Coherent Phase Control Research Articles

Phonon-induced anomalous gauge potential for photonic isolation in frequency space

Photonic gauge potentials are crucial for manipulating charge-neutral photons like their counterpart electrons in the electromagnetic field, allowing the analogous Aharonov–Bohm effect in photonics and paving the way for critical applications such as photonic isolation. Normally, a gauge potential exhibits phase inversion along two opposite propagation paths. Here we experimentally demonstrate phonon-induced anomalous gauge potentials with noninverted gauge phases in a spatial-frequency space, where near-phase-matched nonlinear Brillouin scatterings enable such unique direction-dependent gauge phases. Based on this scheme, we construct photonic isolators in the frequency domain permitting nonreciprocal propagation of light along the frequency axis, where coherent phase control in the photonic isolator allows switching completely the directionality through an Aharonov–Bohm interferometer. Moreover, similar coherent controlled unidirectional frequency conversions are also illustrated. These results may offer a unique platform for a compact, integrated solution to implement synthetic-dimension devices for on-chip optical signal processing.

High-power ultrafast fiber lasers for materials processing

Abstract State-of-the-art fiber-laser systems can deliver femtosecond pulses at average powers beyond the kilowatt level and multi-mJ pulse energies by employing advanced large-mode-area fiber designs, chirped-pulse amplification, and the coherent combination of parallel fiber amplifiers. By using sophisticated coherent phase control, one or even several output ports can be modulated at virtually arbitrary power levels and switching speeds. In addition, an all-fiber setup for GHz-burst generation is described allowing to access an even wider range of laser parameters. The combination of all these approaches together with the robustness, efficiency, and excellent beam quality inherent to fiber-laser technology has the potential to strongly improve existing materials-processing applications.

Coherent Phase Control of Ultrashort Electron Pulses by Traveling Optical Waves and Whispering-gallery Modes

Coherent Phase Control of Ultrashort Electron Pulses by Traveling Optical Waves and Whispering-gallery Modes

Molecular Auger Interferometry

SynopsisWe propose a theory of interferometric measurement of a normal Auger decay width in molecules. Molecular Auger interferometry is based on the coherent phase control of Auger dynamics in a two-colour (ω/2ω) laser field. We show that, in contrast to atoms, in oriented molecules of certain point groups (e.g. CH3F) the relative ω/2ω phase modulates the total ionisation yield. A simple analytical formula is derived for the extraction of the widths of Auger-active states from a molecular Auger interferogram, avoiding the need of either attosecond or high-resolution spectroscopy.

Coherent Control of Light for Non-Line-of-Sight Imaging

Non-line-of-sight (NLOS) imaging relies on collecting light that is rendered incoherent from the multiple scattering events and is then post-processed to provide an estimate of the hidden scene. Here we employ coherent phase control of the outgoing laser beam phase front so as to refocus the beam behind the obscuring obstacle and then use the speckle memory effect to scan the focused spot across the scene. The back-reflected light intensity provides a direct measurement of the scene with a signal-to-noise ratio that is greatly improved when measured using a temporally gated detector. A spatial resolution of less than 1 mm is demonstrated, opening the way to high-resolution NLOS imaging.

Molecular Auger Interferometry.

We introduce and present a theory of interferometric measurement of a normal Auger decay lifetime in molecules. Molecular Auger interferometry is based on the coherent phase control of Auger dynamics in a two-color (ω/2ω) laser field. We show that, in contrast to atoms, in oriented molecules of certain point groups the relative ω/2ω phase modulates the total ionization yield. A simple analytical formula is derived for the extraction of the lifetimes of Auger-active states from a molecular Auger interferogram, circumventing the need in either high-resolution or attosecond spectroscopy. We demonstrate the principle of the interferometric Auger lifetime measurement using inner-valence decay in CH_{3}F.

Coherent control of acoustic phonons by seeded Brillouin scattering in polarization-maintaining fibers.

Coherent excitation of phonons by optical waves, one of the most important channels for light-matter interactions, provides a promising route for optical manipulation of microscopic acoustic phonons for quantum opto-mechanic and phononic devices. Prior research, such as stimulated Brillouin scattering (SBS) in fibers, mainly emphasized phonon amplitude modulation; however, coherent phase control of these phonons has not yet been well explored. Here we experimentally demonstrate a new mechanism to coherently control acoustic phonon phases by a seeded SBS scheme in an optical fiber. Interference between acoustic phonons enables either nearly total transmission or enhanced reflection of optical waves, effectively controlled by phase modulation. This new technique addresses the crucial problem of phase-controlled phonon generation, paving the way for important applications in quantum opto-mechanic and phononic devices.

Coherent phase control of population transfer through ladder system in two laser pulses with ω and nω

The ladder transitions controlled by two harmonic pulses are investigated theoretically using a time-dependent quantum wave packet method for the ground electronic state of HF molecule. By choosing [Formula: see text], [Formula: see text] and [Formula: see text] schemes, the population can be transferred to target states [Formula: see text]. The population distribution can be controlled by the average amplitude of total electric field which depends on the relative phase of two pulses. With the variation of the relative phase between [Formula: see text] and [Formula: see text] pulses, the variation of population has a period of [Formula: see text]. For [Formula: see text] and [Formula: see text] schemes, the population distributions show oscillation behavior with a period of [Formula: see text] by varying the relative phase. The two harmonic pulses can realize a nearly complete population transfer to the target state.

Ionization of argon by two-color laser pulses with coherent phase control

We present a joint experimental and theoretical study of ionization of argon atoms by a linearly polarized two-color laser field (${\ensuremath{\lambda}}_{1}=800$ nm, ${\ensuremath{\lambda}}_{2}=400$ nm). Changing the relative phase $\ensuremath{\varphi}$ between the two colors, the forward-backward asymmetry of the doubly differential momentum distribution of emitted electrons can be controlled. We find excellent agreement between the measurements and the solution of the time-dependent Schr\"odinger equation in the single-active electron approximation. Surprisingly we also find good agreement between the quantum and classical calculations of electron momentum distributions generated by lasers at optical wavelengths.

Coherent phase control of internal conversion in pyrazine.

Shaped ultrafast laser pulses were used to study and control the ionization dynamics of electronically excited pyrazine in a pump and probe experiment. For pump pulses created without feedback from the product signal, the ion growth curve (the parent ion signal as a function of pump/probe delay) was described quantitatively by the classical rate equations for internal conversion of the S2 and S1 states. Very different, non-classical behavior was observed when a genetic algorithm (GA) employing phase-only modulation was used to minimize the ion signal at some pre-determined target time, T. Two qualitatively different control mechanisms were identified for early (T < 1.5 ps) and late (T > 1.5 ps) target times. In the former case, the ion signal was largely suppressed for t < T, while for t ≫ T, the ion signal produced by the GA-optimized pulse and a transform limited (TL) pulse coalesced. In contrast, for T > 1.5 ps, the ion growth curve followed the classical rate equations for t < T, while for t ≫ T, the quantum yield for the GA-optimized pulse was much smaller than for a TL pulse. We interpret the first type of behavior as an indication that the wave packet produced by the pump laser is localized in a region of the S2 potential energy surface where the vertical ionization energy exceeds the probe photon energy, whereas the second type of behavior may be described by a reduced absorption cross section for S0 → S2 followed by incoherent decay of the excited molecules. Amplitude modulation observed in the spectrum of the shaped pulse may have contributed to the control mechanism, although this possibility is mitigated by the very small focal volume of the probe laser.

Pair Creation of Scalar Particles in Intense Bichromatic Laser Fields

The creation of scalar particle pairs due to the decay of a highly energetic gamma quantum traveling in a laser field comprising two independent modes is considered. Employing the framework of strong-field scalar QED, detailed calculations aiming at two different aspects of the process are presented. We first investigate interference effects between reaction channels with absorption of different numbers of laser photons with commensurate frequency and demonstrate the possibility of coherent phase control. Then we study numerically a scheme of dynamical assistance, which offers prospects for an experimental realization of a Schwinger-like pair production process.

Coherent phase control of resonance-mediated two-photon absorption in rare-earth ions

We theoretically and experimentally demonstrate the quantum coherent control of the resonance-mediated two-photon absorption in rare-earth ions by the phase-shaped femtosecond laser pulse. Our theoretical results show that the resonance-mediated two-photon absorption can be effectively controlled, but the control efficiency depends on the laser repetition rate in real experiment due to the long lifetime and the short decoherence time of the excited state, and the larger laser repetition rate yields the lower control efficiency. These theoretical results are experimentally confirmed in glass sample doped with Er3+ by utilizing the femtosecond lasers with low repetition rate of 1 kHz and high repetition rate of 80 MHz.

Resolution enhancement of photoelectron spectra by coherent phase control

In this paper, we study theoretically, the coherent phase control of (2+1) resonantly enhanced multiphoton ionization photoelectron spectra in cesium atom system by shaping the femtosecond pulse with π phase step modulation under weak laser field. Our results show that the photoelectron spectra can be greatly narrowed and the photoelectron intensity can be enhanced after π phase step modulation by precisely setting the phase step position. Also, we show that the high-resolution photoelectron spectra and the fine energy-level diagram of the two closed excited states can be obtained.

Coherent one-photon phase control in closed and open quantum systems: A general master equation approach

The underlying mechanisms for one photon phase control are revealed through a master equation approach. Specifically, two mechanisms are identified, one operating on the laser time scale and the other on the time scale of the system-bath interaction. The effects of the secular and non-secular Markovian approximations are carefully examined.

Weak-field multiphoton femtosecond coherent control in the single-cycle regime

Weak-field coherent phase control of atomic non-resonant multiphoton excitation induced by shaped femtosecond pulses is studied theoretically in the single-cycle regime. The carrier-envelope phase (CEP) of the pulse, which in the multi-cycle regime does not play any control role, is shown here to be a new effective control parameter that its effect is highly sensitive to the spectral position of the ultrabroad spectrum. Rationally chosen position of the ultrabroadband spectrum coherently induces several groups of multiphoton transitions from the ground state to the excited state of the system: transitions involving only absorbed photons as well as Raman transitions involving both absorbed and emitted photons. The intra-group interference is controlled by the relative spectral phase of the different frequency components of the pulse, while the inter-group interference is controlled jointly by the CEP and the relative spectral phase. Specifically, non-resonant two- and three-photon excitation is studied in a simple model system within the perturbative frequency-domain framework. The developed intuition is then applied to weak-field multiphoton excitation of atomic cesium (Cs), where the simplified model is verified by non-perturbative numerical solution of the time-dependent Schrödinger equation. We expect this work to serve as a basis for a new line of femtosecond coherent control experiments.

Erratum: “Conditions for one-photon coherent phase control in isolated and open quantum systems” [J. Chem. Phys. 133, 151101 (2010)

Coherent control of observables using the phase properties of weak light that induces one-photon transitions is considered. Measurable properties are shown to be categorizable as either class A, where control is not possible, or class B, where control is possible. Using formal arguments, we show that phase control in open systems can be environmentally assisted.

Coherent Phase Control in e-Ar Scattering in a Bichromatic Laser Field in the Second Born Approximation

We study the elastic scattering of atomic argon by an electron in the presence of a bichromatic laser field in the second Born approximation. The target atom is approximated by a simple screening potential. We explore the dependences of the differential cross sections on the relative phase ϕ between the two components of the radiation field and discuss the influence of the number of photons exchanged on the phase-dependence effect. Moreover, for different scattering angles and incident electron energies, the differential cross sections are notably different.

Positron-impact ionisation of atomic hydrogen in the presence of a bichromatic laser field

The positron impact-ionisation of atomic hydrogen in the presence of a linearly polarised bichromatic field is investigated in the first Born approximation. The field is composed of a fundamental frequency and its second harmonic. The state of positron in the field is described by the Volkov wavefunction, and the continuum state of the ejected electron is described by the Coulomb-Volkov wavefunction. The dressed ground state of target is a first order time-dependent perturbative wavefunction. The triple differential cross sections and their dependencies on laser field parameters are discussed and compared with the results modified by a monochromatic field. Numerical results show that the coherent phase control is significant and the laser-assisted ionisation cross sections caused by positron and electron are different.

Communication: Conditions for one-photon coherent phase control in isolated and open quantum systems

Coherent control of observables using the phase properties of weak light that induces one-photon transitions is considered. Measurable properties are shown to be categorizable as either class A, where control is not possible, or class B, where control is possible. Using formal arguments, we show that phase control in open systems can be environmentally assisted.

Coherent phase control of (2+1) resonantly enhanced multiphoton ionization photoelectron spectroscopy

In this work, we report a theoretical study of the coherent phase control of typical (2+1) resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) in an atomic system. By properly designing the spectral phase of a femtosecond pulse, we can realize the manipulation of the photoelectron energy, the photoelectron spectral bandwidth and the photoelectron intensity at a certain kinetic energy. Moreover, we can also obtain a high-resolution photoelectron spectrum and fine energy-level structure of the excited states for a complicated quantum system in spite of the broad-width spectrum of the femtosecond pulse. The theoretical results have promising applications for investigating the molecular structure and understanding the laser-induced ionization dynamics.