Döne Yilmaz

Asymmetric light propagation in chirped photonic crystal waveguides

We report numerical and experimental investigations of asymmetric light propagation in a newly designed photonic structure that is formed by creating a chirped photonic crystal (PC) waveguide. The use of a non-symmetric distribution of unit cells of PC ensures the obtaining of asymmetric light propagation. Properly designing the spatial modulation of a PC waveguide inherently modifies the band structure. That in turn induces asymmetry for the lights followed path. The investigation of the transmission characteristics of this structure reveals optical diode like transmission behavior. The amount of power collected at the output of the waveguide centerline is different for the forward and backward propagation directions in the designed configuration. The advantageous properties of the proposed approach are the linear optic concept, compact configuration and compatibility with the integrated photonics. These features are expected to hold great potential for implementing practical optical rectifier-type devices.

Theoretical and experimental investigations of asymmetric light transport in graded index photonic crystal waveguides

To provide asymmetric propagation of light, we propose a graded index photonic crystal (GRIN PC) based waveguide configuration that is formed by introducing line and point defects as well as intentional perturbations inside the structure. The designed system utilizes isotropic materials and is purely reciprocal, linear, and time-independent, since neither magneto-optical materials are used nor time-reversal symmetry is broken. The numerical results show that the proposed scheme based on the spatial-inversion symmetry breaking has different forward (with a peak value of 49.8%) and backward transmissions (4.11% at most) as well as relatively small round-trip transmission (at most 7.11%) in a large operational bandwidth of 52.6 nm. The signal contrast ratio of the designed configuration is above 0.80 in the telecom wavelengths of 1523.5–1576.1 nm. An experimental measurement is also conducted in the microwave regime: A strong asymmetric propagation characteristic is observed within the frequency interval of 12.8 GHz–13.3 GHz. The numerical and experimental results confirm the asymmetric transmission behavior of the proposed GRIN PC waveguide.

Design of a Wavelength Selective Medium by Graded Index Photonic Crystals

In this paper, a novel technique for spatial wavelength division using graded index photonic crystals (GRIN PCs) is proposed. The designed GRIN PC structure with hyperbolic-secant refractive index profile at a fixed frequency generates spatial shifts in the longitudinal direction with respect to different incident wavelengths so that distinct path trajectories are followed by propagating beams through the graded medium. Due to gradual variation of the refractive index profile (incremental decrease), an incident beam with an appropriately selected wavelength undergoes continuing refraction and finally, total internal reflection (TIR) occurs at the turning point. Due to the wavelength-dependency characteristic of GRIN PCs, TIR takes place at different positions for different wavelengths and thus, light wave emanates from the GRIN medium at different locations with different exit angles. It is revealed that the designed structure can work in three distinct frequency regimes and is able to separate a certain number of incident beams with different wavelengths, viz. from three to seven different wavelengths. This condition enables the deployment of the GRIN PC configuration for the design of wavelength selective media.

Biosensing With Asymmetric High Refractive Index Contrast Gratings

We propose a novel label-free optical biosensor configuration based on a spectral refractive index sensing method by utilizing asymmetric high index contrast dielectric gratings. The characterization and performance evaluation of the designed biosensor are carried out by using 2D and 3D finite-difference time-domain methods. The designed sensor is illuminated in-plane with transverse-electric polarized light wave. The salient properties of the device are compactness, having a simple measurement technique, easy excitation scenario, operating label-free, and possessing high sensitivity value reaching up to a level of 450 nm/RIU. The proposed sensor configuration may bring new impetus to the field of alternative biochemical sensing approaches.

On-chip photonic filter design via objective-first algorithm

We propose and demonstrate compact on-chip optical filters with ultra-high efficiencies, which are designed via the recently-emerged objective-first inverse design algorithm. The all-dielectric high-pass, low-pass and all-pass filters presented in this study operate within the telecommunication wavelength spectrum of 1260 - 1625 nm. The high-pass filter shows an outstanding performance for the transmission efficiency higher than the frequency of 210 THz and an effective suppression of the other spectral region. Also, the proposed low-pass filter exhibits high transmission and significant blocking at the ranges of 170 Thz-205 Thz and 205 Thz-240 Thz, respectively. Furthermore, the designed allpass filter possesses notable spectral transmissivity and blocking characteristics. The promising features of the structures were also verified via the finite-difference time-domain method. Such manufacturable optical filters with great device performances are favorable candidates for next-generation photonic applications.

Strong light confinement of tunable resonances in low symmetric quasicrystal through orientational variations

We propose octagonal quasi-crystal designs providing effective light confinement for different resonance frequencies through the structural modification with the utilization of low-symmetric photonic unit cells. The effect of rotational symmetry reduction on the cavity resonance appearing in the corresponding photonic bandgap of each structure has been investigated. Relatively small dielectric cylinders have been additionally located at discrete angular positions with particular distances from the center of the each core cylinder and the noteworthy resonance peaks have been observed to emerge in the bandgaps. Rotational symmetry of the proposed structures is to be modified by varying the angular displacement of the smaller quasi-crystalline rods with the angle θ in terms of the x-axis of the small rod. The successful demonstration of tunable resonance modes has been achieved numerically and experimentally for the first time by tailoring the positional parameters and reducing the crystalline symmetry. Strongly localized modes in the proposed quasi-crystals have great potential for various slow light applications along with other technologies such as sensors, lasers and memory units.

Backward propagation of surface slow light in photonic crystals through morphological diversity

We propose and demonstrate photonic crystals (PCs) providing backward-directional propagation of surface slow waves, which is significant for potential PC-based photonic applications. An effective pathway for backward directing of surface slow light along with the modification of other important characteristics is presented via implementing surface morphological diversity in PCs. With the surface orientation angle varying from 900 to 300, the newly appearing bands inside the band gap shift to higher frequencies, and negative group indices up to -100 are observed as the strong indication of backward propagation. Furthermore, dependence of the propagation direction on the surface corrugation angle has been verified via detailed time-domain analyses and microwave experiments using dipole source. As obtained from both the numerical and experimental results, for instance, the structure with 600 provides a well-defined backward propagation. In addition, normalized-delay-bandwidth-product can easily be modified by varying the surface orientation angle in the proposed structures according to the necessities of the application. Furthermore, the group velocity dispersion spectra extracted for each periodic structure exhibit considerably high-range near-zero values as 0.139 ps2/m at 900 for the range of 495.54-501.25 nm and 0.176 ps2/m at 85° for the range of 495.66-501.25 nm. Third order dispersion spectra also obtained for the proposed PCs show near-zero values as 0.098 ps3/m at 900 and 0.113 ps3/m at 85° in the corresponding frequency regimes. Facile control of the key characteristics such as backward-directed surface wave propagation in the periodic dielectric structures having morphological diversity serves a great potential for nextgeneration photonic applications.

Parametric study of wavelength demultiplexers designed via objective-first algorithm

We demonstrate efficient and compact 1xN wavelength-demultiplexing by using objective-first inverse design algorithm. Ultra-high device performances were achieved for the certain designs of 1x2, 1x3, and 1x4 demultiplexers with very small footprints at the orders of a few microns. The presented 1x2, 1x3, and 1x4 devices operate at the wavelength sets of 1.31μm, 1.55 μm; 1.31 μm, 1.47 μm, 1.55 μm, and; 1.31 μm, 1.39 μm, 1.47 μm, 1.55 μm, respectively. The transmission efficiencies at the corresponding target wavelengths of each vertically or horizontally aligned channels were obtained mostly to be near-unity together with very small crosstalk ranges of around 0.01% - 1.2%. The inverse design approach allowing the implementation of more than four output channels together with the novel functionalities will pave the way for compact and manufacturable 1xN couplers, which is of ultimate significance for integrated photonics.