Infrared Pump Pulse Research Articles

Differentiating Thermal Conductances at Semiconductor Nanocrystal/Ligand and Ligand/Solvent Interfaces in Colloidal Suspensions.

Infrared-pump, electronic-probe (IPEP) spectroscopy is used to measure heat flow into and out of CdSe nanocrystals suspended in an organic solvent, where the surface ligands are initially excited with an infrared pump pulse. Subsequently, the heat is transferred from the excited ligands to the nanocrystals and in parallel to the solvent. Parallel heat transfer in opposite directions uniquely enables us to differentiate the thermal conductances at the nanocrystal/ligand and ligand/solvent interfaces. Using a novel solution to the heat diffusion equation, we fit the IPEP data to find that the nanocrystal/ligand conductances range from 88 to 135 MW m-2 K-1 and are approximately 1 order of magnitude higher than the ligand/solvent conductances, which range from 7 to 26 MW m-2 K-1. Transient nonequilibrium molecular dynamics (MD) simulations of nanocrystal suspensions agree with IPEP data and show that ligands bound to the nanocrystal by bidentate bonds have more than twice the per-ligand conductance as those bound by monodentate bonds.

Optical stark effect on CdSe nanoplatelets with mid-infrared excitation for large amplitude ultrafast modulation

The optical Stark effect is a universal response of the electronic structure to incident light. In semiconductors, particularly nanomaterials, the optical Stark effect achieved with sub-band gap photons can drive large, narrowband, and potentially ultrafast changes in the absorption or reflection at the band gap through excitation of virtual excitons. Rapid optical modulation using the optical Stark effect is ultimately constrained, however, by the generation of long-lived excitons through multiphoton absorption. This work compares the modulation achievable using the optical Stark effect on CdSe nanoplatelets with several different pump photon energies, from the visible to mid-infrared. Despite expected lower efficiencies for spectrally-remote pump energies, infrared pump pulses can ultimately drive larger sub-picosecond optical Stark shifts of virtual excitons without creation of real excitons. The CdSe nanoplatelets show subpicosecond shifts of the lowest excitonic resonance of up to 22 meV, resulting in change in absorption as large as 0.32 OD (49% increase in transmission), with a long-lived offset from real excitons less than 1% of the peak signal.

Spintronic terahertz emitter

While the technology of microwave and infrared sources is quite mature and has been widely used in our daily life for decades, sources that can work well across the terahertz (THz) range are still lagging behind, which is often referred to as the “THz gap.” As one of the most pioneering THz setups, terahertz time-domain spectroscopy has been a vital tool to explore the properties of materials as well as their underlying physics. The mechanism is to use an ultrafast infrared pump pulse for exciting rapidly decaying currents inside either a nonlinear or a photoconducting medium, known as a THz emitter, which produces free-space coherent THz radiation. Most recently, a novel THz emitter emerges and rises, which is based on the spin-related effects in magnetic/nonmagnetic nanofilms and can cover the full range of the THz band, named as spintronic THz emitter (STE). This perspective aims to elucidate the unique features and advantages of STE as well as its capability and potential to develop novel applications. We summarize the multidisciplinary efforts that have been made to improve the performance and function of STE, including but not limited to spintronics, optics, and electromagnetics. Distinct THz setups based on STE are reviewed, which may inspire various “real world” applications in the near future.

LiSAF: an efficient and durable nonlinear material for supercontinuum generation in the ultraviolet

We have experimentally studied supercontinuum generation in an undoped LiSAF crystal using ultraviolet, visible and near infrared pump pulses provided by fundamental and second harmonics of amplified femtosecond Ti:sapphire and Yb:KGW lasers. We have found that the optical degradation of the crystal, manifested by gradual narrowing of the supercontinuum spectrum, starts much faster using infrared pump pulses. This is attributed to the role of impact ionization, which increases with increasing the pump wavelength. The most reliable operation is achieved with the shortest pump wavelength of 400 nm (the second harmonic of a Ti:sapphire laser), where LiSAF produces a stable, almost 1.3 octave-spanning supercontinuum spectrum with a short-wavelength cut-off at 252 nm and shows no apparent optical degradation for one hour of continuous operation at 500 Hz repetition rate without crystal translation.

Possible experimental test of the nonlinear phononics interpretation of light-induced superconductivity

Experimental evidence for a transient enhancement of the superconducting critical temperature of YBa2Cu3O6+x in the presence of an intense THz or infrared pump pulse was ascribed to nonlinear phononics effects. Here, I introduce a simple phenomenological Ginzburg-Landau model of this phenomenon, to explore further consequences and possible experimental tests of this interpretation. This treatment predicts that, upon cooling below Tc in the absence of pumping, (a) an abrupt softening of a Raman-active mode frequency and (b) a spontaneous lattice distortion, growing linearly with (Tc−T) should occur. Numerical estimates for YBa2Cu3O6+x indicate that the frequency softening could likely be observable, whereas the lattice distortion may be too small. A comparison with Raman experiments for the relevant phonon modes in YBa2Cu3O6+x does not lend support to the nonlinear phononics interpretation. On the other hand, however, a very large (up to 18%) phonon frequency softening just below Tc, qualitatively similar to the predictions of the present model for its size and abrupt temperature dependence, was observed over 20 years ago in HgBa2Ca3Cu4O10+x; its explanation in terms of Josephson plasmons has been controversial. Light-induced superconductivity still needs to be investigated in this material and it may be of interest to explore if it is present and possibly connected, via the mechanism discussed here, to the observed anomalous phonon behavior.Received 12 March 2020Revised 20 August 2020Accepted 21 August 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.033384Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDynamical phase transitionsSuperconducting phase transitionTransition temperatureCondensed Matter, Materials & Applied Physics

Infrared-pump electronic-probe of methylammonium lead iodide reveals electronically decoupled organic and inorganic sublattices

Organic-inorganic hybrid perovskites such as methylammonium lead iodide (CH3NH3PbI3) are game-changing semiconductors for solar cells and light-emitting devices owing to their defect tolerance and exceptionally long carrier lifetimes and diffusion lengths. Determining whether the dynamically disordered organic cations with large dipole moment benefit the optoelectronic properties of CH3NH3PbI3 has been an outstanding challenge. Herein, via transient absorption measurements employing an infrared pump pulse tuned to a methylammonium vibration, we observe slow, nanosecond-long thermal dissipation from the selectively excited organic mode to the inorganic sublattice. The resulting transient electronic signatures, during the period of thermal-nonequilibrium when the induced thermal motions are mostly concentrated on the organic sublattice, reveal that the induced atomic motions of the organic cations do not alter the absorption or the photoluminescence response of CH3NH3PbI3, beyond thermal effects. Our results suggest that the attractive optoelectronic properties of CH3NH3PbI3 mainly derive from the inorganic lead-halide framework.

Time-resolved Hall conductivity of pulse-driven topological quantum systems

We address the question of how the time-resolved bulk Hall response of a two dimensional honeycomb lattice develops when driving the system with a pulsed perturbation. A simple toy model that switches a valley Hall signal by breaking inversion symmetry is studied in detail for slow quasi-adiabatic ramps and sudden quenches, obtaining an oscillating dynamical response that depends strongly on doping and time-averaged values that are determined both by the out of equilibrium occupations and the Berry curvature of the final states. On the other hand, the effect of irradiating the sample with a circularly-polarized infrared pump pulse that breaks time reversal symmetry and thus ramps the system into a non-trivial topological regime is probed. Even though there is a non quantized average signal due to the break down of the Floquet adiabatical picture, some features of the photon-dressed topological bands are revealed to be present even in a few femtosecond timescale. Small frequency oscillations during the transient response evidence the emergence of dynamical Floquet gaps which are consistent with the instantaneous amplitude of the pump envelope. On the other hand, a characteristic heterodyining effect is manifested in the model. The presence of a remnant Hall response for ultra-short pulses that contain only a few cycles of the radiation field is briefly discussed.

Time‐domain measurement of pure rotational Raman collisional linewidths of ethane C2H6

AbstractThe pure rotational Raman linewidths of the symmetric‐top molecule C2H6 are determined from a time‐resolved femtosecond experiment at room temperature and moderate pressure in a pump‐probe scheme. After excitation by a strong 100 fs‐duration nonresonant near infrared pump pulse, the molecular sample exhibits periodic post‐pulse alignment revivals that produce an anisotropic angular distribution of the molecular axis. Such molecular motion induces in turn a time‐dependent optical birefringence, which can be probed using a weak time‐delayed probe pulse. The Fourier transform of the temporal signal, recorded over a wide time delay range (several hundred picoseconds) for high frequency resolution, is directly related to the rotational Raman spectrum of ethane molecules. The Raman lines are analyzed with a frequency lineshape taking into account the collisional relaxation and the apparatus function. The linewidths are measured for rotational quantum numbers J=3 to J=30 in the pressure range 0.69–1.06 bar. A decrease of the collisional broadening coefficient is observed as J increases. These first measurements of pure rotational Raman linewidths of ethane are compared with previously reported infrared and far‐infrared values.

Efficient semiconductor multicycle terahertz pulse source

Multicycle THz pulse generation by optical rectification in GaP semiconductor nonlinear material is investigated by numerical simulations. It is shown that GaP can be an efficient and versatile source with up to about 8% conversion efficiency and a tuning range from 0.1 THz to about 7 THz. Contact-grating technology for pulse-front tilt can ensure an excellent focusability and scaling the THz pulse energy beyond 1 mJ. Shapeable infrared pump pulses with a constant intensity-modulation period can be delivered for example by a flexible and efficient dual-chirped optical parametric amplifier. Potential applications include linear and nonlinear THz spectroscopy and THz-driven acceleration of electrons.

Distinguishing Population and Coherence Transfer Pathways in a Metal Dicarbonyl Complex Using Pulse-Shaped Two-Dimensional Infrared Spectroscopy.

Collection of two-dimensional infrared (2DIR) spectra using two ultrafast, broadband infrared pump pulses followed by an ultrafast probe pulse optimizes the experimental time and frequency resolution, but can also introduce quantum beat and coherence transfer pathways. The associated coherent dynamics create intensity oscillations and add extra features to 2DIR spectra. We describe a method to suppress these pathways using pump-pulse shaping, without significantly degrading the time and spectral resolution. We illustrate the method for a rhodium dicarbonyl complex, acetylacetonato dicarbonyl rhodium (RDC), to establish the relative importance of coherence and population transfer between carbonyl symmetric and asymmetric stretching modes. Our technique effectively suppresses the quantum beats. Comparison of peak intensities obtained with shaped and unshaped pump pulses demonstrates that coherence transfer does not play a significant role in the 2DIR spectrum of RDC in this spectral region.

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range.

Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of -0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals.

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.

Gigahertz Modulation of Femtosecond Time-Resolved Surface Sum-Frequency Generation Due to Acoustic Strain Pulses

We study the properties of aqueous solution/air interfaces with time-resolved surface sum-frequency generation (SFG) spectroscopy. Using this technique, we monitor the change in SFG signal following the excitation of the water hydroxyl stretch vibrations with a strong ultrashort infrared pump pulse. We observe that, in addition to the excitation-induced changes in the signal resulting from vibrational excitation and relaxation on a time scale of picoseconds, the infrared excitation also induces a modulation of the SFG signal with a period of ∼180 ps. This modulation is caused by the interference between the SFG field directly radiated away from the air/liquid interface and the SFG field reflected or generated at the acoustic strain wavefront that is created by the infrared excitation pulse. The modulation is observed to be much stronger for aqueous salt solutions than for pure water, and the phase of the oscillations depends on the nature of the dissolved salt. This dependence of the phase can be explained from the differences in surface propensities of the ions of the different salts, giving rise to different net orientation of the interfacial water species.

Attosecond pulses with sophisticated spatio-spectral waveforms: spatio-spectral Airy and auto-focusing beams

We propose a scheme for producing attosecond pulses with sophisticated spatio-spectral waveforms. The profile of a seed attosecond pulse is modified and its central frequency is up-converted through interaction with an infrared pump pulse. The transverse profile of the infrared beam and a spatiotemporal shift between the seed and infrared pulses are used for manipulating the spatio-spectral waveform of the generated pulse beam. We present several examples of sophisticated isolated attosecond pulse beam generation, including spatio-spectral Airy beam that exhibits prismatic self-bending effect and a beam undergoing auto-focusing to a sub-micron spot without the need of a focusing lens or nonlinearity.

Single-shot picosecond interferometry with one-nanometer resolution for dynamical surface morphology using a soft X-ray laser

Using highly coherent radiation at a wavelength of 13.9 nm from a Ag-plasma soft X-ray laser, we constructed a pump-and-probe interferometer based on a double Lloyd's mirror system. The spatial resolutions are evaluated with a test pattern, showing 1.8-mum lateral resolution, and 1-nm depth sensitivity. This instrument enables a single-shot observation of the surface morphology with a 7-ps time-resolution. We succeeded in observing a nanometer scale surface dilation of Pt films at the early stage of the ablation process initiated by a 70 fs near infrared pump pulse.

Coherent nonlinear emission from a single KTP nanoparticle with broadband femtosecond pulses

We demonstrate that the intensity of the second harmonic (SH) generated in KTiOPO(4) nanoparticles excited with femtosecond laser pulses increases with decreasing duration of the infrared pump pulses. The SH intensity scales, approximately, as the inverse of the laser pulse duration ranging between 13 fs and 200 fs. The SH intensity enhancement requires careful compensation of the high-order spectral phase, being achieved with a genetic algorithm. Using ultrashort laser pulses improves the signal-to-noise ratio and will allow the detection of 10-nm size particles. Finally, we demonstrate that the spectrum of broadband (100 nm) pulses can be shaped to generate non-degenerate sum-frequency mixing. This opens up access to the polarization degrees of freedom of this second-order nonlinear process at the nanoscale.

Picosecond Melting of Ice by an Infrared Laser Pulse: A Simulation Study

X-ray lasers bring us into a new world in photon science by delivering extraordinarily intense beams of x-rays in very short bursts that can be more than ten billion times brighter than pulses from other x-ray sources. These lasers find applications in sciences ranging from astrophysics to structural biology, and could allow us to obtain images of single macromolecules when these are injected into the x-ray beam. A macromolecule injected into vacuum in a microdroplet will be affected by evaporation and by the dynamics of the carrier liquid before being hit by the x-ray pulse. Simulations of neutral and charged water droplets were performed to predict structural changes and changes of temperature due to evaporation. The results are discussed in the aspect of single molecule imaging. Further studies show ionization caused by the intense x-ray radiation. These simulations reveal the development of secondary electron cascades in water. Other studies show the development of these cascades in KI and CsI where experimental data exist. The results are in agreement with observation, and show the temporal, spatial and energetic evolution of secondary electron cascades in the sample. X-ray diffraction is sensitive to structural changes on the length scale of chemical bonds. Using a short infrared pump pulse to trigger structural changes, and a short x-ray pulse for probing it, these changes can be studied with a temporal resolution similar to the pulse lengths. Time resolved diffraction experiments were performed on a phase transition during resolidification of a non-thermally molten InSb crystal. The experiment reveals the dynamics of crystal regrowth. Computer simulations were performed on the infrared laser-induced melting of bulk ice, giving a comprehension of the dynamics and the wavelength dependence of melting. These studies form a basis for planning experiments with x-ray lasers.

Observing the Stimulated Raman Gain Spectra of Solutions Using an Infrared Pump Pulse with Narrow Linewidth and a Low-Noise CW Probe Laser

Stimulated Raman gain (SRG) spectroscopy using infrared pump pulses with narrow linewidth and a low-noise cw probe infrared laser was proposed. High-resolution Raman spectra of solutions were obtained. The SRG spectra of crystal GaP, benzene, and toluene were measured to confirm the spectral resolution and sensitivity over the terahertz (THz) region. We discuss the polarization dependence of the spectral measurement of carbon tetrachloride. Our system can detect organic molecules in aqueous solutions.

Transient hole-burning spectroscopy of associated ethanol molecules in the infrared: Structural dynamics and evidence for energy migration

Hydrogen-bonded ethanol molecules in internal positions of oligomers are investigated in solutions of carbon tetrachloride (concentration 0.17 M). After resonant excitation of the OH-stretching vibration by a 2-ps infrared pump pulse at 3340 cm−1, transient spectral hole burning within the inhomogeneously broadened band is observed with a delayed, independently tunable probing pulse of 1 ps duration. Evidence is presented that the vibrational excitation migrates along the oligomer chain with subsequent breaking of H bonds so that additional ethanol dimers and trimers are formed. Correspondingly, the number of ethanol monomers or hydroxilic groups with proton acceptor function is found to increase. Several time constants describing the various processes, e.g., the proposed migration of vibrational quanta along the H bonded chain, breaking of hydrogen bonds, and the reassociation of the generated shorter species, are determined by a comparison of the experimental data with model computations.

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.