Mid-infrared Transitions Research Articles

Unipolar optical transitions in nanoclusters of ellipsoidal geometry

The quantum states of an ellipsoidal nanocluster of a heterophase system InAs/GaAs are studied using an exact analytical approach, in contrast to the generally accepted theoretical model based on the adiabatic approximation. It is shown that the spectrum of a nanoobject is formed from local groups, consisting of discrete levels, separated by terahertz frequency intervals. A double random degeneration of certain spectrum states is revealed. Allowed mid-infrared (IR) optical transitions between different spectral states are analyzed. The role of dimensional parameters and features of the shape of a nanoobject in the characteristics of unipolar transitions is assessed. The absorption spectrum for the transition in the lower part of the substructure spectrum is calculated.

Multimode vibrational strong coupling in direct laser written mid-IR plasmonic MIM nano-patch antennas

Strong coupling of mid-infrared (mid-IR) vibrational transitions to optical cavities provides a means to modify and control a material’s chemical reactivity and offers a foundation for novel chemical detection technology. Currently, the relatively large volumes of the mid-IR photonic cavities and weak oscillator strengths of vibrational transitions restrict vibrational strong coupling (VSC) studies and devices to large ensembles of molecules, thus representing a potential limitation of this nascent field. Here, we experimentally and theoretically investigate the mid-IR optical properties of 3D-printed multimode metal–insulator–metal (MIM) plasmonic nanoscale cavities for enabling strong light–matter interactions at a deep subwavelength regime. We observe strong vibration-plasmon coupling between the two dipolar modes of the L-shaped cavity and the carbonyl stretch vibrational transition of the polymer dielectric. The cavity mode volume is half the size of a typical square-shaped MIM geometry, thus enabling a reduction in the number of vibrational oscillators to achieve strong coupling. The resulting three polariton modes are well described by a fully coupled multimode oscillator model where all coupling potentials are non-zero. The 3D printing technique of the cavities is a highly accessible and versatile means of printing arbitrarily shaped submicron-sized mid-IR plasmonic cavities capable of producing strong light–matter interactions for a variety of photonic or photochemical applications. Specifically, similar MIM structures fabricated with nanoscopic voids within the insulator region could constitute a promising microfluidic plasmonic cavity device platform for applications in chemical sensing or photochemistry.

Charge separation in the photosystem II reaction center resolved by multispectral two-dimensional electronic spectroscopy

The photosystem II reaction center (PSII RC) performs the primary energy conversion steps of oxygenic photosynthesis. While the PSII RC has been studied extensively, the similar time scales of energy transfer and charge separation and the severely overlapping pigment transitions in the Qy region have led to multiple models of its charge separation mechanism and excitonic structure. Here, we combine two-dimensional electronic spectroscopy (2DES) with a continuum probe and two-dimensional electronic vibrational spectroscopy (2DEV) to study the cyt b559-D1D2 PSII RC at 77 K.This multispectral combination correlates the overlapping Qy excitons with distinct anion and pigment-specific Qx and mid-infrared transitions to resolve the charge separation mechanism and excitonic structure. Through extensive simultaneous analysis of the multispectral 2D data, we find that charge separation proceeds on multiple time scales from a delocalized excited state via a single pathway in which PheoD1 is the primary electron acceptor, while ChlD1 and PD1 act in concert as the primary electron donor.

Narrow homogeneous linewidths and slow cooling dynamics across infrared intra-band transitions in n-doped HgSe colloidal quantum dots.

Intra-band transitions in colloidal quantum dots (QDs) are promising for opto-electronic applications in the mid-IR spectral region. However, such intra-band transitions are typically very broad and spectrally overlapping, making the study of individual excited states and their ultrafast dynamics very challenging. Here, we present the first full spectrum two-dimensional continuum infrared (2D CIR) spectroscopy study of intrinsically n-doped HgSe QDs, which exhibit mid-infrared intra-band transitions in their ground state. The obtained 2D CIR spectra reveal that underneath the broad absorption line shape of ∼500cm-1, the transitions exhibit surprisingly narrow intrinsic linewidths with a homogeneous broadening of 175-250cm-1. Furthermore, the 2D IR spectra are remarkably invariant, with no signof spectral diffusion dynamics at waiting times up to 50ps. Accordingly, we attribute the large static inhomogeneous broadening to the distribution of size and doping level of the QDs. In addition, the two higher-lying P-states of the QDs can be clearly identified in the 2D IR spectra along the diagonal with a cross-peak. However, there is no indication of cross-peak dynamics indicating that, despite the strong spin-orbit coupling in HgSe, transitions between the P-states must be longer than our maximum waiting time of 50ps. This study illustrates a new frontier of 2D IR spectroscopy enabling the study of intra-band carrier dynamics in nanocrystalline materials across the entire mid-infrared spectrum.

Mid-infrared intersubband absorption in strain-balanced non-polar (In)AlGaN/InGaN multi-quantum wells

Mid-infrared intersubband transitions in strain-balanced m-plane (In)AlxGa1-xN/In0.16Ga0.84N (0.19≤x≤0.3) multi-quantum wells are reported for the first time in the range of 3.4–5.1 µm (244–360 meV). Direct and attenuated total-reflection infrared absorption measurements are interpreted using structural information revealed by high-resolution x-ray diffraction and transmission electron microanalysis. The experimental intersubband energies are better reproduced by calculations using the local-density approximation than the Hartree-Fock approximation for the exchange-correlation correction. The effect of charge density, quantum well width, and barrier alloy composition on the intersubband transition energy is also investigated to evaluate the potential of this material for practical device applications.

Structural and optical properties of nonpolar m- and a-plane GaN/AlGaN heterostructures for narrow-linewidth mid-infrared intersubband transitions

Mid-infrared intersubband transitions are investigated in nonpolar m-plane and a-plane GaN/AlGaN multi-quantum well heterostructures. Nominally identical heterostructures were grown by ammonia molecular-beam epitaxy on free-standing m-plane and a-plane GaN substrates. A total of 12 well- and barrier-doped samples with intersubband transition energies in the range of 220–320 meV (wavelength range 3.8–5.6 μm) were grown. The intersubband absorption lines of the m-plane samples were 10–40% narrower than those of the a-plane samples, and a very narrow intersubband absorption linewidth of 38 meV (full width at half maximum) at a transition energy of approximately 250 meV (5 μm wavelength) was observed in an m-plane sample. Narrower intersubband absorption linewidths of m-plane samples can be explained by more abrupt heterostructure interfaces revealed by structural characterization, which is attributed to a higher stability of the m-plane compared to the a-plane. No significant difference in the intersubband absorption linewidth was observed between the barrier- and well-doped samples.

(Invited) Structural Origins of Electronic Properties and Transport States in Halide Perovskites

The Asbury group has developed ultrafast infrared spectroscopy techniques to examine electronic processes in materials for photocatalysis and energy harvesting with particular focus on the influence that underlying molecular structures and structural fluctuations have on those processes. This talk will focus on the application of ultrafast infrared spectroscopy to examine the coupling of temperature-dependent lattice fluctuations on electronic transport states in perovskite photovoltaic and photocatalytic materials. The transition of photogenerated charges from free carrier states to large polarons to trapped states is directly observed through the corresponding mid-infrared transitions. Temperature dependent amplitudes of lattice fluctuations are examined through dephasing of vibrational modes and correlated with the nature of the transport states. Ligands attached to surface states provide unique insights into the nature of charge traps and help define pathways for their elimination. The ability to link underlying molecular structures with electronic properties provides unique insights that lead to new design rules in support of continued materials development efforts.

Single-Ended Sensor for Thermometry and Speciation in Shock Tubes Using Native Surfaces

A single-ended laser absorption sensor for time-resolved measurements of temperature and H2O, CO2, and CO concentrations was designed and deployed on a cylindrical shock tube operating at combustion-relevant conditions. The sensor transmitted four monochromatic laser beams probing strong species-specific mid-infrared absorption transitions through a single optical port and measured the backscattered laser radiation from the native surface across the 13.97-cm-diameter shock tube. Despite the non-ideal concave reflective surface and the long optical path relative to the port diameter (1.37 cm), an optimal optical configuration that maximized signal-to-noise ratio and was free of physical blockage was found and implemented using a numerical ray-tracing optimization algorithm. Wavelength-modulation spectroscopy was employed to further compensate for interference sources in the shock tube environment. The four lasers were sinusoidally injection-current modulated at various frequencies near 100 kHz to yield a measurement rate of 44 kS/s. Demonstration measurements involving non-reacting mixtures of the target species and reacting mixtures of ethane and oxygen in a N2 bath were performed in a shock tube facility at temperatures between 1100 and 1800 K and pressures between 2 and 9 atm, with measured quantities demonstrating excellent agreement with the expected values. Despite experiencing strong vibrations induced by the dynamic shock tube environment, the sensor resolved sub-millisecond transients during ethane oxidation and detected H2O, CO2, and CO mole fractions as low as 0.025%, 0.012%, and 0.005%, respectively, representing highly sensitive detection limits suitable for single-port low-concentration multi-species sensing in shock tubes and other flow channels of comparable dimensions.

The water-carbon monoxide dimer: new infrared spectra, ab initio rovibrational energy level calculations, and an interesting in-termolecular mode.

Bound state rovibrational energy level calculations using a high-level intermolecular potential surface are reported for H2O-CO and D2O-CO. They predict the ground K = 1 levels to lie about 20 (12) cm-1 above K = 0 for H2O-CO (D2O-CO) in good agreement with past experiments. But the first excited K = 1 levels are predicted to lie about 3 cm-1 below their K = 0 counterparts in both cases. Line strength calculations also indicate that mid-infrared transitions from the K = 0 ground state to this seemingly anomalous excited K = 1 state should be observable. These predictions are strikingly verified by new spectroscopic measurements covering the C-O stretch region around 2200 cm-1 for H2O-CO, D2O-CO, and HOD-CO, and the O-D stretch region around 2700 cm-1 for D2O-CO, HOD-CO, and DOH-CO. The experiments probe a pulsed supersonic slit jet expansion using tunable infrared quantum cascade laser or optical parametric oscillator sources. Discrete perturbations in the O-D stretch region give an experimental lower limit to the binding energy D0 of about 340 cm-1 for D2O-CO, as compared to our calculated value of 368 cm-1. Wavefunction plots are presented to help understand the intermolecular dynamics of H2O-CO. Coriolis interactions are invoked to explain the seemingly anomalous energies of the first excited K = 1 levels.

Dynamic Disorder Dominates Delocalization, Transport, and Recombination in Halide Perovskites

Summary We investigate the origins of exceptionally long charge-carrier lifetimes of halide perovskites by using temperature-dependent and time-resolved mid-infrared spectroscopy. We report direct observations of large polarons through their distinct mid-infrared electronic transitions and quantitatively correlate their formation with enhancement of electron-phonon coupling that quenches photoluminescence. The temperature-dependent study reveals a branching between free-carrier and large-polaron states in association with thermally induced dynamic disorder of the anharmonic inorganic framework, which is examined through local vibrational modes of the perovskite films. This dynamic disorder localizes charge carriers and enhances electron-phonon coupling, which both quenches photoluminescence and promotes large-polaron formation. Fortunately, such localization extends the charge recombination lifetime by nearly an order of magnitude. This work highlights the interplay between charge-carrier mobility and lifetime and suggests design rules about how cation and anion substitution can be used to tune the structural dynamics that underpin their relationship.

Single-ended mid-infrared laser-absorption sensor for time-resolved measurements of water concentration and temperature within the annulus of a rotating detonation engine

Abstract A novel single-ended mid-infrared laser-absorption sensor for time-resolved measurements of water mole fraction and temperature was developed and deployed within the annulus of a hydrogen/air-fed rotating detonation engine (RDE). The sensor transmitted two laser beams targeting mid-infrared water transitions through a single optical port on the outer wall of the cylindrical RDE annulus and measured the backscattered radiation from the RDE inner surface using a photodetector for a round-trip path of 1.52 cm. Optimizing the sensor's optical arrangement using numerical ray tracing to minimize interference from optical emission, beam steering, and scattered laser light from window surfaces was essential to sensor performance. Scanned-wavelength-modulation spectroscopy with second-harmonic detection and first-harmonic normalization was implemented to allow for frequency-domain multiplexing of the two lasers and to suppress non-absorbing interference sources such as beam-steering and emission. Tunable diode lasers near 2551 and 2482 nm were modulated at 100 and 122 kHz, respectively, and sinusoidally scanned across the peaks of their respective water transitions at 10 kHz to provide a measurement rate of 20 kHz and detection limit of 0.5% water by mole. Experimentally derived spectroscopic parameters enabled water and temperature sensing with respective uncertainties of 7.3% and 5.3% relative to the measured values. Time-resolved and time-averaged sensor measurements of gas temperature and water vapor mole fraction allow quantitative evaluation of the combustion progress at the measurement location and thus provide a design tool for RDE optimization. Broadly, this single-ended laser sensor should find applications in other combustion systems where optical access is limited.

Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216.

A discrepancy exists between the abundance of ammonia (NH3) derived previously for the circumstellar envelope (CSE) of IRC+10216 from far-IR submillimeter rotational lines and that inferred from radio inversion or mid-infrared (MIR) absorption transitions. To address the discrepancy described above, new high-resolution far-infrared (FIR) observations of both ortho- and para-NH3 transitions toward IRC+10216 were obtained with Herschel, with the goal of determining the ammonia abundance and constraining the distribution of NH3 in the envelope of IRC+10216. We used the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel to observe all rotational transitions up to the J = 3 level (three ortho- and six para-NH3 lines). We conducted non-LTE multilevel radiative transfer modelling, including the effects of near-infrared (NIR) radiative pumping through vibrational transitions. The computed emission line profiles are compared with the new HIFI data, the radio inversion transitions, and the MIR absorption lines in the ν2 band taken from the literature. We found that NIR pumping is of key importance for understanding the excitation of rotational levels of NH3. The derived NH3 abundances relative to molecular hydrogen were (2.8 ± 0.5) × 10-8 for ortho-NH3 and [Formula: see text] for para-NH3, consistent with an ortho/para ratio of 1. These values are in a rough agreement with abundances derived from the inversion transitions, as well as with the total abundance of NH3 inferred from the MIR absorption lines. To explain the observed rotational transitions, ammonia must be formed near to the central star at a radius close to the end of the wind acceleration region, but no larger than about 20 stellar radii (1σ confidence level).

Low-threshold terahertz molecular laser optically pumped by a quantum cascade laser

We demonstrate a low-threshold, compact, room temperature, and continuous-wave terahertz molecular laser optically pumped by a mid-infrared quantum cascade laser. These characteristics are obtained, thanks to large dipole transitions of the active medium: NH3 (ammonia) in gas state. The low-power (<60 mW) laser pumping excites the molecules, thanks to intense mid-infrared transitions around 10.3 μm. The molecules de-excite by stimulated emission on pure inversion “umbrella-mode” quantum transitions allowed by the tunnel effect. The tunability of the quantum cascade laser gives access to several pure inversion transitions with different rotation states: we demonstrate the continuous-wave generation of ten laser lines around 1 THz. At 1.07 THz, we measure a power of 34 μW with a very low-threshold of 2 mW and a high differential efficiency of 0.82 mW/W. The spectrum was measured showing that the linewidth is lower than 1 MHz. To our knowledge, this is the first THz molecular laser pumped by a solid-state source and this result opens the way for compact, simple, and efficient THz source at room temperature for imaging applications.

Optical pumping and readout of bismuth hyperfine states in silicon for atomic clock applications.

The push for a semiconductor-based quantum information technology has renewed interest in the spin states and optical transitions of shallow donors in silicon, including the donor bound exciton transitions in the near-infrared and the Rydberg, or hydrogenic, transitions in the mid-infrared. The deepest group V donor in silicon, bismuth, has a large zero-field ground state hyperfine splitting, comparable to that of rubidium, upon which the now-ubiquitous rubidium atomic clock time standard is based. Here we show that the ground state hyperfine populations of bismuth can be read out using the mid-infrared Rydberg transitions, analogous to the optical readout of the rubidium ground state populations upon which rubidium clock technology is based. We further use these transitions to demonstrate strong population pumping by resonant excitation of the bound exciton transitions, suggesting several possible approaches to a solid-state atomic clock using bismuth in silicon, or eventually in enriched 28Si.

Mid infrared lasers for remote sensing applications

To accurately measure the concentrations of atmospheric gasses, especially the gasses with low concentrations, strong absorption features must be accessed. Each molecular species or constituent has characteristic mid-infrared absorption features by which either column content or range resolved concentrations can be measured. Because of these characteristic absorption features the mid infrared spectral region is known as the fingerprint region. However, as noted by the Decadal Survey, mid-infrared solid-state lasers needed for DIAL systems are not available. The primary reason is associated with short upper laser level lifetimes of mid infrared transitions. Energy gaps between the energy levels that produce mid-infrared laser transitions are small, promoting rapid nonradiative quenching. Nonradiative quenching is a multiphonon process, the more phonons needed, the smaller the effect. More low energy phonons are required to span an energy gap than high energy phonons. Thus, low energy phonon materials have less nonradiative quenching compared to high energy phonon materials. Common laser materials, such as oxides like YAG, are high phonon energy materials, while fluorides, chlorides and bromides are low phonon materials. Work at NASA Langley is focused on a systematic search for novel lanthanide-doped mid-infrared solid-state lasers using both quantum mechanical models (theoretical) and spectroscopy (experimental) techniques. Only the best candidates are chosen for laser studies. The capabilities of modeling materials, experimental challenges, material properties, spectroscopy, and prospects for lanthanide-doped mid-infrared solid-state laser devices will be presented.

Mid-infrared optical transitions of a hydrogenic impurity in the conduction band of a Cd1 − xZnxSe/ZnSe parabolic quantum dot

Theoretical investigations on the absorption spectra for the intersubband optical transitions in a Cd1 − xZnxSe/ZnSe quantum dot are performed. Calculations on the electronic states are investigated using a variational scheme within the single band effective mass approximation and the optical gain is calculated using the compact density matrix approach with the iterative procedure. 2p-1s like transition energy and the spontaneous transition lifetime of the shallow donor hydrogenic impurity with the geometrical confinement and the Zn alloy content in a Cd1 − xZnxSe/ZnSe quantum dot are computed. The optical gain as a function of incident photon energy with the effects of spatial confinement and the Zn composition is found. The results show the dependence of the transition energy on the electron density in the Cd0.8Zn0.2Se/ZnSe quantum dot and the magnitude of the optical absorption coefficient enhances when the electron density increases. It is observed that a 1.55 μm wavelength is achieved for a 22 Å radius of a Cd1 − xZnxSe/ZnSe quantum dot. This band gap tunability brings out the possibility for near-infrared detectors, fiber optical communication systems and absorption modulators.

Infrared laser absorption sensors for multiple performance parameters in a detonation combustor

Laser absorption sensors for temperature and H2O near 1.4 and 2.5μm, CO2 near 2.7μm, and CO near 4.8μm were developed, validated, and deployed in an ethylene-fueled pulse-detonation combustor (PDC). Each sensor was fiber-coupled to enable remote light delivery to the PDC and photo-voltaic detectors were mounted directly to the PDC for improved light collection. Measurements were acquired simultaneously along two orthogonal lines-of-sight (LOS) in both the PDC combustion chamber and the throat of a converging–diverging nozzle located at the PDC exit. Measurements in the nozzle throat were combined with a choked-flow assumption to calculate the time-resolved enthalpy flow rate exiting the PDC. All sensors used first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f) to account for non-absorbing transmission losses and emission. Furthermore, strong mid-infrared absorption transitions were used to enable improved measurement accuracy and precision. These sensors were validated in non-reactive shock-tube experiments at temperatures and pressures up to 2700K and 50atm. There, these sensors exhibited a nominal accuracy of 3 to 5% with bandwidths from 2 to 20kHz. Measurements in the PDC indicated peak temperatures near 3500K with near-stoichiometric proportions of H2O and incomplete conversion of CO to CO2. The stagnation enthalpy flow rate was highly transient, but repeatable with peak flow rates near 25MW.

Fine-structure collision strengths and line ratios for [Ne v] in infrared and optical sources

New collisions’ strengths for the mid-infrared (mid-IR) and optical transitions in Ne v are presented. Breit–Pauli-R-Matrix calculations for electron impact excitation are carried out with fully resolved near-threshold resonances at very low energies. In particular, the fine-structure lines at 14 and 24 μm due to transitions among the ground state levels 1s22s22p3 3P0, 1, 2, and the optical/near-ultraviolet lines at 2973, 3346 and 3426 Å transitions among the 3P0, 1, 2, 1D2, 1S0 levels are described. Maxwellian-averaged collision strengths are tabulated for all forbidden transitions within the ground configuration. While some significant differences are found for both the far infrared and the optical transitions compared to previous results, computed line emissivity ratios are in good agreement, but change rapidly in the low temperature range Te < 10 000 K. An analysis of the 14/24 μm ratio in low-energy-density (LED) plasma conditions reveals considerable variation; the effective rate coefficient may be dominated by the very low energy behaviour rather than the Maxwellian-averaged collision strengths. Computed values suggest a possible solution to the anomalous mid-IR ratios found to be lower than theoretical limits observed from planetary nebulae and Seyfert galaxies. While such LED conditions may be present in infrared sources, they might be inconsistent with photoionization equilibrium models.

Superlattice formed by quantum-dot sheets: Density of states and infrared absorption

Low-energy continuous states of electron in a heterosrtucture with periodically placed quantum-dot sheets are studied theoretically. The Green's function of an electron is governed by the Dyson equation with the self-energy function which is determined the boundary conditions at quantum-dot sheets with weak damping in the low-energy region. The parameters of a superlattice formed by quantum-dot sheets are determined using the short-range model of quantum dot. The density of states and spectral dependencies of the anisotropic absorption coefficient under midinfrared transitions from doped quantum dots into miniband states of a superlattice strongly depend on dot concentration and on the period of sheets. These dependencies can be used for the characterization of the multilayer structure and they determine the parameters of different optoelectronic devices exploiting the vertical transport of carriers through quantum-dot sheets.

N2-broadening coefficients of methyl chloride at room temperature

Methyl chloride is of interest for atmospheric applications, since this molecule is directly involved in the catalytic destruction of ozone in the lower stratosphere. In a previous work [Bray et al. JQSRT 2011;112:2446], lines positions and intensities of self-perturbed 12CH335Cl and 12CH337Cl have been studied into details for the 3.4μm spectral region. The present work is focused on measurement and calculation of N2-broadening coefficients of the 12CH335Cl and 12CH337Cl isotopologues. High-resolution Fourier Transform spectra of CH3Cl–N2 mixtures at room-temperature have been recorded between 2800 and 3200cm−1 at LADIR (using a classical source) and between 47 and 59cm−1 at SOLEIL (using the synchrotron source on the AILES beamline). 612 mid-infrared transitions of the ν1 band and 86 far-infrared transitions of the pure rotational band have been analyzed using a multispectrum fitting procedure. Average accuracy on the deduced N2-broadening coefficients has been estimated to 5% and 10% in the mid- and far-infrared spectral regions, respectively. The J- and K-rotational dependences of these coefficients have been observed in the mid-infrared region and then a simulation has been performed using an empirical model for 0≤J≤50, K≤9. The 12CH335Cl–N2 line widths for 0≤J≤50 and K≤10 of the ν1 band and for 55≤J≤67 and K≤15 of the pure rotational band have been computed using a semi-classical approach involving exact trajectories and a real symmetric-top geometry of the active molecule. Finally, a global comparison with the experimental and theoretical data existing in the literature has been performed. Similar J- and K-rotational dependences have been appeared while no clear evidence for any vibrational or isotopic dependences have been pointed out.