A novel biosensor design using a triangular resonator

Ring resonators are very attractive components for a range of devices due to their promise of compact size and versatility. Applications include optical filters, lasers, switches, modulators, wavelength converters, optical logic, memory, and biosensors.¹ As a result of their very sharp, high-Q-factor resonances, photonic microring resonators have gained increasing interest for photonic integrated circuit (PIC) and biological sensing applications.²

To form a microring cavity, an optical waveguide must form a complete loop. This is typically achieved by deep etching, which allows a ring with a small radius to be fabricated without excessive radiation loss from the increased curvature. Deep etching provides strong lateral confinement, but it is also important to fabricate smooth sidewalls to decrease propagation loss. Another approach is to use total internal reflection (TIR) mirrors, which can be combined with regular optical waveguides to greatly reduce the size of PICs. The TIR mirror fabrication requires a deep etch to create TIR at the semiconductor–air interface. We demonstrated very low loss TIR mirrors in our previous work.³

Triangular resonators with TIR mirrors (see Figure 1), also referred to as ring resonators, can be very useful in application. We fabricated one such resonator integrated with a multimode-interference (MMI) coupler, tapers, TIR mirrors, and semiconductor optical amplifiers (SOAs).^4,5 The ridge waveguide of the ring resonator, which has a width of 3μm, contains active material that forms the SOA. The input and output waveguides are connected through short 15μm-long tapers with linearly decreasing widths. The resonator itself consists of two TIR mirrors positioned such that the angle of incidence is large, and a third TIR mirror with a sharp incident angle. This design has the advantage that it avoids direct coupling between the two access waveguides, as this can cause problems when the distance separating them is too small. The TIR mirror positioned with a sharp angle of incidence can be used for effective optical coupling and sensing, since the sharp angle gives rise to interference, which may result in a phase shift known as the Goos-Hänchen (GH) effect. This phase shift is dependent on external variables, allowing it to be used as a sensing indicator. The sharp incident angle at the TIR mirror also increases the evanescent field due to the GH shift,^4,6 which can also increase the optical coupling efficiency of the PIC system so that it can be used as an effective sensing device for quantitative monitoring of biological events.

Figure 1. Micrograph of the triangular ring resonator integrated with a multimode-interference (MMI) coupler, tapers, total-internal-reflection (TIR) mirrors, and a semiconductor optical amplifier (SOA).

In reflection, the plane wave penetrates into the lower index region a finite distance before changing directions. Therefore the path of light at a TIR mirror can be thought of as reflection from a perfect mirror located at a position offset some distance behind the physical interface. In addition, the different plane wave components of the optical mode in the semiconductor diffract in different directions because not every plane wave component experiences total internal reflection and the position of the perfect reflector is different for each plane wave. The diffraction of some plane wave components miss the output waveguide, resulting in loss. We calculated the mirror offset as a function of the incident angle for both the transverse electric (TE) and the transverse magnetic (TM) polarizations by using the finite-difference time-domain (FDTD) method, the details of which have been reported previously.⁶ The mirror offset is as great as 0.8μm for TE and 2.0μm for TM polarization at an incident angle of 18°. We found that the evanescent field is larger when the incident angle is 18° than when it is 30°.

For the sensitivity analysis, we calculated the variation in the transmittance and resonance shift of the triangular resonator with change in the refractive index of the outer region of the TIR mirror. The resonance shift as the refractive index changes by 10⁻⁵ at an angle of incidence of 18° is 104pm for the TM polarized light and 8pm for the TE polarized light.

We also measured the resonance characteristics of the triangular resonator with TIR mirrors (see Figure 2). When there is no bias, resonance characteristics are not observed. This is due to the strong absorption of the SOA inside the resonator. When a bias current of 15mA is first applied to the SOA inside the resonator, transparency is reached and then a net gain is generated. As a result, very-well-developed band-stop characteristics are observed, where frequencies in a specific range are greatly attenuated. The slight variation of the extinction ratio, or ‘on-off value’, between adjacent resonances is due to the difficulty in locating the exact null position in experiments. The characteristic envelope shape of the maxima is due to the spectral shape of the SOA emission outside the resonator. The measured extinction ratio of the triangular ring resonator was about 6dB near 1550nm, when the incident angle at the TIR mirror inside the resonator was 18°.

Figure 2. Resonance characteristics inside the triangular resonators at 0 and 15mA semiconductor optical amplifier bias currents, indicated by dotted and straight lines respectively.

In summary, we investigated the properties of a novel triangular resonator using TIR mirrors with a long evanescent field around the critical angle. These characteristics of the triangular ring resonator can be used to measure biological events that affect the refractive index of the surrounding medium, such as the presence of a specific biomolecule or the number of biological pathogens in a medium of interest. We are currently developing the triangular sensor to detect individual molecules in the small area at the TIR mirror.

This work was supported by the National Research Foundation of Korea Grant funded by the Korea Government: No. 2010-0016326 and No. 2010-0014612.