Kalle Koskinen

Dipole limit in second-harmonic generation from arrays of gold nanoparticles

We present a multipolar tensor analysis of second-harmonic generation from arrays of noncentrosymmetric gold nanoparticles. In contrast to earlier results, where higher multipoles and symmetry-forbidden signals arising from sample defects play a significant role, the present results are completely dominated by symmetry-allowed electric-dipole tensor components. The result arises from significant improvement in sample quality, which suppresses the higher-multipole effects and enhances the overall response by an order of magnitude. The results are a prerequisite for metamaterials with controllable nonlinear properties.

Atomic layer deposited second-order nonlinear optical metamaterial for back-end integration with CMOS-compatible nanophotonic circuitry

We report the fabrication of artificial unidimensional crystals exhibiting an effective bulk second-order nonlinearity. The crystals are created by cycling atomic layer deposition of three dielectric materials such that the resulting metamaterial is noncentrosymmetric in the direction of the deposition. Characterization of the structures by second-harmonic generation Maker-fringe measurements shows that the main component of their nonlinear susceptibility tensor is about 5 pm/V, which is comparable to well-established materials and more than an order of magnitude greater than reported for a similar crystal [Appl. Phys. Lett.107, 121903 (2015)APPLAB0003-695110.1063/1.4931492]. Our demonstration opens new possibilities for second-order nonlinear effects on CMOS-compatible nanophotonic platforms.

Recognition of multipolar second-order nonlinearities in thin-film samples

We use two-beam second-harmonic generation to address thin films of silicon nitride (SiN). This technique is able to distinguish between the dipolar and higher-multipolar (magnetic and quadrupolar) contributions to the nonlinearity, as earlier shown for bulk samples. Our results for the SiN films exhibit strong multipolar signatures. Nevertheless, the results can be fully explained by the strong dipolar response of SiN once multiple reflections of the fundamental and second-harmonic fields within the film are properly taken into account. The results show that the recognition of multipolar nonlinearities requires extreme care for samples typically used for the characterization of new materials.

On the determination of Χ(2) in thin films: A comparison of one-beam second-harmonic generation measurement methodologies

The determination of the second-order susceptibility (χ(2)) of thin film samples can be a delicate matter since well-established χ(2) measurement methodologies such as the Maker fringe technique are best suited for nonlinear materials with large thicknesses typically ranging from tens of microns to several millimeters. Here we compare two different second-harmonic generation setups and the corresponding measurement methodologies that are especially advantageous for thin film χ(2) characterization. This exercise allows for cross-checking the χ(2) obtained for identical samples and identifying the main sources of error for the respective techniques. The development of photonic integrated circuits makes nonlinear thin films of particular interest, since they can be processed into long waveguides to create efficient nonlinear devices. The investigated samples are ABC-type nanolaminates, which were reported recently by two different research groups. However, the subsequent analysis can be useful for all researchers active in the field of thin film χ(2) characterization.

Bulk second-harmonic generation from thermally evaporated indium selenide thin films

We investigate bulk second-order nonlinear optical properties of amorphous indium selenide thin films fabricated by thermal evaporation. Such films are shown to exhibit strong and photostable second-harmonic generation (SHG). We report strong thickness dependence of the second-harmonic signals as characterized by the Maker-fringe method. The absolute value of the nonlinear susceptibility tensor of the film is addressed by analyzing the interference of SHG signals from the film and the glass substrate. The value of the joint non-diagonal component of the susceptibility is found to be 4 pm/V, which is comparable to that of widely used second-order nonlinear materials.

Multipolar second-harmonic generation from films of chalcogenide glasses

Chalcogenide glasses are amorphous semiconductors with a number of interesting properties required for photonic devices. Particularly, their optical properties can be tuned through the change of the glass composition. We investigate second-order nonlinear optical properties of chalcogenide glass (Ge27Se64Sb9) thin films fabricated by thermal evaporation. The strong second-harmonic generation observed for the samples investigated is analyzed as a function of incident polarization. Furthermore, the role of multipole effects in second-harmonic generation is also studied by using two beams at the fundamental frequency. Our results suggest that the higher-multipole effects are present and contribute significantly to the second-harmonic response of chalcogenide the samples.

Metamaterials with Tailorable Nonlinear Optical Properties

Second-order nonlinear processes such as second-harmonic generation (SHG) require noncentrosymmetric structures. The development of second-order metamaterials, however, has been hampered by symmetry breaking due to sample defects and shape distortions. The resulting outcomes can be interpreted in terms of effective higher-multipole (magnetic and quadrupole) effects that strongly modify the radiative properties of the samples.

The important role of reflections in multipolar nonlinear optical characterization of thin films

The main limitation of second-order nonlinear optical materials is the requirement of non-centrosymmetry within the electric-dipole approximation. However, higher multipole effects (such as magnetic-dipole and electric-quadrupole) do not suffer from such restriction. Thus, multipole effects can provide an interesting path towards novel second-order materials. Although multipole effects have been already used in nanostructured materials, the design guidelines for strong multipolar responses in bulk of materials are poorly understood and such responses are difficult to address reliably in experiments.

Study of second-harmonic generation from CdS nanostructured thin film

We investigate second-order nonlinear optical properties of a nanostructured cadmium sulfide thin film by optical second-harmonic generation. The relative values of the components of the second-order susceptibility tensor are found to be Xxxz=0.14 and Xzxx=0.07

Ways to optimize the second-harmonic response from metamaterials

Metamaterials consisting of metal nanoparticles offer the possibility to engineer the nonlinear optical properties by varying their plasmonic resonances. The resonances depend on the size, shape, and dielectric environment of the particles as well as their mutual ordering. We show several ways of controlling and optimizing the second-harmonic response of plasmonic metamaterials.