Non-resonant Absorption Research Articles

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.

Plasmonic terahertz modulator based on a grating-coupled two-dimensional electron system

Electrically driven broadband modulator with large modulation depth and high speed is in high demand to meet the technical advancing and applications in terahertz fields recently. So far, the single-particle non-resonant absorption mechanism described by the Drude conductivity has been utilized in most of the related researches but is still not efficient enough. Here we proposed and demonstrated a terahertz modulator based on the collective electron plasma excitations (plasmons) in a grating-coupled two-dimensional electron gas in GaN/AlGaN heterostructure. By switching between the resonant and non-resonant conditions of the 2D plasmon excitation enabled by applying proper gate biases, the transmission of terahertz electromagnetic waves can be efficiently manipulated. Taking advantage of its resonant characteristic combined with the strong electric field enhancement in the active region, we experimentally achieved a maximum intensity modulation depth of 93%, a 3 dB operation bandwidth of ∼400 kHz, and a small required driving voltage amplitude of 2 V at a cryogenic temperature of 8.7 K. Owing to its excellent performances, this active plasmon-based terahertz modulator may offer some promising solutions in several fields of terahertz technology in the future.

Femtosecond Laser-Induced Upconversion Luminescence in Rare-Earth Ions by Nonresonant Multiphoton Absorption.

The upconversion luminescence of rare-earth ions has attracted considerable interest because of its important applications in photoelectric conversion, color display, laser device, multiplexed biolabeling, and security printing. Previous studies mainly explored the upconversion luminescence generation through excited state absorption, energy transfer upconversion, and photon avalanche under the continuous wave laser excitation. Here, we focus on the upconversion luminescence generation through a nonresonant multiphoton absorption by using the intense femtosecond pulsed laser excitation and study the upconversion luminescence intensity control by varying the femtosecond laser phase and polarization. We show that the upconversion luminescence of rare-earth ions under the intense femtosecond laser field excitation is easy to be obtained due to the nonresonant multiphoton absorption through the nonlinear interaction between light and matter, which is not available by the continuous wave laser excitation in previous works. We also show that the upconversion luminescence intensity can be effectively controlled by varying the femtosecond pulsed laser phase and polarization, which can open a new technological opportunity to generate and control the upconversion luminescence of rare-earth ions and also can be further extended to the relevant application areas.

Spectroscopy underlying microwave remote sensing of atmospheric water vapor

The paper presents a spectroscopist’s view on the problem of recovery of the atmosphere humidity profile using modern microwave radiometers. Fundamental equations, including the description of their limitations, related to modeling of atmospheric water vapor absorption are given. A review of all reported to date experimental studies aimed at obtaining corresponding numerical parameters is presented. Best estimates of these parameters related to the Voigt (Lorentz, Gross, Van Vleck – Weisskopf and other equivalent) profile based modeling of the 22- and 183-GHz water vapor diagnostic lines and to non-resonance absorption as well as corresponding uncertainties are made on the basis of their comparative analysis.

Optics of a single ultrasharp groove in metal.

Optical properties of a single ultrasharp groove of subwavelength width cut in an otherwise flat metal surface are examined theoretically. We calculate optical extinction, scattering, and absorption cross-section spectra for a wide range of groove profiles, establishing several fundamental trends. As grooves are made sharper, amplitudes of oscillations in cross-section spectra and their period decrease, while the absorption level increases, leading eventually to efficient broadband (nonresonant) absorption. Oscillations in scattering spectra are generally more pronounced than those in corresponding absorption spectra. For ultrasharp grooves, oscillations in all spectra can be suppressed by increasing the groove depth. Finally, the level of absorption relative to that of scattering increases as the top groove width decreases, a trend that is analogous to that found when decreasing the size of metal nanoparticles.

Atmospheric absorption model for dry air and water vapor at microwave frequencies below 100 GHz derived from spaceborne radiometer observations

AbstractThe Liebe and Rosenkranz atmospheric absorption models for dry air and water vapor below 100 GHz are refined based on an analysis of antenna temperature (TA) measurements taken by the Global Precipitation Measurement Microwave Imager (GMI) in the frequency range 10.7 to 89.0 GHz. The GMI TA measurements are compared to the TA predicted by a radiative transfer model (RTM), which incorporates both the atmospheric absorption model and a model for the emission and reflection from a rough‐ocean surface. The inputs for the RTM are the geophysical retrievals of wind speed, columnar water vapor, and columnar cloud liquid water obtained from the satellite radiometer WindSat. The Liebe and Rosenkranz absorption models are adjusted to achieve consistency with the RTM. The vapor continuum is decreased by 3% to 10%, depending on vapor. To accomplish this, the foreign‐broadening part is increased by 10%, and the self‐broadening part is decreased by about 40% at the higher frequencies. In addition, the strength of the water vapor line is increased by 1%, and the shape of the line at low frequencies is modified. The dry air absorption is increased, with the increase being a maximum of 20% at the 89 GHz, the highest frequency considered here. The nonresonant oxygen absorption is increased by about 6%. In addition to the RTM comparisons, our results are supported by a comparison between columnar water vapor retrievals from 12 satellite microwave radiometers and GPS‐retrieved water vapor values.

Protecting and accelerating adiabatic passage with time-delayed pulse sequences.

Using numerical simulations of two-photon electronic absorption with femtosecond pulses in Na2 we show that: (i) it is possible to avoid the characteristic saturation or dumped Rabi oscillations in the yield of absorption by time-delaying the laser pulses; (ii) it is possible to accelerate the onset of adiabatic passage by using the vibrational coherence starting in a wave packet; and (iii) it is possible to prepare the initial wave packet in order to achieve full state-selective transitions with broadband pulses. The findings can be used, for instance, to achieve ultrafast adiabatic passage by light-induced potentials and understand its intrinsic robustness.

Electrodynamic characteristics of berillium oxide in the submillimeter-infrared band

Single-crystal beryllium oxide has been studied by submillimeter-infrared spectroscopy and nonlinear optics methods in the wavenumber range of 2–5000 cm–1 in the temperature interval of 80–300 K. With the aid of computation models, the dispersion parameters of phonons and nonresonance absorption bands have been determined. The mechanisms responsible for dielectric loss in the terahertz band have been revealed. It has been shown that the decisive contribution is made by low-frequency dipole excitations, including two-phonon difference processes, and this contribution exceeds the phonon contribution by two orders of magnitude. Coherent anti-Stokes Raman scattering has been used to determine the coefficients of absorption by polaritons of the low-frequency dispersion branch.

Optical visualization and polarized light absorption of the single-wall carbon nanotube to verify intrinsic thermal applications

The predicted extraordinary properties of carbon nanotubes (CNTs) from theoretical calculations have great potential for many applications. However, reliable experimental determination of intrinsic properties at the single-tube level is currently a matter of concern, and many challenges remain because of the unhandled and nanoscale size of individual nanotubes. Here, we demonstrated a prototype to detect the intrinsic thermal conductivity of the single-wall carbon nanotube (SWCNT) and verify the significant non-resonant optical absorption behavior on tiny nanotubes by integrating the nanotube and ice into a new core-shell design. In particular, a reversible optical visualization method based on the individual suspended ultra-long SWCNT was first developed by wrapping a nanotube with ice in the cryogenic air environment. The light-induced thermal effect on the hybrid core-shell structure was used to melt the ice shell, which subsequently acted as a temperature sensor to verify the intrinsic thermal conductivity of the core-like nanotube. More interestingly, we successfully determined for the first time the thermal response phenomenon of the tiny absorption cross section in SWCNT in the vertical-polarization configuration and the significant non-resonant absorption behavior in the parallel-polarization configuration. These investigations will provide a better understanding for the unique optical behaviors of CNT and enable the detection of intrinsic properties of various one-dimensional nanostructures such as nanotubes, nanowires, and nanoribbons. Growing ice on a carbon nanotube not only enables the nanotube to be optically imaged but also permits its thermal properties to be measured. Isolated single-walled carbon nanotubes with diameters of just a few nanometres are usually impossible to observe using an optical microscope. Now, Xiao Zhang and other scientists in China have discovered a non-destructive and reversible way to optically observe carbon nanotubes and measure their thermal properties. Their method involves growing a few-micrometre-thick layer of ice on a suspended nanotube by placing it in a cryogenic environment. The researchers then melted the ice by irradiating the ice-coated nanotube with a laser beam, while inferring the nanotube s temperature from a shift in its Raman signal. From this information, they could determine the intrinsic thermal conductivity and the light absorption behavior of the nanotube.

Polarization control of intermediate state absorption in resonance-mediated multi-photon absorption process

We theoretically and experimentally demonstrate the control of the intermediate state absorption in an (n + m) resonance-mediated multi-photon absorption process by the polarization-modulated femtosecond laser pulse. An analytical solution of the intermediate state absorption in a resonance-mediated multi-photon absorption process is obtained based on the time-dependent perturbation theory. Our theoretical results show that the control efficiency of the intermediate state absorption by the polarization modulation is independent of the laser intensity when the transition from the intermediate state to the final state is coupled by the single-photon absorption, but will be affected by the laser intensity when this transition is coupled by the non-resonant multi-photon absorption. These theoretical results are experimentally confirmed via a two-photon fluorescence control in (2 + 1) resonance-mediated three-photon absorption of Coumarin 480 dye and a single-photon fluorescence control in (1 + 2) resonance-mediated three-photon absorption of IR 125 dye.

(Invited) Non-Resonant Absorption in Carbon Nanotubes

Absorption spectroscopy of large ensembles of carbon nanotubes show marked resonances, attributed to the different exitonic levels of each chiral species, as well as a relatively smooth background. This signal consists of a large number of different intrinsic and extrinsic contributions, such as phonon sidebands, amorphous carbon absorption, light scattering... It is therefore challenging to extract any quantitative information from these direct absorbance measurements. In contrast, photoluminescence excitation measurements in semi-conducting nanotubes give access to the intrinsic non-resonant absorption contribution between the excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component that shows up as 0.5 eV wide plateaus between the excitonic resonances. By comparison with the recently reported excitonic absorption cross-section on the S22 resonance, we found a universal non-resonant absorption which turns out to be of the order of one half of that of an equivalent graphene sheet. This value as well as the absorption line-shape in the non-resonant window is in excellent agreement with microscopic calculations. This non-resonant absorption cross-section is the same (within our resolution) for all the chiral species we measured in this study.

Non-resonant below-bandgap two-photon absorption in quantum dot solar cells

We study the optically nonlinear sub-bandgap photocurrent generation facilitated by an extended tailing distribution of states in an InAs/GaAs quantum dots (QDs) solar cell. The tailing states function as both the energy states for low energy photon absorption and the photocarriers extraction pathway. One of the biggest advantages of our method is that it can clearly differentiate the photocurrent due to one-photon absorption (1PA) process and two-photon absorption (2PA) process. Both 1PA and 2PA photocurrent generation efficiency in an InAs/GaAs QD device operated at 1550 nm have been quantitatively evaluated. A two-photon absorption coefficient β = 5.7 cm/GW is extracted.

Time-dependent simulation of lower hybrid current drive in JET discharges

In this paper we report on simulations of lower hybrid current drive (LHCD) in JET closely comparing the simulation results to the available experimental data. The simulations are performed all over the relevant discharge duration by ASTRA. The LHCD module, FRTC, is based on a standard ray-tracing Fokker–Planck model. The purpose of the paper is to understand the present LHCD experiments issues within the limit of the LH linear propagation model. These issues are: (i) analysis of non-resonant collisional absorption (NRCA) of LH wave power in the main JET plasma during the current ramp-up phase and in steady-state (SS) scenarios, (ii) the lack of penetration of LHCD in high-density plasmas, (iii) current diffusion during the LHCD-assisted current ramp-up and (iv) assessment of the current profile alignment in JET SS discharges in the presence of LHCD.In recent experiments from FTU, JET and C_MOD, LHCD effects at high plasma density are either completely absent or less than expected. It has been shown, both in FTU and ALCATOR-C_MOD, that NRCA of LH wave power can be responsible for that. Indeed NRCA is estimated to be small in JET plasmas, at least in the main heating phase and therefore it is not expected to be responsible for the lack of penetration of LHW in high-density JET plasmas, however here we show for the first time that it can be effective during the early phase of the current ramp-up, when the plasma is still collisional. On the contrary it is suggested that the reduction of LHCD effects at high density may be attributed at least partially to the loss of accessibility of the n‖ spectrum effectively launched into the plasma. Furthermore it is shown that the linear propagation model provide very broad and stable LH current density profiles, with no need to include any non-linear spectral broadening.The current diffusion during the LHCD-assisted current ramp-up is investigated and a careful comparison between the simulated q-profiles and the measured ones is performed over time. The implications of the observed difference are discussed.Finally the important question of the alignment of all the current profile components is analysed in JET SS discharges at high βN.

Universal nonresonant absorption in carbon nanotubes

Photoluminescence excitation measurements in semi-conducting carbon nanotubes show a systematic non-resonant contribution between the well known excitonic resonances. Using a global analysis method, we were able to delineate the contribution of each chiral species including its tiny non-resonant component. By comparison with the recently reported excitonic absorption cross-section on the $S_{22}$ resonance, we found a universal non-resonant absorbance which turns out to be of the order of one half of that of an equivalent graphene sheet. This value as well as the absorption line-shape in the non-resonant window is in excellent agreement with microscopic calculations based on the density matrix formalism. This non-resonant absorption of semi-conducting nanotubes is essentially frequency independent over 0.5~eV wide windows and reaches approximately the same value betweeen the $S_{11}$ and $S_{22}$ resonances or between the $S_{22}$ and $S_{33}$ resonances. In addition, the non-resonant absorption cross- section turns out to be the same for all the chiral species we measured in this study. From a practical point of view, this study puts firm basis on the sample content analysis based on photoluminescence studies by targeting specific excitation wavelengths that lead to almost uniform excitation of all the chiral species of a sample within a given diameter range.

Synthesis, Structure, and Properties of a Copper Bromide Coordination Compound with 3-Amino-4-EthoxyCarbonylPyrazole. Nature of the Nonresonant and Ferromagnetic Absorption Observed by EPR

Synthesis, structure, and magnetic properties of copper bromide CuL2Br2 with 3-amino-4-ethoxycarbonylpyrazole (L) are presented. The CuL2Br2 is a 1D polymer complex and crystal structure has triclinic symmetry with space group P-1. The examined compound has thermochromic properties (russet ↔ light red), and its magnetic properties are changed from paramagnetic to nonresonance and ferromagnetic ones after heat treatment (300 → 77 → 300 K) or pressure treatment (1 → 3,000 → 1 atm).

Observation and symmetry-based coherent control of transient two-photon absorption: the bright side of dark pulses.

We present and experimentally demonstrate for the first time the observation and femtosecond coherent control over the temporal evolution of a transient population that is excited via nonresonant two-photon absorption. Based on symmetry properties of the two-photon absorption process, the exciting femtosecond pulses are phase-shaped to photoinduce different evolutions of the transient excited population for a given final excited population. As a study case, we focus here on the attractive case of two-photon dark pulses that, although inducing zero final population (hence, the terminology of "dark pulses"), they induce a transient excited population during the pulse irradiation that can significantly deviate from zero. This nonzero transient population can be viewed as the bright side of such dark pulses. The symmetry-based coherent control is demonstrated first with dark pulses that we shape to induce transient excited population that at all times is kept below different target levels. Then, it is further demonstrated with pairs of dark pulses where one is rationally shaped to induce temporal evolution of the transient excited population that is the inverse of the evolution induced by the other. The work is conducted in the weak-field regime with the sodium atom as the model system. The approach developed here is general, conceptually simple, and very effective.

Extremely confined gap surface-plasmon modes excited by electrons.

High-spatial and energy resolution electron energy-loss spectroscopy (EELS) can be used for detailed characterization of localized and propagating surface-plasmon excitations in metal nanostructures, giving insight into fundamental physical phenomena and various plasmonic effects. Here, applying EELS to ultra-sharp convex grooves in gold, we directly probe extremely confined gap surface-plasmon (GSP) modes excited by swift electrons in nanometre-wide gaps. We reveal the resonance behaviour associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to ~5 nm. We argue that excitation of this mode, featuring very strong absorption, has a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.

Quantum focusing and coherent control of nonresonant two-photon absorption in frequency domain.

We theoretically investigate the nonresonant two-photon absorption (TPA) process in a two-level atom induced by a weak chirped pulse in the frequency domain. According to the extremum condition of the two-photon transition probability (TPTP) at the transition center frequency, we propose a Fresnel-inspired pulse tailoring scheme for TPA that is significantly different from that of Broers etal. [Phys. Rev. A46, 2749 (1992)]. Using this scheme, the TPTP can be focused or eliminated completely by constructively or destructively modulating various pathways of the quantum interference. Our results are a significant improvement on those obtained by Broers etal. and will have potential applications in selective two-photon microscopy and spectroscopy.

New results in high-resolution X-ray fluorescence spectroscopy

We present some recent results dealing with resonant inelastic X-ray scattering (RIXS) on atomic targets in the 3-5 keV enegy region. In this so-called tender spectral region, the K-shell fluorescence branching ratios become reasonably large, but a full vacuum enclosure is still preferable to avoid detection efficiency loss due to the sizeable arms of high resolution crystal spectrometers. By squeezing energy resolution in the fluorescence decay channel, one may improve the spectral resolution of photoabsorption, enable separation of multielectron excitation and relaxation channels, and completely eliminate the need to scan across the selected energy range of the photon probe in order to acquire the photoabsorption spectrum. On the other hand, the spectra may be untrivially modified by effects such as interference of absorption-emission paths or structured relaxation modes, and a more elaborated modelling is needed to understand the emitted signal. We illustrate these aspects by presenting four cases: the reconstruction of Ar KM and Ar KL absorption edges from a series of highly resolved emission spectra recorded at different probe energies, the reconstruction of the Xe L3 edge from a single X-ray emission spectrum, and the analysis of the radiative Ar K-MM Auger decay preceeded by the resonant or nonresonant photon absorption.

Dynamic optical properties in graphene: Length versus velocity gauge

The dynamic optical properties of graphene are theoretically investigated in both length gauge and velocity gauge in the presence of ultrafast optical radiation field. The two gauges present different results of dynamic photo-induced carriers and optical conductance due to distinct dependencies on electric field and non-resonant optical absorption, while the two gauges give identical results in the steady state time. It shows that the choice of gauge affects evidently the dynamic optical properties of graphene. The velocity gauge represents an outcome of a real physical experiment.