Infrared Pulses Research Articles

Advances in timing and control of ultrafast molecular dynamics: from XUV to infrared

Abstract With the availability of modern laser and detection technologies, the investigation of ultrafast molecular dynamics induced by intense laser pulses has become a routine practice. In this Topical Review, we present a survey of recent progress in the timing and control of ultrafast molecular dynamics, encompassing processes initiated by both extreme ultraviolet and near infrared pulses. Prospects and perspectives of this field are given. This Review underscores the remarkable potential for further advances in understanding and harnessing ultrafast molecular processes.

Atomic-force-microscopy-based time-domain two-dimensional infrared nanospectroscopy.

For decades, infrared (IR) spectroscopy has advanced on two distinct frontiers: enhancing spatial resolution and broadening spectroscopic information. Although atomic force microscopy (AFM)-based IR microscopy overcomes Abbe's diffraction limit and reaches sub-10 nm spatial resolutions, time-domain two-dimensional IR spectroscopy (2DIR) provides insights into molecular structures, mode coupling and energy transfers. Here we bridge the boundary between these two techniques and develop AFM-2DIR nanospectroscopy. Our method offers the spatial precision of AFM in combination with the rich spectroscopic information provided by 2DIR. This approach mechanically detects the sample's photothermal responses to a tip-enhanced femtosecond IR pulse sequence and extracts spatially resolved spectroscopic information via FFTs. In a proof-of-principle experiment, we elucidate the anharmonicity of a carbonyl vibrational mode. Further, leveraging the near-field photons' high momenta from the tip enhancement for phase matching, we photothermally probe hyperbolic phonon polaritons in isotope-enriched h10BN. Our measurements unveil an energy transfer between phonon polaritons and phonons, as well as among different polariton modes, possibly aided by scattering at interfaces. The AFM-2DIR nanospectroscopy enables the in situ investigations of vibrational anharmonicity, coupling and energy transfers in heterogeneous materials and nanostructures, especially suitable for unravelling the relaxation process in two-dimensional materials at IR frequencies.

Temporal contrast enhancement of a Ti:Sapphire laser by nonlinear Fourier filtering

Temporal cleaning of high-power infrared (IR) pulses generated by a Ti:Sapphire system is demonstrated by the use of the Nonlinear Fourier Filtering (NFF) method. In a proof-of-principle experiment suppression of up to 1000 is achieved for the temporal pedestal prior to the main pulse, with a moderate (20-25%) overall throughput. This includes the same suppression ratio for the picosecond coherent pedestal in the direct vicinity of the main pulse. Based on the instantaneous, intensity-dependent and high-order switching characteristics of NFF, excellent pulse cleaning performance is observed. The efficient, high-contrast removal of the coherent pedestal from the foot of the main pulse even if its duration is shorter than 100 fs is compared with the capability of the plasma mirror technique. Calculations are also performed, supporting the experimentally observed sharp intensity dependence of the switching process, pointing out the dominant role of the ionization-based refractive index change.

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures.

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Channel separation of secondary generated radiation induced by orthogonal XUV and IR pulses.

The secondary generated radiation induced by orthogonal linearly polarized extreme ultraviolet (XUV) and infrared (IR) pulses is analyzed for the spectral region of the second XUV harmonic. The polarization-filtering-based method is utilized to separate two spectrally overlapping and competing channels, which are the XUV second harmonic generation (SHG) by IR-dressed atom and XUV-assisted recombination channel of high-order harmonic generation in the IR field [Phys. Rev. A98, 063433 (2018)10.1103/PhysRevA.98.063433]. We demonstrate the use of the separated XUV SHG channel for accurately retrieving the IR-pulse waveform and find the range of IR-pulse intensities for which this retrieving is applicable.

First time-resolved measurement of infrared scintillation light in gaseous xenon

Xenon is a widely used detector target material due to its excellent scintillation properties in the ultraviolet (UV) spectrum. The additional use of infrared (IR) scintillation light could improve future detectors. However, a comprehensive characterization of the IR component is necessary to explore its potential. We report on the first measurement of the time profile of the IR scintillation response of gaseous xenon. Our setup consists of a gaseous xenon target irradiated by an alpha particle source and is instrumented with one IR- and two UV-sensitive photomultiplier tubes. Thereby, it enables IR timing measurements with nanosecond resolution and simultaneous measurement of UV and IR signals. We find that the IR light yield is in the same order of magnitude as the UV yield. We observe that the IR pulses can be described by a fast and a slow component and demonstrate that the size of the slow component decreases with increasing levels of impurities in the gas. Moreover, we study the IR emission as a function of pressure. These findings confirm earlier observations and advance our understanding of the IR scintillation response of gaseous xenon, which could have implications for the development of novel xenon-based detectors.

Enhanced Extreme Ultraviolet Free Induction Decay Emission Assisted by Attosecond Pulses

We demonstrate the extreme ultraviolet free induction decay emission that can be significantly enhanced by employing isolated attosecond pulses. The near infrared pulses are applied to excite the neon atoms into Rydberg states coherently, and isolated attosecond pulses are used to manipulate populations of the Rydberg states and the subsequent free induction decay process. The time resolved experimental measurement of dependence of the resonance emission yield would help to understand the buildup dynamics of population of excited states. The enhancement assisted by attosecond pulses can serve as a mechanism to develop high-flux extreme ultraviolet light sources.

Controlled compression, amplification and frequency up-conversion of optical pulses by media with time-dependent refractive index

Abstract Control over the time dependence of the refractive index of a material allows one to modify and manipulate the properties of light propagating through it. While metamaterials provide a promising avenue in this context, another route has been extensively explored by the ultrafast community — the so-called molecular modulators. Indeed, impulsively-aligned diatomic molecules provide a unique medium, where periodic rotational revivals induced by a pump pulse persist for tens of picoseconds, offering an excellent opportunity for the controlled modification of the refractive index and, therefore, of femtosecond laser pulses propagating through these media. Here we present an analytical theory which describes this process and stumble across a novel mechanism revealing exponential transformations of the probe pulse — its compression, amplification and frequency up-conversion. In particular, our analytical results predict the generation of amplified ultrashort (about 20 fs) ultraviolet pulses centered around 550 nm, starting with near infrared input pulses centered on 1 μm of about 30 fs duration, under very realistic experimental conditions.

Abstract Number ‐ 54: First Fully Automated Robotic Transcranial Doppler Optimization of Cerebral Blood Flow During Cardiac Revascularization Surgery

Introduction Current technologies including EEG and near infrared pulse oximetry for non‐invasive monitoring of cerebral blood flow during cardiac surgery have proven inadequate for perioperative stroke risk mitigation. Cerebral arterial flow velocity is an excellent indicator of cerebral blood flow making transcranial doppler ultrasound (TCD) an excellent alternative technology for operating room continuous cerebral monitoring. To date, TCD utilization in the operating room has been limited due to frequent movement‐related loss of signal requiring nearly continuous manual probe readjustment. The recent development of fully autonomous robotic TCD (aTCD) has overcome these technical limitations, opening the door for real‐time continuous noninvasive cerebral blood flow monitoring with simultaneous emboli detection. Methods A 47‐year‐old female patient was found to have preoperative complete occlusion of the proximal brachiocephalic artery producing a double cerebral steal phenomenon with retrograde flow in the right internal carotid and vertebral arteries into the subclavian. A preoperative aTCD study disclosed baseline markedly diminished blood flow in the right middle cerebral artery (MCA) fed exclusively by small‐diameter collaterals via the circle of Willis.Intraoperatively, robotic aTCD was used to continuously monitor blood flow in the bilateral MCAs and perform emboli detection. In response to movement‐related signal loss, the robotic aTCD autonomously adjusted the probe position to maintain continuous MCA insonation. Results Shortly after intubation, a small drop in mean arterial pressure led to a near‐total loss of cerebral perfusion to the entire right hemisphere. This perfusion decrease was detected by aTCD, triggering the anesthesiologist to rapidly correct blood pressure to ensure adequate cerebral perfusion as confirmed by the robotic system. The patient was then placed on bypass pump with deployment of a temporary Y‐graft from the pump to the subclavian artery distal to the vertebral branch point and to the aorta to temporarily reverse the double cerebral steal. Coronary bypass was then performed, throughout which cerebral flow velocities were monitored continuously with aTCD and maintained in the safe cerebral perfusion range by adjusting pump pressures. Following a three‐vessel coronary bypass graft, permanent brachiocephalic artery bypass grafts were placed from the proximal aorta to the right subclavian and the proximal common carotid artery.Postoperative aTCD disclosed correction of the previously diminished blood flow detected in the right MCA preoperatively. Brain MRI postoperatively confirmed the absence of acute stroke. Conclusions This case represents the first use of robotic aTCD for real time continuous cerebral blood flow monitoring and optimization during complex cardiac surgery. Our experience suggests that aTCD may improve patient outcomes, particularly during high‐risk cardiac surgical and interventional procedures. Further studies of this promising technology are now needed.

High-order harmonic generation in orthogonal IR and XUV pulses: XUV-initiated channel separation and polarization control

Single-atom high-order harmonic generation (HHG) induced by an intense linearly polarized infrared (IR) field and assisted by a perturbative extreme ultraviolet (XUV) short pulse with linear polarization is analyzed in perpendicular geometry. It is shown that for the case of the initial $p$ state, the two independent projections of the laser-induced dipole are formed by two independent physical channels, which are responsible for IR-induced HHG and XUV-initiated HHG. This property of the laser-induced dipole is utilized for polarization control of the generated radiation. An alternative scheme for studying XUV-initiated HHG is proposed.

Second-harmonic generation of a short XUV pulse interacting with an IR-laser-dressed atom

The nonlinear response of an atomic system interacting with short extreme ultraviolet (XUV) and infrared (IR) pulses is considered within the adiabatic approach (for an IR field) and the perturbation theory (for an XUV pulse). The second-order perturbation theory in the XUV field is developed for the IR-dressed atom, and the laser-induced generation of a pulse with a doubled carrier XUV frequency is analyzed. We study the yield of harmonics in the frequency band of the generated doubled frequency XUV pulse as a function of the time delay between initial XUV and IR pulses. Our analytical and numerical results show that this dependence carries the time dependence of the intense IR pulse and can be utilized to retrieve the waveform of the IR pulse.

Fourier-Transform Atomic Force Microscope-Based Photothermal Infrared Spectroscopy with Broadband Source.

The mechanical detection of photothermal expansion from infrared (IR) absorption with an atomic force microscope (AFM) bypasses Abbe's diffraction limit, forming the chemical imaging technique of AFM-IR. Here, we develop a Fourier transform AFM-IR technique with peak force infrared microscopy and broadband femtosecond IR pulses. A Michelson interferometer creates a pair of IR pulses with controlled time delays to generate photothermal signals transduced by AFM to form an interferogram. A Fourier transform is performed to recover IR absorption spectra. We demonstrate the Fourier transform AFM-IR microscopy on a polymer blend and hexagonal boron nitride. An intriguing observation is the vertical asymmetry of the interferogram, which suggests the presence of multiphoton absorption processes under the tip-enhancement and femtosecond IR lasers. Our method demonstrates the feasibility of time-domain detection of the AFM-IR signal in the mid-IR regime and paves the way toward multiphoton vibrational spectroscopy at the nanoscale below the diffraction limit.

Ionization phase retrieval by angular streaking from random shots of XUV radiation

We demonstrate an accurate phase retrieval of extreme ultraviolet (XUV) atomic ionization by streaking the photoelectron in a circularly polarized infrared (IR) laser field. Our demonstration is based on a numerical solution of the time-dependent Schr\"odinger equation. We test this technique using the hydrogen atom ionized by isolated attosecond XUV pulses across a wide range of the photon energies. Importantly, the proposed method works in a random shot mode when the time delay between the ionizing and streaking XUV and IR pulses may vary from shot to shot. This is a significant advantage over the existing interferometric techniques which require a systematic and controllable scan of the XUV and IR delay in one set of measurements. Such a scan may not be possible to take because of the arrival time jitter inherent to the stochastic nature of self-amplified free-electron laser radiation.

Ensemble effects on the reconstruction of attosecond pulses and photoemission time delays

A crucial prerequisite for a detailed interpretation of the experimental results obtained with the most common attosecond spectroscopic techniques is a careful characterization of the attosecond extreme-ultraviolet (XUV) and femtosecond infrared (IR) pulses used in the measurements. A commonly adopted approach is based on the measurement of the spectra of the photoelectrons produced by the interaction of the attosecond pulses with a noble gas in the presence of a delayed IR pulse. Feeding the resulting spectrogram to reconstruction algorithms, it is then possible to retrieve the temporal properties of the XUV and IR pulses. To date, all reconstruction techniques are based on the assumption that the spectrogram is produced by the interaction of a single atom with a two-color (XUV-IR) field. In this work, we numerically investigate the effect of the actual XUV and IR beam spatial distributions, and we analyze their impact on the retrieval of the temporal characteristics of the XUV and IR pulses and on the determination of the photoemission time delays. We show that the impact of the ensemble effects can be severe, leading to notable variation of the photoelectron spectrograms, depending on the ratio between the XUV and IR beam spot sizes and on the IR peak intensity. We demonstrate that the photoemission time delay can be retrieved with great accuracy even in the presence of large deformations of the photoelectron spectrograms by employing suitable reconstruction procedures.

Attosecond Transient Absorption Below the Excited States

In this study, the attosecond transient absorption (ATA) spectrum below the excited states of the helium atom was investigated by numerically solving the fully three-dimensional time-dependent Schrödinger equation. Under single-active electron approximation, the helium atom was illuminated by a combined field comprising of extreme ultraviolet (XUV) and delayed infrared (IR) fields. The response function demonstrates that the absorption near the central frequency (ωX) of the XUV field is periodically modulated during the overlapping between the XUV and IR pulses. Using the time-dependent perturbation, the absorption near ωX is attributed to the wavepacket excited by the XUV pulse. The wave function oscillating at the frequency of the XUV pulse was obtained. Furthermore, the chirp-dependent absorption spectrum near ωX potentially provides an all-optical method for characterizing the attosecond pulse duration. Finally, these results can extend to other systems, such as solids or liquids, indicating a potential for application in photonic devices, and they may be meaningful for quantum manipulation.

Time-Domain Vibrational Action Spectroscopy of Cryogenically Cooled, Messenger-Tagged Ions Using Ultrafast IR Pulses.

Herein, we present the initial steps toward developing a framework that will enable the characterization of photoinitiated dynamics within large molecular ions in the gas phase with temporal and energy resolution. We combine the established techniques of tag-loss action spectroscopy on cryogenically trapped molecular ions with ultrafast vibrational spectroscopy by measuring the linear action spectrum of N2-tagged protonated diglycine (GlyGlyH+·N2) with an ultrafast infrared (IR) pulse pair. The presented time-domain data demonstrate that the excited-state vibrational populations in the tagged parent ions are modulated by the ultrafast IR pulse pair and encoded through the messenger tag-loss action response. The Fourier transform of the time-domain action interferograms yields the linear frequency-domain vibrational spectrum of the ion ensemble, and we show that this spectrum matches the linear spectrum collected in a traditional manner using a frequency-resolved IR laser. Time- and frequency-domain interpretations of the data are considered and discussed. Finally, we demonstrate the acquisition of nonlinear signals through cross-polarization pump-probe experiments. These results validate the prerequisite first steps of combining tag-loss action spectroscopy with two-dimensional IR spectroscopy for probing dynamics in gas-phase molecular ions.

Temporal characterization of electron dynamics in attosecond XUV and infrared laser fields.

We use a Wigner distribution-like function based on the strong field approximation theory to obtain the time-energy distributions and the ionization time distributions of electrons ionized by an XUV pulse alone and in the presence of an infrared (IR) pulse. In the case of a single XUV pulse, although the overall shape of the ionization time distribution resembles the XUV-envelope, its detail shows dependence on the emission direction of the electron and the carrier-envelope phase of the pulse, which mainly results from the low-energy interference structure. It is further found that the electron from the counter-rotating term plays an important role in the interference. In the case of the two-color pulse, both the time-energy distributions and the ionization time distributions change with varying IR field. Our analysis demonstrates that the IR field not only modifies the final electron kinetic energy but also changes the electron's emission time, which is attributed to the change of the electric field induced by the IR pulse. Moreover, the ionization time distributions of the photoelectrons emitted from atoms with higher ionization energy are also given, which show less impact of the IR field on the electron dynamics.

Time-frequency mapping of two-colour photoemission driven by harmonic radiation

The use of few-femtosecond, extreme ultraviolet (XUV) pulses, produced by high-order harmonic generation, in combination with few-femtosecond infrared (IR) pulses in pump–probe experiments has great potential to disclose ultrafast dynamics in molecules, nanostructures and solids. A crucial prerequisite is a reliable characterization of the temporal properties of the XUV and IR pulses. Several techniques have been developed. The majority of them applies phase reconstruction algorithms to a photoelectron spectrogram obtained by ionizing an atomic target in a pump–probe fashion. If the ionizing radiation is a single harmonic, all the information is encoded in a two-color two-photon signal called sideband (SB). In this work, we present a simplified model to interpret the time-frequency mapping of the SB signal and we show that the temporal dispersion of the pulses directly maps onto the shape of its spectrogram. Finally, we derive an analytical solution, which allows us to propose a novel procedure to estimate the second-order dispersion of the XUV and IR pulses in real time and with no need for iterative algorithms.

Advanced time-resolved absorption spectroscopy with an ultrashort visible/near IR laser and a multi-channel lock-in detector

Ultrashort visible-near infrared (NIR) pulse generation and its applications to ultrafast spectroscopy are discussed. Femtosecond pulses of around 800 nm from a Ti:sapphire laser are used as a pump of an optical parametric amplifier (OPA) in a non-collinear configuration to generate ultrashort visible (500–780 nm) pulses and deep-ultraviolet (DUV, 259–282 nm) pulses. The visible-NIR pulses and DUV pulses were compressed to 3.9 fs and 10.4 fs, respectively, and used to elucidate various ultrafast dynamics in condensed matter with a sub-10 fs resolution by pump-probe measurements. We have also developed a 128-channel lock-in amplifier. The combined system of the world-shortest visible pulse from the OPA and the lock-in amplifier with the world-largest channel-number can clarify the sub-10 fs-dynamics in condensed matter. This system clarified structural changes in an excited state, reaction intermediate, and a transition state. This is possible even during molecular vibration and reactions via a real-time-resolved vibronic spectrum, which provides molecular structural change information. Also, ultrafast dynamics in exotic materials like carbon nanotubes, topological insulators, and novel solar battery systems have been clarified. Furthermore, the carrier-envelope phase in the ultrashort pulse has been controlled and measured.

Extreme ultraviolet photon effect on ionization and recombination of high-order harmonic generation

The extreme ultraviolet (XUV) pulse effect on the ionization and recombination of the high-order harmonic generation has been theoretically studied. It is shown that (i) with the introduction of a shorter duration XUV pulse to a fundamental infrared (IR) pulse, the XUV photon can only be absorbed during the electron ionization process. Thus, the harmonic signal can be enhanced due to the XUV resonance transition and ionization. (ii) With the introduction of a longer-duration XUV pulse to a fundamental IR pulse, the XUV photon can be absorbed not only in the electron ionization process, but also in the electron recombination process. As a result, not only the harmonic signal can be improved, but also the harmonic cutoff can be extended with an additional XUV photon energy region (. Moreover, as the XUV intensity increases, much more XUV photons can be absorbed in the electron recombination process, thus leading to the harmonic cutoff extension with the extra photon energy region of .