Multistability at arbitrary low optical intensities in a metal-dielectric layered structure.

We show that a nonlinear metal-dielectric layered slab of subwavelength thickness and very small average dielectric permittivity displays optical multistable behavior at arbitrary low optical intensities. This is due to the fact that, in the presence of the small linear permittivity, one of the multiple electromagnetic slab states exists no matter how small is the transmitted optical intensity. We prove that multiple states at ultra-low optical intensities can be reached only by simultaneously operating on the incident optical intensity and incidence angle. By performing full wave simulations, we prove that the predicted phenomenology is feasible and very robust.

Transmissivity directional hysteresis of a nonlinear metamaterial slab with very small linear permittivity.

We investigate propagation of a transverse magnetic field through a nonlinear metamaterial slab of subwavelength thickness and with a very small and negative linear dielectric permittivity. We prove that, for a given input intensity, the output intensity is a multivalued function of the field incidence angle so that the transmissivity exhibits angular multistability and a pronounced directional hysteresis behavior. The predicted directional hysteresis is a consequence of the fact that the linear and nonlinear contributions to the overall dielectric response can be comparable so that the electromagnetic matching conditions at the output slab boundary allow more than one field configuration within the slab to be compatible with the transmitted field.

Highly nonparaxial (1+1)-D subwavelength optical fields.

A general approach for describing (1+1)-D subwavelength optical field whose waist is much smaller than the wavelength is presented. Exploiting the vectorial Rayleigh-Sommerfeld diffraction theory, a suitable expansion in the ratio between the beam waist and the wavelength allows us to prove the a (1+1)D highly nonparaxial field is generally the product of a cylindrical wave carrier and an envelope which is angularly slowly varying. We apply our general approach to the case of highly nonparaxial Hermite-Gaussian beams whose description is fully analytical.

Transverse power flow reversing of guided waves in extreme nonlinear metamaterials.

We theoretically prove that electromagnetic beams propagating through a nonlinear cubic metamaterial can exhibit a power flow whose direction reverses its sign along the transverse profile. This effect is peculiar of the hitherto unexplored extreme nonlinear regime where the nonlinear response is comparable or even greater than the linear contribution, a condition achievable even at relatively small intensities. We propose a possible metamaterial structure able to support the extreme conditions where the polarization cubic nonlinear contribution does not act as a mere perturbation of the linear part.

Electro-activation and electro-morphing of photorefractive funnel waveguides.

We demonstrate the electro-activation of funnel waveguides through the quadratic electro-optic effect in paraelectric potassiumlithium- tantalate-niobate. This allows us to achieve electro-optic intensity modulation in a single optical beam, a 1x2 switch, and finally the electrically controlled morphing of a single waveguide into a 1x2 and a 1x4 divider.

Photorefractive solitons of arbitrary and controllable linear polarization determined by the local bias field.

We discuss and experimentally demonstrate a scheme to achieve photorefractive solitons of arbitrary linear polarization using the quadratic electro-optic effect and describe the observation of the self-trapping of a set of linear polarized beams in different positions of a paraelectric photorefractive crystal of potassium-lithium-tantalate-niobate (KLTN) biased by the inhomogeneous field produced by two miniaturized top electrodes. The polarization of the single solitons of the set is determined by the local electrostatic configuration and the underlying tunable anisotropy, which is detected through zero-field electro-activation.

Counterpropagating spatial solitons in reflection gratings with a longitudinally modulated Kerr nonlinearity.

We investigate quasi-Bragg-matched counterpropagating spatial solitons in a reflection grating in the presence of a longitudinally modulated Kerr nonlinearity. The physical interplay of linear reflection and Kerr self-focusing with the modulation in the nonlinearity yields a variety of elaborate self-action mechanisms. We first analytically predict the existence of symmetric soliton pairs supported by a pure Kerr-like effective nonlinearity. We then analytically derive two families of solitons, associated with the linear grating eigenmodes, supported by an effective "incoherent" Kerr-like coupling arising from the exact balance between the modulation in the nonlinearity and the Kerr interaction due to beam interference.

Counterpropagating nondiffracting beams through reflection gratings.

We investigate the counterpropagation of paraxial non-diffracting optical beams through a medium hosting a bulk reflection grating in the quasi-Bragg matching condition. The impact of the relative magnitude of the Bragg detuning parameter and the grating depth on the plane wave dispersion relation allows us to identify three distinct regimes where counterpropagation and interaction of nondiffracting beams show qualitatively different features, encompassing longitudinally invariant, periodic or exponential intensity profiles. In one of the identified regimes the dispersion relation is not monotonic and the consequent "longitudinal degeneracy" allows the investigation of new class of nondiffracting beams characterized by a double spectral ring profile.

Separating polarization components through the electro-optic read-out of photorefractive solitons.

Analyzing the propagation dynamics of a light beam of arbitrary linear input polarization in an electro-activated photorefractive soliton we are able to experimentally find the conditions that separate its linear polarization components, mapping them into spatially distinct regions at the crystal output. Extending experiments to the switching scheme based on two oppositely biased solitons, we are able to transform this spatial separation into a separation of two distinct guided modes. The result is a miniaturized electro-optic polarization separator.

Role of charge saturation in photorefractive dynamics of micron-sized beams and departure from soliton behavior.

Experimental and theoretical results indicate that miniaturized micron-sized nonlinear beam phenomenology in photorefractives leads to a regime qualitatively distinct from solitonlike propagation on consequence of the specific role of space-charge saturation. In the highly modulated conditions typical of beams, this contribution amounts to an effective electron self-action.

Longitudinal-mode selectivity and perturbation sensitivity of multimirror laser cavities.

Some relevant features of a particular type of multimirror laser cavity, i.e., those cavities terminating in highfinesse reflective multipass interferometers, are theoretically investigated with the aim of highlighting the dependence of the level of spectral control achievable with these cavities on their geometrical and optical characteristics. It is shown that cavities equipped with suitably designed reflective multipass interferometers are not only capable of forcing a laser operation on a single longitudinal mode but can act at the same time as effective emission frequency stabilizers because of their inherently low sensitivity to any perturbation affecting the cavity optical length. This feature is particularly attractive for intrapulse and pulse-to-pulse stabilization of high power pulsed lasers. Criteria are given also for the design of optimized cavities that could simultaneously provide narrow linewidth laser emission and minimize the effects of perturbations.

Control of intrapulse frequency chirping in long-pulse CO(2) lasers employing perturbation-insensitive optical cavities.

We present the results of intrapulse frequency chirping measurements that were carried out on a TEA CO(2) laser equipped with various conventional and unconventional frequency-selective optical cavities. We experimentally demonstrate that there is a close relationship between the causes and the extent of intrapulse frequency chirping on the one hand and the type of optical configuration that is used for controlling the laser mode behavior on the other. In particular, it is shown that intrapulse chirping of the emitted frequency can be significantly reduced by employing laser cavities terminating in high-finesse reflective multipass interferometers for single-mode selection. In fact, this type of cavity can be designed in such a way to minimize the effects on the emitted frequency of perturbations affecting the cavity optical length.

Reflective high-finesse interferometers as effective intrapulse laser frequency stabilizers.

The effect of optical cavity length perturbations on the intrapulse frequency stability of laser cavities terminating in reflective multipass interferometers is investigated. We demonstrate both theoretically and experimentally, in a long-pulse single-mode TEA CO(2) laser, that intrapulse frequency chirping can be strongly reduced by using relatively long high-finesse interferometers. These results can be applied to the design of single-mode lasers with negligible chirping.

Q switching and cavity dumping of a high-power cw Nd:YAG laser by means of a novel electro-optic configuration.

We present a novel electro-optic configuration that is capable of switching unpolarized light with high efficiency. Its application as the output coupler of a high-repetition-rate, Q-switched (up to 50 kHz), cavity-dumped highpower cw Nd:YAG laser is reported.

Multipass grating interferometer as output coupler for tunable, single-mode operation of large-bandwidth lasers.

A novel multipass grating interferometer is presented that is capable of operating as an efficient, very-narrow-band output coupler for high-power, large-bandwidth lasers. Its application for tunable (up to 3.5 GHz), single-mode operation of a TEA CO(2) laser is discussed.

Multipass-prism interferometer for fine-frequency-tuning, single-mode operation of TEA CO(2) lasers.

A novel interferometric configuration has been used as an efficient, narrow-band output coupler for high-power lasers. Its application to a TEA CO(2) laser has resulted in extremely reliable (100%) single transverse- and longitudinal-mode operation of this laser with a fine-frequency tunability of up to 3 GHz.