Control Of Harmonic Generation Research Articles

Second Harmonic Generation Control in 2D Layered Materials: Status and Outlook

AbstractSecond harmonic generation (SHG) as an essential nonlinear optical effect, has gradually shifted its research trend toward the integration and miniaturization of photonic and optoelectronic on‐chip devices in recent years. 2D layered materials (2DLMs) open up a new research paradigm of nonlinear optics due to their large second‐order susceptibility, atomically thin structure, and perfect phase‐matching. However, 2DLMs are facing a bottleneck of weak SHG conversion efficiency limit caused by short light–matter interaction lengths at a nanoscale. Moreover, advances in integrated on‐chip SHG devices based on 2DLMs rely on the continuing development of novel strategies with tunable and efficient SHG responses. Here, this review provides a comprehensive overview of recent progress in exploring highly efficient and tunable SHG responses in 2DLMs. Various modulation and enhancement strategies for the SHG response of 2DLMs are extensively studied and systematically discussed, which can be classified into two categories: symmetry breaking and light‐matter interaction enhancement. Moreover, remaining challenges and outlooks toward further extending and realizing the practical applications of 2DLMs in nonlinear on‐chip integrated devices with SHG modulation and enhancement characteristics are discussed.

Control of Harmonic Generation by the Time Delay Between Two-Color, Bicircular Few-Cycle Mid-IR Laser Pulses.

We study control of high-order harmonic generation (HHG) driven by time-delayed, few-cycle ω and 2ω counterrotating mid-IR pulses. Our numerical and analytical study shows that the time delay between the two-color pulses allows control of the harmonic positions, both those allowed by angular momentum conservation and those seemingly forbidden by it. Moreover, the helicity of any particular harmonic is tunable from left to right circular without changing the driving pulse helicity. The highest HHG yield occurs for a time delay comparable to the fundamental period T=2π/ω.

Control of harmonic generation in a quantum well with two-colour few-cycle pulses

We investigate the spectra of two-colour few-cycle pulses in a semiconductor quantum well (QW) with consideration of quantum size effects. Our results show that, in the few-cycle regime, the spectral peak at 4ω0 can be achieved even under the condition of a two-colour ω0 − 3ω0 pulse combination. Moreover, when the quantum size is modified, the enlargement or suppression of the spectral peak at 4ω0 or 5ω0 can be selectively controlled by the relative phase of the two-colour few-cycle pulses.

Changing and control of harmonic generation in superconducting films

The generation of high-order harmonics from ion implanted YBCO films has been investigated by monitoring of its response to an alternating magnetic field. These harmonics are generated because of the high nonlinearity caused by sudden magnetic flux penetration above the first critical field. Harmonics up to 100th order and interference effects were observed.

Spectral Phase Distribution Retrieval through Coherent Control of Harmonic Generation

The temporal intensity distribution of the third harmonic of a Ti:sapphire laser generated in Xe gas is fully reconstructed from its spectral phase and amplitude distributions. The spectral phases are retrieved by cross correlating the fundamental laser frequency field with that of the third harmonic, in a three laser versus one harmonic photon coupling scheme. The third harmonic spectral amplitude distribution is extracted from its field autocorrelation. The measured pulse duration is found to be in agreement with that expected from lowest order perturbation theory both for unstretched and chirped pulses.

Control of harmonic generation using two-colour femtosecond–attosecond laser fields: quantum and classical perspectives

We investigate high-order harmonic generation in atoms and molecules using two-colour laser pulses: an infrared (IR) laser pulse of a few cycles superposed on an attosecond ultraviolet (UV) or vaccum ultraviolet laser pulse. We adapt a standard semiclassical model to this process. We show that enhancement and additional control of harmonics via time delay between these pulses are possible because of a different ionization mechanism from the one-colour case. The electron is moved to the continuum via one-photon absorption of a UV photon (instead of tunnelling through the potential barrier) at a specific phase of the IR pulse (determined by the time delay between two pulses), with a non-zero initial velocity, leading to new possibilities for the control of harmonic generation.

Control of harmonic generation in a free-electron laser with a quasiperiodic wiggler configuration

A quasiperiodic wiggler configuration made from two periodic wigglers of irrational period ratio is considered for application to free-electron lasers (FEL's). With an analytical formulation of its spontaneous emission, spectral behavior of a quasiperiodic wiggler is studied. It is found that the nonlinear coupling between the two constituent wigglers of the quasiperiodic wiggler configuration leads to significant spontaneous emission at both the usual FEL harmonic frequencies and new frequencies that are irrational multiples of the fundamental FEL radiation frequency. By adjusting the relative field strengths of the two constituent wigglers and their period ratio, the usual FEL harmonics can be suppressed considerably. This unique spectral characteristic of quasiperiodic wigglers may be used to relieve the problem of mirror damage in some free electron lasers operating in ultraviolet and x-ray wavelengths.

Control of harmonic generation by initial-state preparation

A procedure to control harmonic generation is proposed using initial state control. We show that an initial molecular state can be prepared before shining a strong laser pulse to maximize the output harmonic generation. We demonstrate the method by maximizing emission for the sixth harmonic on a given initial IR pulse in a model H 2 + system using five initial vibrational states.

High-power cycloconverter drive for double-fed induction motors

Two cycloconverters independently supply stator and rotor windings of a wound rotor induction machine. The control strategy to stabilize this inherently unstable system is derived from the double-fed machine matrix (DFM) and the resulting algorithm is implemented in hardware with a simple field calculator. A practical implementation of a 2 kW drive illustrates the feasibility of the drive, and practical performances of full speed in both directions, as well as speed ramps of up to 700 r/min, are realized. The system benefits from the simplicity of line-commutated converters associated with a relatively high frequency of operation of each individual cycloconverter, thereby avoiding saturation effects appearing at low frequencies, combined with control of harmonic generation using the extra degree of freedom provided by the DFM. >