Juha M. Kontio

Second-Harmonic Generation Imaging of Metal Nano-Objects with Cylindrical Vector Beams

We introduce an imaging technique based on second-harmonic generation with cylindrical vector beams that is extremely sensitive to three-dimensional orientation and nanoscale morphology of metal nano-objects. Our experiments and second-harmonic field calculations based on frequency-domain boundary element method are in very good agreement. The technique provides contrast for structural features that cannot be resolved by linear techniques or conventional states of polarization and shows great potential for simple and cost-effective far-field optical imaging in plasmonics.

Nanoimprint fabrication of gold nanocones with ~10 nm tips for enhanced optical interactions

We show that nanoimprint lithography combined with electron-beam evaporation provides a cost-efficient, rapid, and reproducible method to fabricate conical nanostructures with very sharp tips on flat surfaces in high volumes. We demonstrate the method by preparing a wafer-scale array of gold nanocones with an average tip radius of 5 nm. Strong local fields at the tips enhance the second-harmonic generation by over 2 orders of magnitude compared with a nonsharp reference.

Third- and second-harmonic generation microscopy of individual metal nanocones using cylindrical vector beams

We demonstrate third- (THG) and second-harmonic generation (SHG) microscopy of individual silver nanocones using tightly focused cylindrical vector beams (CVBs). Although THG is expected to be a weaker process than SHG, the yield for THG with radial polarization was higher than for SHG. We also found an excellent correlation between the imaging properties of THG and SHG, suggesting that both are governed by the same overall features of the individual nanocone. We also found that the transverse spatial resolution of THG with CVBs, particularly RP, exceeds that of SHG. Our work establishes the potential of THG microscopy with CVBs for structure-sensitive imaging of three-dimensional (3D) metal nano-objects.

Broadband infrared mirror using guided-mode resonance in a subwavelength germanium grating

We demonstrate a broadband mirror for the IR wavelength region comprising a subwavelength grating made of germanium. We design and optimize the guided-mode resonances in the structure for TM-polarized incident light by rigorous electromagnetic simulations. The grating structure is realized by nanoimprint lithography and dry etching. The reflectivity of the mirror is over 95% for the wavelength range between 2245 and 3080nm.

Inhomogeneities in the nonlinear tensorial responses of arrays of gold nanodots

We use second- and third-harmonic-generation microscopy to address the tensorial nonlinear responses of individual particles in an array of cylindrical gold nanodots. The responses in both orders exhibit widely-variable, polarization-dependent differences between individual nanodots and thereby indicate tensorial inhomogeneities in the sample. The result provides clear evidence that the second-order response, which is forbidden by symmetry for ideal particles, must arise from small-scale, symmetry-breaking features. A similar result for the third-order response, which is allowed for ideal particles, suggests that both nonlinear responses are dominated by strong variations in field localization around the small-scale features differing among individual nanodots.

Soft stamp ultraviolet-nanoimprint lithography for fabrication of laser diodes

We investigate a novel nanofabrication process called soft ultraviolet (UV) nanoimprint lithography (NIL), for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Because of the cost-effective full-wafer NIL, these lasers are attractive in various applications where low-cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes and improves the quality of the transferred patterns. To the best in our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.

Low Temperature Gold-to-Gold Bonded Semiconductor Disk Laser

We present a gold-to-gold bonding method that combines features of surface activated bonding and capillary bonding. The process is performed at a relatively low temperature of 150°C and therefore allows the integration of materials with highly mismatched coefficients of thermal expansion. In this letter, the potential of this technique is illustrated by assembling a high-power flip chip semiconductor disk laser utilizing a chemical vapor deposition diamond heat spreader. The laser produces up to 14 W of output power at 15°C gain element temperature with a nearly diffraction-limited output beam. Further scaling of bonding area to wafer-level could make this method useful in the packaging of various optoelectronic and microelectronic components.

Tip-enhanced Raman scattering from bridged nanocones.

We present two silver nanocones separated by 450 nm, well beyond the typical gap spacing of coupled nanoantennas, and connected by a metal bridge to facilitate plasmonic coupling between them. The tip-enhanced Raman scattering from crystal violet molecules is found to be almost an order of magnitude higher from the bridged cones than from individual cones. This result is supported by local-field calculations of the two types of structures. The bridged nanocones are easily fabricated by a nanoimprint-based process, thus offering a faster and simpler approach compared to other fabrication techniques.

Soft stamp UV-nanoimprint lithography for fabrication of laser diodes

In this paper, we investigate a novel nanofabrication process called soft UV nanoimprint lithography, for nanopatterning of compound semiconductors. We use flexible stamps with three layers and analyze their performance with wafers composed of III-V semiconductors. The developed stamp configuration is in many ways advantageous for the fabrication of precise gratings for various applications in photonics. We describe how to handle the deformation in both lateral and vertical directions by tuning the softness of the stamp and using a two step imprint process. As an application of the UV-NIL, we demonstrate a fabrication process for a laterally corrugated distributed feedback laser. Our laser fabrication process is free from regrowth and therefore easily adaptable to various material compositions and emission wavelengths. Due to the cost effective full wafer NIL, these lasers are attractive in various applications where low cost, single-mode laser diodes are required. Our development work improves the design freedom of the NIL fabrication process of the laser diodes, and improves the quality of the transferred patterns. To the best of our knowledge, this is the first demonstration of a single-mode laser diode fabricated by soft UV-NIL.

Nanoimprint Lithography - Next Generation Nanopatterning Methods for Nanophotonics Fabrication

Nanophotonics is wide field covering many interesting applications branching from cutting edge science including plasmonics, metamaterials, cavity quantum electrodynamics in high-Q cavities all the way to applied sciences like silicon nanophotonics for on chip optical interconnections and single frequency semiconductor light sources. Most of the practical device demonstrations in these fields utilize nanopatterned surfaces. Applications require patterning of nanoscopic gratings, photonic crystals, waveguides and metal structures. There are many wonderful demonstrations of nanotechnology-based lasers and other photonic components. However, difficult questions related to fabrication need to be addressed before these components enter any market. Demonstrations in the scientific literature have relied heavily on the use of direct writing lithography methods, such as electron beam lithography or focused ion beam lithography. These methods, although excellent for scientific studies, cannot be scaled up to allow cost effective production of nanophotonics. Lithography solutions developed for integrated circuits can produce extremely narrow linewidths and deliver high precision but are difficult to transfer to photonics fabrication. There exist many alternative lithography methods, but their scale-up to cost effective volume production is challenging. Since the introduction of nanoimprint lithography (NIL) in 1995 (Chou et al. 1995), there has been widespread interest in the development of NIL for various applications. As early as in 2003 NIL had gained substantial support and was chosen as one of MITs Technology Review’s “10 Emerging Technologies That Will Change the World” (Technology Review 2003). The selection was justified by the fact that NIL can bridge the gap between lab level nanotechnology research and production level manufacturing requirements. In this chapter, we briefly review the state-of-the-art lithography methods and introduce nanoimprint lithography (NIL), a very cost effective lithography method for nanophotonics applications. In section two we will introduce soft UV-NIL, an imprint method using soft and flexible stamps, as a method for patterning compound semiconductor optoelectronics. Finally, in section three, we highlight some NIL activities based on soft-UV-NIL.