M. Thakur

Single-pass thin-film electro-optic modulator based on an organic molecular salt

Single crystal films of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) with excellent optical quality have been prepared by modified shear method. Electro-optic modulation for light propagating perpendicular to the film surface was measured at a wavelength (720 nm) where absorption is small. Very large electro-optic modulation (20%) was demonstrated in a 3 μm thick film at a low ac field (1 V/μm, at 4 kHz). Electro-optic modulation in such thin films is suitable for a wide range of applications in free-space interconnects and various modulator structures with light propagating perpendicular to the film. The magnitude of the electro-optic coefficient at 720 nm, r11∼530u200apm/V, is significantly larger than that of bulk crystals.

Junction devices based on sulfonated polyaniline

Schottky diodes were fabricated using aluminum/neutralized-sulfonated-polyaniline (SPAN) junctions. I–V and C–V measurements were made, and the barrier height (ΦB) and the background concentration (NB) were determined to be 0.8 V and 4×1017/cm3, respectively. Using these diodes as gate control, depletion-mode thin-film transistors were fabricated with a source and drain made of gold Ohmic contacts. The transistors were characterized by I–V measurements, and the carrier mobility determined from devices operating in the |VG|>|VDS| “linear” regime was about 0.01u2009cm2/Vu2009s. This high value of mobility could be attributed to the spherulitic (partially ordered) structures observed in the SPAN thin films. Field-effect transistors were also fabricated on SPAN films deposited on an n-doped silicon substrate acting as the gate electrode with a thermally grown oxide layer. A reasonably high on/off ratio (∼4×103) was measured in these devices.

Quadratic electro-optic effect in a nano-optical material based on the nonconjugated conductive polymer, poly(β-pinene)

Large quadratic electro-optic effect has been observed in a nano-optical material based on a nonconjugated conductive polymer, poly(β-pinene). Doping of poly(β-pinene) leads to subnanometer size charge-transfer sites with positive charges (holes). The observed large electro-optic effect has been attributed to these confined systems with special structures. The magnitude of the Kerr constant increases with the concentration of the dopant or charge-transfer sites. The maximum Kerr coefficient at 633nm which is close to resonance is about 50 times that of nitrobenzene.

Electro-optic modulation using an organic single crystal film in a Fabry–Perot cavity

Electro-optic modulation has been demonstrated using single crystal film of an organic electro-optic material, [N-(4-nitrophenyl)-L-prolinol] (NPP), placed in a Fabry–Perot cavity. The transverse geometry was used for electro-optic modulation. The cavity used had a finesse of about 50. The modulation depth achieved was about 0.7% for a low ac field of 0.5 V/μm applied along the charge-transfer (polar) axis on the film. The results are promising for applications of such films in high speed optical signal processing.

Polarized optical absorption and photoluminescence measurements in single-crystal thin films of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate

Single-crystal thin films of the anhydrous (red) and the hydrated (orange) phases of the organic salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate were grown by a modification of the shear method. The optical absorption coefficients of the films were measured with light polarized along and normal to the dipole/molecular axis at both resonant and off-resonant wavelengths, and a strong dichroism was observed at the resonant wavelengths. The absorption measurements are important considering potential applications of these films (red phase) in high-speed single-pass thin-film electro-optic modulators [M. Thakur, J. Xu, A. Bhowmik, and L. Zhou, Appl. Phys. Lett. 74, 635 (1999)] and other photonic devices. Highly polarized photoluminescence (PL) has been observed in these films. The PL efficiencies of the red- and orange-phase single-crystal films were measured to be about 12% and 14%, respectively, which are significantly higher than the maximum PL efficiency measured in solution (3%).

Two-photon absorption in quantum dots based on a nonconjugated conductive polymer

Two-photon absorption has been measured in a nano-optical material (quantum dots) based on the nonconjugated conductive polymer, iodine-doped poly(β-pinene). The measurement has been made using open-aperture z scan at 730–860nm with 150fs pulses. Exceptionally large two-photon absorption coefficient has been observed with a peak of magnitude ∼2.6cm∕MW that appeared at ∼1.53eV which is close to half of the optical gap corresponding to the charge-transfer transition involving the isolated double bond. Saturation of absorption was observed at lower intensities and shorter wavelengths. The results have been attributed to the quantum dots (subnanometer size) formed upon doping.

SECOND-HARMONIC GENERATION IN SINGLE-CRYSTAL THIN FILMS OF 4-AMINOBENZOPHENONE

Single-crystal thin films of 4-aminobenzophenone have been prepared by a modification of the shear method, and the significant elements of the second-order optical polarizability tensor have been measured by the transverse second-harmonic generation (SHG) technique. Films with thickness less than the coherence length allowed measurements without significant effects due to phase-mismatch. Detailed SHG experiments with different polarization conditions were performed to determine the d coefficients. The magnitudes of the d coefficients at the fundamental wavelength of 1064 nm were found to be d23=7.3±0.4u200apm/V and d22=0.73±0.04u200apm/V. Directions for both type-I and type-II phase-matched propagation parallel to the film were identified. The results showed that guided-wave phase-matched SHG (type I and type II) should be possible using the films.

Highly efficient mirrorless laser-like emission in an organic molecular salt

Unusually high conversion efficiency in laser-like emission has been observed in solutions of an organic molecular salt, styrylpyridinium cyanine dye, at a low threshold input pulse energy. Pumped with frequency-doubled 55 ps pulses from a Nd:YAG laser, a conversion efficiency as high as 40% was achieved without incorporating external mirrors. The spectrally narrowed output beam was observed to be polarized and having a low divergence. The exceptionally low threshold (<1 μJ with excitation area ∼5 mm2) and high conversion efficiency at a high molar concentration in spite of a very small photoluminescence quantum efficiency (<0.3%) can be attributed to the large dipole moments associated with the charge-transfer transitions of these molecules favoring strong cooperative emission.

Electro-optic modulation using single-crystal film of an organic molecular salt in a Fabry-Perot cavity

Electro-optic modulation using single-crystal film of an organic molecular salt placed in a Fabry-Perot cavity has been measured at 633 and 750nm. Large modulation depth (80%) at a low applied field (0.5V∕μm) has been observed in the field-induced birefringence geometry (mirror reflectivity ∼70%) for a bias point which was controlled by moving one of the mirrors. A modulation of about 60% with a higher throughput was obtained when no analyzer was used and the incident polarization was parallel to the dipole-axis.

Temporal measurement on and using pulses from spectrally narrowed emission in styrylpyridinium cyanine dye

Highly efficient spectrally narrowed emission (SNE) was observed in the solution of strylpyridinium cyanine dye (SPCD) pumped by fundamental and second harmonic of a picosecond Nd:YAG laser in two separate arrangements. A highly directional emission was observed in both the pumping arrangements without incorporating any mirrors. The pulse duration of the SNE was measured by background free SHG intensity autocorrelation technique. The measured duration of the pulses was 40 ps. These pulses, having a spectral linewidth of 10 nm (full width at half maximum), were used as a probe to measure the transient changes in the transmission in SPCD solution using a pump–probe setup. The transient optical transmission indicated a gain at the overlap and no gain was observed beyond a delay of 40 ps.