Recent efforts of transverse gradient forces to manipulate micro/nano-scale mechanical objects have spawned a number of new types of micro/nano-scale opto-mechanical systems. The optical force could be further enhanced by coupling the nanomechanical devices with high-finesse cavities or slow-light structures because of the highly concentrated electrical field in these nanostructures. Coupling force between two closed guided electromagnetic waves and guided wave with substrate would help to manipulate resonators for various types of applications such as optical memory and optical switch. Tunability of optical devices has shown its potential in the on-chip all-optical circuits while MEMS based tuning mechanisms offer much larger tuning range than pure optical approaches as well as the ability to control the tuning amount precisely. 1-D photonics cavity on a suspended nanobeam leverages the tunable air gap in between the PhC structures, demonstrating a tunable optical device in nanometer scale. The use of piezoelectric material AlN as guiding and tuning waveguides will demonstrate GHz tuning speed and a mechanically tunable nano-scale optical filter, while the actuation voltage and electrical power consumption are expected to be very small, e.g. 1 volt.

 

Micro-ring resonators, which consist of a ring waveguide sandwiched by two straight waveguides i.e., bus and drop waveguides, have been reported by us as good sensing micro-structures that exhibit high sensitivity due to the high quality factor of the resonant peak in the output spectra.When light is input into our designed dual nanoring (DNR) structure, it will be coupled to the waveguide opposite from the input. Slight alteration of the distance between nanorings can lead to a significant change of resonance condition. This phenomenon can be extended to the sensing of various physical attributes such as force or pressure.

In the area of tunable nanophotonics, we have investigated a novel hexagonal nano-ring based resonator made of silicon slab of photonic crystals (PhCs). The resonant behavior are studied so as to achieve resonance wavelength tunability. More specifically, the wavelength shift of resonant peak in the output spectrum of the DNR channel drop filter under various force loads is the key parameter to be explored. Microcantilever, the widely used MEMS structure, is used as the mechanical tunable device with the DNR resonator located at the junction edge of cantilever beam and substrate.

This idea is further extended to pressure sensing, where a triple-nano-ring (TNR) resonator is fabricated on a diaphragm structure. When pressure is applied to the diaphragm surface, the dimensions of the air holes changes. This alters the effective refractive index of the PhCs, which will in turn cause a change in the resonance wavelength that is coupled into backward drop (BD). The sensitivity of the TNR pressure sensor was found to be 0.0094nm/MPa in the pressure range of 5MPa-20MPa, and 0.024nm/MPa for range of 20-40MPa. Therefore, the corresponding minimum detectable pressure is calculated as 10.64MPa and 4.17MPa respectively. This once again emphasizes the ability of PhC as a highly sensitive and compact pressure sensor.

Based on our studies, we have demonstrated and elaborated the ring-ring coupling behavior under Si strain effect in a dual-ring system. The benefits of such structure include ease in tuning and wide linearity range. In addition, the micrometer dimensions of the resonator greatly increases the possible applications that such structures can be used in. Present designs reveal interesting device configurations for optical resonance tuning and pressure/ force sensing with ultra-compact footprint, as the sensing area can go to as small as 10μm².

Selected Publications:

1. Bo Li and Chengkuo Lee, NEMS diaphragm sensors integrated with triple nano ring resonator, Sens. Actuators A: Phys., vol. 172, no. 1, pp. 61-68, 2011. [PDF] [DOI]
2. Bo Li, Fu-Li Hsiao and Chengkuo Lee, Configuration analysis of sensing element for photonic crystal based NEMS cantilever using dual nano-ring resonator, Sens. Actuators A: Phys., vol.169, p352-361, 2011. [PDF] [DOI]
3. Chengkuo Lee, Jayaraj Thillaigovindan, Chii-Chang Chen, Xian Tong Chen, Ya-Ting Chao, Shaohua Tao, Wenfeng Xiang, Aibin Yu, Hanhua Feng, and G. Q. Lo, Si nanophotonics cantilever sensor, Appl. Phys. Lett., vol. 93, 113113, Sept. 2008. [PDF] [DOI]
4. Chengkuo Lee, Rohit Radhakrishnan, Chii-Chang Chen, Jing Li, Jayaraj Thillaigovindan and N. Balasubramanian, Design and Modeling of A Nanomechanical Sensor Using Silicon Photonic Crystals, IEEE J. Lightwave Technol., vol. 26, no.7, pp. 839-846, Apr. 2008. [PDF] [DOI]

Cantilever beam with dimension in micro/nanometer scale is a well-adopted chemical and biological MEMS/NEMS sensor due to the high sensitivity of cantilever deflection. By using bio-compatible photonic crystal cantilever, the size of the device can be reduced significantly as it offers very well optical confinement due to ultra-low bending loss. This is in line with the idea of requiring only a small amount of the sample, while not compromising on the sensitivity of the sensor. In our designed cantilever structure, input light are coupled into the resonator via the waveguide, causing the resonator to be excited when the input light is matched with particular modes. The wavelength of resonant light is dominated by the geometrical parameter, refractive index of resonator and ambience. A Si-based cantilever sensor with 4-holes defect packaged in a microfluidic channel PhC resonator is used as readout for chemical sensing and analysis. Solution containing molecules are made to flow through this microfluidic channel such that the targeted molecules we wish to investigate are bounded on to cantilever surface due to the biomolecule selective binding, e.g., antigens to antibodies, When this happens, the cantilever undergoes deflection due to a differential stress caused by the forces involved. The induced cantilever deflection is characterized in terms of the resonant wavelength shift as a result from the deformation of the PC resonator. Due to the minute size of the cantilever, the structure can be extended to detect very small biomolecules such as cells and proteins. The holes at which the biological species are attached to will result in a difference to the extent of resultant wavelength shift. Such characteristics allow us to enhance the sensing capability of the sensor and thus boosts the flexibility of the whole device.

Si-based nanoring structure is also another interesting way to detect biomolecules. When the biological species, such as, DNAs are manipulated to the vicinity of point defects of a particular photonic crystal, the targeted molecules are bound onto the surface of the ring resonator due to the biomolecule selective binding. This will modify effective refractive index of the resonator and in turn changes the local electromagnetic field. This will then cause a shift of the resonant wavelength, allowing monitoring of molecules binding to be done in situ. For nanoring structures, the resonant wavelength red shifts during biomolecule detection. In our design, the minimum detectable biomolecule weight in a sensing hole for a nano-ring resonator of two-hole coupling distance is derived as 0.23fg. This result demonstrates promising applications which demands detection of biomolecules down to the level of single copy of DNA.

Selected Publications:

1. Chong Pei Ho, Bo Li, Aaron J. Danner and Chengkuo Lee, Design and modeling of 2-D photonic crystals based hexagonal triple-nano-ring resonators as biosensors, Microsyst. Technol., vol. 19, no. 1, pp. 53-60, 2013. [PDF] [DOI]
2. Trong Thi Mai, Fu-Li Hsiao, Chengkuo Lee, Wenfeng Xiang, Chii-Chang Chen, and W. K. Choi, Optimization and comparison of photonic crystal resonators for silicon microcantilever sensor, Sensors & Actuators A, vol. 165, no. 1, pp.16-25, 2011. [PDF] [DOI]
3. Fu-Li Hsiao and Chengkuo Lee, Computational study of photonic crystals nano-ring resonator for biochemical sensing, IEEE Sensors Journal, vol. 10, no.7, pp.1185-1191, 2010. [PDF] [DOI]
4. Wenfeng Xiang and Chengkuo Lee, Nanophotonics sensor based on microcantilever for chemical analysis, IEEE J. Selected Topics in Quantum Electronics, vol. 15, no. 5, pp. 1323-1326, Sep/Oct, 2009. [PDF] [DOI]

 

Electrical interconnects which are normally employed in filters result in a huge bottleneck due to their limited bandwidth and throughput which has led to considering optical interconnects, but on scaling to nano-range, they are restricted by the phenomenon of diffraction which has made plasmonics a viable technology. Plasmonics offers better integration for interfacing with nanoscale electronics and good efficiency as it relies on the excitation of surface plasmon polaritons (SPPs) coupled to the oscillations of free electrons in a metal. These surface plasmon polaritons allow a stronger intrinsic confinement due to their maximum electromagnetic field intensity at the surface that decays away from the metal-dielectric interface, thus enabling the use of this technique in integrated nanophotonic circuits.

Among the many plasmonic waveguides based on the principle of SPPs, Metal-Insulator-Metal (MIM) waveguides were found to offer good confinement and propagation length, where the mode is confined to the dielectric core in the form of a coupled gap-SPP between the two interfaces. In this research, we have proposed a nanoplasmonic optical filter that uses a complementary square split-ring resonator (CSSRR) and it was seen that by including a nano-wall in the structure, it is possible to excite the non-integer modes as well. The CCSRR structure is formed by positioning a metallic nano-wall inside a MIM circular-ring resulting in a split-ring resonator (SRR). The two dimensional finite-difference time-domain (FDTD) method is employed to investigate the influence of the nano-wall on the filter characteristics and it was seen that the wall positioning offers a control over the excitation of the resonant modes. In contrast to the normal CSSRR principles, the structure proposed here does not consider the perturbation effect of the corners and the newly excited non-integer modes were found to be highly sensitive to the wall dimensions as opposed to high-order modes.The results from the proposed CSSRR structures apart from offering great flexibility to integrated optical circuits also provide an alternative way for designing tunable multi-channel filtering devices based on an analogue of electromagnetically induced transparency.

Selected publications:

1. Fusheng Ma and Chengkuo Lee, Optical Nanofilters Based on Meta-Atom Side-Coupled Plasmonics Metal- Insulator-Metal Waveguides, IEEE J. Lightw. Technol., vol. 31, no. 17, pp. 2876-2880, 2013. [PDF] [DOI]

 

Fabry-Perot (FP) filters have been widely employed in optical devices and optical telecommunication systems for their wavelength selective properties, wide frequency spectrum range (FSR) and wide dynamic range. Some of the commonly employed techniques to enhance the finesse (F) and Quality factor (Q-factor) of FP filters are waveguides, photonics crystals cavity and ring resonators, but all these methods suffer from high propagation losses. Providing a high refractive index (RI) contrast in the dielectric stack of the FP cavity is found to increase the Q-factor.

We propose a design that will reduce the propagation losses normally encountered while using distributed Bragg reflector (DBR) mirrors by allowing the light to resonate effectively in the FP cavity and propagate through the resonators. We propose a design where we integrate antireflection grating structures (ARG) on the inner walls of the FP resonator. Our proposal includes FP resonators that use rectangular grating (RG) and triangular grating (TG) structures to achieve high Q-factor and the results have been compared to the conventional FP resonator with a slot. The results show that the intensity of the FP resonator with RG was enhanced 1.9-fold higher when compared with that of the FP resonator with slot, while the intensity of the FP resonator with TG was enhanced 4.5-fold and 2.3-fold higher than those of FP resonator with slot and RG, respectively. Also, Q-factor of the hybrid FP resonator with TG and two slots was enhanced 9.5-fold and 4.7-fold better than those values of FP resonator with one slot and FP resonator with TG, respectively. These filters can also be tuned using microelectromechanical systems to offer better tuning range and they can be realized for wavelength division multiplexing, sensors, and nonlinear optics applications.

Selected publications:

1. Yu-Sheng Lin, Chong Pei Ho, Kah How Koh and Chengkuo Lee, Fabry-Perot filter using grating structures, Opt. Lett., vol. 38, no. 6, pp. 902-904, 2013. [PDF] [DOI]

 

 

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