M. J. Soileau

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.

Energy band-gap dependence of two-photon absorption

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

Optical switching and n2 measurements in CS2

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

Color-center generation in silicate glasses exposed to infrared femtosecond pulses

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.

Optical power limiter with picosecond response time

We present measurements of the two-photon absorption coefficients beta(2) of 10 different semiconductors having band-gap energies between 1.4 and 3.7 eV. We find that beta(2) varies as E(g)(-3), as predicted by theory. In addition, the absolute values of beta(2) agree with theory, which includes the effect of nonparabolic bands, the average difference being less than 26%. This agreement permits confident predictions of two-photon absorption coefficients of other materials at other wavelengths.

Self-protecting semiconductor optical limiters

Abstract Optical switching of picosecond pulses in CS 2 has been observed to occur at the second critical power for self-focusing. This has been confirmed in an independent direct measurement of the nonlinear refractive index at 1.06 μm and 0.53 μm. The measurement technique involves observations of far field spatial distortions of gaussian beams after passing through a nonlinear material.

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

The optical properties of silicate glasses under high-power, 850-nm femtosecond laser irradiation have been studied. Photoinduced processes occurred at irradiances well below the threshold for laser-induce damage. Laser spectral line broadening leading to supercontinuum generation in the visible and UV spectral regions was observed in all the glasses studied. Color-center generation and intrinsic luminescence were found in boro-silicate and alkali silicate glasses. It is believed that these processes result from linear and two-photon absorption of the short-wavelength component of the supercontinuum, causing ionization of the glass matrix. No color-center absorption in the visible region was observed in fused silica at irradiances up to the laser-damage threshold.

Picosecond air breakdown studies at 0.53 μm

Optical self-action in CS 2 and other liquids was used to make a power-limiting device having a picosecond response time. This device uses self-focusing in liquids to produce phase aberrations and laser-induced breakdown, which in turn limit the transmitted power. This device has near-unity transmission for input power below P c , which is on the order of the critical power for self-focusing, and limits the transmitted power to a nearly constant value for input power greater than P c . The onset of nonlinear transmission was adjusted by mixing various liquids to adjust the nonlinear refractive index. Experimental results using linearly and circularly polarized 40 ps (FWHM) pulses at 1.06 μm are presented.

Measurement of the optical damage threshold in fused quartz

We present a detailed characterization of passive, picosecond optical-power-limiting devices using tightly focused beams in thick semiconductor samples. This study of limiting in ZnSe with 30-psec, 532-nm pulses shows that the resulting internal self-action (two-photon absorption plus free-carrier self-defocusing) protects the bulk material from optical damage. Simple scaling relations were determined from our results that link the limiting energy and the dynamic range to the focusing geometry and sample dimensions. These relations were used to design a monolithic optical limiter, optimized to have maximum dynamic range and minimum limiting energy. This device limits at an input energy of 10 nJ (300 W) and has a dynamic range greater than 10(4).

Diffusion of color centers generated by two-photon absorption at 532 nm in cubic zirconia

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.