Compression of single-cycle mid-infrared pulses by Raman-active molecular modulators

We show theoretically how high-order stimulated Raman scattering in the impulsive pump-probe regime can be used for generation of single mid-infrared (MIR) single-cycle pulses. The propagation of MIR probe pulses in a hollow waveguide filled with a Raman-excited gaseous medium, with a probe delay in the maximum of the molecular oscillations, results in spectral broadening covering almost 2 octaves. The spectral phases of this broadening can be compensated for by use of an output glass window with anomalous dispersion in the MIR. The spectral and temporal characteristics of the output pulses and the mechanism of pulse compression are studied by use of numerical and analytical solutions, and compression of a 70-fs input pulse at 4 microm to a single-cycle 6.5-fs output pulse is shown.     

Generation of a single-cycle optical pulse

We make use of coherent control of four-wave mixing to the ultraviolet as a diagnostic and describe the generation of a periodic optical waveform where the spectrum is sufficiently broad that the envelope is approximately a single-cycle in length, and where the temporal shape of this envelope may be synthesized by varying the coefficients of a Fourier series. Specifically, using seven sidebands, we report the generation of a train of single-cycle optical pulses with a pulse width of 1.6 fs, a pulse separation of 11 fs, and a peak power of 1 MW.     

Ionization-assisted guided-wave pulse compression to extreme peak powers and single-cycle pulse widths in the mid-infrared

Ionization-assisted spectral broadening of high-energy 10.6 μm laser pulses in a gas-filled hollow waveguide is shown to yield single-cycle pulses with multiterawatt peak powers in the mid-IR. While the highest quality of pulse compression is achieved in the regime of weak ionization, careful management of complex ionization-assisted spectral broadening of guided-wave fields is the key to compressing the output of advanced high-power mid-IR laser sources to single-cycle pulse widths.     

Amplitude and chirp characterization of high-power laser pulses in the 5-fs regime

Frequency-resolved optical gating (FROG) based on second-harmonic generation has been demonstrated to be capable of high-fidelity measurement of the electric-field envelope and of the temporal evolution of the instantaneous carrier frequency of 0.1-TW 5-fs pulses without the need for any correction for systematic experimental errors. At a 1-kHz repetition rate, pulse energies of a few microjoules are sufficient for reliable FROG characterization of pulses with durations down to the single-cycle regime. The results obtained reveal that carefully designed hollow-fiber chirped-mirror compressors are able to deliver high-power sub-10-fs pulses with a smooth Gaussianlike leading edge that has an intensity contrast of approximately 10(-2) .     

Pulse synthesis in the single-cycle regime from independent mode-locked lasers using attosecond-precision feedback

We report the synthesis of a nearly single-cycle (3.7?fs), ultrafast optical pulse train at 78?MHz from the coherent combination of a passively mode-locked Ti:sapphire laser (6?fs pulses) and a fiber supercontinuum (1-1.4?μm, with 8?fs pulses). The coherent combination is achieved via orthogonal, attosecond-precision synchronization of both pulse envelope timing and carrier envelope phase using balanced optical cross-correlation and balanced homodyne detection, respectively. The resulting pulse envelope, which is only 1.1 optical cycles in duration, is retrieved with two-dimensional spectral shearing interferometry (2DSI). To our knowledge, this work represents the first stable synthesis of few-cycle pulses from independent laser sources.     

Generation of 30 microJ single-cycle terahertz pulses at 100 Hz repetition rate by optical rectification

We report the generation of 30 microJ single-cycle terahertz pulses at 100 Hz repetition rate by phase-matched optical rectification in lithium niobate using 28 mJ femtosecond laser pulses. The phase-matching condition is achieved by tilting the laser pulse intensity front. Temporal, spectral, and propagation properties of the generated terahertz pulses are presented. In addition, we discuss possibilities for further increasing the energy of single-cycle terahertz pulses obtained by optical rectification.     

Gouy shift and temporal reshaping of focused single-cycle electromagnetic pulses

We discuss exact solutions of Maxwell's equations that describe the evolution of single-cycle electromagnetic pulses. The solutions are applied to recent observations of the diffraction transformation of terahertz pulses. In particular, we elucidate the role of the Gouy shift in the temporal reshaping and polarity reversals of single-cycle terahertz pulses.     

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.     

Nonlinear cross-phase modulation with intense single-cycle terahertz pulses

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.     

宽带光学参变啁啾脉冲放大系统的色散控制

在设计宽带光学参变啁啾脉冲放大系统时,对色散源进行了理论分析和讨论。对光学参变啁啾脉冲放大系统的色散量以及各个过程中的高阶色散对脉冲时域和频域特性的影响进行了计算。结果表明展宽器引入了非常大的色散,放大器中信号光相位变化产生的色散较大,参量晶体本身引入的色散相对较小可以忽略不计。输出脉冲宽度主要受二阶色散影响,而三阶和四阶色散主要影响脉冲的波形和信噪比,对频域影响很小。     

Simulation of Ultrashort Laser Pulse Propagation in Silica Fibre by FDTD+

The finite-difference time-domain algorithm is extended to solving the Maxwell's equations with nonlinear terms which include all exact Sellmeier-fitting values in this paper. The results obtained by the presented method are in good agreement with those from experiments, and this method is even better than solving the generalized nonlinear Schrödinger equation by split-step Fourier method. The spectral phase of propagated pulses is also obtained by the presented method, which is important for investigation of single-cycle optical pulse generation via phase compensation of the nonlinear-chirped in silica fibers.     

相干原子系统的失谐对脉冲调制的影响

研究了一对非共振脉冲在相干介质中的时空传播,着重考究了失谐对相干脉冲相位调制的影响。计算结果表明当脉冲在介质中稳定传播时,脉冲的最终振幅与双光子脉冲共振的情况下相同,只由介质的初始状态决定,与失谐无关。而失谐仅仅只对脉冲的相位进行调制。     

Nonlinear Dynamics of Wave Fields in Nonresonant Media: From Envelope Solitons toward Video Solitons

We analyze a new class of soliton solutions for a wave field, which describes propagation of soliton-like structures of a circularly polarized electromagnetic field comprising a finite number of field-oscillation periods in a transparent nonresonant medium. The considered solutions feature a smooth transition from the soliton solutions of Schröodinger type, which correspond to long pulses with a large number of field oscillations, to extremely short, virtually single-cycle video pulses. We show that such solutions can also be important for linearly polarized laser fields. The structural stability of few-optical-cycle solitons is demonstrated numerically, including the case of their collision. Based on stability analysis and with allowance for the genealogic relation between the obtained wave solitons and the solitons of the nonlinear Schröodinger equation, we argue that the former solitons can play the same fundamental role in the nonlinear dynamics of the considered wave fields. In particular, it is shown by numerical simulations that the few-optical-cycle solutions turn out to be the basic elementary components of such a dynamical process as the temporal compression of an initially long pulse to a pulse of very short duration. In this case, the minimum duration of a compressed pulse is determined by soliton structures of about minimal duration.     

Solitons evolving toward few-and single-cycle pulses in photonic-crystal fibers

Numerical analysis of soliton dynamics in highly nonlinear photonic-crystal fibers reveals intriguing scenarios enabling efficient pulse compression down to single-cycle field waveforms. We derive simple analytical expressions relating the pulse width and the energy of ultrashort pulses generated through such a field-evolution dynamics to the fiber dispersion and nonlinearity.     

Tunable optimal compression of ultrabroadband pulses by cross-phase modulation

We show how cross-phase modulation between two pulses, combined with optimal pulse shaping at the input of a dielectric medium, can be used to generate nearly single-cycle pulses that are tunable from the ultraviolet to the mid-infrared at the output of the medium, precompensating for dispersion to all orders.     

Spectral-interferometric method for retrieving femtosecond pulse envelope

We propose a new, spectral-interferometric, technique of complete characterization of ultrashort pulses. The technique is based on employing, as a reference, a sub-parabolic pulse with a known phase formed in a single-mode fiber from the pulse under study. The proposed method has been tested experimentally with femtosecond pulses. Initial phases and temporal profiles of intensity of tested pulses have been restored.     

Noncausal time response in frustrated total internal reflection?

Tunneling of photons in frustrated total internal reflection has been studied in the time domain with single-cycle femtosecond pulses. It is seen that both the phase and energy of the pulse travel faster than the speed of light in vacuum. Theoretical analysis of the experiments shows that the time-response function for electromagnetic waves propagating in the air gap is noncausal. However, it is found that superluminal signal propagation is not possible in this case because of the inevitable diffractive spreading of the signal beam.     

Scattering of an ultrashort electromagnetic radiation pulse by an atom in a broad spectral range

The scattering of an ultrashort electromagnetic pulse by atomic particles is described using a consistent quantum-mechanical approach taking into account excitation of a target and nondipole electromagnetic interaction, which is valid in a broad spectral range. This approach is applied to the scattering of single- and few-cycle pulses by a multielectron atom and a hydrogen atom. Scattering spectra are obtained for ultrashort pulses of different durations. The relative contribution of “elastic” scattering of a single-cycle pulse by a hydrogen atom is studied in the high-frequency limit as a function of the carrier frequency and scattering angle.     

Strong field ionization by ultrashort laser pulses: Application of the Keldysh theory

Analytical expressions and numerical results describing ionization of atoms by intense linearly polarized ultrashort laser pulses are obtained in the frame of the Keldysh approach. Photoelectron spectra and total ionization probabilities are presented for several analytical models of a single-cycle laser pulse. In particular, strong left-right asymmetry of the spectra is shown for the case of odd pulses.     

Spectral shifts as a signature of the onset of diffusion of broadband terahertz pulses

We describe measurements of polarization dynamics as a probe of multiple scattering of photons in a random medium by use of single-cycle terahertz pulses. We measure the degree of polarization and correlate it directly with the single-scattering regime in the time domain. We also measure the evolution of the temporal phase of the radiation and show that the average spectral content depends on the state of polarization. In the case of broadband radiation, this effect can be used to distinguish photons that have been scattered a few times from those that are propagating diffusively.