In this paper, we report the theoretical study of polymer-based photonic crystals for laser beam steering
which is based on the superprism effect as well as the experiment fabrication of the two dimensional
photonic crystals for the laser beam steering. Superprism effect, the principle for beam steering, was
separately studied in details through EFC (Equifrequency Contour) analysis. Polymer based photonic
crystals were fabricated through double exposure holographic interference method using SU8-2007. The
experiment results were also reported.
We experimentally demonstrate a novel slot photonic crystal waveguide for guiding light with low group velocity in a
100-nm-wide low-index region. The unique optical property and structural features of the slotted photonic crystals best
match the requirements for active material-based silicon devices. We integrate the novel photonic crystal waveguide with
a multimode interference-based coupling structure and measure a 20dB efficiency enhancement compared with direct
coupling configuration. The measured transmission spectra are in good agreement with simulated band diagram.
We experimentally demonstrate an all-silicon optical transmission controller based on a
semiconductor-oxide-semiconductor capacitor embedded in a slot photonic-crystal waveguide. We incorporate a
multimode interference-based structure to reduce the coupling loss induced by the waveguide mode mismatch. We
perform a detailed DC characterization of the electro-optic device including the DC modulation test and the
evaluation of the resistance-capacitance constant. The measured modulation curve is in good agreement with our
theoretical analysis. Calculation of the effective index change indicates as much as 30 times efficiency enhancement
compared with the slotted silicon rib waveguide. Such a waveguide layer can serve as the active layer for fully
embedded optical interconnect architecture with minimum power consumption.
An analysis of superprism effect in low index contrast polymer photonic crystal is presented. It
shows extremely sensitivity to the wavelength and angle of the incident light due to the strong
anisotropy of photonic band structures. Two-dimensional (2-D) polymer photonic crystals with
triangular lattice structure were fabricated by soft lithography using elastomeric
polydimethylsiloxane (PDMS) templates. Dense two dimensional photonic crystal superprism
structures with feature sizes of 150-500nm and aspect ratios of up to 1.25 were successfully
replicated by soft lithography. Large field size and easy fabrication are two major advantages when
compared with other imprint technology. Atomic Force Microscopy images showed that the molded
structures had high fidelity to the masters. Such an effective, low cost, and high throughput soft
lithography technique could find wide use in making photonic crystal based nanostructures.
An ultra-compact silicon Mach-Zehnder interferometer (MZI) modulator featuring p-i-n-diode-embedded photonic crystal
waveguides has been fabricated. As carrier injection is the only practical option for optical modulation in silicon, a lower limit of
current density (~104A/cm2) exists for achieving gigahertz modulation in the widely employed p-i-n diode configuration. Electrical
simulations have been performed to design and analyze the device. The device interaction length was reduced by one order of
magnitude compared to the conventional waveguide based MZI modulators by taking advantage of the slow group velocity exhibited
by photonic crystal waveguides (PCWs). A maximum modulation depth of 93% has been obtained under an injected current of 7.1
mA. High-speed optical modulation at 1 Gbit s-1 in the 1.55 micron wavelength region was experimentally demonstrated. To our
knowledge, this is the fastest speed ever achieved for a p-i-n diode based integrated silicon MZI modulator.
Silicon nanophotonics has recently attracted great attention since it offers an opportunity for low cost opto-electronic
solutions based on silicon complementary metal oxide semiconductor (MOS) technology. Photonic crystal (PhC)
structures with slow photon effect are expected to play a key role in future large-scale ultra-compact photonic
integrated circuits. A novel vertical-MOS-capacitor-based silicon PhC waveguide structure was proposed to achieve
active transmission control via the free carrier plasma dispersion effect. We designed and fabricated a single-arm
PhC waveguide with MOS gate defect using silicon-on-insulator (SOI) substrate and demonstrated that a defect
mode was present in the infrared region. Plane wave expansion (PWE) method based simulation indicated that high
group index of the fabricated PhC waveguide could be achieved near the transmission band edge. Further
investigation demonstrated that such PhC MOS capacitor would be a good candidate to realize ultra-compact
transmission control.
Photonic crystal based superprism offers a new way to design new optical components for beam steering and DWDM application. 3D photonic crystals are especially attractive as they could offer more control of the light beam based on the needs. A polygonal prism based holographic fabrication method has been demonstrated for a three-dimensional face-centered-cubic (FCC)-type submicron polymer photonic crystal using SU8 as the photo-sensitive material. Therefore antivibration equipment and complicated optical alignment system are not needed and the requirement for the coherence of the laser source is relaxed compared with the traditional holographic setup. By changing the top-cut prism structure, the polarization of the laser beam, the exposure and development conditions we can achieve different kinds of triclinic or orthorhombic photonic crystals on demand. Special fabrication treatments have been introduced to ensure the survivability of the fabricated large area (cm2) nano-structures. Scanning electron microscopy and diffraction results proved the good uniformity of the fabricated structures. With the proper design of the refraction prism we have achieved a partial bandgap for S+C band (1460-1565nm) in the [111] direction. The transmission and reflection spectra obtained by Fourier transform infrared spectroscopy (FTIR) are in good agreement with simulated band structure. The superprism effects around 1550nm wavelength for the fabricated 3D polymer photonic crystal have been theoretically calculated and such effects can be used for beam steering purpose.
Ultra-compact silicon-photonic-crystal-waveguide-based thermo-optic and electro-optical Mach-Zehnder interferometers
have been proposed and fabricated. Thermal and electrical simulations and optical characterizations have been performed.
Experimental results were in good agreement with the theoretical predictions.
Photonic crystals (PhCs) provide a promising nanophotonic platform for developing novel optoelectronic devices with significantly reduced device size and power consumption. Silicon nanophotonics is anticipated to play a pivotal role in the future nano-system integration owing to the maturity of sub-micron silicon complementary metal oxide semiconductor (CMOS) technology. An ultra-compact silicon modulator was experimentally demonstrated based on silicon photonic crystal waveguides. Modulation operation was achieved by carrier injection into an 80-micron-long silicon PhC waveguide of a Mach-Zehnder interferometer (MZI) structure. The driving current to obtain a phase shift of pi across the active region was as low as 0.15 mA, owing to slow light group velocity in PhC waveguides. The modulation depth was 92%. The electrode between the two waveguide arms of the MZI structure was routed to the space outside the MZI. In real devices, this planarized routing design would be essential to integrating the silicon modulator with electrical driving circuitry on a single silicon chip. For laboratory test, this routing scheme also eliminated the need of placing a bulky pad between the two arms and gave our modulator a distinctive slim profile and a much smaller footprint. Polymeric photonic crystals were designed for superprism based laser beam steering applications, and were fabricated by nano-imprint and other techniques.
An ultra-compact silicon electro-optic modulator was experimentally demonstrated based on highly dispersive silicon photonic crystal (PhC) waveguides. Modulation operation was demonstrated by carrier injection into an 80 μm-long silicon PhC waveguide of a Mach-Zehnder interferometer (MZI) structure. The π phase shift driving current, Iπ, across the active region is as low as 0.15 mA, which is equivalent to a Vπ of 7.5 mV when a 50 Ω impedance-matched structure is applied. The modulation depth is 92%. Highly dispersive PhC fibers were previously proposed to reduce the payload of true-time delay (TTD) modules for phased-array antenna (PAA) systems. The payload reduction factor is proportional to the enhanced dispersion of highly dispersive PhC fibers. An ultra-large dispersion of -1.1×104 ps/nm•km with the full width at half maximum (FWHM) of 40 nm was numerically simulated from a dual core PhC fibers. The payload reduction factor of the TTD module is as high as 110 compared to that using conventional dispersion compensation fibers (D = -100 ps/nm•km).
Si nanophotonics is anticipated to play a critical role in the future ultra-compact system integration due to the maturity of sub-micron silicon complementary metal oxide semiconductor (CMOS) technology. Photonic crystals (PhCs) provide a promising platform for developing novel optoelectronic devices with significantly reduced device size and power consumption. The active control of photonic crystal waveguides (PCWs) incorporated in Mach-Zehnder interferometers has been investigated in this paper. We designed and fabricated a PCW based silicon thermo-optic (TO) switch operating at 1.55 μm. A novel device structure was proposed to enhance the heat exchange efficiency between the source and the active PCW region, which resulted in a faster switching time (< 20μs) compared with the conventional structure. The required π phase shift between the two arms of the MZI has been successfully achieved within an 80 μm interaction distance. The maximum modulation depth of 84% was demonstrated for switching power of 78mW. For high-speed applications, a p-i-n structure based PCW electro-optical (EO) MZI modulator was proposed. The transient performance of such a device was evaluated using a two-dimensional semiconductor device simulator MEDICI. The simulated structure demonstrated a great potential to realize high-speed ultra-compact Si modulators in the GHz region.
Continuously tunable optical true time-delay (TTD) modules based on dispersion-enhanced photonic crystal fibers (DEPCFs) are demonstrated to provide continuous microwave squint-free beam scanning for an X-band (8 to 12 GHz) phased-array antenna (PAA) system. The dispersion of the fabricated photonic crystal fibers (PCFs) is as high as –600 ps/nm km at 1550 nm. By employing PCFs to increase the dispersion, the TTD module size can be proportionally reduced. The time delay is continuously tunable from –31 ps to 31 ps between adjacent delay lines by tuning the laser wavelength continuously from 1528 to 1560 nm. The far-field radiation patterns of a 1×4 subarray were measured from –45 to 45 deg scanning angles. Squint-free operation is experimentally confirmed. Wavelength conversion is also demonstrated to confirm that time-delay information can be successfully transferred from one wavelength to the other without being changed, which is suitable to be implemented in 2-D phased-array antenna systems. The TTD formation idea presented is suitable for not only the X band, but also the other higher microwave frequencies, such as the K band.
Nanophotonics including photonic crystals promises to have a revolutionary impact on the landscape of photonics technology. Photonic crystal line defect waveguides show high group velocity dispersion and slow photon effect near transmission band edge. By using photonic crystal waveguides to build true time delay based phased array antenna or other optical signal processing systems, the length of the tunable true time delay lines can be dramatically reduced inversely proportional to group velocity dispersion in dispersion enhanced system architecture or reduced inversely proportional to group index in slow photon enhanced system architecture. The group index of the fabricated silicon photonic crystal line defect waveguide is experimentally demonstrated as high as 40 at optical wavelength around 1569 nm. The group velocity dispersion of the fabricated silicon photonic crystal line defect waveguide is as high as 50 ps/nm∙mm at wavelength around 1569 nm, which is more than 107 times the dispersion of the standard telecom fiber (D = 3 ps/nm∙km). Due to the integration nature of photonic crystals, system-on-chip integration of the true time delay modules can be easily achieved.
High diffraction efficiency and large diffraction angle are two major concerns in designing a liquid crystal (LC) phase grating for its applications in beam diffractive devices. High-spatial-frequency grating is capable of providing a large diffraction angle. However, fringing-field effect becomes more severe when the grating pitch size decreases, which imposes a limitation on the phase modulation depth and the diffraction efficiency of the LC grating. In this paper, a novel LC grating with striped electrodes patterned on both the top and bottom sides was proposed and fabricated. By using a specified biasing configuration, vertical electric fields are generated and well confined between the facing electrodes. Meanwhile, horizontal electrical fields are created between adjacent electrodes which help reducing the undesirable deformation of the LC director axis resulting from the fringing filed. Computer simulations show, in our novel structure, a maximum phase modulation depth of 4.15 rad (for 1.55 μm) can be achieved, which is large enough to satisfy the 1.17 π phase-shift requirement for maximum first order diffraction in sinusoidal phase gratings. Both the conventional single-sided and the novel double-sided LC gratings were fabricated and tested. Measurements showed, there was an efficiency enhancement of 77 times achieved by the double-sided structure comparing the conventional structure. A first order diffraction with diffraction angle at 14.5o and diffraction efficiency of ~31% is experimentally achieved, of which the efficiency approaches the theoretical upper limit at 33.8% for a sinusoidal phase grating.
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