Frequency shift of even-order high harmonic generation in monolayer MoS2.

Sub-optical-cycle electron dynamics in materials driven by intense laser fields can be investigated by high harmonic generation. We observed frequency shift of high harmonic spectrum near the band gap of monolayer MoS2 experimentally. Through semi-classical quantum trajectory analysis, we demonstrated that the phase of transition dipole moment varies according to the recombination timing and momentum of tunneled electrons. It results in either blue- or red-shift of harmonic frequencies, determined by the modulated energy gap by transition dipole phases (TDPs) and Berry connections. Our finding reveals the effect of TDPs on high harmonic frequency in non-central symmetric materials.

Generation of necklace-shaped high harmonics in a two-color vortex field.

We numerically studied gas high-harmonic generation in a two-color vortex laser field using the non-adiabatic Lewenstein model. Macroscopic responses were calculated by numerically solving the three-dimensional propagation equation in cylindrical coordinates. It was confirmed that unique high-harmonic signals with necklace-like shapes exhibit orbital angular momentum (OAM). The azimuthally distributed necklace harmonics exhibit periodic modulation as a function of laser frequency and topological charges of the driving field. Phase investigation showed that the OAM of the necklace harmonics is attributable to the tuning of the relative intensity of the two driving pulses. These findings provide a new dimension for high-harmonic manipulation in the vortex field. The two-color vortex field is the first scheme proposed for manipulating the intensity profile of high harmonics.

Controlling of the harmonic generation induced by the Berry curvature.

High-order harmonic generation in solid state has attracted a lot of attentions. The Berry curvature (BC), a geometrical property of the Bloch energy band, plays an important role for the harmonic generation in crystal. As we all know, the influence of BC on the harmonic emission has been investigated before and BC is simplified as a 1D structure. However, many other materials including MoS2 are 2D materials. In this work, we extend the investigation for BC to 2D structure and get a generalized equation, which not only gives a new method to control the harmonic emission with BC, but also gives a deeper understanding for the influence of the BC. We show the ability to control the harmonic emission related to the BC using the orthogonal two-color (OTC) laser field. By tuning the delay of OTC laser field, one can steer the trajectory of electrons and modulate the emission of harmonics. This study can provide us a deeper insight into the role of the BC which is difficult to be measured experimentally.

Infrared-laser-induced ultrafast modulation on the spectrum of an extreme-ultraviolet attosecond pulse.

We present a theoretical investigation of the isolated extreme-ultraviolet (XUV) attosecond pulse propagating in the ionizing gas induced by a synchronized strong infrared (IR) laser, with the numerical solution of the nonadiabatic one-dimensional propagation model. Upon scanning the relative delay between the XUV and the IR pulse, it is found that the delay-dependent XUV transmission spectrogram exhibits the unique pattern that is controllable by the chirp of the XUV pulse. Furthermore, we demonstrate that this kind of spectrum modulation can be attributed to the term of the light energy loss involved in the propagation equation. The characteristics of the spectrum modulation dependent on the XUV chirp might provide an all-optical way for the reconstruction of the XUV spectral phase.

Collisional dynamics in laser-induced plasmas: evidence for electron-impact excitation.

We have experimentally investigated the collisional dynamics in femtosecond-laser-induced plasmas and presented the evidence for electron-impact excitation through enhanced high-order harmonic (HH) generation. The measurements were carried out by using an elliptically polarized pump pulse to induce the underdense plasmas and by using a time-delayed linearly polarized probe pulse to drive the HH generation from the plasmas. We found that the rise time of this enhanced HH generation was insensitive to the ellipticity degree (ED) of pump pulse but sensitive to its laser intensity (LI). With further comparison between physical scenarios and qualitative analysis, we demonstrated that the atomic excitation causing the HH enhancement should be attributed to the electron-impact excitation, i.e., the excitation from the collision between neutral atoms and electrons during the lifetime of the underdense plasma.

Probing electron-atom collision dynamics in gas plasma by high-order harmonic spectroscopy.

Plasma is a complex system involving diverse collisional processes and interactions, such as electron-impact excitation, ionization, recombination, etc. One of the most important methods for studying the properties and dynamics of plasma is to analyze the radiations from plasma. Here, we demonstrate the high-order harmonic (HH) spectroscopy for probing the complex electron-atom collision (EAC) dynamics in a laser-induced gas plasma. These measurements were carried out by using an elliptically polarized pump and a time-delayed linearly polarized probe. The HH spectra from argon and krypton plasmas were recorded by scanning the time delay up to hundreds of picoseconds. We found that the delay-dependent HH yield contains three distinct regions, i.e., the first enhancement, the subsequent suppression, and the final restoration regions. A qualitative analysis shows that these features are clear signatures of the EAC processes and interactions involved in the delay-dependent HH spectroscopy.

Attosecond chirp effect on the transient absorption spectrum of laser-dressed helium atom.

We theoretically investigate the attosecond transient absorption spectrum of helium atom in the presence of an infrared-dressed laser pulse upon scanning their relative delay, with the particular emphasis on the chirp effect of the attosecond pulse. By numerically solving the fully three-dimensional time-dependent Schrödinger equation, we identify the attoscecond chirp can induce the temporal shift of the absorption spectrogram along the delay axis. Additionally, it is found that the extent of the temporal shift is dependent on both the position of the absorption line and the infrared pulse wavelength, which is well confirmed and reproduced by a three-level model. Moreover, we demonstrate that the observed features can be quantitatively explained in terms of the indirect two-photon absorption processes through some virtual states. This effect might provide a way to measure the chirp of attosecond pulse in an all-optical way.

Effect of nuclear motion on spectral broadening of high-order harmonic generation.

High-order harmonic generation (HHG) in molecular targets is experimentally investigated in order to reveal the role of the nuclear motion played in the harmonic generation process. An obvious broadening in the harmonic spectrum from the H2 molecule is observed in comparison with the harmonic spectrum generated from other molecules with relatively heavy nuclei. We also find that the harmonic yield from the H2 molecule is much weaker than the yield from those gas targets with the similar ionization potentials, such as Ar atom and N2 molecule. The yield suppression and the spectrum broadening of HHG can be attributed to the vibrational motion of nuclear induced by the driving laser pulse. Moreover, the one-dimensional (1D) time-dependent Schrödinger equation (TDSE) with the non-Born-Oppenheimer (NBO) treatment is numerically solved to provide a theoretical support to our explanation.

Mapping the spectral phase of isolated attosecond pulses by extreme-ultraviolet emission spectrum.

An all-optical method is proposed for the measurement of the spectral phase of isolated attosecond pulses. The technique is based on the generation of extreme-ultraviolet (XUV) radiation in a gas by the combination of an attosecond pulse and a strong infrared (IR) pulse with controlled electric field. By using a full quantum simulation, we demonstrate that, for particular temporal delays between the two pulses, the IR field can drive back to the parent ions the photoelectrons generated by the attosecond pulse, thus leading to the generation of XUV photons. It is found that the generated XUV spectrum is notably sensitive to the chirp of the attosecond pulse, which can then be reliably retrieved. A classical quantum-path analysis is further used to quantitatively explain the main features exhibited in the XUV emission.

Enhanced high-order harmonic generation from spatially prepared filamentation in argon.

We experimentally demonstrate enhanced high-order harmonic generation (HHG) from spatially prepared filamentation in Argon. Upon shifting the focus position of an elliptically polarized laser pulse over the filament induced by a linearly polarized laser pulse, an obvious enhancement of harmonic yield by nearly one order of magnitude is observed. The result could be interpreted in terms of the double contributions from both the excited states of target atom and the phase-matching effect of harmonic beam. In contrast to the enhancement phenomena, an obvious suppression of harmonic yield is also presented, which could be attributed to both the ground-state depletion and the plasma effect.

Carrier-envelope phase effects of a single attosecond pulse in two-color photoionization.

The attosecond streak camera method is usually implemented to characterize the temporal phase and amplitude of isolated attosecond pulses produced by high-order harmonic generation. This approach, however, does not provide any information about the carrier-envelope phase of the attosecond pulses. We demonstrate that the photoelectron spectra generated by an attosecond waveform and an intense synchronized infrared field are sensitive to the electric field of the attosecond pulse. The dependence on the carrier-envelope phase of the attosecond pulse is understood in terms of the coherent superposition of two photoelectron wave packets. This effect suggests an experimentally feasible method for complete reconstruction of attosecond waveforms.

Molecular high harmonic generation in a two-color field.

We experimentally investigate the high harmonic generation (HHG) from CH4 molecules and Xe atoms in a two-color field (using the 800 nm laser and the tunable laser with the longer wavelength from 1500 nm to 1900 nm), and observe that the longer wavelength component can destructively suppress the HHG from CH4 molecules. By controlling the time delay between the two color laser pulses or tuning the laser intensity of the longer wavelength component, the suppressions of the HHG from CH4 molecules and the enhancements of the HHG from Xe atoms at the same laser condition are observed. The results indicate that the longer wavelength component around the molecular infrared absorption can suppress the molecular HHG process.

Nonadiabatic propagation effect for generating isolated sub-100 as pulses in the high-order harmonic plateau.

A robust scheme is proposed to generate an isolated sub-100 as pulse by using a 5fs/800nm driving pulse in combination with a macroscopic propagation effect. We demonstrate that one of the generated attosecond bursts in the high-order harmonic plateau can be efficiently dominant only when both the gas pressure and the gas cell length are properly chosen. We also show that this method can relax the dependence on the stabilization of the carrier-envelope phase (CEP) of the driving pulse. A single sub-100as pulse with only 10as duration variation can be produced even for about 50% fluctuation of the CEP. Under optimized conditions, an isolated 60as pulse can be directly generated without any additional phase compensation.

Multi-quantum-path interference in high harmonic generation driven by a chirped laser pulse.

We theoretically investigate the quantum-path interference during the high harmonic generation in argon and observe the multi-quantum- path-interference (MQPI) effect related to the chirp of driving laser pulse. We successfully identify the interference originated from four significantly contributing quantum paths in the interaction of "isolated" atom with the driving laser field. Moreover, the MQPI effect is further demonstrated beyond the single atom response by spatial filtering which is used for angle-selective detection of transmitted light when three-dimensional propagation is considered. It implies that the role of high-order electron returning trajectories during high harmonic generation can be observed in the experiment.

Wavelength effect on atomic and molecular high harmonic generation driven by a tunable infrared parametric source.

We experimentally investigate the wavelength effect on high-order harmonic generation (HHG) in CH(4) molecules and Xe atoms driven by a tunable infrared parametric source, and observe that the molecular HHG around the vibrational resonance is more sensitive to the driver wavelength than HHG from an atomic gas with comparable ionization potential. The results can be attributed to the light nuclear motion induced by the driving laser field, and it becomes possible to control the proton vibration in the molecular HHG by tuning the infrared wavelength of the driving laser.

Low-power laser therapy for carpal tunnel syndrome: effective optical power.

Low-power laser therapy has been used for the non-surgical treatment of mild to moderate carpal tunnel syndrome, although its efficacy has been a long-standing controversy. The laser parameters in low-power laser therapy are closely related to the laser effect on human tissue. To evaluate the efficacy of low-power laser therapy, laser parameters should be accurately measured and controlled, which has been ignored in previous clinical trials. Here, we report the measurement of the effective optical power of low-power laser therapy for carpal tunnel syndrome. By monitoring the backside reflection and scattering laser power from human skin at the wrist, the effective laser power can be inferred. Using clinical measurements from 30 cases, we found that the effective laser power differed significantly among cases, with the measured laser reflection coefficient ranging from 1.8% to 54%. The reflection coefficient for 36.7% of these 30 cases was in the range of 10-20%, but for 16.7% of cases, it was higher than 40%. Consequently, monitoring the effective optical power during laser irradiation is necessary for the laser therapy of carpal tunnel syndrome.