Antonella Milani

High-Pressure Bridgman Grown CdZnTe for Electro-Optic Applications

The linear electro-optic response of high pressure Bridgman growth semi-insulating Cd0.9Zn0.1Te bulk samples has been characterized. Measurements have been performed in the third optical window for telecommunications around = 1.5 m. The dependence of the Pockels figure of merit on the modulation frequency and on the intensity of the optical probe beam is presented and discussed. Despite the residual optical absorption, Cd0.9Zn0.1Te is not affected by photo-generated auto-inhibition of the Pockels effect. This can be attributed to the action of an efficient intragap recombination center. It is, therefore, a suitable basic material for electro-optic switching of near-infrared beams at low frequency and quasi-continuous wave (cw) regimes. The implementation of a Cd0.9Zn0.1Te-based cross-bar switch for optical communication applications is also presented, which reaches −30 dB of extinction ratio and sub millisecond response time.

Measurements of second-order susceptibility at λ=1.5 μm in CdTe-based ternary alloys for efficient wavelength conversion

We have characterized the second-order optical nonlinear response of II-VI semiconductor ternary compounds Cd0.8Zn0.2Te and Cd0.78Mn0.22Te at λ=1.5 μm. A spectrally resolved phase-mismatch second-harmonic generation (SHG) technique has been used on bulk single crystals, exploiting 10−13 s optical pulses and multichannel detection. The nonlinear d coefficient has been measured and chromatic dispersion parameters have been validated. By normalizing SHG results in view of applications to all-optical wavelength conversion, the tested compounds prove to be interesting alternatives to more renowned AlxGa1−xAs.

Two-dimensional mapping of residual stress-induced birefringence in differently-grown semiconductors for optical communication applications

Abstract Semiconductor-based bulk optical modulators and switches demand material homogeneity and lack of residual birefringence, to guarantee low attenuation and noise performances. The fulfillment of such requirements strongly depends on crystal growth technique and machinery process. We have developed a polarimetric two-dimensional mapping technique at λ =1.5 μm, that comparatively characterizes the residual stress-induced birefringence of semiconductor modulator rods from a functional point of view, by evaluating the distribution of effective birefringence and estimating the overall noise figure of merit (cross-talk) for optical switching applications. We have examined high-resistivity CdTe rods grown by the horizontal Bridgman technique, and compared results to data from GaAs single crystals grown by horizontal Bridgman, liquid encapsulated Czochralski and vapor pressure controlled Czochralski. The birefringence has been measured with a resolution in the Δ n ≈10 −7 range; spatial resolution can be adjusted, reaching 10 μm. The performance of CdTe samples is strongly affected by the presence of dislocations and related localized stress fields, and acceptable performance is reached only for large diameter optical beams, which average birefringent effects on the cross-sectional area of the rod. High volume density of dislocations is intrinsic to single crystals grown by the Bridgman method. Growing techniques from the vapor phase usually result in more homogeneous samples. Therefore, we think that the development of a suitable vapor phase growth method is the key to obtain semi-insulating CdTe bulk crystals to be used for high-performance optical modulation and switching.

Sub-nanosecond all-optical switching in CdZnTe

We demonstrate the possibility of using high resistivity CdZnTe as an active material for all-optical switching of signals in the third telecommunication window, with sub-nanosecond response times. A wide range of IR control wavelengths are effective. The physical mechanism relies on joint exploitation of Pockels and optically generated field shielding effect. Fast recovery confirms the presence of an efficient recombination center.

Near-IR Comparative Characterization of Optical Second-Order Nonlinearities in Te-Based Semiconductors

We have systematically characterized the second harmonic generation efficiency of Te-based binary and ternary II–VI compounds in the near-IR spectral region, namely at=1.5 m and=1.9 m. A phase mismatched technique has been employed, as a translation into the spectral domain of the Maker fringes method. By using sub-picosecond optical pulses, reliable results for the value of non second-order linear coefficientd and of chromatic dispersion have been obtained. We have performed a comparative evaluation of nonlinear wavelength conversion efficiency of Te-based semiconductors and GaAs. The ternary alloys Cd0.5Zn0.5Te and Cd0.78Mn0.22Te can be considered as the best performing basic materials for optical wavelength converters working in the third communication window, since they possess at the same time a highd coefficient, good transparency, and low chromatic dispersion.

Optically induced switching in CdZnTe

All-optical switching performances of the semiconductor ternary alloy CdZnTe have been characterized, operating on a signal at /spl lambda/=1.5 /spl mu/m. The physical switching mechanism exploits both the electrooptic effect and near-infrared photoconductivity. The use of CdZnTe improves the recovery time constant by six orders of magnitude when compared to previously tested CdTe-based modules. Control fluences of less than 0.02 nJ//spl mu/m/sup 2/ and extinction ratios as high as 23 dB have been demonstrated when operating in the nanosecond temporal regime.