Intensity Dependent Reflection Research Articles

Indium phosphide metasurface with enhanced nonlinear absorption

We solve the nonlinear Maxwell equations in an InP-based dielectric metamaterial, considering both two-photon absorption and photo-induced free-carrier absorption. We obtain the intensity-dependent reflection, absorption, and effective permittivity and permeability of the metamaterial. Our results show that nonlinear absorption dampens both the electric and magnetic Mie resonance, although the magnetic resonance is more affected because it occurs at longer wavelengths where the free-carrier absorption cross section is larger. Owing to field concentration in the metamaterial at resonance, the threshold intensity for nonlinear absorption is reduced by a factor of about 30 compared to a homogeneous layer of the same thickness. Our results have implications on the use of dielectric metamaterials for nonlinear applications such as higher harmonic generation, optical limiting, and ultrafast modulation.

Nonlinear mirror mode-locked Pr:YAlO3 laser

Mode-locking of a praseodymium based solid-state laser by a nonlinear mirror is reported, for the first time to the best of our knowledge. Pr:YAlO3 crystal having 0.6 at.% of Pr3+ ions was used as an active medium. For mode-locking, a nonlinear BBO crystal together with a properly designed dichroic mirror were employed to form an output coupler with intensity dependent reflection. A continuous-wave mode-locked laser operation with a mean output power of 100 mW was achieved under 3.5 W InGaN laser diode pumping. The laser emitted 250 ps pulses with a repetition rate of 105 MHz at a center oscillation wavelength of 747 nm.

Shrinkage of picosecond laser beam by plasma reflection in super-resolution near-field structure of SiN/Sb/SiN thin film

The transmittive and reflective Z-scan technique is used with a 10 Hz, frequency doubled, Q-switched, and mode-locked Nd:YAG laser to verify that the reflectivity of the super-resolution near-field structure of an SiN/Sb/SiN thin film increases as incident intensity decreases. This intensity-dependent reflection, called nonlinear reflection, reflects a TEM(00) mode laser beam more strongly at its periphery than at its center and so shrinks the transmitted laser beam. The observed nonlinear reflection is attributed to laser-induced change of carrier densities in Sb, to justify quantitatively the experimental results.

Passive mode-locking of a diode-pumped Nd:YVO_4 laser by intracavity SHG in PPKTP

Experimental results on passive mode-locking of a Nd:YVO(4) laser using intracavity frequency doubling in periodically poled KTP (PPKTP) crystal are reported. Both, negative cascaded chi((2)) lensing and frequency doubling nonlinear mirror (FDNLM) are exploited for the laser mode-locking. The FDNLM based on intensity dependent reflection in the laser cavity ensures self-starting and self-sustaining mode-locking while the cascaded chi((2)) lens process contributes to substantial pulse shortening. This hybrid technique enables generation of stable trains of pulses at high-average output power with several picoseconds pulse width. The pulse repetition rate of the laser is 117 MHz with average output power ranging from 0.5 to 3.1 W and pulse duration from 2.9 to 5.2 ps.

Post-growth tailoring of quantum-dot saturable absorber mirrors by chemical etching

We have designed and grown a resonant, low-finesse quantum-dot saturable absorber mirror and subsequently modified the important parameters using chemical etching. The modulation depth and saturation fluence at the design wavelength of 1064 nm were modified by etching the sample to tune the cavity resonance. The device properties were characterised using normal incidence spectroscopic reflectivity measurements, intensity dependent reflectivity measurements and modelled using a transfer matrix approach. The saturable absorber mirror was used to facilitate self-starting, passively mode locked pulses in a neodymium vanadate laser operating at 1064 nm. The etching was found to affect the duration of the pulses, leading to temporal width tuning over a range of 94 ps. The shortest pulse duration of 84 ps was achieved for the cavity resonance close to 1064 nm, with an output power of 3 W. This method is an effective technique for post-growth engineering of the properties of semiconductor saturable absorber mirrors (SESAMs) with nanometre precision.

DYNAMICS OF SPATIAL SOLITONS AT PHASE-MISMATCH INTERFACES

We studied the dynamics of spatial solitons at a phase-mismatch interface in quadratic nonlinear waveguides. We present here a simple formalism to analyze intensity-dependent reflection, refraction and emission of beams. Experimental demonstrations at a phase-mismatch PPLN/LiNbO3 interface are reported.

FEL-like interaction regime in the up-shifted scattering of radiation by the laser-plasma wake

We study the process of resonant scattering of radiation by the laser excited plasma wake in the wavebreaking regime. The resonant interaction arises between the cusp electrons and the slow wave of ponderomotive potential which is formed by the incident and scattered electromagnetic waves. Scattered wave is amplified when the density cusp is moving slightly faster than ponderomotive wave, and is absorbed in the opposite case. This FEL-like interaction leads to nonlinearity in reflection, the intensity-dependent reflection coefficient is calculated.

Influence of leaky waves on free-electron-induced nonlinear reflection properties of highly doped n-GaAs layer at medium IR-wavelengths

A multi-layer structure was theoretically elaborated to characterize the free-electron-induced optical nonlinearity in a thin sheet of a multi-valley semiconductor-like highly doped n-GaAs at medium-IR wavelengths. The multi-layer structure was designed to enhance the electric field and hence the absorption process inside this thin highly doped n-GaAs layer. The field enhancement was due to the excitation of a leaky TM polarized waveguide mode. An adequate nonlinear propagation model was derived taking into account the energy balance equation of conduction electrons distributed between the Γ- and L-valleys of highly doped n-GaAs. The angular and intensity dependent reflection properties were analysed. The influence of the intervalley deformation potential ΛLL on the nonlinear reflection properties was highlighted. The optical sensitivity of the leaky mode structure could be increased by ten times with respect to that of a bulk structure: reflection variations as high as factor of 3 for an intensity of 1.5 MW cm-2 could be simulated for a doping density of 7.7×1018 cm-3.

Single-longitudinal-mode, phase-conjugate ring master oscillator power amplifier using external stimulated-Brillouin-scattering Q switching

A phase-conjugate ring laser operating on a single longitudinal mode and with good beam quality (TEM00) is reported. Phase conjugation is achieved with stimulated Brillouin scattering by CS2. The system acts as an oscillator–two-pass amplifier system, and the intensity-dependent reflectivity of stimulated-Brillouin scattering, positioned outside the ring oscillator, acts as a Q switch.

Effects of Group-velocity Dispersion on the Evolution of Optical Pulses in a Mode-locked Laser Using a Single SHG Crystal

The effects of group-velocity dispersion in an SHG crystal on the evolution of optical pulses in a mode-locked laser by a nonlinear mirror are analysed. The pulse duration is determined by the balance between pulse stretching due to the group-velocity dispersion of the SHG crystal and pulse shortening due to the intensity-dependent reflection coefficient of the nonlinear mirror. Theoretical calculation shows that the final pulse duration is mainly confined by the group-velocity mismatch of the fundamental and the second harmonic in the nonlinear crystal.

Dispersive subband-resonant nonlinearity in amorphous Si/SiO/sub 2/ quantum well structures

Dispersive nonlinearity in amorphous Si/SiO/sub 2/ quantum well structures (QW's) has been investigated. The refractive index changes obtained from the intensity-dependent reflection spectra are nonlinearly dependent on the excitation intensity and can be described by the model of the saturating nonlinearity at low pump intensities. The nonlinear refractive index reveals resonant behavior associated with the subband structure of the QW's. The saturated nonlinear index and the saturation intensity have been obtained as /spl Delta/n/sub s/=-0.11 and I/sub s/=1.9 MW/cm/sup 2/ at the transitions between the lowest subbands, and /spl Delta/n/sub s//spl sim/0.3 and I/sub s//spl sim/0.5 MW/cm/sup 2/ at the transitions between the second subbands of the valence and conduction bands. The nonlinearity for the second subband transitions has been found high enough to provide potentially bistable operation, but the bistability is not expected at the transitions between the ground subbands. Carrier lifetime less than 1 ps restricting the switching time of the nonlinearity has been estimated from the saturation intensity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reflectivity and oscillation conditions of a gain medium in a self-conjugating loop geometry

The intensity-dependent reflectivity of a gain medium in a self-phase-conjugating loop geometry is numerically modeled by use of exact integration equations without restriction on the degree of gain saturation. High conjugate reflectivity and high energy extraction efficiency of the gain medium are predicted. A weak saturation analysis is also presented to derive useful expressions for the threshold input intensity for oscillation of the backward conjugate mode and the resonant oscillation frequencies.

Resonant passive mode-locked Nd:YLF laser

The authors review the theory of resonant passive mode locking (RPM) lasers and present experimental results for an improved Nd:YLF RPM laser that produced stable pulses as short as 3.7 ps at a repetition rate of 250 MHz. The average output power was 550 mW with 1.3-W average pump power from a Ti:sapphire laser. It was possible to couple 85% out of the nonlinear coupled cavity and still maintain stable mode locking. In RPM and amplitude nonlinearity such as absorption bleaching in a semiconductor reflector introduces an intensity-dependent reflectivity which strongly mode locks the laser. The reduced carrier lifetime in a low-temperature MBE-grown (LT) InGaAs-GaAs quantum-well reflector produces a sufficiently fast saturable absorber in the coupled cavity. >

Analysis of optical bistability in a nonlinear distributively coupled resonator

A nonlinear distributively coupled resonator (NDCR) used as a bistable element is studied. The resonator is an integrated optics implementation of a nonlinear Fabry-Perot resonator with a variable reflectivity mirror. Under certain conditions, the intensity-dependent reflectivity and tuning of the resonator contribute constructively and enhance nonlinear properties of the device. Numerical methods are used to optimize NDCR design parameters and show that the critical intensity required for onset of bistability is reduced by 30% relative to a comparable regular Fabry-Perot. Similarly, the minimum required index change is reduced by 10%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The fluctuation model of a mode-locked laser using a mirror with intensity-dependent reflection coefficient

A fluctuation model is presented for a mode-locked laser incorporating a mirror with intensity dependent reflection coefficient. Computer simulation with this model is executed for some nonlinear crystals and for varied parameters. The optimum parameters and conditions under which good mode-locking will be achieved is predicted according to the simulation calculation.

THEORETICAL CALCULATION OF A NEW METHOD OF MODE-LOCKING FOR LASERS

A fluctuation model is suggested in this paper for the mode-locked laser incorporating a nonlinear mirror with intensity dependent reflection coefficient. Computer simulation with this model is conducted for varied parameters of Nd: YAG lasers and for two kinds of nonlinear crystal (KTP and KDP). The optimum parameters and conditions under which perfect mode-locking will be achieved are predicted according to the simulation calculation.

Modelocking using mirrors with intensity dependent reflection coefficients

The steady state modelocking of a laser incorporating a mirror with an intensity dependent reflection coefficient is analysed. The pulse length is determined by the balance between pulse stretching due to gain narrowing and pulse shortening due to the nonlinear mirror. The final pulse length can approach the limit set by the gain bandwidth of the active medium.

Pulse shortening by a nonlinear mirror mode locker

A device, composed of a nonlinear crystal and a dichroic mirror, features intensity-dependent reflection and may be used as a passive mode locker. The pulse shortening of a Gaussian light pulse due to reflection by this nonlinear mirror has been calculated and compared to that of a saturable absorber used for mode locking.

A new mode-locking technique using a nonlinear mirror

Abstract A novel simple technique of mode-locking is demonstrated. It uses a nonlinear mirror with intensity-dependent reflection coefficient. The mirror is constituted by a combination of a SHG crystal and a dichroic mirror with high reflectivity for the second harmonic and partial transmission for the fundamental. In a preliminary experiment with a Nd:YAG laser a train of ultrashort pulses has been obtained with individual pulse duration at the second harmonic wavelength of approx. 100 ps.

A mirror with an intensity-dependent reflection coefficient

A device exhibiting a power-dependent reflection coefficient with fast time response and no frequency shift is proposed and analysed. It utilizes a combination of a nonlinear crystal for second harmonic generation and a dichroic mirror with high reflectivity for the second harmonic and partial transmission for the fundamental wavelenth. This device has been successfully used in a preliminary experiment to modelock a Nd:YAG laser.