Infrared Pump Research Articles

Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics.

Several hybrid halide 2D-perovskite species emit light with an emergent and controversial broadband emission Stokes-shifted down from the narrow band emission. This paper uncovers the sub- and above-bandgap emission and absorption characteristics of PEA2PbI4 prepared with gap states introduced during single crystal growth. Here, gap states led to coexistent intrinsic and heterostructured electronic frameworks that are selectively accessible with ultraviolet (UV) and infrared (IR) light, respectively, resulting in the phenomenon of photoluminescence (PL) switching from narrowband green to broadband red. Electron-energy dependent cathodoluminescence shows a relative increase in the broadband red PL intensity as the electron penetration depth increases from 30 nm to 2 μm, confirming the heterostructured framework is formed in the bulk of the crystal. Excitation-emission power slope of 2.5 and up-conversion pump transient absorption (TA) spectra suggest that the IR up-conversion excitation with red photoluminescence, peaked at 655 nm, is a multiphoton process occurring in the heterostructured framework through a nonlinear optical response. The energetic pathways toward the dual emission bands are revealed by pump-probe transient absorption spectroscopy, showing energetically broad gap states with high sensitivity to an IR pump are upconverted and subsequently quickly relax from high to low energy levels within 4 ps. Furthermore, the up-conversion red PL demonstrates a linear polarization with magnetic field effects, thus affirming that the band-like heterostructured framework is crystallographically aligned with characteristics of spatially extended charge-transfer states.

Unveiling the warm and dense ISM in z > 6 quasar host galaxies via water vapor emission

Water vapor (H2O) is one of the brightest molecular emitters after carbon monoxide (CO) in galaxies with high infrared (IR) luminosity, allowing us to investigate the warm and dense phase of the interstellar medium (ISM) where star formation occurs. However, due to the complexity of its radiative spectrum, H2O is not frequently exploited as an ISM tracer in distant galaxies. Therefore, H2O studies of the warm and dense gas at high-z remain largely unexplored. In this work, we present observations conducted with the Northern Extended Millimeter Array (NOEMA) toward three z > 6 IR-bright quasars J2310+1855, J1148+5251, and J0439+1634 targeted in their multiple para- and ortho-H2O transitions (312 − 303, 111 − 000, 220 − 211, and 422 − 413), as well as their far-IR (FIR) dust continuum. By combining our data with previous measurements from the literature, we estimated the dust masses and temperatures, continuum optical depths, IR luminosities, and star formation rates (SFR) from the FIR continuum. We modeled the H2O lines using the MOLPOP-CEP radiative transfer code, finding that water vapor lines in our quasar host galaxies are primarily excited in the warm, dense (with a gas kinetic temperature and density of Tkin = 50 K, nH2 ∼ 104.5 − 105 cm−3) molecular medium with a water vapor column density of NH2O ∼ 2 × 1017 − 3 × 1018 cm−3. High-J H2O lines are mainly radiatively pumped by the intense optically-thin far-IR radiation field associated with a warm dust component at temperatures of Tdust ∼ 80 − 190 K that account for < 5 − 10% of the total dust mass. In the case of J2310+1855, our analysis points to a relatively high value of the continuum optical depth at 100 μm (τ100 ∼ 1). Our results are in agreement with expectations based on the H2O spectral line energy distribution of local and high-z ultra-luminous IR galaxies and active galactic nuclei (AGN). The analysis of the Boltzmann diagrams highlights the interplay between collisions and IR pumping in populating the high H2O energy levels and it allows us to directly compare the excitation conditions in the targeted quasar host galaxies. In addition, the observations enable us to sample the high-luminosity part of the H2O–total-IR (TIR) luminosity relations (LH2O − LTIR). Overall, our results point to supralinear trends that suggest H2O–TIR relations are likely driven by IR pumping, rather than the mere co-spatiality between the FIR continuum- and line-emitting regions. The observed LH2O/LTIR ratios in our z > 6 quasars do not show any strong deviations with respect to those measured in star-forming galaxies and AGN at lower redshifts. This supports the notion that H2O can be likely used to trace the star formation activity buried deep within the dense molecular clouds.

Two-photon IR pumped UV–Vis transient absorption spectroscopy of Dirac fermions in the topological insulator Bi2Se3

It is often taken for granted that in pump-probe experiments on the topological insulator (TI) Bi2Se3 using IR pumping with a commercial Ti:sapphire laser [∼800 nm (1.55 eV photon energy)], the electrons are excited in the one-photon absorption regime, even when pumped with absorbed fluences in the mJ cm−2 range. Here, using UV–Vis transient absorption (TA) spectroscopy, we show that even at low-power Infrared (IR) pumping with absorbed fluences in the μJ cm−2 range, the TA spectra of the TI Bi2Se3 extend across a part of the UV and the entire visible region. This observation suggests unambiguously that the two-photon pumping regime accompanies the usual one-photon pumping regime even at low laser powers applied. We attribute the high efficiency of two-photon pumping to the giant nonlinearity of Dirac fermions in the Dirac surface states (SS). On the contrary, one-photon pumping is associated with the excitation of bound valence electrons in the bulk into the conduction band. Two mechanisms of absorption bleaching were also revealed since they manifest themselves in different spectral regions of probing and cause the appearance of three different relaxation dynamics. These two mechanisms were attributed to the filling of the phase-space in the Dirac SS and bulk states, followed by the corresponding Pauli blocking.

Super-Resolution Far-Field Infrared Imaging by Photothermal Heterodyne Imaging.

Infrared (IR) imaging provides chemical-specific information without the need for exogenous labels. Conventional far-field IR imaging techniques are diffraction limited, which means an effective spatial resolution of >5 μm with currently available optics. In this article, we present a novel far-field IR imaging technique based on photothermal heterodyne imaging (IR-PHI). In our version of IR-PHI, an IR pump laser excites the sample, causing a small temperature rise that is detected by a counterpropagating visible probe beam. Images and spectra of several different types of soft matter systems (polystyrene beads, thin polymer films, and single Escherichia coli bacterial cells) are presented to demonstrate the sensitivity and versatility of the technique. Importantly, the spatial resolution in the IR-PHI measurements is determined by the visible probe beam: a spatial resolution of 0.3 μm was achieved with a 0.53 μm probe wavelength and a high numerical aperture focusing objective. This is the highest spatial resolution reported to date for far-field IR imaging. Analysis of the experiments shows that for polymer beads in a dry environment, the magnitude of the IR-PHI signal is determined by the scattering cross section of the nano-object at the probe wavelength. This is in contrast to conventional PHI experiments in a heat-transfer medium, where the signal scales as the absorption cross section. This different scaling can be understood through the optical theorem. Our analysis also shows that both thermal expansion and changes in the refractive index of the material are important and that these two effects, in general, counteract each other.

Antenna-Enhanced Nonlinear Infrared Spectroscopy in Reflection Geometry

We propose and demonstrate antenna-enhanced nonlinear infrared (IR) spectroscopy in reflection geometry. Our approach uses resonant metal nanoantennas to enhance near-fields and to amplify the interaction between molecular vibrations and IR light. We successfully obtain amplified nonlinear vibrational signals in backscattering light of nanoantennas using IR pump energy of 10 nJ, with local signal enhancement of more than 7 orders of magnitude. This ultrasensitive and reflection-type method is useful for characterizing the structure and dynamics of minute volumes of molecules, monolayer materials, and biomolecules in aqueous environments, with additional benefit of surface sensitivity.

Vibrational energy flow in photoactive yellow protein revealed by infrared pump-visible probe spectroscopy.

Vibrational energy flow in the electronic ground state of photoactive yellow protein (PYP) is studied by ultrafast infrared (IR) pump-visible probe spectroscopy. Vibrational modes of the chromophore and the surrounding protein are excited with a femtosecond IR pump pulse, and the subsequent vibrational dynamics in the chromophore are selectively probed with a visible probe pulse through changes in the absorption spectrum of the chromophore. We thus obtain the vibrational energy flow with four characteristic time constants. The vibrational excitation with an IR pulse at 1340, 1420, 1500, or 1670 cm(-1) results in ultrafast intramolecular vibrational redistribution (IVR) with a time constant of 0.2 ps. The vibrational modes excited through the IVR process relax to the initial ground state with a time constant of 6-8 ps in parallel with vibrational cooling with a time constant of 14 ps. In addition, upon excitation with an IR pulse at 1670 cm(-1), we observe the energy flow from the protein backbone to the chromophore that occurs with a time constant of 4.2 ps.

Solute–Solvent Interactions of Benzonitrile in Solutions Studied by Sub-Picosecond Infrared Pump–Probe Spectroscopy

Abstract The vibrational energy relaxation of the CN stretching mode of benzonitrile (BN) in solution has been investigated by infrared (IR) pump–probe spectroscopy. The peak wavenumber, line shape, and cross section of the ground state absorption band due to the CN stretching mode depend on the solvent and concentration. From the experimental results of IR spectroscopy and calculations with density functional theory, the structures of solute–solute or solute–solvent complexes are suggested. In hexane, BN exists as a monomer or a dimer in which the two CN groups are arranged antiparallel. In ethanol (EtOH), it is suggested that BN forms the antiparallel dimer and a BN–EtOH hydrogen-bonded complex. In dimethyl sulfoxide (DMSO), BN probably forms a solute–solvent complex. From the results of IR pump–probe spectroscopy, the decay time constant of the pump–probe signal shows a probe wavenumber dependence in hexane and DMSO, which indicates inhomogeneity of the microscopic environment around BN in these solvents. In EtOH, the wavenumber dependence was not observed. This suggests that the VER processes for the BN dimer and the BN–EtOH complex are accelerated from that for the monomer, and the VER times are almost identical among the two structures.

WATER VAPOR IN NEARBY INFRARED GALAXIES AS PROBED BY HERSCHEL

We report the first systematic study of the submillimeter water vapor rotational emission lines in infrared (IR) galaxies based on the Fourier Transform Spectrometer (FTS) data of {\it Herschel} SPIRE. Among the 176 galaxies with publicly available FTS data, 45 have at least one H$_2$O emission line detected. The H$_2$O line luminosities range from $\sim 1 \times 10^5$ $L_{\odot}$ to $\sim 5 \times 10^7 L_{\odot}$ while the total IR luminosities ($L_\mathrm{IR}$) have a similar spread ($\sim 1-300 \times 10^{10} L_{\odot}$). In addition, emission lines of H$_2$O$^+$ and H$_2^{18}$O are also detected. H$_2$O is found, for most galaxies, to be the strongest molecular emitter after CO in FTS spectra. The luminosity of the five most important H$_2$O lines is near-linearly correlated with $L_\mathrm{IR}$, no matter whether strong active galactic nucleus signature is present or not. However, the luminosity of H$_2$O($2_{11}-2_{02}$) and H$_2$O($2_{20}-2_{11}$) appears to increase slightly faster than linear with $L_\mathrm{IR}$. Although the slope turns out to be slightly steeper when $z\sim 2-4$ ULIRGs are included, the correlation is still closely linear. We find that $L_\mathrm{H_2O}/L_\mathrm{IR}$ decreases with increasing $f_{25}/f_{60}$, but see no dependence on $f_{60}/f_{100}$, possibly indicating that very warm dust contributes little to the excitation of the submillimeter H$_2$O lines. The average spectral line energy distribution (SLED) of the entire sample is consistent with individual SLEDs and the IR pumping plus collisional excitation model, showing that the strongest lines are H$_2$O($2_{02}-1_{11}$) and H$_2$O($3_{21}-3_{12}$).

Photoemission electron microscopy using extreme ultraviolet attosecond pulse trains

We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.

Vibrational Dynamics of Hydrogen-Bonded Complexes in Solutions Studied with Ultrafast Infrared Pump−Probe Spectroscopy

In aqueous solution, the basis of all living processes, hydrogen bonding exerts a powerful effect on chemical reactivity. The vibrational energy relaxation (VER) process in hydrogen-bonded complexes in solution is sensitive to the microscopic environment around the oscillator and to the geometrical configuration of the hydrogen-bonded complexes. In this Account, we describe the use of time-resolved infrared (IR) pump-probe spectroscopy to study the vibrational dynamics of (i) the carbonyl CO stretching modes in protic solvents and (ii) the OH stretching modes of phenol and carboxylic acid. In these cases, the carbonyl group acts as a hydrogen-bond acceptor, whereas the hydroxyl group acts as a hydrogen-bond donor. These vibrational modes have different properties depending on their respective chemical bonds, suggesting that hydrogen bonding may have different mechanisms and effects on the VER of the CO and OH modes than previously understood. The IR pump-probe signals of the CO stretching mode of 9-fluorenone and methyl acetate in alcohol, as well as that of acetic acid in water, include several components with different time constants. Quantum chemical calculations indicate that the dynamical components are the result of various hydrogen-bonded complexes that form between solute and solvent molecules. The acceleration of the VER is due to the increasing vibrational density of states caused by the formation of hydrogen bonds. The vibrational dynamics of the OH stretching mode in hydrogen-bonded complexes were studied in several systems. For phenol-base complexes, the decay time constant of the pump-probe signal decreases as the band peak of the IR absorption spectrum shifts to lower wavenumbers (the result of changing the proton acceptor). For phenol oligomers, the decay time constant of the pump-probe signal decreases as the probe wavenumber decreases. These observations show that the VER time strongly correlates with the strength of hydrogen bonding. This acceleration may be due to increased coupling between the OH stretching mode and the accepting mode of the VER, because the low-frequency shift caused by hydrogen bond formation is very large. Unlike phenol oligomers, however, the pump-probe signals of phenol-base complexes did not exhibit probe frequency dependence. For these complexes, rapid interconversion between different conformations causes rapid fluctuations in the vibrational frequency of the OH stretching modes, and these fluctuations level the VER times of different conformations. For the benzoic acid dimer, a quantum beat at a frequency of around 100 cm(-1) is superimposed on the pump-probe signal. This result indicates the presence of strong anharmonic coupling between the intramolecular OH stretching and the intermolecular stretching modes. From a two-dimensional plot of the OH stretching wavenumber and the low-frequency wavenumber, the wavenumber of the low-frequency mode is found to increase monotonically as the probe wavenumber is shifted toward lower wavenumbers. Our results represent a quantitative determination of the acceleration of VER by the formation of hydrogen bonds. Our studies merit further evaluation and raise fundamental questions about the current theory of vibrational dynamics in the condensed phase.

Vibrational Energy Dynamics of Normal and Deuterated Liquid Benzene

Ultrafast Raman spectroscopy with infrared (IR) excitation is used to study vibrational energy dynamics of ambient temperature liquids benzene and benzene-d(6). After IR pumping of a CH-stretch or CD-stretch parent excitation, the redistribution of vibrational energy is probed with anti-Stokes Raman. Ten benzene or 12 benzene-d(6) vibrations out of 30 total have large enough cross sections to be observed. The pathways, quantum yields, and lifetimes for energy transfer among these vibrations are quantified. Using a CCl(4) molecular thermometer, we demonstrate an ultrafast Raman calorimetry method which allows measurement of the rate that benzene vibrational energy is dissipated into the bath. On the basis of energy conservation, we then determine the time-dependent dissipation of aggregate vibrational energy from the unobserved, "invisible" vibrations. During the approximately 1 ps IR excitation process, vibrational energy is coherently redistributed to several vibrational modes ("coherently" means the rate is faster than (T(2))(-1) of the pumped transition). This energy is then further redistributed in an incoherent intramolecular vibrational relaxation process with a 6 ps T(1) time constant. The subsequent dynamics involve energy transfer processes accompanied by vibrational energy dissipation to the bath. This vibrational cooling process has a half-life of 30 ps in benzene and 20 ps in benzene-d(6), and thermalization is complete in approximately 100 ps. The observed strongly Raman-active vibrations have about the same amount of energy per mode as the invisible vibrations. The invisible vibrational energy in benzene decays somewhat faster than the observed energy. These two decay rates are about the same in benzene-d(6).

A Revisory Mathematical Model on Optically Pumped Far-Infrared Laser

In an optically pumped far-infrared laser (OPFIRL) system, the infrared (IR) pumping laser with collision broadening which was closer to the practice was considered. A more precise mathematical model was set up. When the IR pumping laser line had the Lorentz function structure, by solving the semi-classic density matrix equations, the generalized form of FIR signal gain Gs and IR pumping signal absorption coefficient Gp were obtained.

Nonlinear IR absorption in europium- and thulium-doped oxide glasses

Infrared (IR)-induced two-photon absorption (TPA) in synthesized PbO–Ga2O3–Bi2O3–CdO (PGBC) glasses doped by Eu3+ and Tm3+ rare earth (RE3+) ions was investigated. Temperature-dependent measurements of TPA were carried out in the mid-IR spectral range. A CO pulsed laser (λ = 5.5 μm, energy power density up to 3.8 GW/cm2 per pulse with picosecond pulse duration) was applied as the source of photoinducing IR radiation. An optical parametrical generator cut from a proustite crystal was used as the source of the fundamental TPA beam within the 3–7.5 μm spectral range. Absolute values of the TPA coefficients were more than 13% higher than corresponding values obtained for other similar glasses (As2Te3–CaCl2–PbCl2 [1] or Sb2Se3–BaCl2–PbCl2 [2]). The PGBC system investigated possessed a shorter time response (about 12 ps) compared with other IR nonlinear optical glasses. It was also established that all nonlinear optical susceptibilities were dependent on the type of RE3+ ion. A maximum value of the TPA was achieved for the glasses doped by Tm3+. The TPA values increased significantly below 45 K. Ab initio molecular dynamics and quantum chemical simulations were performed in order to evaluate the possible role of electron–phonon anharmonic interactions in the observed phenomena. The influence of the external CO photoinduced pump beam through the photoinduced electrostricted anharmonic electron–phonon interactions was simulated. A decrease of picosecond delaying time response compared with other glasses was achieved. To obtain independent confirmation of the observed dependencies, measurements were carried out of χxxxx(E) during the external IR pumping. The measured and theoretically calculated dependencies of the third-order susceptibilities were also compared. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesized rare-earth doped oxide glasses for nonlinear optics

Photostimulated nonlinear optical effects in synthesized PbO–Ga2O3–Bi2O3–CdO (PGBC) glasses doped by Yb3+, Er3+, and Dy3+ rare-earth (RE3+) ions were discovered. Temperature-dependent measurements of optical photoinduced second-harmonic generation (PISHG) and two-photon absorption were performed in the infrared (IR) range. CO pulse laser (λ=5.5 μm, energy power density up to 3.8 GW/cm2 per pulse) was applied as a source of IR-photoinducing and probing (fundamental) light. Absolute values of the PISHG were more than 22% higher than corresponding values obtained for other glasses: As2Te3–CaCl2–PbCl2 [1] or Sb2Se3–BaCl2–PbCl2 [2] type. The investigated PGBC system possesses a shorter time response (about 18 ps), compared with other IR nonlinear optical glasses. We have also established that all nonlinear optical susceptibilities are dependent on the type of RE3+ ion. A maximal value of the PISHG is achieved for the glasses doped by Yb3+. The PISHG values increase significantly below 25 K. We have carried out ab initio molecular dynamics and quantum chemical simulations in order to evaluate the possible contribution of electron–phonon anharmonic interactions in the observed phenomena. We have modeled the influence of the external CO photoinduced beam through the photoinduced anharmonic electron-phonon interactions. A decrease of the delaying time response is achieved. To obtain independent confirmation of the observed dependencies, we have carried out measurements of the ε2(E) during the external IR pumping. We have also compared the measured and theoretically calculated dependencies of the IR-induced effects.

Photovoltaic effect in coupled quantum wells under intersubband excitation

The photovoltaic effect in tunnel-coupled double quantum wells (DQWs) due to resonant infrared (IR) excitation of the electrons from the lower ground state to the excited state is considered. Intra- and interwell relaxation of electrons are studied self-consistently, taking into account modification of the electron states in DQWs due to the upper ground state occupation under IR pumping. The photoinduced potential is calculated for different energies of IR excitation, and the correlation between this potential and modification of the ground states in DQWs is discussed.

Far infrared laser assignments and predictions for CO-stretching and CH 3-rocking states of optically pumped 13CH 3OH

High-resolution Fourier transform studies of the CO-stretching and CH 3-rocking bands of 13CH 3OH have been applied to the assignment of far-infrared (FIR) laser transitions optically pumped by CO 2 infrared (IR) lasers. The assignments are based on spectroscopic identification of the IR pump transitions at the reported offsets from the CO 2 laser lines and agreement between the FIR laser wavenumbers and values calculated from IR/FIR combination-loop relations. Wavenumbers are also predicted for further potential new FIR laser lines in transition systems that could be pumped by CO 2 and 13CO 2 cw and waveguide lasers.

Ultrafast infrared spectroscopy of vibrational CO-stretch up-pumping and relaxation dynamics of W(CO) 6

The ultrafast infrared (IR) vibrational up-pumping and energy relaxation dynamics are reported of the T 1u CO-stretching manifold of W(CO) 6 in room temperature n-hexane solution. The frequency and energy content of the tunable 2 ps IR pump pulse strongly influences the distribution of vibrational level population. With broadband transient IR absorption spectroscopy, vibrational T 1 lifetimes of the overtone states of W(CO) 6 were directly determined to be 140 ± 20 (1 σ) ps for ν = 1, 75 ± 20 ps for ν = 2 and 30 ± 15 ps for ν = 3. T 1u to E g intermode energy transfer occurs on the 10–20 ps timescale.

Negative submillimeter absorption in tunnel-coupled quantum wells under resonant infrared excitation

We have studied the redistribution of the electrons between the tunnel-coupled ground levels in double quantum wells (DQWs), caused by the resonant infrared (IR) excitation on the next level and relaxation of the photoexcited electrons due to spontaneous emission of the optical phonons. The condition for the inversion of the electron populations in the tunnel-coupled pair of ground states is presented as a function of the DQWs parameters. Negative absorption coefficient of the submillimeter radiation under intensive (2–4 kW/cm2) IR pumping and commonly realized parameters of DQWs is of order 20–50 cm−1.

Assignment in methanol-D 1 of the far-infrared laser emission optically pumped by the 10P(46) CO 2 line and associated infrared spectrum

High-resolution Fourier transform spectroscopy of the infrared (IR) OCD bending mode of methanol-D 1, has led to the identification of the 374 μm far-infrared (FIR) laser transition optically pumped by the 10P(46) CO 2 laser line in CH 2DOH. The IR pump transition coincident with the 10P(46) line is assigned as the R(17), K = 4 transition of the σ $ ́ = 0 torsional species in the torsional ground state. The 374 μm FIR laser line is then assigned to the J K = 18 4→17 4 transition in the excited OCD-bend state. The assignment scheme is supported by combination relations among R, P and Q-branch IR transitions in the OCD-bending band. This is believed to be the first FIR laser line assignment for an asymmetrically-deuterated methanol species, as well as the first reported high-resolution study of the IR spectrum.

Application of high-resolution Fourier transform spectroscopy to Fermi resonance and the assignment of far-infrared laser lines in CD 3OH

High-resolution Fourier transform spectra of the infrared (IR) CO stretch band and the farinfrared (FIR) torsion-rotation band of CD 3OH have been analyzed to find new assignments of reported optically-pumped FIR laser lines and to confirm previous assignments proposed by other workers. The identified CD 3OH FIR laser emission includes a substantial group of transitions involving CO stretch levels which are significantly shifted by Fermi resonance with high-lying torsional levels of the vibrational ground state. The results are supported by frequency combination relations for closed IR FIR transition loops, which also yield improved accuracies for a number of observed or predicted FIR laser frequencies. The previous assignments have generally been confirmed, with modifications for two of the pump systems. Offsets of the assigned IR pump transitions from the corresponding CO 2 laser lines are also presented, as determined from the spectroscopic data.