Prevention of decoherence by two femtosecond chirped pulse trains

We study the vibrational energy relaxation (VER) and collisional dephasing as channels of coherence loss in a vibrational mode that is selectively excited using chirped pulse adiabatic passage in the Raman configuration. Based on the dressed state picture analysis we propose a method to reduce decoherence using femtosecond chirped pulse trains. When applied with a period close to the VER time, the pulse trains allow one to sustain high coherence in the selected vibrational mode.     

Quantum beats of vibrational modes in both the low and high wavenumber regions studied by fs-CARS

We have studied the quantum beats of vibrational modes in different phase samples by employing femtosecond time-resolved coherent anti-Stokes Raman scattering (fs-CARS) technique. The temporal chirped white-light continuum (WLC) is used for the Stokes pulse, we have achieved the selective excitation of vibrational modes in both the low and high wavenumber regions without complicated laser system, and observed quantum coherence between vibrational modes. This work is of special significance to enhance our understanding of quantum coherence and develop the applications of quantum technologies.     

Ultrashort-pulse modulation in adiabatically prepared Raman media

We demonstrate the efficient modulation of an approximately 100-femtosecond pulse in a Raman medium coherently prepared by nanosecond pulses. Raman sidebands of the ultrashort pulse spanning 360 THz are generated with an efficiency of >5%. We show that the mechanism permitting the sidebands to be generated is the preparation of a significant vibrational coherence in the medium that is robust to disturbance by an intense short pulse. If the observed sidebands were phase compensated, they would form a short train of approximately ten 3-fs pulses. Focusing would permit the realization of a peak intensity of >10(13) W cm(-2).     

Mixed quantum-classical treatment of vibrational decoherence

The mixed quantum-classical method based on the Bohmian formulation of quantum mechanics [J. Chem. Phys. 113, 9369 (2000)]] is applied to study the process of vibrational decoherence of I2 in a dense helium environment. Specifically, the revival of vibrational wave packets, detectable by pump-probe spectroscopy, is a quantum phenomena which depends sensitively on the coherence between the vibrational levels excited by the pump pulse. The time-dependent pump-probe revival signal is a very sensitive way of detecting vibrational dephasing induced by an environment. The very good agreement between previous experimental signals and calculations presented in ths work confirms the theoretical approach and provides a promising basis for the prediction and interpretation of future experiments exploiting quantum revivals as a probe of decoherence.     

复自相干度度量超连续谱相干性

引入复自相干度来度量同一脉冲产生的超连续谱的相干性, 并对其进行了实验研究. 采用Mach-Zehnder干涉仪, 测量700 ps脉冲抽运光子晶体光纤产生的超连续谱的相干性. 实验测得超连续谱不同波长成分的相干长度均大于40 μm, 在长波区可达225 μm. 超连续谱的整个光谱区域各谱成分的相干度有差异, 但复自相干度的模平均值为0.461, 相干性较好, 可以满足如光测量、光学传感等很多应用.     

Vibrational relaxation dynamics in transient grating spectroscopy studied by rate equations based on time-dependent correlation function

A modified model, a set of rate equations based on time-dependent correlation function, is used to study vibrational relaxation dynamics in transient grating spectroscopy. The dephasing, the population dynamics, and the vibrational coherence concerning two vibrational states are observed respectively in organic dye IR780 perchlorate molecules doped polyvinyl alcohol matrix. The result shows that in addition to the information concerning system-environment interac- tion and vibrational coherence, the vibrational energy transfer can be described by this modified model.     

利用双光子吸收效应测量激光脉冲的相干时间

研究了利用反向泵浦－探测双光子吸收材料的方法来测量脉冲激光的相干时间，并测量量了２３ｐｓ，５３２ｎｍ Ｎｄ：ＹＡＧ激光脉冲的相干时间。实验和理论分析表明，光束相干作用增加了双光子吸收，利用此方法可测量其它脉宽激光的相干时间。     

Vibrational dynamics of nitromethane mixed with IR780 dye studied by coherent anti‐stokes Raman spectroscopy

Vibrational coupling between different kinds of molecules in liquid mixture is studied by multiplex coherent anti‐Stokes Raman spectroscopy (CARS). To identify vibrational coherence, fs‐probe with high time resolution and narrowband‐probe with high spectral resolution are adopted in CARS experiments. Using liquid nitromethane (NM) mixed with organic dye IR780 perchlorate as the sample, we can clearly observe the interference between different vibrational modes. The intermolecular vibrational interaction between NM and IR780 molecules results in the vibrational coherence transfer (VCT) in the form of a change of phase correlation. Compared with symmetric bending vibration of NO₂, coherence transfer is found to be easier to take place between C―N bond of NM and vibrations of IR780, which indicates the selectivity of intermolecular vibrational interaction. The selectivity is deduced to be related to the coordination between intramolecular and collective motion of molecules. Copyright © 2016 John Wiley & Sons, Ltd.     

Multiple-quantum relaxation in the magic-angle-spinning NMR of 13C spin pairs.

We determine the decay rate constants of zero-, double- and single-quantum coherence for 13C spin pairs in magic-angle-spinning solid-state NMR. The double-quantum coherence is excited by a C7 pulse sequence and converted into zero-quantum coherence by a frequency-selective pair of pi/2 pulses. The zero-quantum coherence is reconverted into observable magnetization by a second pair of pi/2 pulses followed by a second C7 sequence. In a magnetically dilute system where the 13C-13C distance is 0.296 nm, the relaxation rate constants are consistent with a model of uncorrelated random fields at the two labeled 13C sites. In a fully-labelled system with a short 13C-13C distance of 0.153 nm, the measured rate constants are inconsistent with the uncorrelated random field model.     

Coherence properties of the supercontinuum source

Mutual–temporal coherence of the supercontinuum (SC) generation has been experimentally investigated using a diffraction grating based interferometer. A broadband SC generation was produced in a pyrex glass plate with a 1.6-μm coherence length at 550-nm center wavelength. The degree of mutual coherence of 0.34 was measured for a wavefront shift for zero and first diffraction orders of half of the beam diameter. A design of SC source with the capability of manipulating the amplitudes and phases of optical frequency components within the SC pulse is presented for coherence applications.     

Ultrahigh-resolution coherence spectroscopy by means of periodic excitation with picosecond pulses

The feasibility of ultrahigh-resolution coherence spectroscopy with a train of picosecond light pulses from a synchronously pumped mode-locked and cavity-dumped dye laser is demonstrated. Narrow resonances of periodic pulse excitation are observed whenever the pulse repetition rate of the laser of a higher harmonic coincides with the frequency splitting of coherently excited levels. The zero-field hyperfine splitting of the Na-ground state (1771.6 MHz) could be measured in the 2100th order of the excitation rate with a resonance halfwidth of 800 Hz.     

Tracking coherent low frequency vibrational information of Rh101 in ground and excited electronic states by broadband transient grating spectroscopy

Time-and frequency-resolved broadband transient grating(BB-TG) spectroscopy is used to distinguish between ground-and excite-electronic state vibrational coherence at different wavelengths. Qualitative theoretical analysis using double-sided Feynman diagrams indicates that a superposition of ground and excited state vibrational coherence are contained in the ground state absorption(GSA) and stimulated emission(SE) overlap band, while only the excited state is contained in the excited state absorption(ESA) band. The TG experiment, in which a white light continuum(WLC) is adopted as a probe, is conducted with rhodamine101(Rh101~+) as the target molecule. Fourier analysis of TG dynamics in a positive delay time range at specific wavelengths enables us to distinguish the low-frequency vibrational modes of Rh101 in ground-and excite-electronic states.     

Long-term carrier-envelope phase coherence

The carrier-envelope phase of the pulse train emitted by a 10-fs mode-locked laser has been stabilized such that carrier-envelope phase coherence is maintained for at least 150 s (measurement limited). The phase coherence time was measured independently of the feedback loop.     

A new approach for simultaneous measurement of ADC and T2 from echoes generated via multiple coherence transfer pathways

The study of rotational and translational diffusion requires the measurement of both T2 and apparent diffusion coefficient (ADC), quantities that are typically measured in separate experiments. The exploitation of echoes generated via multiple coherence transfer pathways offers an opportunity for measuring T2 and ADC values simultaneously in a single experiment. A series of RF pulses can generate multiple echoes via different coherence pathways with each one being uniquely encoded. Here, we demonstrate one pulse sequence that uses an initial theta; RF pulse to generate three coherence orders (C = 0, -1, +1). In the particular version of the method discussed here only two are used (C = 0, +1). Each order is encoded with a different b value from which the ADC is derived. The coherence order echo C = 0 is refocused to quantify T2. The performance of the method--dubbed simultaneous measurement of ADC and relaxation time (SMART)--is demonstrated on a set of samples differing in T2 and ADC achieved by varying the relative volume fractions in mixtures of gadolinium-doped H2O and D2O. The regional SMART derived T2 and ADC agree well with those obtained with conventional double-spin-echo and pulsed gradient spin-echo methods.     

Control of de-excitation to selected vibrational levels in the ground state of NaH molecule using two broadband ultrashort pulses

We show that the de-excitation to different vibrational levels of the ground state in NaH molecule can be controlled by using two delayed ultrashort pulses (4 fs Gaussian). A vibrational wave packet generated on the excited A¹Σ⁺ state by the first pulse is de-excited back to the ground state by a second pulse after a time delay. The cross-section for de-excitation of the wave packet to different vibrational levels of the ground electronic state can be controlled by controlling the delay time between the two pulses as well as by choosing a pulse duration much shorter than the vibrational period of the molecule, such that the de-excited wave packet remains localized in the Franck–Condon region of a particular vibrational level of the ground state. Hence, the de-excitation to a particular vibrational level can be enhanced by suppressing that in others. In spite of the large bandwidth of the pulse which includes nine vibrational levels of the upper state and five vibrational levels of the ground state, one can selectively de-excite the molecule to any one or two vibrational levels of the ground state by carefully choosing the delay time between the pulses and the pulse duration. We are designing the wave packet in the ground state by two short pulses and selectively distributing the population in one or two levels at various values of the delay time. In light molecules having small vibrational period, this selectivity in de-excitation to one or two vibrational levels in the ground state can be achieved only by using ultrashort (4 fs) pulses in the presence of which the localization of the wave packet in the Franck–Condon region of the vibrational levels are particularly possible. It has been shown that the de-excitation cross-section to a particular vibrational level oscillates with delay between the pulses which can be realized as a time-dependent quantum gate.     

幅度调制器对宽带低相干光时频特性的影响

通过改变马赫-曾德干涉型幅度调制器的射频系数和偏置电压,调制光脉冲的强度。研究了幅度调制器对宽带低相干光时频特性影响的规律,分析调制后的光脉冲时域波形分布、光谱和复相干度模值曲线,结果表明,射频系数对光脉冲的光谱成分和时间相干性无明显的调制,射频系数存在最佳工作区间使得输出光脉冲的波形保真度最佳。当偏置电压处于半波电压时,光脉冲的时域波形保真度最好,时间相干性最低,但光谱成分会缺失。理论仿真了调制器的臂长差、偏置电压对宽带低相干光频域特性的影响,与由实测光谱计算出调制器的臂长差,实验结果进行了对比,结果基本符合。由于实际的电光重叠积分因子随加载电压值变化,因而模拟与实测结果存在误差,但研究得出的规律将为低相干脉冲精密整形系统提供更为明确的方向。     

高非线性光纤正常色散区利用皮秒脉冲产生超连续谱的相干特性

使用复互相干度的定义对时域光波分裂前后以及不同输入噪声、不同初始啁啾和波形下抽运脉冲在高非线性光纤中产生的超连续谱的相干度进行了数值计算,得到了光波分裂前后和不同输入噪声下生成的超连续谱的演化和相干性变化,结果表明:皮秒脉冲在高非线性光纤正常色散区产生超连续谱的相干性主要受到系统中噪声占比的影响,其中由光波分裂生成的频谱旁瓣的相干度低于由自相位调制生成的中心频谱的相干度:抽运脉冲啁啾和波形对在高非线性光纤正常色散区产生超连续谱的相干性的影响不明显,若想获得高相干的超连续谱,需要采用低噪声的脉冲进行抽运;若获得大谱宽高相干度的超连续谱,则需要合理选择皮秒脉冲的功率。     

Investigations on coherence of stimulated Brillouin scattering excited by a single-mode-pulsed laser

We experimentally studied the temporal coherence of stimulated Brillouin scattering (SBS) excited by a single-longitude-mode laser. We found that the temporal coherence of SBS is variable but not fixed. In a certain range above the threshold pump energy in which the pulse of the Stokes component is Fourier limited, the coherence length is considerably long, although the temporal coherence decreases with increasing pulse compression rate. If the Fourier transform condition is not satisfied for the pulse of the Stokes component, the temporal coherence decreases rapidly, and the coherence length is much shorter.     

Role of coherence in interaction of opticalradiation with macromolecules

Based on analysis of the properties of macromolecules in a coherent optical radiation field and taking into account experimentally established data on the specificity of the interaction between laser radiation and biomolecules (the dependence of the efficacy of the interaction on the coherence length, the presence of an effect in a region of the spectrum far away from the absorption band), we propose a mechanism for wave interaction of coherent optical radiation with macromolecules, and we construct a very simple mathematical model for such interaction. Using the mathematical model, we calculate the dependence of the vibrational energy of the macromolecule in a coherent radiation field on the coherence time and the intramolecular relaxation rate. We show that the increase in the vibrational energy of the macromolecules strongly depends on the radiation coherence length. When exposed to incoherent radiation, the vibrational energy of the biomolecules remains practically constant, while when exposed to laser radiation (coherence length ≈3 cm), the vibrational energy of the atoms increases by 2–4 orders of magnitude, leading to a change in the conformation of the biomolecules and the activity of enzymes. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 225–231, March–April, 2006.     

Full Bandwidth Measurement of Supercontinuum Spectral Phase Coherence in Long Pulse Regime

Abstract

This article reports that spectral phase coherence in the supercontinuum in long pulse regime can be measured simply and effectively by using an interference technique with the help of a Mach–Zehnder interferometer. It is also demonstrated that chromatic dispersion on the fringe visibility of interference spectral patterns is overcome in the setup. The technique is applied to characterize supercontinuum spectral phase coherence in a highly non-linear optical fiber with different input conditions: unseeded, coherent seeded, and incoherent seeded picosecond pumps. The results confirm the phase coherence characteristic predicted theoretically in previous studies.