Editorial for Special Issue on Multiphoton Processes

With the rapid development of ultrafast intense laser technologies, the interaction of intense laser radiation with mat- ter has been a frontier for few decades. The International Conference on Multiphoton Processes （ICOMP）, initiated in 1977, covers the latest advances in the field every three years. The special issue is based on the spirit of the 13th International Conference on Multi-Photon Processes, ICOMP13, which was held in Shanghai, organized by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, on Dec. 7-10, 2014

Ground state cooling of an optomechanical resonator with double quantum interference processes

We present a cooling scheme with a tripod configuration atomic ensemble trapped in an optomechanical cavity.With the employment of two different quantum interference processes,our scheme illustrates that it is possible to cool a resonator to its ground state in the strong cavity-atom coupling regime.Moreover,with the assistance of one additional energy level,our scheme takes a larger cooling rate to realize the ground state cooling.In addition,this scheme is a feasible candidate for experimental applications.

Laser Induced Photoelectron Impact Ionization In Multiphoton Ionization Process

Multiply charged ions of Ar and NO were observed in MPI experiment Of NO/Ar with TOF-MS. A delayable pulsed acceleration field wn applied tO investigate the effect of the photoelectrons on the formation of the multiply charged ions. The multiply charged ions were suggested to be produced by photoelectron impact ionization, in the region bentween the extractor grid and the repeller plate, step by step, from neutral species and lower charged ions. The 50-60ns of FWHM of the ion peaks implies that the pulse width of the photoelectrons should be shorter considering the broadening effect during the ionization process.

Two-photon Upconversion Luminescence of Eu^(3+):Ca_2Al_2SiO_7 Crystal Irradiated by Infrared femtosecond Laser

Infrared to visible upconversion luminescence was demonstrated in trivalent Europium doped Ca2Al2SiO7 crystal （Eu^3＋：Ca2Al2SiO7） irradiated by focused infrared femtosecond laser. The upconversion luminescence originated from 5D0 to 7Ej （j= 1, 2） transitions of Eu^3＋ The relationship between the upconversion luminescence intensity and the pump power indicated that the upconversion from near-infrared to red is dominated by a two-photon absorption process of Eu^3＋ Analysis suggested that two-photon simultaneous absorption induced population inversion should be the predominant frequency upconversion mechanism.

Estimation of temperature rise at the focus of objective lens at the 1700 nm window

Optical microscopy of biological tissues at the 1700 nm window has enabled deeper penetration,due to the combined advantage of relatively small water absorption and tissue scattering at this wavelength.Compared with excitation at other wavelengths,such as the commonly used 800 nm window for two-photon microscopy,water absorption at the 1700nm window is more than one order of magnitude higher.As a result,more temperature rise can be expected and can be potentially detrimental to biological tissues.Here,we present theoretical estimation of temper-ature rise at the focus of objective lens at the 1700nm window,purely due to water absorption.Our calculated result shows that under realistic experimental conditions,temperature rise due to water absorption is still below 1 K and may not cause tissue damage during imaging.

Upconversion Luminescence of Ce^(3+) Doped BK7 Glass by Femtosecond Laser Irradiation

Near-infrared to visible upconversion luminescence was observed in a multicomponent silicate （BK7） glass containing Ce^3 ＋ ions under focused infrared femtosecond laser irradiation. The emission spectra show that the upconversion luminescence comes from the 4f-5d transition of the Ce^3 ＋ ions. The relationship between the intensity of the Ce^3 ＋ emission and the pump power reveals that a three-photon absorption predominates in the conversion process from the near-infrared into the blue luminescence. The analysis of the upconversion mechanism suggests that the upconversion luminescence may come from a three-photon simultaneous absorption that leads to a population of the 5d level in which the characteristic luminescence occurs.

Theoretical analysis on the efficiency of optical-optical double-color double-resonance multiphoton ionization

Analytic formula of the efficiency of optical-optical double-color double-resonance multi-photon ionization （OODR-MPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the influence of characteristic of the pump and probe laser on the ionization efficiency of （1＋2＋1） OODR-MPI process is simulated theoretically. It is shown that the pump laser will affect the ionization efficiency by the number control of the molecules excited to the first resonance state. The ionization efficiency is decided by the probe laser directly. Both of the excited molecules and ionization efficiency increase with the intensity and pulse duration of the laser until saturation. It is also found that the longer the delay time of the probe laser to the pump one is, the lower the ionization efficiency would be. The delay time ought to be smaller than the lifetime of the excited molecule in the practical use of the OODR-MPI technique.

Influence of laser intensity on the double-resonance multiphoton ionization process of NO molecule

The analytic formula of the ionization efficiency in the process of double resonance enhanced multi-photon ionization （DREMPI） is derived from the dynamic rate equation about the interaction of photon and material. Based on this formula, the ionization efficiency and the laser power index versus laser intensity in the DREMPI process of NO molecule, via A2E and S2E intermediate resonant states, is numerically simulated. It is shown that the ionization efficiency of NO molecule increases with the laser intensity until getting saturation, while the laser power index decreases with the enhancement of the laser intensity and changes to zero at last. The variation of the laser power index with the laser intensity indicates that the ionization efficiency reaches saturation in the one, two, and three excitation steps respectively. It is also found that the narrower the laser pulse duration is, the higher becomes the laser intensity for saturation.

Quantum sensing of control errors in three-level systems by coherent control techniques

The quantum coherent control of a quantum system with high fidelity is rather important in quantum computation and quantum information processing.Many control techniques are used to reach these targets,such as resonant excitation,adiabatic passages,shortcuts to adiabaticity,and composite pulses.However,for a single pulse to realize population transfer,a tiny external error has a slight influence on the final population.The repeated application of the same pulse will greatly amplify the error efect,making it easy to be detected.Here,we propose to measure small control errors in three-level quantum systems through a coherent amplification of their efects using several coherent control techniques.For the two types of Hamiltonian with an SU(2)dynamic symmetry,we analyze how the fidelity of the population transfer is afected by the Rabi frequency error and static detuning deviation based on the pulse sequence with alternating and same phases,respectively.The results show that the sensitivity of detecting these errors can be efectively amplified by control pulse sequences.Furthermore,we discuss the efficiency of sensing the two errors with the control techniques by comparing the full width at half maximum of the population profiles.The results provide an accurate and reliable way for detecting the weak error in three-level quantum systems by repeatedly applying the coherent control pulse.

Monte Carlo simulation of photon migration path in turbid media

A new method of Monte Carlo simulation is developed to simulate the photon migration path in a scattering medium after an ultrashort-pulse laser beam comes into the medium. The most probable trajectory of photons at an instant can be obtained with this method. How the photon migration paths are affected by the optical parameters of the scattering medium is analyzed. It is also concluded that the absorption coefficient has no effect on the most probable trajectory of photons.

Doping transition metal ions as a method for enhancement of ablation rate in femtosecond laser irradiation of silicate glass

We present a doping method to improve the femtosecond laser ablation rate and promote ablation selectivity. Doping transition metal ions, Co2＋ or Cu2＋, in silicate glass apparently change absorption spectroscopy and induce resonant absorption at wavelengths of 600 and 800 nm, respectively. Comparing with femtosecond laser processing of the same glass without doping, we find that the threshold fiuenee decreases and the ablation rate increases in resonant absorption in doped silicate glass. Resonant absorption effectively increases multiphoton ionization for seed-free electron generation, which in turn enhances avalanche ionization.

Experimental observation of the linewidth narrowing of electromagnetically induced transparency resonance

We report an experimental observation of the variation in linewidth of the electromagnetically induced transparency （EIT） resonance in a three-level A-type system for several laser bandwidths in a Rb vapor cell, with and without a buffer gas. It is found, using narrow bandwidth （about 20 kHz） diode laser for both coupling and probe beams, that the linewidth of the EIT resonance can be significantly narrowed in the Rb vapor cell with the buffer gas. The results are in good qualitative agreement with a simple theoretical calculation.

Diode-pumped passively Q-switched Nd:YVO_4 laser with GaAs saturable absorber

The intracavity photon density is assumed to be of Gaussian spatial distributions and its longitudinal variation is also considered in the rate equations for a laser diode (LD) end-pumped passively Q-switched Nd:YVO4 laser with GaAs saturable absorber. These space-dependent rate equations are solved numerically. The dependences of pulse width, pulse repetition rate, single-pulse energy, and peak power on incident pump power are obtained. In the experiment, the LD end-pumped passively Q-switched Nd:YVO4 laser with GaAs saturable absorber is realized and the experimental results are consistent with the numerical solutions.

Theoretical analysis on two-photon absorption spectroscopy in a confined four-level atomic system

We investigate theoretically two-photon absorption spectroscopy modified by a control field in a confined Y-type four-level system. Dicke-narrowing effect occurs both in two-photon absorption lines and the dips of transparency against two-photon absorption due to enhanced contribution of slow atoms. We also find that the suppression and the enhancement of two-photon absorption can be modified by changing the strength of the control field and the detuning of three laser fields. This control of two-photon absorption may have some applications in information processing and optical devices.

Quantitative deviation of the two-photon absorption coefficient based on three laser pulse models

The pulse profile influence of excitation light on the two-photon absorption coefficient β is theoretically and numerically studied. Based on Gaussian spatial and temporal laser, we obtain an expansion formula of energy transmission. As compared with a plain beam and a pulse beam that is rectangular in time but Gaussian in space, the relative deviations of β turn out to be about 214% and 47%, respectively. These differences indicate that a smaller β may be obtained than the real one in usual nonlinear transmission. Our result suggests that by taking real pulse profile into account, a more exact β can be derived in energy transmission measurement.

Compact and high-power broadband terahertz source based on femtosecond photonic crystal fiber amplifier

We present a review of the development of a compact and high-power broadband terahertz （THz） source optically excited by a femtosecond photonic crystal fiber （PCF） amplifier.The large mode area of the PCF and the stretcher-free configuration make the pump source compact and very efficient.Broadband THz pulses of 150 μW extending from 0.1 to 3.5 THz are generated from a 3-mm-thick GaP crystal through optical rectification of 12-W pump pulses with duration of 66 fs and a repetition rate of 52 MHz.A strong saturation effect is observed,which is attributed to pump pulse absorption;a Z-scan measurement shows that three-photon absorption dominates the nonlinear absorption when the crystal is pumped by femtosecond pulses at 1 040 nm.A further scale-up of the THz source power is expected to find important applications in THz nonlinear optics and nonlinear THz spectroscopy.

Non-classical correlation between a single photon and the collective spin excited state of a cold atomic ensemble

We experimentally establish a non-classical correlation between a single Stokes photon and the collective spin excited state of a cold atomic ensemble by using a spontaneous Raman scattering process. The correlation between them can be proved by transferring the spin excited state of the atomic ensemble into an anti-Stokes photon and checking the Cauchy-Schwarz inequality between the Stokes and the anti-Stokes photons. The non-classical correlation can be kept for at least 300 ns.

Theoretical and experimental investigations on the threshold of a laser-induced breakdown of air

The threshold of a laser-induced breakdown of air is determined experimentally and theoretically. We find that the ionization of air has two steps： the first step is a multi-photon ionization process, which provides enough ＂seed electrons＂ to initiate the next step, and the second one is predominated by cascade ionization, which con- tinues to produce free electrons geometrically until the critical free-electron density for breakdown is reached. So a two-step model based on the Morgan ionization model is established to describe the breakdown process. It is found that the time node dividing the two steps is about 9.8 ns in atmospheric air, and the threshold derived from the two-step model proposed here is more consistent with the experimental results than traditional ionization model.

New phenomena in laser-assisted scattering of an electron by a muon

The scattering of an electron by a muon in the presence of a linearly polarized laser field is investigated in the first Born approximation. The theoretical results reveal the following： i） At medium and large scattering angles, many multiphoton processes occur during scattering, and these nonlinear phenomena may predict the resonant state of the electron and the muon formed in the collision process, ii） The photoabsorption （inverse brerasstrahlung） dominates the photoemission （bremsstrahlung）, causing the cross section to increase, iii） When the laser polarization deviates from the incident direction, the laser- modified total cross section depends considerably on the azimuthal angle of the scattered electron. The dependence of the cross section on the field strength, polarization direction, and electron-impact energy are studied.

Electron-Positron Pair Production in Strong Fields Characterized by Conversion Energy

It is demonstrated that the production mechanism of a pair which is produced from vacuum under an external field can be characterized by its conversion energy, a quantity defined as total mass-energy of this pair. The value of this quantity is checked with quantum field theoretical simulations for several field configurations and it is found that conversion energy can show all the production channels and give the yields of each channel specifically. We detect signatures of effective mass, combination of different photons as well as dynamically assisted Schwinger mechanism and represented more features of these processes in the view of conversion energy.