Elsevier

Applied Surface Science

Volume 355, 15 November 2015, Pages 1186-1191
Applied Surface Science

Surface topography to reflectivity mapping in two-dimensional photonic crystals designed in germanium

https://doi.org/10.1016/j.apsusc.2015.07.218Get rights and content

Highlights

  • Laser ablation is used for drilling a periodic 2D photonic structure.

  • Confinement of radiation is revealed by infra-red spectromicroscopy correlated with numerical calculations.

  • Telecommunication range is accessible upon tuning conveniently the processing parameters.

Abstract

Light confinement in a two dimensional photonic crystal (2D PhC) with hexagonal symmetry is studied using infra-red reflectance spectromicroscopy and numerical calculations. The structure has been realized by laser ablation, using a pulsed laser (λ = 775 nm), perforating an In-doped Ge wafer and creating a lattice of holes with well-defined symmetry.

Correlating the spectral signature of the photonic gaps recorded experimentally with the results obtained in the finite difference time domain and finite difference frequency domain calculations, we established the relationship between the geometric parameters of the structure (lattice constants, shape of the hole) and its efficiency in trapping and guiding the radiation in a well-defined frequency range. Besides the gap in the low energy range of transversal electric modes, a second one is identified in the telecommunication range, originating in the localization of the leaky modes within the radiation continuum. The emerging picture is of a device with promising characteristics as an alternative to Si-based technology in photonic device fabrication with special emphasize in energy storage and conversion.

Introduction

Since 1987, when the first theoretical [1] and experimental [2] reports on photonic crystals (PhCs) were published, many interesting properties have been studied in all of the three dimensions. In order to achieve a proper manipulation of light in optoelectronic devices routinely used in telecommunication and information technology, extensive efforts are conducted to the development of PhCs and of devices with PhCs [3]. These artificially created periodic dielectric materials possess a photonic bandgap (PBG), preventing light from being transmitted in a range of frequencies [4], [5], diffract light in UV [6], visible [7], and near infrared [8] regions and are capable of photon localization. The possibility of obtaining PBGs for certain PhCs designs, i.e. an energy gap between adjacent bands where no photonic mode exists in a certain propagation direction, is encoded in their photonic band structure (the relationship between each photonic mode energy and wave vector). Therefore such periodic structures allow the control of the allowed or suppressed optical modes. The range of potential applications is very generous, including many optical devices, such as optical filters [9], [10], waveguides [11], [12], optical switches [13], sensors [14], PhC fibers [15], and coupled devices [16]. By tuning the design of the system, such as refractive index, lattice constant and sample thickness, various heterostructures can be achieved which tailor the optical response of these structures. Till now, few researches have been made on germanium-based (Ge) two-dimensional (2D) PhCs because of the low theoretical transmittance of Ge, which decreases as the temperature increases, and due to a higher refractive index temperature coefficient (dn/dT) compared to other infrared optical materials. Despite of these drawbacks, Ge possesses many advantages, such as the highest refractive index (n  4.0) of any of the infrared bulk transmitting substrate materials and low dispersion properties across a wide range of temperatures. Germanium is also non-hygroscopic, non-toxic and possesses good thermal conductivity. Therefore, Ge offers both a wide range of well established and potential technological applications of high relevance and impact. For example, in microelectronics, due to the large mobility of both electrons and holes in bulk germanium, the speed and drive current of CMOS based logic devices could be increased [17]. Even if silicon has been established as the material of choice for this industry, in photonics it has some constrains related to the limited degree of freedom in material design combined with the indirect bandgap. On the other hand, germanium-based metal-oxide-semiconductor field-effect transistors (MOSFETs) had some limitations since it lacks a stable native oxide for gate insulation (in contrast to SiO2 on Si). However, the progress on the device miniaturization on high-k dielectrics opened the door to potential high mobility replacements for Si as the channel material, germanium having the advantage of direct CMOS compatibility. Due to its direct compatibility with the silicon microelectronics platform and to its strong interband absorption at near-infrared optical communication wavelengths, Ge it is known to be a good photodetector material for use in on-chip data distribution [18], [19], as well as in other applications like lasers [20], solar cells [21] and emitters [22].

In this work we present the results of a pilot-study aiming to establish the feasibility of using Ge as substrate for applications in photonics. This potential is illustrated through the study of 2D PhCs. We have investigated experimentally and numerically the spectral reflectivity of 2D triangular PhCs designed in a germanium matrix. The holes were drilled by ultra-short pulses laser for patterning the Ge surface with sub-micrometer resolution. Laser ablation is a direct writing method, suitable for a large class of materials, no corrosive chemicals being used and no mask being required. Compared to the classical lithography and other more complex fabrication techniques, it is a versatile and an environmental friendly technique, being a useful method for producing periodical systems, as well as any computer designed arrangements required by various optical devices based on photonic crystals [23], [24]. A number of publications are already available on photonic or plasmonic structures obtained by laser ablation on silicon [25], chalcogenide glasses [26], [27] or metallic films [28]. Nevertheless, to the best of our knowledge, no report exists up to now on two-dimensional hexagonal photonic crystals prepared by femtosecond-laser ablation of Ge.

The paper is organized as follows: basic features of the experimental setups are described in Section 2. In Section 3 we introduce the geometry problem and represent a brief description of finite-difference time-domain (FDTD) method as our numerical tool in this study. In Section 4 we present and study in detail the experimental and numerical results followed by the concluding remarks.

Section snippets

Experimental details

The holes in the Ge(0 0 1) samples, cut from a p-type Ge wafer (In-doped), were performed by a femtosecond laser ablation setup (see Fig. 1). Direct laser writing (DLW) using ultrafast lasers became a popular technique for the accurate structuring of materials in two dimensions [29], [30], [31]. The optical source for fabrication the periodic structure was a femtosecond regenerative amplifier (CPA-2101 system from Clark-MXR), which delivers pulses at 2 kHz repetition rate, with 200 fs pulse

Numerical details

The theoretical computations performed with the MIT Photonic Bands (MPB) Package [32] and MIT Electromagnetic Equation Propagation (MEEP) [33] provide very accurate prediction on the photonic bands dispersion, mode localization and localization/propagation across a defect or into an optical cavity.

MPB is an open source software package, which directly computes eigenstates and eigenvalues of the Maxwell equations (1) in the frequency domain [34] for arbitrary electromagnetic (EM) sources:×1ε(r)

Results and discussion

A scanning electron microscope image of the 2D photonic structure investigated within our work is presented in Fig. 3. It consists in a triangular lattice of air cylinders drilled into the Ge matrix, with lattice constant a = 1.5 μm. We approximate the holes as infinitely long air cylinders with radius R = 0.375a as established from the SEM images. The static dielectric permittivity of Ge matrix was taken as ɛ = 16.2 [37]. The imaginary component of the dielectric was assumed to be zero, as it does

Conclusions

A two dimensional photonic system with triangular symmetry and a = 1.5 μm lattice constant was designed in In-doped Ge wafer by femtosecond laser ablation. The laser irradiation parameters were optimized in order to achieve a periodic photonic structure. It was further experimentally and theoretically investigated by means of infra-red reflectance spectromicroscopy, scanning electron microscopy and numerical calculations. The finite difference time domain and finite difference frequency domain

Acknowledgments

This work was financed by the Romanian UEFISCDI Agency under Contract PN 2 – Partnerships No. 152/2011.

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