2 ab-plane. The cutting surfaces were finely polished for the c-axis polarization measurement. The optical reflectance measurements with E ‖ c-axis were performed on a Bruker IFS 66v/s spectrometer in the frequency range from 50 to 25000 cm − $^{1}$. An in situ gold and aluminium overcoating technique was used to get the reflectivity R( ω ). The real part of conductivity σ 1 ( ω ) is obtained by the Kramers-Kronig transformation of R( ω ). A Hagen-Rubens relation was used for low fre- quency extrapolation. A ω − 0 . 5 dependence was used for high frequency extrapolation up to 300000 cm − $^{1}$, above which a ω − 4 dependence is employed. Figure 1 shows the c-axis R( ω ) and σ 1 ( ω ) at room temperature over a broad frequency range up to 20000 cm − $^{1}$for BaFe 2 As 2 . For a comparison, we also plot the optical spectra with E ‖ ab-plane[7]. We can see that the overall R( ω ) along the c-axis is quite similar to that in the ab-plane except for relatively lower values. This is dramatically different from the optical spectra of some high-Tc cuprates[2] and other layered compounds, for example, layered ruthenates[20], where the c-axis R( ω ) shows much lower values and quite different frequency- dependent behavior from the ab-plane. This observation suggests that the band structure of Fe-pnictide should be quite three-dimensional (3D), in contrast to the expecta- tion based on its layered crystal structure. The differ- ence in σ 1 ( ω ) spectra between ab-plane and the c-axis seems to become larger at low frequencies. An extrapo- lation to zero frequency, or the dc conductivity, shows an anisotropy ratio of about 2.8. This matches well with the anisotropy ratio determined directly from the dc resis- tivity measurement by a careful Montgomery technique (about 3 ± 1)[21]. We noticed that the c-axis reflectance values obtained here are much higher than the data pre- sented by Wu et al. on a EuFe 2 As 2 sample[10]. The re- flectance values in their measured frequency range (above The temperature dependences of the R( ω ) and σ 1 ( ω ) spectra of BaFe 2 As 2 below 7000 cm − $^{1}$( ∼ 1eV) are plotted in Fig. 2 (a) and (b), respectively. In the mid-infrared region (near 4000 cm − $^{1}$), there is a strong suppression feature in both R( ω ) and σ 1 ( ω ) spectra. Essentially, the same feature is seen in the ab-plane optical response at slightly higher frequencies (near 5000 cm − $^{1}$). For a com- parison we also plot the in-plane R( ω ) and σ 1 ( ω ) spectra at 300 K and 10 K in the figure.[7] The suppressed spec- tral weight in σ 1 ( ω ) is largely transferred to the high frequencies above 5000 cm − $^{1}$for both polarizations. The suppression is a common feature for Fe-pnictide mate- rials and is not directly related to the SDW order as it is seen well above the structural/magnetic transition temperature.[7, 13, 16] Its origin remains to be explored. Most remarkably, the reflectance is strongly suppressed below ∼ 320 cm − $^{1}$upon entering the SDW ordered state. A sharp upturn appears at lower energy scale. The strong suppression in R( ω ) results in a prominent gap structure in σ 1 ( ω ). The sharp upturn at lower frequencies in R( ω ) 700 cm − $^{1}$) are already below 0.4, leading to extremely low values of conductivity (below 100 Ω − $^{1}$cm − 1 at 700 cm − $^{1}$). We remark here that we repeated measurement of E ‖ c on another crystal grown from a different batch and achieved almost identical result. FIG. 2: The c-axis R( ω ) (a) and σ$_{1}$ ( ω ) (b) at different temper- atures below 7000 cm − $^{1}$for BaFe$_{2}$ As$_{2}$. Inset: expanded plot of the low- ω R( ω ) and σ$_{1}$ ( ω ) spectra. The ab-plane R( ω ) and σ$_{1}$ ( ω ) at 300 K and 10 K are included for comparison. The two upward arrows indicate the anomalies associated with the SDW order. The very pronounced peak in σ$_{1}$ ( ω ) for E ‖ ab- plane becomes extremely weak for E ‖ c . FIG. 1: The c-axis R( ω ) and σ$_{1}$ ( ω ) at 300 K for BaFe$_{2}$As$_{2}$ over a broad frequencis up to 20000 cm − $^{1}$. The ab-plane spectra were also included for a purpose of comparison.[7] The conductivity anisotropy at low frequency limit is about 2.8. 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