Mrinal Thakur

Measurement of ultrafast optical nonlinearities using a modified Sagnac interferometer

A method for the measurement of fast, intensity-dependent refractive-index changes with the use of a modified Sagnac ring interferometer is presented. The measurement is not degraded by slowly responding background index changes. Nonlinear refractive-index changes in an undoped silicon wafer, and in poly-bis toluene sulfonate polydiacetylene and dye-doped polymethyl methacrylate waveguides, were measured with the use of a cw mode-locked Nd:YAG laser.

Self-phase modulation in polydiacetylene single crystal measured at 720–1064 nm

Detailed measurement of spectral broadening in a poly-[2, 4 hexadiyne-1, 6 diol-bis-(p -toluene sulfonate)] (PTS) single crystal owing to self-phase modulation was performed as a function of wavelength by use of a Ti:sapphire laser producing 200-fs pulses at 720-920 nm and a Nd:YAG laser producing 50-ps pulses at 1064 nm. The nonlinear refractive index (n(2)) of PTS at these wavelengths was determined from the measured phase shift. Group-velocity dispersion was estimated and found to have a negligible effect on the observed spectral broadening. The two-photon absorption coefficient (alpha(2)) over this wavelength range was determined from nonlinear transmission measurements. The largest magnitude of n(2) observed at 720 nm was 3.9x10(-5)cm (2)/MW . The results show that the magnitude of n(2) monotonically decreases as wavelength is increased away from resonance, and two-photon absorption does not make a significant contribution to n(2) at off-resonant wavelengths up to 1064 nm.

Enhanced photoluminescence in a novel photobleached dithienylbenzo[c]thiophene oligomer derivative

Photoluminescence of a novel dithienylbenzo[c]thiophene oligomer has been studied in detail. Significant enhancement of photoluminescence has been observed in the material after photobleaching. The bleached oligomer was observed to be stable over time and had a photoluminescence peak significantly blue shifted compared to the fresh material. UV-visible and FTIR spectroscopic analyses indicated photooxidation to be the origin of bleaching. Photoluminescence efficiencies have been measured for both fresh and photobleached solutions. The PL quantum efficiencies have been determined to be 92% for the photobleached material, and 9% for the fresh solution. Spectral narrowing was observed in solid films of the bleached oligomer for excitation with 1 nJ, 200 fs pulses at 400 nm.

Single-crystal thin films of organic molecular salt may lead to a new generation of electro-optic devices

Single-crystal films of an organic molecular salt, 4-dimethylamino- N -methyl-4-stilbazolium tosylate (DAST) with exceptional optical quality are prepared by a modification of the shear method. Electro-optic modulation for light propagating perpendicular to the film is measured at 720 and 750 nm wavelengths with electric fields over the frequency range of 2 kHz to 100 MHz. A modulation depth of 80% was measured at 750 nm for a field of 4 V/μm in a 4-μm-thick film. The measured electro-optic response is flat over the frequency range used (2 kHz to 100 MHz), indicating electronic origin of the electro-optic effect. The results show that these films may lead to a new generation of electro-optic devices.

Second-harmonic generation in single-crystal films of an organic material.

Single-crystal films of 2-cyclooctylamino-5-nitropyridine have been prepared by the shear method, and the second-order susceptibility has been measured by second-harmonic generation. Polarization-dependent measurements in an a plate show that the maximum value of deff (~56 pm/V) is approximately 10 times larger than that of LiNbO3 and that the ratio d32/d33 is approximately −4 at a 1064-nm wavelength. This material has a phase-matching direction (type I only) for propagation parallel to the plane of the a plate. The results of these measurements are important for phase-matched second-harmonic generation in waveguides of this material.

Electroabsorption in single-crystal film of a second-order optical material

In this letter, we report results of electroabsorption and resonant electro-optic measurements in single-crystal film of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). The electroabsorption measurement was made by recording the changes in transmission through the film as an ac field was applied along the dipole axis. The electro-optic measurement was made using the field-induced birefringence method. The measurements were made in the absorptive domain, at 633 and 488nm. The modulation (ΔT∕T) due to electroabsorption decreased as the angle between the incident polarization and the dipole axis was increased. The observed electroabsorption is due to the imaginary part of the electro-optic coefficient. The real part of the electro-optic coefficient (r11) as measured at 633nm is 770pm∕V. The imaginary parts of the electro-optic coefficients at 633 and 488nm are 104 and 259pm∕V, respectively.

Electro-optic modulation at 1.5 GHz using single-crystal film of an organic molecular salt

Electro-optic modulation using single-crystal film of an organic molecular salt with light propagating perpendicular to the film (transverse configuration) has been recently reported. In this letter, we report results of measurements at high speed (up to 1.5 GHz) using such films in the same configuration. Excellent signal-to-noise ratio has been observed even at a low applied voltage (1 V across 15 μm gap) for a 3 μm thick film. The magnitudes of the electro-optic coefficients are: r11=445 pm/V and r21=148 pm/V at 750 nm. A wide range of applications of these films are predicted.

Photo-induced Charge-transfer and Photovoltaic Effect in a Composite Involving a Nonconjugated Conductive Polymer and C60

Photo-induced charge-transfer and resulting photovoltaic effect in a composite involving a nonconjugated conductive polymer, poly(β -pinene) and C60 is reported. The photovoltaic cell was fabricated using indium-tin-oxide coated glass as one electrode and aluminum as the other, with poly(β -pinene)-C60 composite film sandwiched in-between. A Nitrogen laser (325 nm) and an illuminant white-light-source (300–700 nm) were used and the photo-voltage produced was found to have a linear dependency on light intensity. The photoluminescence of poly(β -pinene) (at 360 nm) was quenched when C60 was added to form the composite. Therefore, the photovoltaic effect is a result of excited-state electron-transfer from poly(β -pinene) to C60.

Photovoltaic Cells Involving the Nonconjugated Conductive Polymer Iodine-Doped Styrene-Butadiene-Rubber (SBR)

Novel photovoltaic cells involving a nonconjugated conductive polymer have been fabricated using titanium dioxide/doped styrene-butadiene-rubber/carbon on ITO coated PET substrates. Photocurrents and photo-voltages for different intensities of light (emission at 300–700 nm) have been measured. These cells have shown significantly higher photocurrents and photo-voltages compared to previous reports. A photocurrent density of about 0.25 mA/cm2 and a photo-voltage of 0.74 V have been measured for a light intensity of ∼4 mW/cm2.

Photovoltaic Cells Involving the Nonconjugated Conductive Polymer, Iodine-Doped Poly(β-pinene)

Novel photovoltaic cells involving a nonconjugated conductive polymer have been fabricated using titanium dioxide/doped poly(β-pinene)/carbon on ITO glass-substrates. Photocurrents and photo-voltages for different intensities of light (emission at 300-700 nm) have been measured. Use of iodine-doped nonconjugated conductive polymer (poly(β-pinene)) film has led to significant enhancement of photocurrent compared to previous reports involving undoped polymer in a different device-structure. A photocurrent density of about 0.03 mA/cm2 and a photo-voltage of 0.65 V have been measured for a light intensity of ∼5 mW/cm2. These cells being the first generation of its kind the results are promising for applications.