Shekhar Guha

Two Photon Absorption, Nonlinear Refraction, And Optical Limiting In Semiconductors

Two-photon absorption coefficients /32 of ten direct gap semiconductors with band-gap energy Eg varying between 1 .4 and 3.7 eV were measured using 1.06 µm and 0.53 um picosecond pulses.

Self-defocusing in CdSe induced by charge carriers created by two-photon absorption.

2 was found to scale as E43, as predicted by theory for the samples measured. Extension of the empirical relationship between

Studies of the Nonlinear Switching Properties of Liquid Crystals with Picosecond Pulses

2 and Eg to InSb with Eg = 0.2 eV also provides agree-ment between previously measured values and the predicted 02. In addition, the absolute values of

Verification of the scaling rule for two-photon absorption in semiconductors

2 are in excellent agreement (the average difference being <26%) with recent theory, which includes the effects of nonparabolic bands. The nonlinear refraction induced in these materials was monitored and found to agree well with the assumption that the self-refraction originates from the two-photon-generated free carriers. The observed self-defocusing yields an effective nonlinear index as much as two orders of magnitude larger than CS2 for comparable irradiances. This self-defocusing, in conjunction with two-photon absorption, was used to construct a simple, effective optical limiter that has high transmission at low input irradiance and low transmission at high input irradiance. The device is the optical analog of a Zener diode.

Nonlinear optical Properties of Liquid Crystals In The Isotropic Phase

We observe self-defocusing of picosecond, 1.06-microm pulses in CdSe. The effective nonlinear refraction can be 2 orders of magnitude larger than that of CS(2). We obtain good agreement with the theory presented here, which assumes that the self-refraction is caused by charge carriers created by two-photon absorption.

Passive Optical Limiting With Picosecond Response

Abstract A comparative study of self-focusing in seven liquid crystals using picosecond 0.53 and 1.06 μm pulses is presented. MEBBA was found to have the highest nonlinearity at 0.53 μm as determined in an optical power limiting experiment. This limiting appears to be due to nonlinear refraction enhanced by two-photon absorption.

Two-Photon Absorption And Nonlinear Refraction In Isotropic Liquid Crystals

The simple parametric scaling rules for the two-photon absorption coefficients, g2, predicted by recent theory, have been experimentally confirmed by the measurement of g2 in ten different semiconductors using carefully characterized picosecond 1 and 0·5 wm pulses. We find g2 (cm/GW) = (3·1 - 0·5) 2 103 i EpF2(2Ey/Eg)n m2E m3 g, where Ep xad 21 eV for the semiconductors studied, Eg is the energy gap in eV, n the refractive index and F2(x) = (x m 1)3/2/x5 . This relation allows the prediction of g2 in other materials at other wavelengths, which is useful in the design of nonlinear optical elements. The strong self-defocusing observed is consistent with nonlinear refraction by the two-photon-generated free carriers.

Passive Optical Limiting

Abstract Picosecond nonlinear absorption and nonlinear refraction were studied for several classes of isotropic phase liquid crystals, including Schiff base and ester compounds. Materials studied exhibit a large two-photon absorption coefficient (j3) at 532 nm.Values of j3 were found to be -0.6 cm/GW in several of the compounds studied. Nonlinear refraction was also observed and the nonlinear refractive index, n2 was measured for each material at 1.06 pm using an external self-focusing mangement. n2 ranged from 6 × 10-13 esu to 2 × 10-l2 esu. The combination of nonlinear absorption and nonlinear refraction in these materials result in optical limiting for input energies as low as 0.15 microjoules for 30 psec pules at 532 nm.

Nonlinear Absorption and Nonlinear Refraction Studies in MEBBA

We describe a completely passive technique for limiting the power of light beams. The techniques we use are based on optical self-action (self-focusing or defocusing) and non-linear absorption or a combination of both. These processes are passive and thus require no external mechanism to induce limiting other than the light beam itself. We demonstrate picosecond turn on times while 10 secresponse times are possible depending on the material. Specifically we demonstrate optical limiting in CS2 and other organic liquids, and in liquid crystals.

Picosecond study of charge-carrier-induced nonlinear refraction in intermediate bandgap semiconductors (A)

Picosecond nonlinear absorption and nonlinear refraction studies are presented for isotropic phase liquid crystals. These materials exhibit a large two-photon absorption coefficient (f3) at 532 nm, with values as large as 0.6 cm/GW. Nonlinear refraction was also observed and the nonlinear refractive index (n9) was measuv.qd at 1064 nm usiAg an external self-focusing arrangement. Values of n2 range from 6 x 10-13 esu to 17 x 10-13 esu.