Second-order Nonlinear Material Research Articles

Nonlinear upconverted thermal emission through difference frequency generation.

Thermal radiation management is of critical importance in energy, sensing, and heat transfer. According to Planck's law, objects at room temperature predominantly emit thermal radiation within the mid- and far-infrared bands. Here, we demonstrated the upconversion of the mid- and far-infrared thermal radiation emitted by second-order nonlinear material to the easily-detectable visible band through a difference frequency process. This nonlinear broad-spectrum upconversion is facilitated by the random quasi-phase-matching technique in the nanoparticle system. Furthermore, we show the temperature measurement of thermal spots using such nonlinear thermal radiation. This scheme paves the way for applications in thermal management and sensing.

Light squeezing enhancement by coupling nonlinear optical cavities

In this paper, we explore the squeezing effect generated by two coupled optical cavities. Each cavity contains a second-order nonlinear material and coherently pumped by a laser. Our results show that light intensity is strongly improved due to the presence of the nonlinearities and mainly depends on the detunings between external laser frequencies and cavity modes. More interestingly, the proposed scheme could enhance light squeezing for moderate coupling between cavities : the squeezing generated by one cavity is enhanced by the other one. For resonant interaction, highest squeezing effect is obtained near resonance. When fields are non resonant, squeezing increases near resonance of the considered cavity, but decreases for large detunings relative to the second cavity. Further, when the dissipation rate of the second cavity is smaller than the first, the squeezing could be improved, attaining nearly the perfect squeezing. While the temperature elevation has a negative impact overall on the nonclassical light, squeezing shows an appreciable resistance against thermal baths for appropriate parameter sets.

Photon blockade in a bimode nonlinear nanocavity embedded with a quantum dot

We study the photon antibunching effect in a bimode nano-optical cavity filled with the second-order nonlinear material and embedded with a quantum dot. Analytical results show that both the conventional photon blockade (CPB) induced by the nonlinear interaction between polaritons and the unconventional photon blockade (UCPB) induced by the quantum interference appear in our model. In particular, we find that CPB and UCPB can simultaneously occur in an overlapped parameter regime, which allows one to prepare a single-photon source with high purity and high brightness in the strong-coupling regime. Furthermore, owing to the multipathway interference in this composite model, the photon antibunching effect is significantly enhanced compared to that in the Jaynes-Cummings model and in the degenerate optical parametric amplifier in the weak-coupling regime.

Tunable Second Harmonic Generation in Twisted Bilayer Graphene

Summary Twisted stacking of van der Waals materials introduces a new way in band-structure engineering and has given rise to numerous extraordinary physical phenomena. Despite the absence of second harmonic generation (SHG) in non-gated monolayer graphene, artificially twisted bilayer graphene (tBLG) possesses more possible point-group symmetries, including those with broken inversion symmetry. Here, we report twist-angle-dependent SHG from tBLG, which is the first demonstration of an elemental material with intrinsically tunable nonlinearity. We show that depending on the twist angle, the susceptibility of the dominant chiral tensor component of tBLGs can vary from 0 to 28 × 104 pm2/V, which is at the same order of magnitude as on-resonance susceptibility of monolayer MoS2. These results shed light on the underlying symmetry of tBLG systems and electronic band structure near van Hove singularities. More importantly, they introduce a new degree of freedom, the twisting, in creating efficient second-order nonlinear material form centrosymmetric constituents.

Enhancement of second-harmonic generation in nonlinear nanolaminate metamaterials by nanophotonic resonances.

Nanolaminate metamaterials recently attracted a lot of attention as a novel second-order nonlinear material that can be used in integrated photonic circuits. Here, we explore theoretically and numerically the opportunity to enhance the nonlinear response from such nanolaminates by exploiting Fano resonances supported in grating-coupled waveguides. The enhancement factor of the radiated second harmonic signal compared to a flat nanolaminate can reach values as large as 35 for gold gratings and even 7000 for MgF2 gratings. For the MgF2 grating, extremely high-Q Fano resonances are excited in such all-dielectric system that result in strong local fields in the nonlinear waveguide layer to boost the nonlinear conversion. A significant portion of the nonlinear signal is also strongly coupled to a dark waveguide mode, which remains guided in the nanolaminate. The strong excitation of a dark mode at the second harmonic frequency provides a viable method for utilizing second-order nonlinearities for light generation and manipulation in integrated photonic circuits.

Enhanced second-harmonic generation from nonlinear optical metamagnetics.

We present a numerical simulation of second-harmonic generation (SHG) from a nonlinear magnetic metamaterial. By inserting a second-order nonlinear material in the high local field area of magnetic metamaterial, which consists of periodic arrays of paired thin silver strips, the convertion efficiency of SHG has been significantly enhanced by almost four orders of magnitude. The corresponding field patterns and further studies on dependance between SHG and symmetry of nonlinear crystal show that the increase of the conversion efficiency is attributed to the local field enhancement caused by the magnetic resonnance of the structure. Our researches provide an additional way to further improve the optical nonlinearity in nanostructures.

Exploring fundamental limits to terahertz generation in electrooptic materials: From bulk to nanolayers

We review the progress made by us on the exploration of the fundamental limits to terahertz (THz) generation from several semiconductor electrooptic materials. Through the measurements of the THz output versus the pump beam in terms of incident angle, polarization, azithumal angle, and pump intensity, we have demonstrated that we can precisely determine the contributions made by the optical rectification and photocurrent surge. When a material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, these two mechanisms often compete with each other, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients are resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also illustrated that, for some materials, two-photon absorption can be one of the fundamental limits to the THz generation.

Power Scaling on Efficient Generation of Ultrafast Terahertz Pulses

We have investigated power scaling for the efficient generation of the broadband terahertz (THz) pulses. These THz short pulses are converted from ultrafast laser pulses propagating in a class of semiconductor electrooptic materials. By measuring the dependence of the THz output on the pump beam in terms of incident angle, polarization, azimuthal angle, and pump intensity, we have precisely determined the contributions made by optical rectification, drift of carriers under a surface or external field, and photo-Dember effect. When a second-order nonlinear material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, the three mechanisms mentioned earlier often compete with one another, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients can be resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be dramatically increased. We have also analyzed how the THz generation is affected by some competing processes such as two-photon absorption.

FROM NONRESONANT TO RESONANCE-ENHANCED THz GENERATION

We have reviewed mechanisms for the efficient conversion of the broadband terahertz (THz) pulses from ultrafast laser pulses propagating in semiconductor electro-optic materials. By measuring the behaviors of the THz output versus the pump beam in terms of incident angle, polarization, and azimuthal angle, we have demonstrated that we can precisely determine the contributions made by optical rectification and photocurrent surge. When a material is pumped below its bandgap, optical rectification is always the mechanism for the THz generation. Above the bandgap, however, these two mechanisms often compete with each other, depending on the material characteristics and pump intensity. At a sufficiently high pump intensity, optical rectification usually becomes the dominant mechanism for a second-order nonlinear material. Our analysis indicates that second-order nonlinear coefficients can be resonantly enhanced when a material is pumped above its bandgap. In such a case, the THz output power and normalized conversion efficiency can be greatly enhanced. We have also analyzed how the THz generation is affected by some competing processes such as two-photon absorption.

Vector solitary optical beam control with mixed type I–type II second-harmonic generation

The four vector equations, appropriate to a mixed type I–type II second-harmonic generation, in a thin planar waveguide, made from a second-order non-linear material, are solved, both approximately and exactly. The solutions are then used to generate possible applications. In this investigation, both the fundamental and the second-harmonic waves have two transverse field components. It is shown that, by controlling the ratio of the components of the fundamental wave, one component of the second-harmonic wave can be rigorously controlled, and even switched off. The ratio of the harmonic field components depends strongly upon the polarization angle of the fundamental wave, with the extinction angle depending strongly upon the phase mismatch parameter. In a second application, an aperture is placed at the output and it is shown that if approximate stationary states are used as an input to a quadratically non-linear medium, then varying the angle of incidence of two input beams produces an excellent output control. A numerical demonstration is given in which a switch from 80% of the input energy arriving at the output port to less than 3% is readily achievable.

Cascading effect on second-harmonic generation in a ring cavity

Recently the cubic behavior of nonlinear second-order materials (the so-called cascading effect) was discussed and demonstrated. Here we discuss the bistability and the chaotic behavior of an optical ring cavity containing a second-order nonlinear material when second-harmonic generation occurs, focusing on the role of the cascading effect. The role in instability played by the phase mismatch Δk for second-harmonic generation is also considered.

Observation of two-dimensional spatial solitary waves in a quadratic medium.

We report the first experimental demonstration of two-dimensional spatial solitary waves in second-order nonlinear optical material. When an intense optical beam is focused into a phase-matchable second-order nonlinear material, the fundamental and generated second-harmonic fields are mutually trapped as a result of the strong nonlinear coupling which counteracts both diffraction and beam walkoff.