Alireza Gharavi

Grating based electro-optic switch with azo nonlinear optical polymers

Depth grating was inscribed into nonlinear optical (NLO) polymers functionalized with azo chromophores by a simple interferometric apparatus using a continuous wave laser within the absorption bands of NLO polymer. An attenuation of more than 99.5% with a full width at half maximum of ∼1.4nm was obtained for a single grating with a length of ∼1mm. By making use of the Pockels effect, the refractive index of the poled polymer was changed, and consequently the Bragg wavelength and filter transmission shifted. An ∼0.17nm shift in an applied field of about 116V∕μm was obtained in a weakly poled material.

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.

Near-Infrared Organic Light-Emitting Diodes Based on Donor-pi-Acceptor Oligomers

This letter reports efficient near-infrared (NIR) electroluminescence (EL) of organic light-emitting diodes (OLEDs) based on two donor-pi-acceptor oligomers. The energy gap can be tuned by changing the conjugated length, strengths of the acceptor component, and nonplanarity. EL with peak emission wavelengths of 775 and 905 nm were observed from the two NIR oligomers. External quantum efficiencies up to 3.54%, electrical-to-optical power efficiencies up to 15.8 mW/W, and turn-on voltage of 4.5 V were achieved in OLEDs based on these NIR emitters.

Photo-switching effect in stilbene organic field effect transistors

In this paper, we have investigated experimentally the photo-response of the organic field effect transistors (OFETs) based on π-conjugated stilbene oligomers with mutual optical and electrical properties. The cis-trans photoisomerization of these oligomers in the active layer of the OFETs disturbs the intermolecular interactions. This perturbation disrupts the charge transport in the conduction channel and turns-off the transistor. We used this phenomenon to modulate the drain current of the OFETs optically. The photo-response activity of the transistors is temperature dependent, in agreement with the isomerization of the stilbenes. Photoluminescent evaluations verify the isomerization of the oligomers in the thin film.

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.

Femtosecond laser direct writing of single mode polymer micro ring laser with high stability and low pumping threshold.

We have demonstrated an optically pumped polymer microring laser fabricated by two photon polymerization (TPP) of SU-8. The gain medium is an organic dye (Rhodamine B) doped in SU-8, and the laser cavity is a double coupled microring structure. Single mode lasing was obtained from the two coupled rings each with 30 µm and 29 µm radii using Vernier effect. Low laser threshold of 0.4 µJ/mm(2) is achieved using 1 µm wide polymer waveguides and the quality factor is greater than 10(4) at 612.4 nm wavelength. The lasing remained stable with pump energies from threshold to energies as high as 125 times the threshold.

Integrated Polymeric All-Optical Switch

An all-optical switch was fabricated using azo polymers in a directional coupler form where the index of the coupling region was change via cis-trans of azo dyes in the polymer matrix. Exposing the coupling region to green or violet light caused switching action with a response time of about 10 s.

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.

Characteristics of New Hybrid Plasmonic Bragg Reflectors Based on Sinusoidal and Triangular Gratings

In this paper, new structures for hybrid plasmonic Bragg reflectors (HPBRs), based on sinusoidal and triangular gratings, are proposed. The proposed structures exhibit better characteristics compared to previously studied rectangular HPBRs. Among these two structures, the triangular HPBR exhibits a better performance in terms of passband ripples and bandwidth than that of the sinusoidal one. Therefore, the structure of a microcavity as a defect in the proposed triangular HPBR is studied and its Q factor is compared with other types of plasmonic microcavities. It is also demonstrated that an apodization technique can reduce the size of the triangular HPBR-based microcavity and improve its Q factor.

Integrated 1

We have designed and fabricated an add/drop multiplexer on an azo-functionalized polymeric film using four slanted gratings. The gratings were placed on the corners of a rectangular formation. The gratings were made using the interference pattern of two overlapping laser beams and were used as 90°reflectors. Two channels were separated from the main beam with centers at 1546 and 1560 nm with a 3 dB bandwidth of 6 nm. These channels were then added together by two other gratings and sent out of the integrated circuit.