Phase-modulated Laser Pulses Research Articles

Eliminating molecular-alignment dephasing with a phase-modulated femtosecond laser pulse

Alignment dephasing caused by centrifugal distortion is a significant effect accompanying laser-induced field-free molecular alignment. Centrifugal distortion is the manifestation of the intrinsic nonrigidity of molecules, which is especially prominent when molecules are excited to high angular velocities. In this work, we show that this type of dephasing can be almost completely eliminated by modulating the phase of the femtosecond laser pulse. Nearly perfect molecular alignment can be achieved as if the molecules were ideal rigid rotors. The dephasing effect can be cancelled in various time windows by tuning the modulation strength parameter. The dephasing cancellation mechanism is explained by considering the relative phases of the eigenstates forming the rotational wave packet. This work is of great significance to the experiment and dynamics studies of molecular alignment.

Propagation of ultra-short dark soliton pulses in an isotropic medium under the influence of third-order linear dispersion and nonlinearity dispersion

The nonlinear Schrödinger equation (NSE) is well-known and is one of the most commonly used in the field of nonlinear optics. Pulses with picosecond and femtosecond duration are very well described by the NSE, but it does not work for attosecond and phase-modulated femtosecond laser pulses, where the spectral width of the pulse is of the order of the carrying frequency. In such cases, it is more convenient to use the general nonlinear amplitude equation (NAE). During the propagation of ultra-short pulses in optical fibers, the effects of the third-order linear dispersion and dispersion of nonlinearity become significant and have to be taken into account. We expanded the NAE by including these two effects. In this paper, we present an analytical study on the influence of third-order linear dispersion and dispersion of nonlinearity on the evolution of ultra-short optical pulses and on the possibility of dark soliton formation under such conditions in a medium with normal dispersion.

Radiation of a Resonant Medium Excited by a Periodically Phase-Modulated Laser in the Regime of Strong Coupling between the Field and the Matter

The radiation of a two-level resonant medium that is placed into a cavity and is excited by a periodically phase modulated laser pulse is studied theoretically. The radiation is analyzed based on the analytical and numerical solutions of the system of Maxwell–Bloch equations under conditions of a strong coupling between the field and the matter. Under these conditions, this system is similar to a polariton laser. The excitation efficiency of a polariton laser by a phase-modulated radiation pulse is shown to be high compared to a pulse without phase modulation of the carrier frequency. It is shown that the main reason for the efficient excitation of polariton modes in the medium is the appearance of difference combination parametric resonance. The obtained results open up new possibilities for the excitation of radiation of polariton lasers by low-power frequency-modulated pump laser radiation.

Optimal and robust control of quantum state transfer by shaping the spectral phase of ultrafast laser pulses.

Achieving fast and efficient quantum state transfer is a fundamental task in physics, chemistry and quantum information science. However, the successful implementation of the perfect quantum state transfer also requires robustness under practically inevitable perturbative defects. Here, we demonstrate how an optimal and robust quantum state transfer can be achieved by shaping the spectral phase of an ultrafast laser pulse in the framework of frequency domain quantum optimal control theory. Our numerical simulations of the single dibenzoterrylene molecule as well as in atomic rubidium show that optimal and robust quantum state transfer via spectral phase modulated laser pulses can be achieved by incorporating a filtering function of the frequency into the optimization algorithm, which in turn has potential applications for ultrafast robust control of photochemical reactions.

Quantum memory in an orthogonal geometry of silenced echo retrieval

We experimentally realize a quantum-memory protocol based on retrieval of silenced echo (ROSE) in Tm3+:Y3Al5O12 crystal in an orthogonal geometry of the signal and control light fields. The silenced echo signal revival efficiency of ~13% with 36 μs storage time is demonstrated. To achieve that we implemented a high-precision atomic coherence control via amplitude- and phase-modulated laser pulses. We also discuss capabilities of this configuration, ways to increase quantum efficiency and to combine it with a single-mode optical cavity.

Tunable second harmonic generation by phase-modulated ultrashort laser pulses

We report on the generation of tunable light around 400 nm by frequency-doubling ultrashort laser pulses whose spectral phase is modulated by a sum of sinusoidal functions. The linewidth of the ultraviolet band produced is narrower than 1 nm, in contrast to the 12 nm linewidth of the non-modulated incident spectrum. The influence of pixellation of the liquid crystal spatial light modulator on the efficiency of the phase-modulated second harmonic generation is discussed.

4 μJ, 3 ns phase-modulated laser pulse from ytterbium-doped polarization-maintaining large-mode-area fiber amplifier

In the current inertial confinement fusion laser drivers, integrated fiber front-end is widely applied to generate the phase-modulated, linearly polarized laser pulses with an output energy of several nano-joules. In this contribution, we design an amplifier with polarization-maintaining optical components. The amplification of the phase-modulated 3 ns, 0.24 nm pulse is experimentally demonstrated in the ytterbium-doped large-mode-area (11 μm-diameter) fiber amplifier. The pulse energy is amplified to around 4 μJ which is free of pulse distortion and stimulated Brillouin scattering. The modulation degree after optimal compensation of waveplates is less than 6%. The output mode field is diffraction-limited.

Efficient pulse compression and frequency conversion of phase-modulated laser pulses in engineered quasi-phase-matching gratings

The simultaneous generation of the 2nd and 3rd harmonics of a frequency-chirped pulse in a quadratically nonlinear medium encompassing a quasi-phase-matched grating with linearly varying inverse domain size has been numerically studied. The efficient pulse compression and energy conversion have been analyzed taking into account the group velocity mismatch and dispersion.

Parallel Drawing of Multiple Bent Optical Waveguides Using a Spatial Light Modulator

Multiple optical waveguides inside glass were drawn in parallel using multiple light spots, which were formed by focusing a spatially phase-modulated femtosecond laser pulse. Because the spatial phase distribution was modulated by a computer-generated hologram (CGH) on a liquid-crystal spatial light modulator (SLM), bent structures were formed in the drawn waveguides by changing the positions of the light spots by switching the CGHs when the waveguides were being drawn. We confirmed that a He–Ne laser beam can be guided through the fabricated waveguides, and we found that the propagation loss of the waveguides was determined by both the steps for changing the beam position and the number of times the CGHs were switched. We also explain the method we used to reduce the time required to calculate the CGHs used to create bent waveguides.

Extremely broadened high-order harmonics generated by the femtosecond pulses propagating through the filaments in air

We demonstrate a considerable broadening of the harmonics generating in laser plasma using the laser pulses propagating through the filaments in air. A fourfold increase of harmonic bandwidth was achieved in the case of phase-modulated laser pulses, alongside with an increase in harmonic conversion efficiency (from 5×10−6 to 1.2×10−5). We also present the studies of harmonic generation using phase-modulated pulses at different ranges of ellipticity of the polarization of laser beam.

Transmission of phase-modulated laser light through a thin semiconductor film in the excitonic spectral range and an optical analog of the feshbach effect

A theoretical and numerical analysis of the transmission of phase-modulated laser pulses through a thin film with excitonic nonlinearity is performed. It is shown that, by choosing the modulation law, one can efficiently control the optical nonlinearity of the film down to the inversion of the sign of nonlinearity.

Nonlinear pulse compression in inhomogeneous photonic crystals upon backward second harmonic generation

Frequency doubling of phase-modulated laser pulses, which is caused by a quasi-synchronous interaction of counterpropagating waves, is studied theoretically in crystals with an aperiodic domain structure. The simultaneous influence of the change in the domain period and the phase-modulation depth of fundamental radiation on the formation of a second-harmonic pulse is analysed in the nonstationary regime. It is shown that there exists an optimal relation between chirps in an aperiodic crystal and the phase modulation of fundamental radiation at which the maximum nonlinear compression of the second-harmonic pulse duration is possible.

The nature of a nonlinear excitation pathway from the Shockley surface state as probed by chirped pulse two photon photoemission

Phase-modulated femtosecond laser pulses are used to study the spectral response of a non-resonant two photon excitation from the Cu(111) Shockley surface state (SS). Controlled variations in the spectral phase of the laser pulse were introduced using a tuneable Fork prism phase modulator and resulted in a shift in the peak-position (of up to 110 meV), variations in the spectral width (up to 88 meV) and changes in the asymmetry of the SS peak as detected by two-photon photoemission. A satisfactory quantitative model of the experimental results can only be achieved if the complete spectral phase up to the third-order dispersion terms is taken into account. Of particular note, we find that a consistent description of this two photon absorption process does not require coupling of the excitation to an intermediate copper bulk state, which contradicts the previous results of Petek et al (1997 Phys. Rev. Lett. 79 4649).

Degenerate parametric frequency conversion of phase-modulated femtosecond laser pulses in crystals with a regular and chirped domain structure

A systematic theoretical analysis of the degenerate parametric frequency conversion in a LiNbO3 crystal with a regular domain structure and a linearly varying domain thickness (chirped crystal) is presented for the pulses of a titanium-sapphire laser with a wavelength of 0.8 μm and durations of 100 and 50 fs in the presence and in the absence of phase modulation. The results are obtained with regard to the difference in the group velocities of interacting pulses and the group velocity dispersion. For an effective frequency conversion of the phase-modulated (PM) pump pulse, it is expedient to employ chirped crystals in which the domain thickness decreases from the entrance to the exit of the crystal. The pump energy is effectively converted into subharmonic energy when the pump carrier frequency decreases with time. It is demonstrated that the efficiency of the energy conversion to a subharmonic of 80% can be realized for PM pumping with a pulse duration of 100 fs in the chirped LiNbO3 crystal. The efficiency of the parametric frequency conversion depends on the pump intensity as well as on the phase modulation of the pulse and the chirp of the crystal. Note that a variation in one of these parameters causes variations in the remaining parameters needed for the maximum efficiency of the parametric frequency conversion.

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 µm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO3 crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO3 PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.

Nonstationary frequency doubling in periodically-poled nonlinear crystals in the presence of self-action effects

The efficiency of the nonstationary frequency doubling of a phase-modulated ultrashort laser pulse under conditions of strong energy exchange is analyzed in detail taking into account the self-action effects in periodically-poled nonlinear crystals. The calculations were conducted for a LiNbO3 crystal irradiated by a laser with wavelength of 1.06 µm and pulse duration of 500 fs. Simultaneous laser-pulse phase modulation and effects of self-action were found to reduce the efficiency of the process severalfold in comparison with the case where the above effects were not taken into account. It was shown that in the nonstationary-pulse-frequency conversion without phase modulation the frequency-doubling effects could develop at higher orders of quasi-phase matching.

Fibonacci-like photonic structure for femtosecond pulse compression

The compression of femtosecond laser pulses by linear quasiperiodic and periodic photonic multilayer structures is studied both experimentally and theoretically. We compare the compression performance of a Fibonacci and a periodic structure with similar total thickness and the same number of layers, and find the performance to be higher in the Fibonacci case, as predicted by numerical simulation. This compression enhancement takes place due to the larger group velocity dispersion at a defect resonance of the transmission spectrum of the Fibonacci structure. We demonstrate that the Fibonacci structure with the thickness of only 2.8 microm can compress a phase-modulated laser pulse by up to 30%. The possibility for compression of laser pulses with different characteristics in a single multilayer is explored. The operation of the compressor in the reflection regime has been modeled, and we show numerically that the reflected laser pulse is subjected to real compression: not only does its duration decrease but also its amplitude rises.

ON THE PRACTICALITY OF ADIABATIC QUANTUM COMPUTING WITH OPTICAL SCHEMES

A robust implementation of quantum logical gates for a multilevel system is possible through decoherence control under the quantum adiabatic method using simple phase modulated laser pulses. Selective population inversion and Hamiltonian evolution with time through ultrafast pulse shaping techniques essentially amount to adiabatic quantum computing (AQC) instead of the standard unitary transformation. An important aspect of the AQC model is in addressing the atomic or molecular ensemble and hence in robust implementation. We argue that experimental demonstrations of selective population transfer through adiabatic rapid passage form useful adiabatic quantum computing logic. Similarly, a simple Hadamard operation can be demonstrated with phase-modulated laser pulses. Finally, we present a framework to efficiently solve approximate Euclidean Traveling Salesman Problem (Approx-TSP) with bounded error in the AQC model. We present an efficient and intuitive encoding for Approx-TSP in a quantum computing paradigm. Optical approaches to quantum computing have the potential to be used in a distributive sense to defray the present caveat of limited resources and scalability. We present how we make use of such schemes towards practicality issues in AQC. As far as we know, our results are the first realistic demonstration of the possibility of using ensemble states for AQC in multilevel systems.

Theoretical model for ultracold molecule formation via adaptive feedback control

We theoretically investigate pump-dump photoassociation of ultracold molecules with amplitude- and phase-modulated femtosecond laser pulses. For this purpose, a perturbative model for light-matter interaction is developed and combined with a genetic algorithm for adaptive feedback control of the laser pulse shapes. The model is applied to the formation of 85Rb2 molecules in a magneto-optical trap. We find that optimized pulse shapes may maximize the formation of ground state molecules in a specific vibrational state at a pump-dump delay time for which unshaped pulses lead to a minimum of the formation rate. Compared to the maximum formation rate obtained for unshaped pulses at the optimum pump-dump delay, the optimized pulses lead to a significant improvement of about 40% for the target level population. Since our model yields the spectral amplitudes and phases of the optimized pulses, the results are directly applicable in pulse shaping experiments.

Adiabatic quantum computing with phase modulated laser pulses

Implementation of quantum logical gates for multilevel system is demonstrated through decoherence control under the quantum adiabatic method using simple phase modulated laser pulses. We make use of selective population inversion and Hamiltonian evolution with time to achieve such goals robustly instead of the standard unitary transformation language.