, consideration of the lifetimes alongside the absorption data of the individual ions indicates that despite the large absorption cross-section of Nd3+ at 808 nm, the concentration needed to yield more absorbed power than utilizing direct 976 nm excitation of Er3+ results in unfavorable lifetimes of the mid-infrared transition. In the end, Pr3+ prevails as the superior co-dopant in terms of the effects on fluorescence lifetimes as well as potential laser system design considerations. In a unique self-doping approach, a reducing melt atmosphere of Ar instead of O2 creates a small fraction of Ti3+. In 5Er2O3-12La2O3-83TiO2 glass, the presence of Ti3+ quenches the 4I13/2 emission about 2.6 times more than the 4I11/2 when lifetimes are compared to an O2 melt environment. As an additional means of increasing the mid-infrared emission, the effect of temperature on the mid- and near- infrared lifetimes of a lightly doped lanthanum titanate composition is investigated between 77-300 K. The mid-infrared lifetime increases by ∼30% while the near-infrared lifetime increases by ∼10%, which suggests in addition to co-doping, active cooling of the gain media will further enhance performance.

Low intensity saturation of an ISB transition by a mid-IR quantum cascade laser

We demonstrate that absorption saturation of a mid-infrared intersubband transition can be engineered to occur at moderate light intensities of the order of 10–20 kW cm−2 and at room temperature. The structure consists of an array of metal–semiconductor–metal patches hosting a judiciously designed 253 nm thick GaAs/AlGaAs semiconductor heterostructure. At low incident intensity, the structure operates in the strong light–matter coupling regime and exhibits two absorption peaks at wavelengths close to 8.9 μm. Saturation appears as a transition to the weak coupling regime—and therefore, to a single-peaked absorption—when increasing the incident intensity. Comparison with a coupled mode theory model explains the data and permits to infer the relevant system parameters. When the pump laser is tuned at the cavity frequency, the reflectivity decreases with increasing incident intensity. When instead the laser is tuned at the polariton frequencies, the reflectivity non-linearly increases with increasing incident intensity. At those wavelengths, the system, therefore, mimics the behavior of a saturable absorption mirror in the mid-IR range, a technology that is currently missing.

Photonic bandgap engineering in (VO2) n /(WSe2) n photonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal–insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO2) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO2 as a strongly correlated phase transition material and tungsten diselenide (WSe2) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO2 with WSe2 in a photonic superlattices structure through the near and mid-infrared (NIR–MIR) thermochromic phase transition regions. By increasing the thickness of the VO2 layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO2 thickness. For the monoclinic case of VO2, the number of the forbidden bands increase with the number of layers of WSe2. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR–MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study’s findings may help understand vanadium oxide’s role in tunable photonic superlattice for infrared switchable devices and optical filters.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Spectroscopic and laser properties of Er3+, Pr3+ co-doped LiYF4 crystal

In this paper, the absorption and fluorescence spectra of Er3+, Pr3+ co-doped LiYF4 (Er,Pr:YLF) crystal were measured and analyzed. The Pr3+ co-doping was proved to effectively enhance the Er3+:I411/2→I413/2 mid-infrared transition at the 2.7 µm with 74.1% energy transfer efficiency from Er3+:I413/2 to Pr3+:F34. By using the Judd–Ofelt theory, the stimulated emission cross section was calculated to be 1.834×10−20 cm2 at 2685 nm and 1.359×10−20 cm2 at 2804.6 nm. Moreover, a diode-end-pumped Er,Pr:YLF laser operating at 2659 nm was realized for the first time, to the best of our knowledge. The maximum output power was determined to be 258 mW with a slope efficiency of 7.4%, and the corresponding beam quality factors Mx2=1.29 and My2=1.25. Our results suggest that Er,Pr:YLF should be a promising material for 2.7 µm laser generation.

Nanospectroscopy of a single patch antenna strongly coupled to a mid-infrared intersubband transition in a quantum well

Scanning-probe-assisted mid-infrared nano-spectroscopy is employed to reveal the polaritonic dispersion of individual MIM (metal-insulator-metal) square patch antennas whose modes can be strongly coupled to a mid-infrared intersubband transition. The patch antenna side length L sets the resonances between λ = 5.5 μm and 12.5 μm. The active region consists of a highly doped AlInAs/InGaAs/AlInAs single quantum well that presents an intersubband transition at 1190 cm−1 (λ = 8.4 μm). When the patch antenna optical resonance approaches and matches the intersubband transition frequency (L ∼ 1.8 μm), a clear anticrossing behavior—evidence of strong coupling—is observed in the near-field scattering phase spectra of individual antennas. The measured Rabi splitting is 4.5 THz. The near-field scattering spectra agree with the far-field extinction spectra acquired on arrays of identical antennas.

Emergence of intraband transitions in colloidal nanocrystals [Invited

The chemistry of nanocrystals enables the receipt of semiconductor nanoparticles with tunable optical properties. So far most scientific efforts have been focused on wide band gap materials to achieve a bright luminescence and a higher solar power conversion efficiency. Their properties in the infrared range of wavelengths are interesting as well. Two strategies can be used to achieve mid-infrared (mid-IR) transition, either interband transition in narrow band gap material or intraband transition in doped material. In this review, we discuss recent progress to achieve stable doped nanocrystals. We focus on mercury chalcogenide compounds since they are so far the only materials that combine mid-IR absorption with photoconductive properties in this range of energies. We discuss the origin of the doping and its tunability as well as how the doping impacts the optical, transport, and photodetection properties. Finally, we discuss Hg-free alternative materials, and present mid-IR transitions.

Static and time-resolved mid-infrared spectroscopy of Hg0.95Cd0.05Cr2Se4 spinel

Static and time-resolved mid-infrared spectroscopy of ferromagnetic single crystal Hg0.95Cd0.05Cr2Se4 was performed below the absorption edge, in order to reveal the origin of the electronic transitions contributing to the magneto-optical properties of this material. The mid-infrared spectroscopy reveals a strong absorption peak around 0.236 eV which formerly was assigned to a transition within the selenide–chromium complexes (Se–Cr2+). To reveal the sensitivity of the transition to the magnetic order, we performed the studies in a temperature range across the Curie temperature and magnetic fields across the value at which the saturation of ferromagnetic magnetization occurs. Despite the fact that the Curie temperature of this ferromagnetic semiconductor is around 107 K, the intensity of the mid-infrared transition reduces substantially increasing the temperature, so that already at 70 K the absorption peak is hardly visible. Such a dramatic decrease of the oscillator strength is observed simultaneously with the strong red-shift of the absorption edge in the magnetic semiconductor. Employing a time-resolved pump-and-probe technique enabled us to determine the lifetime of the electrons in the excited state of this optical transition. In the temperature range from 7 K to 80 K, the lifetime changes from 3 ps to 6 ps. This behavior agrees with the phenomenon of giant oscillator strength described earlier for weakly bound excitons in nonmagnetic semiconductors.

Diode-pumped mid-infrared fiber laser with 50% slope efficiency

Until now, the field of mid-infrared fiber laser research has been constrained by the limitation imposed by the Stokes efficiency limit. The conversion of high-power diode light emission operating at near-infrared wavelengths into mid-infrared light invariably results in the deposition of significant amounts of heat in the fiber. This issue is compounded by the fact that mid-infrared transmitting glasses are thermomechanically weak, which means scaling the output power has been a longstanding challenge. In this report, we show that by cascading the adjacent transitions of the erbium ion at 2.8 and 1.6 μm in combination with a low-loss fluoride fiber, the slope efficiency for emission at 2.8 μm can reach 50%, thus exceeding the Stokes limit by 15%. We also show that by highly resonating the 1.6 μm transition, a highly non-resonant excited-state absorption process efficiently recycles the excitation back to the upper laser level of the mid-infrared transition. This demonstration represents a significant advancement for the field that paves the way for future demonstrations that will exceed the 100 W power level.

Double resonant absorption measurement of acetylene symmetric vibrational states probed with cavity ring down spectroscopy.

A novel mid-infrared/near-infrared double resonant absorption setup for studying infrared-inactive vibrational states is presented. A strong vibrational transition in the mid-infrared region is excited using an idler beam from a singly resonant continuous-wave optical parametric oscillator, to populate an intermediate vibrational state. High output power of the optical parametric oscillator and the strength of the mid-infrared transition result in efficient population transfer to the intermediate state, which allows measuring secondary transitions from this state with a high signal-to-noise ratio. A secondary, near-infrared transition from the intermediate state is probed using cavity ring-down spectroscopy, which provides high sensitivity in this wavelength region. Due to the narrow linewidths of the excitation sources, the rovibrational lines of the secondary transition are measured with sub-Doppler resolution. The setup is used to access a previously unreported symmetric vibrational state of acetylene, ν1+ν2+ν3+ν4 (1)+ν5 (-1) in the normal mode notation. Single-photon transitions to this state from the vibrational ground state are forbidden. Ten lines of the newly measured state are observed and fitted with the linear least-squares method to extract the band parameters. The vibrational term value was measured to be at 9775.0018(45) cm(-1), the rotational parameter B was 1.162 222(37) cm(-1), and the quartic centrifugal distortion parameter D was 3.998(62) × 10(-6) cm(-1), where the numbers in the parenthesis are one-standard errors in the least significant digits.

Studying the evolution of galaxies in compact groups over the past 3 Gyr – II. The importance of environment in the suppression of star formation

We present an in depth study on the evolution of galaxy properties in compact groups over the past 3 Gyr. We are using the largest multi-wavelength sample to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift range of 0.01<z<0.23. To derive the physical properties of the galaxies we rely on ultraviolet (UV)-to-infrared spectral energy distribution modeling, using CIGALE. Our results suggest that during the 3 Gyr period covered by our sample, the star formation activity of galaxies in our groups has been substantially reduced (3-10 times). Moreover, their star formation histories as well as their UV-optical and mid-infrared colors are significantly different from those of field and cluster galaxies, indicating that compact group galaxies spend more time transitioning through the green valley. The morphological transformation from late-type spirals into early-type galaxies occurs in the mid-infrared transition zone rather than in the UV-optical green valley. We find evidence of shocks in the emission line ratios and gas velocity dispersions of the late-type galaxies located below the star forming main sequence. Our results suggest that in addition to gas stripping, turbulence and shocks might play an important role in suppressing the star formation in compact group galaxies.

Doping dependent blue shift and linewidth broadening of intersubband absorption in non-polar m-plane AlGaN/GaN multiple quantum wells

Blue shift and broadening of the absorption spectra of mid-infrared intersubband transition in non-polar m-plane AlGaN/GaN 10 quantum wells were observed with increasing doping density. As the doping density was increased from 6.6 × 1011 to 6.0 × 1012 cm−2 per a quantum well, the intersubband absorption peak energy shifted from 274.0 meV to 302.9 meV, and the full width at half maximum increased from 56.4 meV to 112.4 meV. Theoretical calculations reveal that the blue shift is due to many body effects, and the intersubband linewidth in doped AlGaN/GaN QW is mainly determined by scattering due to interface roughness, LO phonons, and ionized impurities.

Mid-infrared intersubband polaritons in dispersive metal-insulator-metal resonators

We demonstrate room-temperature strong coupling between a mid-infrared (λ = 9.9 μm) intersubband transition and the fundamental cavity mode of a metal-insulator-metal resonator. Patterning of the resonator surface enables surface-coupling of the radiation and introduces an energy dispersion which can be probed with angle-resolved reflectivity. In particular, the polaritonic dispersion presents an accessible energy minimum at k = 0 where—potentially—polaritons can accumulate. We also show that it is possible to maximize the coupling of photons into the polaritonic states and—simultaneously—to engineer the position of the minimum Rabi splitting at a desired value of the in-plane wavevector. This can be precisely accomplished via a simple post-processing technique. The results are confirmed using the temporal coupled mode theory formalism and their significance in the context of the strong critical coupling concept is highlighted.

THE OPTICAL GREEN VALLEY VERSUS MID-INFRARED CANYON IN COMPACT GROUPS

Compact groups of galaxies provide conditions similar to those experienced by galaxies in the earlier universe. Recent work on compact groups has led to the discovery of a dearth of mid-infrared transition galaxies (MIRTGs) in IRAC (3.6 - 8.0 micron) color space (Johnson et al. 2007; Walker et al. 2012) as well as at intermediate specific star formation rates (Tzanavaris et al. 2010). However, we find that in compact groups these mid-infrared (mid-IR) transition galaxies in the mid-infrared dearth have already transitioned to the optical ([g-r]) red sequence. We investigate the optical color-magnitude diagram (CMD) of 99 compact groups containing 348 galaxies and compare the optical CMD with mid-IR color space for compact group galaxies. Utilizing redshifts available from SDSS, we identified new galaxy members for 6 groups. By combining optical and mid-IR data, we obtain information on both the dust and the stellar populations in compact group galaxies. We also compare with more isolated galaxies and galaxies in the Coma cluster, which reveals that, similar to clusters, compact groups are dominated by optically red galaxies. While we find that compact group transition galaxies lie on the optical red sequence, LVL+SINGS mid-IR transition galaxies span the range of optical colors. The dearth of mid-IR transition galaxies in compact groups may be due to a lack of moderately star forming low mass galaxies; the relative lack of these galaxies could be due to their relatively small gravitational potential wells. This makes them more susceptible to this dynamic environment, thus causing them to more easily lose gas or be accreted by larger members.

A promising mid-infrared transition metal laser ion

Themid-infrared (mid-IR) wavelength region (2–5μm) is used in a number of important applications such as environmental sensing, infrared countermeasures against heat-seekingmissiles, and laser radar. It is also extensively used in medicine, spectroscopy, and manufacturing. These applications are all driving a pressing need for robust, compact, solid-state, tunable laser sources operating at room temperature in the mid-IR range. In this context, transition metal ions have always been of great interest from the very beginnings of laser development efforts because of their broadband emission. Recent developments in room temperature Cr2+ lasers have resulted in impressive performance reports, but still face development and power-scaling roadblocks. Ruby (Cr:Al2O3), the first material to demonstrate laser action in 1960,1 turned out to be the exception to the rule that transition metal lasers are broadly tunable. Tunable transition metal laser development started with flashlamp-pumped demonstrations of Ni2+and Co2+-doped fluoride hosts in 1963.2, 3 However, laser tuning in these hosts is often less than the observed fluorescence of their active ions: this restriction in tuning range is now known to be caused by excited state absorption (ESA).4 In addition, these materials were found to operate poorly or not at all at room temperature due to nonradiative quenching of the excited state populations. Tunable chromium ion lasing was only discovered with Cr3+:alexandrite in 1979.5 The wait was even longer for Cr4+ and Cr2+ infrared laser materials. In 1995, researchers at the Lawrence Livermore National Laboratory reported a new class of room-temperature, widely tunable, mid-IR laser materials that did not exhibit the ESA or nonradiative quenching problems of previous transition metal lasers.6, 7 The active ion was Cr2+ and the host materials consisted of II-VI semiconductor compounds. While the initial Livermore report used ZnS and ZnSe as host materials, other researchers, including my group at the Figure 1. Multi-pass cell with thin disk of Cr2+:ZnSe laser material bonded to a copper heat sink. A focusing mirror on the left (with a hole in the center for the output laser beam), prisms, and a return mirror on the right are used to provide multiple re-imaging of the pump beam (green lines) on the laser disk for a total of 16 pump passes. The red line represents the laser beam in a resonator formed by the disk and an external mirror (not shown).

Intersubband χ/sup 3/ in coupled InGaAs-AlGaAs multiple quantum wells

The third-order nonlinearity, /spl chi//sup 3/(/spl omega/,/spl omega/,-/spl omega/), is measured for a mid-infrared intersubband transition in strained InGaAs-AlGaAs multiple quantum wells (MQW's). The high conduction band offset of this system allows an intersubband transition at 3.1 /spl mu/m. The level structure of the quantum well is designed to include a meta-stable trapping level, resulting in a peak saturation intensity of 6 MW/cm/sup 2/ at Brewster's angle, approximately 20 times lower than would be found in a square quantum well with similar linewidth. A near-resonant n/sub 2/ of 8.4/spl times/10/sup -7/ cm/sup 2//W at 3.1 /spl mu/m is calculated. The off-resonant n/sub 2/ is also calculated and shown to be attractive at wavelengths as short as 1.55 /spl mu/m.

Ultrafast heating and cooling of electron plasmas in GaInAs/AlInAs quantum wells after intersubband excitation

Abstract Energy exchange between nonequilibrium carriers and a two-dimensional electron plasma and subsequent carrier cooling are studied in femtosecond pump-probe experiments. (n = 1) electrons excited to the (n = 2) conduction subband via the midinfrared intersubband transition undergo intersubband scattering and subsequently transfer a large amount of energy to unexcited (n = 1) electrons resulting in a hot Fermi distribution within 2 ps. The observed heating rate increases with the plasma density whereas the subsequent cooling proceeds within 40 ps by emission of optical phonons and is found to be density independent.

Observation of a midinfrared fine-structure transition for the single trapped barium ion.

Observation of a midinfrared fine-structure transition for the single trapped barium ion.