High Q-factor Photonic Crystal Cavities on Transparent Polymers Ashfaqul Anwar Siraji and Yang Zhao Department of Electrical and Computer Engineering, Wayne State University Detroit, Michigan

Outline Introduction Planar Photonic Crystal Cavity (PPCC) PPCC using low refractive index materials Design and implementation Results Summary and Conclusions

Planar Photonic Crystal Cavity Photonic crystals (PhC) are a class of artificial material with periodic variation in the refractive index and complete photonic bandgaps. By utilizing the bandgap, it is possible to construct a cavity where photons remain confined within a very small volume. Planar Photonic Crystal cavity (PPCC) is a special case of the PhC cavity with 2 dimensional periodicity. Photonic confinement within the plane of periodicity is due to the bandgap. Along the third dimension, the confinement is provided by total internal reflection

Planar Photonic Crystal Cavity: Applications PPCC have been utilized in many applications including VCSEL planar waveguides lasers or nano-cavity-based photonics devices optical sensing Elaborate

PPCC using low refractive index and transparent materials High Quality Factor (Q) PPCC have been implemented using high refractive index materials such as Si, GaN, GaAs. However, these materials are not transparent at visible wavelengths. Many display and window materials require the use of transparent materials (glass or polymers). Fabrication of PPCC on glass and polymers can lead to many new devices and systems.

PPCC using Low refractive index Materials Since nanofabrication on glass and polymer materials has been challenging, their potential use in nanophotonics has not been thoroughly explored. However, recently methods have been developed to fabricate nanostructures on glass and polymers, making PPCC on these materials feasible. The existence of bandgap in 1D and 2D photonic crystals built on glass and polymers has been previously demonstrated.

PPCC using Low refractive index Materials Our calculations show that glass as well as several transparent polymers including Polyvinyl-N-carbazole (PVK)[n~1.68] are suitable material for PPCC. As a proof of concept , we have previously designed and numerically demonstrated high Q cavities and highly sensitive sensors using PPCC on glass (n = 1.9).

PPCC using Low refractive index Materials Here, we design and demonstrate high Q cavities using PVK as the material (n = 1.68). Finite difference time domain method has been used to numerically solve the Maxwell’s equation for two separate design of PPCC. For each of the devices, we calculated the impulse response. Subsequently the decaying field strength is calculated, from which the Q factor can be calculated. We calculated the sensitivity of the cavities to different stimuli by calculating the resonance with and then without perturbation.

Design We have designed two PPCC devices with proper periodicity (a) and airhole radius (r). The one in Fig. (a) is a twofold defect cavity where the defect in the lattice act as the cavity. The device in Fig. (b) is a bandedge cavity where the low group velocity mode in the center is confined by bandgap mirror around it.

Design The 3D structure of the devices is shown in the figure. The device is an airbridged structure where a free standing thin film of low refractive index material is separated from the substrate by air.

General Scheme We plan to characterized the PPCC by measuring their photoluminescence. A laser of proper wavelength would act as the pump. The induced photoluminescence would be collected through a monochromator into a photodetector. A conventional InGaAs p-n heterojunction can act as the photodetector.

Results As proof of concept, we have designed and analyzed high Q PPCC on low refractive index materials like glass and PVK. Finite difference time domain method has been used to numerically solve the Maxwell’s equations. For device, we calculate the impulse response and subsequently the decaying field strength, from which the Q factor can be obtained. We evaluate the sensitivity of the cavities to different stimuli by calculating the resonance with and without perturbation.

Results: Nanocavity on Glass The resonance spectrum of the defect cavity is shown for two different period (a) of the defect cavity. The base material is glass (n = 1.9)

Results: Nanocavity on Glass The design guideline of a defect cavity of desired Q is shown. The resonant wavelength is set first. Then the period (a) is chosen for a high enough Q. Then the airhole radious (r) is chosen from Q and a.

Results: Nanocavity on Glass The resonant wavelength of the defect cavity on glass depends on the refractive index of the background (background index). This indicates that the cavity can be used as a refractive index sensor.

Results: Nanocavity on PVK In (a), the different periodicity of the PhC required for achieving different resonant wavelength is shown. Linear Scaling is upheld. In (b), The inplane Q factor (Q||) of the PPCC are plotted against the resonant wavelength. The Q|| of the defect cavity is in the range of 103 while that of the bandedge cavity is in the range of 105.

Results: Nanocavity on PVK The dependence of the Q|| factor on the mirror quality. Bandgap mirrors with more period i.e., higher reflectivity results in higher Q|| .

Results: Nanocavity on PVK The refractive index of PVK polymer varies between 1.68~1.7. The Q|| remains appreciably high across the entire range.

Comparison Between glass and PVK Quantity Glass (n = 1.9) PVK (n = 1.68) Q|| 1.2×104 3.162×103 Q 4000 2200 Sensitivity (nm shift in resonance per unit change in background refractive index) 440 nm/RIU 640 nm/RIU

Experimental Implementation We have a general scheme to experimentally study optical nanocavities using PPCC on low refractive index material. The first step is to prepare free standing of transparent polymers. A thin layer of PVK is spincoated on a cleaned glass surface and then annealed to remove solvent. A holder is made out of plastic by drilling a hole in a square piece of transparency. The holder is attached to the thin film by double sided tape. The sample is submerged in water and then peeled off.

Experimental Implementation Standing alone film: thickness has been measured by ellipsometry to be in the range of 400-800nm for spin speed of 500-2500 rpm.

Experimental Implementation The photonic crystal is to be fabricated on the free standing portion of the thin film using focused ion beam. PVK is a photo-luminescent material. We will use the PL based scheme discussed previously to characterize the PPCC. The PPCC will be pumped with laser and the output light will be gathered in a photodetector. The resonant wavelength will be measured by analyzing the output current of the photodetector using a spectrum analyzer. From the spectrum, the resonant wavelength and Q factors will be obtained. The results will be compared with the numerical analysis. A resonant in the 700~1100 nm range and a Q of ~4000 is expected.

Summary and Conclusion Planar photonic crystal cavity devices are highly useful devices for many applications. So far, PPCC devices have been reported on high refractive index materials like Si, GaN etc. Here, we numerically show that high Q devices can be implemented in low refractive index material like transparent polymers. We studied two devices with different confinement mechanism. Both devices showed high Q and scalability, implying versatility in terms of device design. Experimental implementation of these nano-cavities are being carried out.