Effect of intensity dependent higher-order dispersion on femtosecond pulse propagation in quantum well waveguides

We have studied theoretically the dispersion of ultrafast coherent pulses in GaAs/AlGaAs quantum well waveguide structures as a function of optical intensity. Semiconductor Bloch equations are used to obtain the polarization induced in the medium due to an incident Gaussian electromagnetic beam. The partial differential equation describing the pulse propagation in the presence of group velocity dispersion is used to analyze the role of higher-order dispersion on femtosecond pulse propagation in the waveguide. Due consideration has been given to the intensity dependent optical susceptibility of the medium. The results of the numerical analysis manifest significant influence of higher-order dispersion on femtosecond pulse propagation over short waveguide distance.     

Frequency conversion of femtosecond laser pulses in an engineered aperiodic poled optical superlattice

We theoretically propose a procedure based on a cascading genetic algorithm for the design of aperiodically quasi-phase-matched gratings for frequency conversion of optical ultrafast pulses during difference-frequency generation. By designing the sequence of a domain inversion grating, different wavelengths at the output idler pulse almost have the same phase response, so femtosecond laser pulses at wavelength 800 nm can be shifted to other wavelengths without group-velocity mismatch.     

Study of the excess noise associated with demodulation of ultra-short infrared pulses

The demodulation of ultra-short light pulses with photodetectors is accompanied by excess phase noise at the pulse repetition rate and harmonics in the spectrum of the photocurrent. The major contribution to this noise is power fluctuations of the detected pulse train that, if not compensated for, can seriously limit the stability of frequency transfer from optical to microwave domain. By making use of an infrared femtosecond laser, we measured the spectral density of the excess phase noise, as well as power-to-phase conversion for different types of InGaAs photodetectors. Noise measurements were performed with a novel type of dual-channel readout system using a fiber coupled beam splitter. Strong suppression of the excess phase noise was observed in both channels of the measurement system when the average power of the femtosecond pulse train was stabilized. The results of this study are important for the development of low-noise microwave sources derived from optical "clocks" and optical frequency synthesis.     

Gauss-Legendre quadrature method used to evaluate the spatio-temporal intensity of ultrashort pulses in the focal region of lenses

We analyze the spatio-temporal intensity of sub-20 femtosecond pulses with a carrier wavelength of 810 nm along the optical axis of low numerical aperture achromatic and apochromatic doublets designed in the IR region by using the scalar diffraction theory. The diffraction integral is solved by expanding the wave number around the carrier frequency of the pulse in a Taylor series up to third order, and then the integral over the frequencies is solved by using the Gauss-Legendre quadrature method. The numerical errors in this method are negligible by taking 96 nodes and the computational time is reduced by 95% compared to the integration method by rectangles. We will show that the third-order group velocity dispersion (GVD) is not negligible for 10 fs pulses at 810 nm propagating through the low numerical aperture doublets, and its effect is more important than the propagation time difference (PTD). This last effect, however, is also significant. For sub-20 femtosecond pulses, these two effects make the use of a pulse shaper necessary to correct for second and higher-order GVD terms and also the use of apochromatic optics to correct the PTD effect. The design of an apochromatic doublet is presented in this paper and the spatio-temporal intensity of the pulse at the focal region of this doublet is compared to that given by the achromatic doublet.     

Femtosecond laser pulse propagation in a uniaxial crystal

Abstract

The wave equation describing the vector propagation of a femtosecond laser pulse of a few optical cycles in a uniaxial crystal is solved numerically by the method of unidirectional waves. Propagation of the pulse in the direction normal to the optical axis is studied, taking into account both second- and third-order nonlinearities of the crystal. Conversion efficiency as a function of crystal length, pump intensity and pulse duration is studied. As an example, the propagation of femtosecond laser pulse of τ = 10 fs duration at λ = 810 nm in a LiNbO₃ crystal 12 μm thick is studied numerically.     

Mid/far-infrared few-cycle-pulse emission via resonant mixing in semiconductor heterostructures

Abstract

Mid/far-infrared emission from a semiconductor multiple quantum well structure under femtosecond optical pulse excitation is studied. It is shown that resonant nonlinear-wave mixing in the quantum wells can be used for the generation of ultra-short mid/far-infrared pulses with a duration of a few cycles or even a single cycle. Explicit analytical formulas for the mid/far-infrared radiation field and polarization in a simple three-level model of a quantum well are presented and compared with numerical simulations. The power of the mid/far-infrared emission and the down conversion efficiency of the resonant nonlinear-wave mixing are discussed.     

Temporal superresolution: An application of frequency filtering by a grating in the resonance domain

Abstract

A diffraction grating in the resonance domain is known to exhibit significant change in diffraction efficiencies with a small change of the grating parameters. It is proposed that this property can be utilized for frequency filtering, when polychromatic light illuminates the grating. As an example, compression of a femtosecond optical pulse is numerically demonstrated with the concept of superresolution. Suppression of zeroth diffraction order by suitably optimized grating structure induces the pulse width to narrow. This scheme considerably simplifies existing optical pulse shaping systems.     

Seeded femtosecond optical parametric amplification in the mid-infrared spectral region above 3 mum

Femtosecond optical parametric amplification that results in microjoule mid-infrared pulses at wavelengths exceeding 3 mum is demonstrated. Narrow-band quasi-cw seeding at the signal wavelength is applied to ensure the generation of nearly transform-limited femtosecond pulses at the idler wavelength. The broad bandwidth of the parametric amplification provided by pumping with femtosecond pulses from a Ti:sapphire regenerative amplifier at high intensity results in idler pulse durations shorter than the pump pulse length. The potentials of three nonlinear optical crystals that belong to the potassium titanyl phosphate family are comparatively studied. At 1-kHz repetition rate our all-solid-state system produces highly synchronized ~100-fs pulses in the spectral range between 3 and 4 mum.     

Above threshold dissociation in HD+ using frequency chirped laser pulses

We have theoretically studied the dynamics of above threshold dissociation (ATD) in molecular ions HD⁺ using frequency chirped femtosecond laser pulses from numerical solutions of the time-dependent Schrödinger equation by using the three-dimensional time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by an asymptotic-flow expression in momentum space. Linearly positive and negative frequency chirped laser pulses are adopted. It is found that varying frequency chirped parameters can change branching ratios of the 1sσ _g and 2pσ _u dissociations channels. The concept of a light-induced potential is used to interpret the ATD process. The angular resolved energy distributions of the photofragments are also illustrated.     

Short-pulse properties of optical frequency comb generators

An optical frequency comb generator, based on a simple electro-optic modulator in an optical resonator, can produce high-repetition-rate picosecond pulses. Unlike conventional picosecond lasers, the properties of these pulses are greatly affected by detuning the optical cavity and by dispersion caused by the electro-optic crystal. Picosecond pulses were studied in a physical device by numerical simulation and intensity autocorrelation measurements. The pulse width and pulse-to-pulse spacing were greatly affected by detuning the input laser frequency and the resonance of the optical resonator, and the numerical simulations showed that dispersion causes temporal ripples that are antisymmetric between pulse pairs.     

Phase-transitions in GeSb induced by customized ultrafast optical pulses

The use of ultrafast laser pulses to initiate solid-state phase-transitions in certain materials has shown promise in achieving sub-nanosecond phase changes with different optical properties. These phase changes have been well studied using pulse durations between femtoseconds and nanoseconds to determine the dynamics for the reversible phase changes on multiple time scales. In this study femtosecond pulse shaping techniques, driven by evolutionary algorithms, were used to obtain optimized temporally shaped ultrashort laser pulses to induce and control permanent phase changes in GeSb thin-films. Through monitoring the pulse effects it has been determined that the crystalline-to-amorphous phase transition is minimized using optical pulses with pulse widths less than the electron-phonon coupling time. It is maximized by using pulses longer than the time required for energy transfer from the excited carriers to the lattice.     

Diffraction characteristics of spatial and temporal Gaussian-shaped femtosecond laser pulse by rectangle reflection grating

The exact intensity distribution expression for the spatial and temporal Gaussian-shaped femtosecond laser pulse diffracted by a rectangle reflection grating is derived. The spatial and temporal diffraction characteristics are theoretically investigated in detail, and a criterion for judging whether or not the diffraction pulse is just split into two independent pulses in the temporal domain is obtained. The results show that the diffraction intensity in the temporal domain consists of three parts: the intensity diffracted by the upper reflection surface of the grating, the intensity diffracted by the nether reflection surface, and their temporal coherent intensity. The temporal coherent intensity becomes weaker, even is zero, for the higher height from the nether surface to the upper surface of the grating. The principal maximum becomes more sharply bright for the bigger waist width of the femtosecond laser pulse in the spatial domain.     

All-optical synchronization of self-pulsating laser diodes with frequency multiplication and division

Abstract

We investigate optical synchronization with frequency multiplication and division of self-pulsating laser diodes. Using a simple rate equation model to describe self-pulsation we show that synchronization with frequency multiplication can be accompanied by large variations in the period of the emitted laser pulses over the injected pulse cycle. These variations are not obvious in the frequency domain treatment of synchronization. By applying a frequency division synchronizing signal to our model, we also show that the sensitivity of the phase difference between the injected signal and laser emission to power increases with increasing frequency division ratio and leads to the presence of power synchronization ranges.     

Direct measurement of photon number statistics at telecom wavelengths using a charge integration photon detector

In optical quantum information technology, a photon number resolving detector (PNRD) is the basic device for developing photonic quantum computers. The demands for the PNRD are high quantum efficiency and wide dynamic range. We have developed a charge integration photon detector (CIPD) with a quantum efficiency of 80% at telecom wavelengths. The repetition rate of the CIPD is as low as 40 Hz at present, but it can be applied for measurement of short pulses. We report the capability of the CIPD for multiphoton counting over 10 photons, its responsivity to the short pulses, and its high linearity using a binary intensity modulated short pulse (2 ns) train and simultaneous irradiation of two kinds of pulses.     

Complete characterization of a self-mode-locked ti:sapphire laser in the vicinity of zero group-delay dispersion by frequency-resolved optical gating

The intensity and the phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser operating in the vicinity of zero group-delay dispersion (GDD) have been completely characterized by the technique of frequency-resolved optical gating (FROG). For small values of negative GDD, the appearance of a dispersive wave in the pulse spectrum is manifested in the measured FROG trace, and pulse retrieval directly shows its association with a broad leading-edge pedestal. For positive GDD, we confirm previous experimental observations of picosecond pulses with large positive chirp and report a new operating regime in which the output pulses are of picosecond duration but are intensity modulated at 20 THz. The physical origin of this modulation is discussed by analogy with similar effects observed during pulse propagation in optical fibers, and the experimental results are compared with a model of intracavity four-wave mixing about the cavity zero GDD wavelength.     

Intersubband Rabi oscillations in asymmetric nanoheterostructures: implications for a tunable continuous-wave source of a far-infrared and THz radiation

A tunable continuous-wave source of a far-infrared and THz radiation based on a semiconductor nanoheterostructure with asymmetric quantum wells is suggested. It utilizes Rabi oscillations at a transition between quantum well subbands excited by external femtosecond pulses of a mid-infrared electromagnetic field. Due to quantum well broken inversion symmetry the subbands possess different average dipole moments, which enables the creation of polarization at the Rabi frequency as the subband populations change. It is shown that if this polarization is excited so that it is periodic in space, then, though being pulsed, it can produce continuous-wave output radiation. Changing the polarization space period and the time intervals between the exciting pulses, one can tune the frequency of this radiation throughout the far-infrared and THz range. In the present work a concrete multiple quantum well heterostructure design and a scheme of its space-periodic polarization are suggested. It is shown that for existing sources of mid-infrared femtosecond pulses the proposed scheme can provide a continuous-wave output power of order the power of far-infrared and THz quantum cascade lasers. Being added to the possibility of its output frequency tuning, this can make the suggested device attractive for fundamental research and various applications.     

Dynamics and interference of fine-structure wave packets created by strong ultrashort pulses

This work presents a theoretical study of two coherent schemes which allow the manipulation of wave packets created in atomic systems by strong ultrashort pulses. Our three-state system is composed of a ground state and two excited states simultaneously excited by the laser pulses. The corresponding dynamics are described in the bright state-dark state formalism where spectacular effects appear. (1) The wave packet created initially by a first pulse can be completely frozen through the action of a second strong pulse. (2) For pulses with a generalized pulse area equal to (2 p +1)2π (p integer), the population is preferentially transferred from the ground state to the dark state whatever the pulse duration. This is in complete disagreement with the physical interpretation valid in the weak field regime where the wave packet created by an ultrashort pulse is localized in the bright state at the end of the pulse. This effect can be revealed with wave packet interference induced by a second identical pulse. A simple analytical model using squared pulses is used to highlight the physical insight. This model is ‘reinforced’ by numerical simulations on the (4s–4p ² P 1/2,1/3) transitions in potassium atoms excited with Gaussian pulses.     

半导体可饱和吸收镜的光损伤阈值测量

半导体可饱和吸收镜(SESAM)在飞秒脉冲激光器锁模中，是一种非常有潜力的锁模启动器．其损伤闽值的高低与连续锁模阈值比较相近，极易损伤，因而研究在飞秒激光作用下SESAM的损伤阈值很有必要．利用飞秒激光分别对单晶硅、自然生长SESAM及腐蚀后SESAM在50fs、200fs和400fs脉宽下进行了表面烧蚀研究，并且保证每次烧蚀的激光脉；中个数为50个．结果发现单晶硅和自然生长SESAM的损伤阈值要高于腐蚀乓SESAM，随脉宽的增加而逐渐增大；而腐蚀后SESAM的损伤阈值却随脉宽的增加而逐渐减小．     

On the Theory of Stimulated Raman Radiation in an Optical Resonator

Stimulated Raman radiation involving an arbitrary number of Stokes components in an optical resonator is considered for the case when a pumping wave is directly incident on a Raman-active condensed substance at an angle θ to the resonator axis. The expressions are derived for the steady-state radiation intensities of all the Stokes components generated, and the number of the components versus the pumping wave intensity is discussed. The conditions of the effective conversion of the pumping wave into radiation of Stokes components are elucidated. It is shown that in a frequency region close to the point ?² n/? ω² = 0 (n is the effective refractive index of the substance inside the resonator) locking Stokes components is feasible due to a ‘tunnel’ traversal of femtosecond pulses through a dispersive medium, provided the latter occupies a characteristic tunnel length along the resonator axis. For the case of the three components generated, the wave band over which locking occurs is derived and a frequency modulation of the first and the third Stokes components beyond this band is predicted. The possibility of an experimental indication of the band discussed, due to a discontinuous disappearance of the frequency modulation in the band, is noted. For locking four Stokes components the possibility is shown of the change of their stable phase relation on variation of the pumping wave intensity.     

Ultrafast coherent population transfer with a train of weak femtosecond pulse pairs

We investigate ultrafast coherent population transfer in a Λ-type three-level system driven by a train of weak phase-locked pump-Stokes femtosecond pulse pairs, where the pump and Stokes pulses in each pair are applied in counterintuitive order, similar to that in the stimulated Raman adiabatic passage technique. It is shown that due to temporal constructive quantum interference and the subsequent coherent accumulation, complete population transfer can be achieved by a few pulse pairs with suitable interpair and intrapair time delays even if a single pair of pulses in the train is so weak that nearly no population transfer can take place. The method is efficient and robust to moderate variations in the interpair and intrapair time delays, number of pulse pairs, and laser intensity, which is particularly favorable to coherent population transfer in quantum systems with weak oscillator strengths.