Ratnakar Neurgaonkar - Academia.edu (original) (raw)

Papers by Ratnakar Neurgaonkar

$Research paper thumbnail of Photorefractive properties of strontium‐barium niobate$

Journal of Applied Physics, Jul 15, 1987

We have grown and optically characterized strontium-barium niobate crystals, including both undop... more We have grown and optically characterized strontium-barium niobate crystals, including both undoped and cerium-doped crystals having two different Sr/Ba ratios (61/39 and 75/25). By measuring the coupling of two optical beams in the crystals, we have determined the following photorefractive properties: the effective density, sign, and spectral response of the dominant charge carrier, the grating formation rate, dark conductivity, and carrier diffusion length. We find that electrons are the dominant photorefractive charge carriers in all of our samples; the typical density of photorefractive charges is ∼1×1016 cm−3 in the undoped samples. The grating formation rate increases with intensity, with a slope of ∼0.3 cm2/(W s) over an intensity range of ∼1–15 W/cm2 in undoped samples. Cerium doping improves both the charge density (increased by a factor of ∼3) and the response rate per unit intensity (∼5 times faster).

$Research paper thumbnail of Cr3+Sr0.6Ba0.4Nb2O6 single crystals for photorefractive applications$

Materials Research Bulletin, May 1, 1989

Optical quality Cra+-doped Sro. sBao.,Nb20 5 (SBN'60) single crystals have been grown by the Czoc... more Optical quality Cra+-doped Sro. sBao.,Nb20 5 (SBN'60) single crystals have been grown by the Czochralski technique with boules as large as 2 cm in diameter and 5 cm long being ~rown. The doping of 0.01 wt% Cr 3+ on the 6-fold coordinated Nb s site increases the dielectric constant approximately 15% and reduces the phase transition temperature from 75 ° to 72°C. Photorefractive fanning measurements show a response time of 0.9 s at 40 mW/cm 2, a value nearly three times faster than found in Ce3÷-doped SBN:60 crystals.

Proceedings of SPIE, Mar 1, 1991

Ninety six percent grain-orientation was achieved in YBa2Cu3O(7-x) ceramics using the hot-forging... more Ninety six percent grain-orientation was achieved in YBa2Cu3O(7-x) ceramics using the hot-forging technique. X-ray diffraction measurements show that the c-axis is oriented parallel to the direction of pressing while the a and b axes are in the plane perpendicular to the pressing direction. Optical and electron microscopy fully support the X-ray results. Highly grain-oriented samples show a sharp Tc at 92 K and in some samples, Jc was found to be close to 10,000 A/sq cm.

$Research paper thumbnail of <title>Photorefractive self-focusing and defocusing as an optical limiter</title>$

Proceedings of SPIE, Jul 6, 1994

Focusing and defocusing of laser light has been observed for many years. Optical Kerr type materi... more Focusing and defocusing of laser light has been observed for many years. Optical Kerr type materials exhibit this effect only for high intensities. We show experimental evidence that photorefractive materials can also produce dramatic focusing and defocusing. Whereas Kerr materials produce this effect for high intensities, photorefractive materials produce these effects independent of intensity indicating that this effect would be ideal for an optical limiter. We compare the characteristics of Kerr and photorefractive materials, discuss the physical models for both materials and present experimental evidence for photorefractive defocusing. Self-focusing and defocusing was observed for any incident polarization although the effect was more pronounced using extraordinary polarized light. In addition, self-focusing or defocusing could be observed depending on the direction of the applied electric field. When the applied field was in the same direction as the crystal spontaneous polarization, focusing was observed. When the applied field was opposite the material spontaneous polarization, the incident laser light was dramatically defocused.

$Research paper thumbnail of Photorefractive Properties of Undoped and Doped Single Crystal SBN:60$

Proceedings of SPIE, Mar 4, 1986

We present the results of our theoretical and experimental studies of the photorefractive effect ... more We present the results of our theoretical and experimental studies of the photorefractive effect in single crystal SBN:60, SBN:Ce, and SBN:Fe. Specifically, the two-beam coupling coefficients, response times and absorption coefficients of these materials are given. Mathematically, this two-beam coupling may be described in the steady-state as follows:

$Research paper thumbnail of Photorefractive properties of Ce-doped (BaSr)_6Ti_2Nb_8O_30 crystals$

Optics Letters, Jan 15, 1992

The photorefractive effect is demonstrated in single crystals of ferroelectric Ce-doped (BaSr)6Ti... more The photorefractive effect is demonstrated in single crystals of ferroelectric Ce-doped (BaSr)6Ti2Nb8030 for the first time to our knowledge. The maximum intensity gain, measured by two-beam coupling at a 488-nm wavelength, was F-20 cm-'. The photorefractive time response, also measured from beam coupling, was determined to be 0.1 s at an incident intensity of I = 1 W/cm 2. Ce-doped (BaSr)6Ti 2 Nb8s3O (BSTN), which has a Ba-to-Sr ratio of 2:1, is a new material with a crys

Journal of Crystal Growth, Sep 1, 1987

The crystal chemical approach has been shown to be successful in improving the temperature stabil... more The crystal chemical approach has been shown to be successful in improving the temperature stability of the ferroelectric LiNbO 3 and LiTaO3 phases. Results of this investigation show that it is possible to reduce the temperature coefficient of SAW velocity for crystallized Li0 99Na001NbO3 and Li0 99Na001TaO3 thin films grown by the LPE technique. The improvement is approximately 40% for Na~-dopedLiNbO3.

$Research paper thumbnail of Photorefractive gain and response time of Cr-doped strontium barium niobate$

Applied Physics Letters, Jul 24, 1989

$Research paper thumbnail of Photorefractive properties of SBN:60 systematically doped with rhodium$

Journal of The Optical Society of America B-optical Physics, Aug 1, 1992

Strontium barium niobate (SBN) was doped with rhodium to enhance its photorefractive behavior. Cr... more Strontium barium niobate (SBN) was doped with rhodium to enhance its photorefractive behavior. Crystals with six different Rh concentrations, ranging from 0.015 to 0.20 wt. %, were grown and characterized. With a higher Rh concentration the following trends were observed at a wavelength of 514.5 nm: (i) a larger linear absorption coefficient a, (ii) a larger maximum two-beam-coupling coefficient F, (iii) a higher net coupling coefficient (-a), (iv) a longer two-beam-coupling time response, (v) a shorter fixed-level-gain time response, (vi) a constant photoionization cross section, and (vii) a smaller photorefractive sensitivity. With extraordinary polarization used to invoke the large r 3 3 electro-optic coefficient, two-beam-coupling coefficients exceeding 60 cm-' were measured in thin (1 mm) plates of the heavily Rh-doped crystals, which is consistent with the expected coupling by inference from measurements with ordinary polarization in thick (-5 mm) crystals. Contradirectional two-beam coupling in SBN along the c axis (independent of polarization, since r 13 = r 23) gave a coupling coefficient of almost 14 cm-' for the 0.20-wt. % sample; the contradirectional coupling coefficient decreased approximately in proportion with the decreasing Rh-doping concentration in the other crystals. Estimated photorefractive charge densities ranged from-4 x 1016 to-8 X 1017 cm 3 , which constituted approximately 1% of the total Rh-doping concentration.

Optical Society of America Annual Meeting, 1992

Optimizing photorefractive charge density for contradirectional two-beam coupling suggests doping... more Optimizing photorefractive charge density for contradirectional two-beam coupling suggests doping photorefractive crystals with high impurity concentrations. A series of Rh-doped SBN:60 samples with Rh concentrations ranging from 0.015 to 0.20% by weight were grown and fabricated. The corresponding absorption coefficients α, extending from 0.5 to 6.2 cm−1, are a penalty for such high doping levels. These α are non-negligible when compared to the measured contradirectional two-beam-coupling coefficients Γ, which ranged from 0.8 to 13.8 cm−1. In this regime of non-negligible α, the simple analytic expressions derived by Yeh1 are no longer valid. Ja2 has provided an applicable theoretical description giving a numerical solution. We have extended Ja’s work, deriving an approximate expression for the two-beam-coupling gain g in the presence of strong α, which ranged from 0.8 to 13.8 cm−1. In this regime of non-negligible α, the simple analytic exp which is a function of α, Γ, thickness L, and incident pump-probe intensity beam ratio r pp . We will present g versus rpp data for six different Rh concentrations and compare that data to various theoretical predictions.

Physical Review Letters, Apr 14, 1997

Enhancement of Multiplexed Holograms in Cerium-Doped Sr 0.75 Ba 0.25 Nb 2 O 6. Jian Ma, Tallis Y.... more Enhancement of Multiplexed Holograms in Cerium-Doped Sr 0.75 Ba 0.25 Nb 2 O 6. Jian Ma, Tallis Y. Chang, John H. Hong, and Ratnakar R. Neurgaonkar Science Center, Rockwell International Corporation, 1049 Camino Dos Rios, Thousand Oaks, California 91360. ...

$Research paper thumbnail of Contradirectional two-beam coupling in absorptive photorefractive materials: application to Rh-doped strontium barium niobate (SBN:60)$

Journal of The Optical Society of America B-optical Physics, 1995

Contradirectional two-beam-coupling gains are measured in Rh-doped strontium barium niobate (SBN)... more Contradirectional two-beam-coupling gains are measured in Rh-doped strontium barium niobate (SBN) as a function of the Rh concentration and the incident pump-probe beam ratio. These measurements indicate that larger two-beam-coupling coefficients in this contradirectional geometry (as large as ϳ16 cm 21) are typically linked to stronger optical absorption in the more heavily doped samples. The effects of such strong absorption are explicitly included in a theoretical analysis applied to the measured data. In particular, earlier theoretical treatments of this two-beam-coupling process with nonnegligible absorption are extended to yield an approximate expression for the probe gain. The accuracy of this analytic expression is then verified by comparison with exact numerical results as well as with experimental measurements in the Rh-doped SBN samples. Further experimental and theoretical comparisons are made after the contradirectional two-beamcoupling process is related to asymmetric transmission through a photorefractive window (i.e., one-way glass), again in the presence of strong absorption.

$Research paper thumbnail of Photorefractive properties of doped strontium-barium niobate$

Optics Communications, 1991

... 1 January 1991 Photorefractive properties of doped strontium-barium niobate RA Vazquez, MD Ew... more ... 1 January 1991 Photorefractive properties of doped strontium-barium niobate RA Vazquez, MD Ewbank and RR Neurgaonkar Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA Received 28 June 1990; revised manuscript ...

$Research paper thumbnail of Enhanced photorefractive effect in SBN:60:Cr and SBN.60:Rh with external dc fields$

Annual Meeting Optical Society of America, 1989

A major thrust of photorefractive materials research has been to increase the coupling constant Γ... more A major thrust of photorefractive materials research has been to increase the coupling constant Γ and speed up response time. Previous two-beam coupling experiments using SBN:60 have shown that the space charge field Esc, and therefore the coupling constant, can be increased to some degree by optimizing the characteristic fields En and Ed.1,2 However, since Esc tends toward the smaller of EN and Ed for no external field, any attempt to incrase Esc by increasing En will have minimal effect since Esc will be limited by the diffusion field Ed. By applying an external dc electric field E0, the photoexcited carriers will tend to be driven into the traps half a grating period away, and Esc will be limited by En = eNA/ϵK for large E0. Experimentally, Γ has been found to increase by almost a factor of 3 to over 10 cm−1 in Cr-doped SBN:60 with an applied field of 10 kV/cm. In addition, for Rh-doped SBN:60, the response time has been found to get significantly faster with an applied field, decreasing by more than an order of magnitude from ~2 s for E0 = 0 to 0.1 s with E0 = 10. These results suggest potential applications in field-enhanced optical limiting, switching, and thresholding.

Ferroelectrics, 1980

... growth of LiNb03 films for SAW device applications. Crystal Chemistry The limits of stability... more ... growth of LiNb03 films for SAW device applications. Crystal Chemistry The limits of stability of the LiNb03-structure was established by intro-ducing various ions for the Li+ and Nb5+ or Ta5+ positions. ... 3570. 63 Page 2. 64 RR NEURGAONKAR, TC LIM, EJ STAPLES, LE CROSS ...

$Research paper thumbnail of Enhanced photorefractive gain in Cr‐doped strontium barium niobate with an external dc electric field$

Journal of Applied Physics, Feb 1, 1990

We present experimental results of field-enhanced photorefractive two-beam coupling and gain in C... more We present experimental results of field-enhanced photorefractive two-beam coupling and gain in Cr-doped strontium barium niobate crystals and its dependence on dopant concentrations. Increases in the exponential gain factor by more than a factor of 2 were observed by applying a field of up to 10 kV/cm. This property has potential applications such as switchable optical phase conjugators, limiters, and thresholding devices.

$Research paper thumbnail of Order-of-magnitude reduction of the photorefractive response time in rhodium-doped Sr_06Ba_04Nb_2O_6 with a dc electric field$

Optics Letters, 1990

We present what is to our knowledge the first report on the photorefractive properties of Rh-dope... more We present what is to our knowledge the first report on the photorefractive properties of Rh-doped Sr 0 o 6 Ba 0. 4 Nb 2 06 and experimental results showing a reduction of the photorefractive two-beam coupling response time by more than an order of magnitude with an external dc field of 10 kV/cm. Doped SrU. 6 BaO. 4 Nb 2 0 6 (SBN:60) using Fe, Ce, Ce-Ca, and Cr has been shown to be photorefractive with high gain and real-time response. 1-3 Cr-doped SBN:60 crystals have been shown to have response times of near 0.2 sec at 0.25 W/cm 2 , which is approximately an order of magnitude faster than what was measured in Ce-doped samples, but their photorefractive gain ranged from 3 to 6 cm-1 compared with more than 10 cm-1 in Ce-doped SBN:60. Applying an electric field

$Research paper thumbnail of Ferroelectric tungsten bronze crystals and their photorefractive applications$

Ferroelectrics, Feb 1, 1990

The photorefractive properties of tungsten bronze ferroelectric crystals are reviewed with respec... more The photorefractive properties of tungsten bronze ferroelectric crystals are reviewed with respect to host character and dopant. A classification into four groups is proposed for tungsten bronze materials based on their structural, ferroelectric and optical properties. The use of dopants is found to control the speed of response and coupling strength as well as spectral range. In particular, the availability of different crystallographic sites is shown to provide opportunities to separately adjust these properties. The best performance has been established for Ce and Cr doped SBN:60 crystals in the visible and IR regions, respectively.

Applied Physics Letters, Oct 15, 1979

LiNbO3 thin films doped with Na+ and Co2++ Zr4+ have been obtained by the liquid-phase epitaxial ... more LiNbO3 thin films doped with Na+ and Co2++ Zr4+ have been obtained by the liquid-phase epitaxial (LPE) technique from Li2O-V2O5 flux. X-ray diffraction studies indicated that the films had a high single crystallinity with good epitaxy. The Na+ and Co2+ +Zr4+ doped LiNbO3 films showed a reduction in the temperature coefficient of surface acoustic wave velocity.