Infrared Photons Research Articles

Photoelectron angular distributions in infrared one-photon and two-photon ionization of FEL-pumped Rydberg states of helium

The photoelectron angular distributions (PADs) have been investigated for infrared (IR) ionization of He atoms excited to Rydberg states by extreme ultraviolet free-electron laser pulses. The experiment was carried out with two pulses which do not overlap in time. Depending on the intensity of the IR pulses, one IR photon ionization or additionally two-photon above-threshold ionization is observed. For low IR intensity, the PAD is well described by a contribution of s and d partial waves in accordance with early experiments. At high IR intensity, the PAD for two IR photon ionization clearly shows the contribution of higher partial waves. The experimental data are compared with the results of theoretical calculations based on solving the time-dependent Schrödinger equation.

Infrared quantum cutting in Er3+:NaYF4 nanostructured glass ceramics for solar cells

In this paper, Er3+:NaYF4 nanostructured glass ceramics were fabricated by conventional melt-quenching method. The excitation, emission spectra and fluorescence lifetime of samples at room temperature had been measured. Quantum cutting (QC) involving the emission of multiple-photon infrared (IR) photons for each visible photon absorbed was realized, and the mechanism responsible for QC by cross relaxation was analyzed. An effective IR QC was carried out by cross-energy-transfer processes occurring within the Er3+. It was found that decay lifetime of 4S3/2 level decreases rapidly with the concentration increasing of Er3+. The QC efficiency, reached 150.06% for the sample with 4mol% Er3+, was calculated from the decay curves. The analysis results reveal that Er3+:NaYF4 nanostructured glass ceramic is promising for the use in Si–Ge solar cells, and the spectral mismatch losses for Si–Ge solar cells will be greatly reduced.

Dispersed Three-Pulse Infrared Photon Echoes of Nitrous Oxide in Water and Octanol

Dispersed IR three-pulse photon echoes due to the antisymmetric (ν3) stretch mode of N2O dissolved in H2O and 1-octanol at room temperature are reported and analyzed. The experimentally determined transition frequency-frequency correlation function (FFCF) in these two solvents is explained in terms of inertial solvent contributions, hydrogen bond network fluctuations, and, for octanol, the motions of the alkyl chains. The H2O hydrogen bond fluctuations result in 1.5 ps FFCF decay, in agreement with relaxation rates determined from photon echo based measurements of other aqueous solutions including salt solutions. In octanol, hydrogen bond fluctuations decay on a slower time scale of 3.3 ps and alkyl chain motions result in an inhomogeneous broadening contribution to the ν3 absorption spectrum that decays on a 35 ps time scale. Rotational reorientation of N2O is nearly 3 times faster in octanol as compared to water. Although the vibrational ν3 N2O absorption line shapes in water and octanol are similar, the line widths result from different coherence loss mechanisms. A hot band contribution in the N2O in octanol solution is found to have a significant effect on the echo spectrum due to its correspondingly stronger transition moment than that of the fundamental transition. The dephasing dynamics of the N2O ν3 stretch mode is of interest as a probe in ultrafast studies of complex or nanoconfined systems with both hydrophobic and hydrophilic regions such as phospholipids, nucleic acids, and proteins. These results demonstrate the value of the N2O molecule to act as a reporter of equilibrium fluctuations in such complex systems particularly due to its solubility characteristics and long vibrational lifetime.

Strong Magnetophotoelectric Effect in Folded Graphene

We study electronic transport in graphene under the influence of a transversal magnetic field B(r)=B(x)ez with the asymptotics B(x→±∞)=±B0, which could be realized via a folded graphene sheet in a constant magnetic field, for example. By solving the effective Dirac equation, we find robust modes with a finite energy gap which propagate along the fold-where particles and holes move in opposite directions. Exciting these particle-hole pairs with incident (optical or infrared) photons would then generate a nearly perfect charge separation and thus a strong magnetophotoelectric or magnetothermoelectric effect-even at room temperature.

Effect of extended π-conjugation on photovoltaic performance of dye sensitized solar cells based on unsymmetrical squaraine dyes

Aiming toward the far-red to near infra-red (NIR) photon harvesting, three new unsymmetrical squaraine dyes bearing direct ring carboxy functionalized indole as an anchoring moiety with varying donor groups with extended π-conjugation have been successfully synthesized and utilized for dye sensitized solar cell fabrication. Under simulated solar irradiation, dye SQ-8 gave a photoconversion efficiency of 3.3% mainly harvesting photons in the far-red region between 500 and 700 nm. By extending the π-conjugation of the donor moieties in the novel unsymmetrical squaraine dyes, it was possible to extend the light absorption from far-red to NIR wavelength region. In spite of good light absorption up to 900 nm and energetic matching, dye SQ-16 was found to exhibit the decreased photon harvesting, which was explained by the enhanced dye aggregation along with its difficulty in facile electron injection as indicated from electronic absorption spectroscopic and DFT calculation results, respectively.

Molecular recognition using receptor-free nanomechanical infrared spectroscopy based on a quantum cascade laser

Speciation of complex mixtures of trace explosives presents a formidable challenge for sensors that rely on chemoselective interfaces due to the unspecific nature of weak intermolecular interactions. Nanomechanical infrared (IR) spectroscopy provides higher selectivity in molecular detection without using chemoselective interfaces by measuring the photothermal effect of adsorbed molecules on a thermally sensitive microcantilever. In addition, unlike conventional IR spectroscopy, the detection sensitivity is drastically enhanced by increasing the IR laser power, since the photothermal signal comes from the absorption of IR photons and nonradiative decay processes. By using a broadly tunable quantum cascade laser for the resonant excitation of molecules, we increased the detection sensitivity by one order of magnitude compared to the use of a conventional IR monochromator. Here, we demonstrate the successful speciation and quantification of picogram levels of ternary mixtures of similar explosives (trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX), and pentaerythritol tetranitrate (PETN)) using nanomechanical IR spectroscopy.

Nanophotonics and Nanochemistry: Controlling the Excitation Dynamics for Frequency Up- and Down-Conversion in Lanthanide-Doped Nanoparticles

Nanophotonics is an emerging science dealing with the interaction of light and matter on a nanometer scale and holds promise to produce new generation nanophosphors with highly efficient frequency conversion of infrared (IR) light. Scientists can control the excitation dynamics by using nanochemistry to produce hierarchically built nanostructures and tailor their interfaces. These nanophosphors can either perform frequency up-conversion from IR to visible or ultraviolet (UV) or down-conversion, which results in the IR light being further red shifted. Nanophotonics and nanochemistry open up numerous opportunities for these photon converters, including in high contrast bioimaging, photodynamic therapy, drug release and gene delivery, nanothermometry, and solar cells. Applications of these nanophosphors in these directions derive from three main stimuli. Light excitation and emission within the near-infrared (NIR) "optical transparency window" of tissues is ideal for high contrast in vitro and in vivo imaging. This is due to low natural florescence, reduced scattering background, and deep penetration in tissues. Secondly, the naked eye is highly sensitive in the visible range, but it has no response to IR light. Therefore, many scientists have interest in the frequency up-conversion of IR wavelengths for security and display applications. Lastly, frequency up-conversion can convert IR photons to higher energy photons, which can then readily be absorbed by solar materials. Current solar devices do not use abundant IR light that comprises almost half of solar energy. In this Account, we present our recent work on nanophotonic control of frequency up- and down-conversion in fluoride nanophosphors, and their biophotonic and nanophotonic applications. Through nanoscopic control of phonon dynamics, electronic energy transfer, local crystal field, and surface-induced non-radiative processes, we were able to produce new generation nanophosphors with highly efficient frequency conversion of IR light. We show that nanochemistry plays a vital role in the design and interface engineering of nanophosphors, providing pathways to expand their range of applications. High contrast in vitro and in vivo NIR-to-NIR up- and down-conversion bioimaging were successfully demonstrated by our group, evoking wide interests along this line. We introduced trivalent gadolinium ions into the lattice of the nanophosphors or into the shell layer of nanophosphors in a core/shell configuration to produce novel nanophosphors for multimodal (MRI and optical) imaging. We also demonstrate the security and display applications using photopatternable NIR-to-NIR and NIR-to-visible frequency up-conversion nanophosphors with appropriately engineered surface chemistry. In addition, we present preliminary results on dye-sensitized solar cells using up-conversion in fluoride lattice-based nanophosphors for IR photon harvesting.

Time-resolved spectroscopy on epitaxial graphene in the infrared spectral range: relaxation dynamics and saturation behavior

We present the results of pump–probe experiments on multilayer graphene samples performed in a wide spectral range, namely from the near infrared (photon energy 1.5 eV) to the terahertz (photon energy 8 meV) spectral range. In the near infrared, exciting carriers and probing at higher photon energies provides direct evidence for a hot carrier distribution. Furthermore, spectroscopic signatures of the highly doped graphene layers at the interface to SiC are observed in the near-infrared range. In the mid-infrared range, the various relaxation mechanisms, in particular scattering via optical phonons and Auger-type processes, are identified by comparing the experimental results to microscopic modeling. Changes from induced transmission to induced absorption are attributed to probing above or below the Fermi edge of the graphene layers. This effect occurs for certain photon energies in the near-infrared range, where it is related to highly doped graphene layers at the interface to SiC, and in the far-infrared range for the quasi-intrinsic graphene layers. In addition to the relaxation dynamics, the saturation of pump-induced bleaching of graphene is studied. Here a quadratic dependence of the saturation fluence on the pump photon energy in the infrared spectral range is revealed.

Increased Upconversion Response in a-Si:H Solar Cells With Broad-Band Light

High-bandgap solar cells such as a-Si:H with a bandgap around 1.8-eV loose 50% of the incident solar energy due to transmittance of near infrared photons. Part of these photons can be converted into visible photons with a suitable upconverter and in this way can increase the photo response of solar cells in the near infrared. In this contribution, we demonstrate increased photo response of a-Si:H solar cells for sub bandgap light under broad-band light excitation of the upconverter. Broad-band excitation represents daily life and is more effective because simultaneously different wavelengths can be absorbed and converted. Bifacial a-Si:H solar cells were made with and without Gd2O2S: Er3+, Yb3+ upconverter attached at the back of the cells. A set-up was made to concentrate near infrared light from a solar simulator up to 25 times. I-V curves were measured of upconverter and reference solar cells. The upconverter solar cells showed a nonlinear and stronger increase in current with increasing concentration than the reference solar cells.

Incipient chemical bond formation of Xe to a cationic silicon cluster: Vibrational spectroscopy and structure of the Si4Xe+ complex

The size-selective vibrational spectrum of Si4Xe+ in the 240–500cm−1 range has been recorded using infrared (IR) multiple photon dissociation spectroscopy. Comparison to linear IR absorption spectra of Si4+ and Si4Xe+ calculated using density functional theory including dispersion corrections reveals an important influence of the Xe ligand on the structure of the cluster, involving significant charge transfer and formation of an incipient chemical Si–Xe bond with De = 0.3eV.

Sine gating detector with simple filtering for low-noise infra-red single photon detection at room temperature

We present and analyze a gated single photon avalanche detector using a sine gating scheme with a simple but effective low-pass filtering technique for fast low-noise single photon detection at telecom wavelength. The detector is characterized by 130 ps short gates applied with a frequency of 1.25 GHz, yields only 70 ps timing jitter and noise probabilities as low as 7·10−7 per gate at 10% detection efficiency. We show that the detector is suitable for high rate quantum key distribution (QKD) and even at room temperature it could allow for QKD over distances larger than 25 km.

ARCH-type micro-cantilever FPA for uncooled IR detection

Graphical abstractDisplay Omitted Highlights? Analysis of different cantilever micro-fabrication complexity. ? Numerical and experimental evaluation of different thermo-mechanical transducer designs for IR detection. ? Analysis of potential for an micro-cantilever sensor for low-cost IR technology. We report on fabrication, characterization and evaluation of different thermo-mechanical transducer designs serving as uncooled infrared (IR) sensors. The working principle of this transducer-type is based on the bi-metallic effect and the physical background is described. The absorbed IR radiation (long wave: λ=7?14µm) heats the structure locally and due to the mismatch of the coefficients of thermal expansion (CTE) the freestanding structure mechanically deflects. The deflection is a proportional merit for the absorbed IR photon flux. The performance of the classic metal-dielectric sandwich-type cantilever design is compared to our design approach, the so call ARCH-type cantilever. The results are discussed.

Optical excitation of zigzag carbon nanotubes with photons guided in nanofibers

We consider the excitation of electrons in semiconducting carbon nanotubes by photons from the evanescent field created by a subwavelength-diameter optical fiber. The strongly changing evanescent field of such nanofibers requires dropping the dipole approximation. We show that this leads to novel effects, especially a high dependence of the photon absorption on the relative orientation and geometry of the nanotube-nanofiber setup in the optical and near infrared domain. In particular, we calculate photon absorption probabilities for a straight nanotube and nanofiber depending on their relative angle. Nanotubes orthogonal to the fiber are found to perform much better than parallel nanotubes when they are short. As the nanotube gets longer the absorption of parallel nanotubes is found to exceed the orthogonal nanotubes and approach 100% for extremely long nanotubes. In addition, we show that if the nanotube is wrapped around the fiber in an appropriate way the absorption is enhanced. We find that optical and near infrared photons could be converted to excitations with efficiencies that may exceed 90%. This may provide opportunities for future photodetectors and we discuss possible setups.

COSMIC-RAY ELECTRON EVOLUTION IN THE SUPERNOVA REMNANT RX J1713.7–3946

A simple formalism to describe nonthermal electron acceleration, evolution, and radiation in supernova remnants (SNRs) is presented. The electron continuity equation is analytically solved assuming that the nonthermal electron injection power is proportional to the rate at which the kinetic energy of matter swept up in an adiabatically expanding SNR shell. We apply this model to \fermi\ and HESS data from the SNR \rxj, and find that a one-zone leptonic model with Compton-scattered cosmic microwave background (CMB) and interstellar infrared photons has difficulty providing a good fit to its spectral energy distribution, provided the source is at a distance $\sim 1\ \kpc$ from the Earth. However, the inclusion of multiple zones, as hinted at by recent {\em Chandra} observations, does provide a good fit, but requires a second zone of compact knots with magnetic fields $B\sim 16\ \mu$G, comparable to shock-compressed fields found in the bulk of the remnant.

Interferometric autocorrelation in the ultraviolet utilizing spontaneous parametric down-conversion inside an enhancement cavity

Autocorrelation is a common method to estimate the duration of ultrashort laser pulses. In the ultraviolet (UV) regime it is challenging to employ the process of second-harmonic generation, most prominently due to absorption in nonlinear crystals at very short wavelengths. Here we show how to utilize spontaneous parametric down-conversion (SPDC) to generate an autocorrelation signal in the infrared (IR) for UV pulses. Our method utilizes the nth-order emission of the SPDC process, which occurs for low pumping powers proportional to the nth power of the UV intensity. Thus, counting 2n down-converted photons directly yields the nth-order autocorrelation. The method, now with detection of near-IR photons, is applied to the first direct measurement of ultrashort UV pulses circulating inside a UV enhancement cavity.

Broadband downshifting luminescence of Cr3+/Yb3+-codoped fluorosilicate glass

Cr3+/Yb3+-codoped glasses were studied as broadband down shifting materials for improving solar cells’ efficiency. We observed Föster resonance energy transfers (FRET) from Cr3+: 4T2g to Yb3+: 2F5/2. The ET efficiency was up to 84 %, and the measured quantum yield (QY) reached 21.51 %. The glass can convert solar photons in super wide UV–Vis range to near infrared photons with the minimum quantum yields larger than 10%. Thus, such materials can be applied for improving silicon solar cells’ conversion efficiency.

Near Infrared Single Photon Detector Using an InGaAs/InP Avalanche Photodiode Operated with a Bipolar Gating Signal

We report a near infrared (NIR) single photon detector (SPD) using an InGaAs/InP avalanche photodiode (APD) operated with a bipolar rectangular gating signal. The use of the bipolar gating pulse enabled us to operate the APD well below the breakdown voltage during the gate-off time. As a result, it permits to decrease the lifetime of the trapped carriers, and then reduces the after-pulse noise of the detector. At a repetition rate of 200 MHz, the after-pulse probability is 8.2% less comparing to that of conventional gating signal SPD.

Ultrafast vibrational dynamics of SCN− and N3− in polar solvents studied by nonlinear infrared spectroscopy

In this paper, we report on our investigation into the vibrational dynamics of the antisymmetric stretching modes of SCN(-) and N(3)(-) in several polar solvents. We used an infrared (IR) pump-probe method to study orientational relaxation processes. In two aprotic solvents (N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the anisotropy decay shows a bimodal feature, whereas in other solvents the anisotropy decay can be fitted well by a single exponential function. We consider that the relative contribution of fast-decaying components is smaller in the other solvents than in DMF and DMSO. We discuss the possible origins of the different anisotropy decay behavior in different solvents. From the three-pulse IR photon echo measurements for SCN(-) and N(3)(-), we found that the time-correlation functions (TCFs) of vibrational frequency fluctuations decay on two different time scales, one of which is less than 100 fs and the other is approximately 3-6 ps. In aprotic solvents, the fast-decaying components of the TCFs on a <100 fs time scale play an important role in the vibrational frequency fluctuation, although the contribution of collective solvent reorganization in aprotic solvents was clearly observed to have small amplitudes. On the other hand, we found that the amplitude of components that decay in a few picoseconds and/or the constant offset of the TCF in protic solvents is relatively large compared with that in aprotic solvents. With the formation and dissociation of hydrogen bonds between ion solute and solvent molecules, the spectra of different solvated species are exchanged with each other and merged into one band. We considered that this exchange may be an origin of slow-decaying components of the TCFs and that the decay of the TCFs corresponds to the time scales of the exchange for protic solvents such as formamide. The mechanism of vibrational frequency fluctuations for the antisymmetric stretching modes of SCN(-) and N(3)(-) is discussed in terms of the difference between protic and aprotic solvents.

Structure Determination of Anionic Metal Clusters via Infrared Resonance Enhanced Multiple Photon Electron Detachment Spectroscopy

We report vibrational spectra of anionic metal clusters, measured via electron detachment following resonant absorption of multiple infrared photons. To facilitate the sequential absorption of the required large number of photons, the cluster beam interacts with the infrared radiation inside the cavity of a free electron laser. Far-infrared spectra of the bare metal cluster anion Nb10– as well as of the Nb6C– anion are measured in the 130–1000 cm–1 range. The structures of these clusters can be unambiguously determined by comparison with calculated spectra of the putative global minima structures, identified by density functional theory calculations.

Temporal correlation of photons following frequency up-conversion

We demonstrate an approach to measure temporal correlations of photons in the near infrared range using frequency up-conversion. In this approach, the near infrared signal photons are converted into the visible range, in which highly efficient silicon avalanche photodiodes are used to perform the temporal correlation measurements. A coherent light source and a pseudo-thermal light source were used in the experiment. The results are in agreement with theoretical values and those obtained from measurements directly made using superconducting nanowire single photon detectors. We conclude that the temporal correlation (up to 4th order) of photons was preserved in the frequency up-conversion process. We further theoretically and experimentally studied the influence of the dark counts on the measurement. The setup uses commercially available components and achieves high total detection efficiency (~26%).