Nanostructuring of alumina optical waveguides by hot water treatment for tuning sensor output
Introduction
An evanescent wave sensor typically consists of three components: a cover medium with a thin molecular layer that is created by gas or biological molecules, a transducer, and an electronic part. Adsorption and desorption of the target molecules on transducer surfaces at the interface of the transducer and the cover medium alters the effective refractive index of the guided modes (Neff, TE and Neff, TM) as well as the thickness of the molecular layer (tm). Detection of these molecular changes is possible with an optical system. Even though the basic principles or configurations of the optical systems can vary, all systems are based on the generation of an evanescent field on the transducer surface in a cover medium where the molecular activity happens. Depending on the optical configuration, layers of the transducer, and the surface material (dielectric/metallic film), various modes of electromagnetic waves can be excited as surface plasmon (SP) or waveguide modes. The transducer may consist of protective, passive, waveguide, and metallic layers to excite electromagnetic mode(s), adhesion, and other layers in addition to a substrate for the reinforcement of deposited thin films.
The light interacts with the structure and characteristics of the nanostructure, such as its size, shape, spacing, and material type, which acts to modulate the light. Thus, enhancement and control of the light is possible via the optimization of the structure's dimensions, the aspect ratio of these dimensions, and the density of the nano-features. The enhanced light field is more sensitive to small refractive index changes close to the volumes [1]. Therefore, nanostructuring has the potential to improve the sensitivity of the chemical and biological optical waveguides [2] and SP sensors [3]. To achieve better sensitivity, both the optimal dimensions and the aspect ratio of the two major dimensions of the structures (e.g., periodic gratings, pillars, etc.) can be varied depending on the wavelength, optical properties, and the thicknesses of the sensor layers. The light enhancement factor, caused by localized SP on nanostructured metal surfaces, can be up to 108 around the resonance wavelength [4].
Recent developments in advanced fabrication technology have allowed researchers to make nano modifications on thin films and surfaces [5], [6]. There are two major nano-fabrication methods, namely top–down and bottom–up, and the most common method for nanofabrication is lithography, i.e., photolithography, electron beam lithography [7], X-ray lithography [8], and ion beam lithography [9]. Although it is possible to fabricate nano-structures on the surface of the waveguide via lithography using a “top–down” approach that creates surface nanostructures with well-controlled size, shape, and spacing, this procedure requires many steps and is time consuming and very expensive. Unconventional methods used as alternatives to traditional nanofabrication are straight-forward and more economical than other methods [9]. Many uniform, reproducible, and distinctive nanostructures can be fabricated by taking advantage of nanodimensions, judiciously combining materials, manipulating residual stress, and taking advantage of anisotropic elastic properties [9], [10], [11], [12]. It is possible to produce flat films, tubes, coils, rings, and “rug wrinkles” using unconventional fabrication methods. One of these unconventional fabrication approaches, hot water treatment, is well known and the nanostructuring of alumina films in hot water has been used to produce flower-like structures for anti-reflection coatings [13] and water-repellent surfaces [14]. One-dimensional pillar-like structures can form during hot water treatment of alumina films [15] and this nanostructuring can enhance their optical transmission [16]. Surface nanostructures improve transmission behavior when alumina films are hot water treated long enough to form tall and dense flower-like structures on the surface [16]. Even though some studies have used hot water-treated alumina films to create water-repellent surfaces and anti-reflection coatings [13], [14], [16], hot water treatment has never been used for the fabrication of nanostructured alumina waveguide sensors. Characteristics of such alumina nanopillars (i.e., their size, shape, and spacing) may alter spatial properties of the light (i.e., intensity, polarization, phase, and frequency) confined to the waveguide. Thus, when compared to smooth waveguides, nanostructuring potentially (1) promises an improvement in the localized electromagnetic field, (2) provides for the efficient transmission of electromagnetic energy through the nanostructure to the molecular layer (and vice-versa), and (3) allows for the control of the flow of electromagnetic energy (scattered and transmitted components).
In this study, we describe planar integrated optical waveguides (IOWs) initially fabricated from a very homogeneous alumina layer and modified by the growth of dense nano-pillars on the waveguide surface to allow for the control and enhancement of light scattering at the surface. We used a hot water treatment, which is an unconventional method, to form the nano-pillars on the surface of the alumina IOWs. Tuning of the amount of scattered light and of the optical coupling was achieved by altering the dimensions of the surface nanostructures. Atomic force microscopy measurements were made to determine the relationship between the apparent heights of the pillars and the water temperature and treatment duration during waveguide fabrication. Optical transmission spectra were collected from the samples that were immersed in the hot deionized (DI) water and optical coupling tests were conducted for some of the samples. In addition, scanning electron microscopy (SEM) images of nanostructured integrated optical waveguides (n-IOW) cross-sections were taken. Finally, scattering ratios of the coupled light were calculated and plotted versus the average heights of the nanopillars.
Section snippets
Fabrication and characterization of n-IOWs
The development of our n-IOWs consisted of several steps, which included the fabrication of grating couplers, deposition of alumina film waveguides, and nanostructuring of the waveguide surface. The method utilizes a unique and straightforward approach based on a hot water treatment for nano-fabrication of pillars on the waveguide surface. Experimental procedures for all of these fabrication and characterization steps are discussed in detail below.
Typical nanostructure formation
There are several factors that play dominant roles in determining the characteristics (density, height and diameter) of the pillar formation, and these include film grain size and its distribution, grain orientations, film surface texture, deposition temperature, post-process (hot water treatment) temperature, exposure duration, film and substrate thicknesses, and the thermal expansion difference between the film and substrate [26]. In this study, we only investigated the effect of hot water
Conclusions
We present a simple and economic method for the fabrication of n-IOWs consisting of a very homogeneous alumina layer in which the light propagates via total internal reflection (TIR) and nano-pillars on the waveguide's surface where the scattered light can be enhanced and controlled. Hot water treatment creates nanopillars on the surface, while a solid layer of alumina remains as a nanostructured layer and acts as a wave-guiding medium. Results from topology measurements of hot water-treated
Acknowledgment
This work was supported by the Materials Institute, TUBITAK Marmara Research Center. The initial part of the work was done at the University of Louisville with the support of the National Institutes of Health (award #NIH, R21RR022864-02 to Dr. S.B. Mendes). The author would like to thank to Dr. Sergio B. Mendes for his corrections and Dr. Tarık Baykara and Dr. Kerim Allahverdi for their support. Finally, the author is grateful to Dr. Wei-Min Li and Otto Laitinen of Picosun Oy Inc. for coating
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