Mohammad Amin Baghban

Electronically tunable silicon photonic delay lines

A novel type of electronically tunable optical delay lines is proposed based on silicon photonics. The integrable devices can achieve delays as high as ~660 ps with a loss of <; 2.2 dB.

Fabrication of graded index waveguides in azo polymers using a direct writing technique

Fast direct writing of waveguides on polymers using low power continuous-wave lasers has been investigated. Using the cis-trans property of a functionalized sulfonated azo chromophores, we have fabricated graded index waveguides with low loss, which is due to graded index sidewalls. Fabrication is done by exposing the polymer film to a 532nm wavelength laser beam focused to about 5μm spot at the film. Losses were calculated to be 0.48±0.04dB∕cm at 830nm wavelength with no surface deformation. This technique enables us to fabricate integrated optical circuits including directional couplers, dividers, filters, switches, etc., as they are currently investigated in our laboratory.

Add/drop filter using in-plane slanted gratings in azo polymers

We have fabricated in-plane slanted gratings on azo-functionalized polymeric films using a fast, direct-writing method. By properly adjusting the resonance, these gratings can be used as 90 degrees integrated reflectors and add/drop filters in the plane of the film. We have produced an attenuation of 14.8 dB at 1560.2 nm with a FWHM of 6.47 nm. Also, a signal of 1548 nm wavelength was added to the output from a different direction. Any light shifted from the resonance will pass through the filter undisturbed.

Charge and topography patterned lithium niobate provides physical cues to fluidically isolated cortical axons

In vitro devices that combine chemotactic and physical cues are needed for understanding how cells integrate different stimuli. We explored the suitability of lithium niobate (LiNbO3), a transparent ferroelectric material that can be patterned with electrical charge domains and micro/nanotopography, as a neural substrate. On flat LiNbO3 z-surfaces with periodically alternating charge domains, cortical axons are partially aligned with domain boundaries. On submicron-deep etched trenches, neurites are aligned with the edges of the topographical features. Finally, we bonded a bicompartmental microfluidic chip to LiNbO3 surfaces patterned by etching, to create isolated axon microenvironments with predefined topographical cues. LiNbO3 is shown to be an emerging neuron culture substrate with tunable electrical and topographical properties that can be integrated with microfluidic devices, suitable for studying axon growth and guidance mechanisms under combined topographical/chemical stimuli.

Impact of longitudinal fields on second harmonic generation in lithium niobate nanopillars

An optimized focused ion beam process is used to fabricate micrometer-long LiNbO3 nanopillars with diameters varying between 150 and 325 nm. Polarimetric mappings of second harmonic generation from a wavelength of 850 nm demonstrate the ability to modify the polarization features of the nonlinear response through a fine adjustment of the pillar size. The effect is ascribed to the non-negligible contribution of the longitudinal fields associated with sub-wavelength light confinement in the LiNbO3 nanopillars. The results also highlight the importance of a fine control over the nanopillar size in order to effectively engineer their nonlinear response.

Influence of annealing on the photodeposition of silver on periodically poled lithium niobate

The preferential deposition of metal nanoparticles onto periodically poled lithium niobate surfaces, whereby photogenerated electrons accumulate in accordance with local electric fields and reduce metal ions from solution, is known to depend on the intensity and wavelength of the illumination and the concentration of the solution used. Here, it is shown that for identical deposition conditions (wavelength, intensity, concentration), post-poling annealing for 10 h at 200 °C modifies the surface reactivity through the reorientation of internal defect fields. Whereas silver nanoparticles deposit preferentially on the +z domains on unannealed crystals, the deposition occurs preferentially along 180° domain walls for annealed crystals. In neither case is the deposition selective; limited deposition occurs also on the unannealed –z domain surface and on both annealed domain surfaces. The observed behavior is attributed to a relaxation of the poling-induced defect frustration mediated by Li+ ion mobility during annealing, which affects the accumulation of electrons, thereby changing the surface reactivity. The evolution of the defect field with temperature is corroborated using Raman spectroscopy.

Two-dimensional photonic crystal for optical channel separation in azo polymers

We have fabricated a two-dimensional photonic crystal that functions as a two-channel add/drop filter on azo-functionalized polymeric films using a fast, direct-writing method. By properly adjusting the resonance, this structure can be used as a multichannel add/drop filter in the plane of the film. Here, we were able to separate two channels at 1555.7 and 1570.7 nm with attenuation of 13.1 dB and FWHM of 6.7 nm for each channel. The separated channels were directed towards the sides at an approximately 90 degrees angle with respect to the input direction.

Photo-Induced Enhanced Raman from Lithium Niobate on Insulator Template

Photoinduced enhanced Raman spectroscopy from a lithium niobate on insulator (LNOI)-silver nanoparticle template is demonstrated both by irradiating the template with 254 nm ultraviolet (UV) light before adding an analyte and before placing the substrate in the Raman system (substrate irradiation) and by irradiating the sample in the Raman system after adding the molecule (sample irradiation). The photoinduced enhancement enables up to an ∼sevenfold increase of the surface-enhanced Raman scattering signal strength of an analyte following substrate irradiation, whereas an ∼threefold enhancement above the surface-enhanced signal is obtained for sample irradiation. The photoinduced enhancement relaxes over the course of ∼10 h for a substrate irradiation duration of 150 min before returning to initial signal levels. The increase in Raman scattering intensity following UV irradiation is attributed to photoinduced charge transfer from the LNOI template to the analyte. New Raman bands are observed following UV irradiation, the appearance of which is suggestive of a photocatalytic reaction and highlight the potential of LNOI as a photoactive surface-enhanced Raman spectroscopy substrate.

Waveguide gratings in thin-film lithium niobate on insulator

Thanks to its nonlinear-, electro- and acousto-optical and piezo-response properties [1], lithium niobate (LiNbOs, LN) is widely used for many photonic applications. The high refractive index contrast offered by recently available thin film lithium-niobate-on-insulator (LNOI) platforms, provides new possibilities for miniaturizing LN-based integrated optics structures such as submicron waveguides [2] and resonators [3]. Inherent difficulties in patterning LN have led many researchers to pattern other optical layers contiguous to the LNOI instead [3], which results in a looser confinement of light in the LN layer, and hence a less efficient exploitation of its nonlinear and electro-optical properties. The latter features motivate ongoing efforts aiming at the development of a suitable technology for low-loss and high confinement waveguide ridges in LNOI [2, 4], which however still needs to be combined with periodic structures such as photonic crystals [5], grating couplers and Bragg reflectors in order to ultimately provide a fully functional integrated optics toolbox for ultra-compact LN photonics chips for efficient and ultrafast signal processing, modulation and frequency conversion.

Birefringence-free lithium niobate waveguides

Lithium niobate (LiNbO3, LN) is widely used for photonic applications, thanks to its wide transparency range, electro-optic and nonlinear optical properties [1]. Recently available lithium-niobate-on-insulator (LNOI) substrates afford high index contrast and thus tight field confinement in LN photonic devices such as optical modulators and microring resonators [2, 3]. However, similar to other (e.g. SOI) platforms, high-confinement waveguides are strongly birefringent, which leads to e.g. polarization mode dispersion with deleterious effects for delay-based filters, modulators and other signal processing and transmission devices [4]. Here, with vectorial simulations for high-confinement ridge waveguides in LNOI, we show how the interplay of the high birefringence of LN and of the waveguide structure can be employed to achieve birefringence-free designs in a broad spectral range, from 0.8 up to 1.8 μm, through a careful tuning of the LNOI ridge dimensions.