1. Introduction

Chalcogenide glasses are ideally suited for fabrication of thin-film optical devices that operate in the near- to far-infrared spectral regimes [1–4]. The selenide system offers low melting and evaporation temperatures, a wide range of glass-forming compositions that permit both engineering design of the refractive index and high transmittance in the 1-to-12-μm wavelength regime. The GeSbSe system is highly covalent with excellent thermal and other physical properties, and through composition control, can be rendered electrically conductive or photoconductive. Although these chalcogenide glasses are hard to process in bulk without the introduction of water and oxygen impurities (and associated absorption bands in the infrared spectral regime), those problems are highly suppressed in thin films of these materials. It is also worth mentioning that these materials can exhibit phase-change behavior which could introduce other functionalities into chalcogenide-glass-based infrared optical devices.

In a research program to develop the use of chalcogenide glasses for free-space optics in the 1-to-3-μm wavelength regime, we decided to fabricate circular-polarization filters. Circularly polarized light in this regime is commonly generated by transmission of light first through a linear polarizer and then through a quarter wave plate, typically made of a birefringent crystal such as quartz [5]. However, a compact wideband filter that rejects one of the two circular polarization states but allows the other circular polarization state to pass through with sufficient transmittance appears to be uavailable for the 1-to-3-μm wavelength regime. This kind of filter should require the use of a structurally chiral material [6] made of a material that is transparent in that wavelength regime.

Therefore, we decided to fabricate circular-polarization filters for the IR-A (700–1400 nm) and IR-B (1400–3000 nm) spectral regimes by the vacuum deposition of chiral sculptured thin films (STFs) [7] of a commercially available chalcogenide glass with nominal composition Ge₂₈Sb₁₂Se₆₀. This material can be easily evaporated by resistive heating in a vacuum chamber and deposited as a thin film with columnar morphology [8, 9]. Its bulk refractive index is large and it is weakly dissipative in the IR-A and IR-B spectral regimes. Chiral STFs made of chalcogenide glasses have not been reported previously.

A chiral STF is an assembly of parallel and upright helical columns of sufficiently small cross-sectional diameter so that it can be considered as a unidirectionally nonhomogeneous, dielectric continuum whose relative permittivity dyadic [10] rotates in a helical fashion along the direction of nonhomogeneity. It is thus a structurally chiral material and can function as a wideband-rejection filter for circularly polarized radiation of the same handedness as that of its own helical columns [11]. Once an appropriate bulk material has been chosen, the spectral regime of operation can be engineered quite simply though the structural period (i.e., the pitch) and the angle of rise of the helical columns. A chiral STF with a central phase defect can function as a narrowband spectral-hole filter for circularly polarized radiation of the same handedness as that of its own helical columns [12].

The plan of this paper is as follows. Section 2 provides a brief description of the theory underlying the performance of both types of circular-polarization filters. The vacuum deposition process adopted is presented in Sec. 3, and the fabricated filters are characterized in Sec. 4. Many different types of central phase defects [12] can be implemented, but the one we have chosen here is the central 90°-twist defect [13–15].

2. Theory in brief

For our purposes, a chiral STF is viewed as an electromagnetic continuum that completely occupies the region between the planes z = −L and z = L. Whereas the relative permeability of the chiral STF is unity, its permittivity dyadic is stated as follows [7]:

The dyadic

Provided that circularly polarized radiation is normally incident on the chiral STF, the ratio N = L/2Ω is sufficiently large, and |γ⁺ − γ⁻|modπ = 0, the circular Bragg phenomenon may be exhibited as follows: If the handedness of the incident radiation is the same as the structural handedness of the chiral STF, and the free-space wavelength of the incident radiation lies within a spectral regime called the circular Bragg regime, the reflectance is very high and the reflected radiation is mostly co-polarized [7, 11]. The reflectance is very low when the two handednesses are not the same. With the twin assumptions that dispersion of the constitutive parameters is sufficiently weak in the neighborhood of the circular Bragg regime and that dissipation is weak enough to be ignored, the circular Bragg regime may be estimated [17] as ${\lambda }_0\in \left[2\Omega \sqrt{{\varepsilon }_c},2\Omega \sqrt{{\varepsilon }_d}\right]$, where

The calculated transmittances T_RR, T_LL, T_LR, and T_RL are plotted in Fig. 1 as functions of the free-space wavelength λ₀ for a chiral STF without a central phase defect. The notation T_RL indicates the efficiency of transmission of an incident LCP plane wave as an RCP plane wave. The relevant parameters of the chiral STF are as follows: N = 8, h = +1, Ω = 350 nm, ɛ_c = 2.89, ɛ_d = 3.24, and γ⁺ − γ⁻ = 0. Within the circular Bragg regime λ₀ ∈ [1190, 1260] nm, an incident right-circularly polarized (RCP) plane wave will suffer high reflection, but an incident left-circularly polarized (LCP) plane wave will undergo high transmission, both without significant change of polarization state. Accordingly, T_RR, T_LR, and T_RL must be very weak in the circular Bragg regime, whereas T_LL must have a high magnitude, which is confirmed by the plots in Fig. 1. Thus, a chiral STF without a central phase defect is a wideband rejection filter for co-handed circularly polarized radiation, which has already been demonstrated experimentally in the visible regime [11].

 

Fig. 1 Calculated transmittances T_RR, T_LL, T_LR, and T_RL as functions of the free-space wavelength λ₀ when a plane wave is normally incident on a chiral STF without a central phase defect. The relevant parameters of the chiral STF are as follows: N = 8, h = +1, Ω = 350 nm, ɛ_c = 2.89, ɛ_d = 3.24, and γ⁺ − γ⁻ = 0. Interchange the subscripts L and R in the transmittances for h = −1.

Download Full Size | PDF

Suppose next that γ⁺ − γ⁻ = 90°, so that the chiral STF has a central 90°-twist defect. The spectral characteristics exhibited in Fig. 1 do not change qualitatively except in the central part of the circular Bragg regime. If N is sufficiently but not excessively large, a reflection hole appears in the center of the circular Bragg regime in the response of the chiral STF to incident RCP (LCP) plane waves for h = +1 (h = −1), as can be deduced from Fig. 2, and a very weak similar effect also appears in the response to incident LCP (RCP) plane waves. Thus, a weakly dissipative chiral STF with a central 90°-twist defect is a narrowband transmission filter for co-handed circularly polarized radiation, which has also been demonstrated experimentally in the visible regime [13].

 

Fig. 2 Same as Fig. 1, except that γ⁺ − γ⁻ = 90°.

Download Full Size | PDF

Parenthetically, when N is very large and γ⁺ − γ⁻ = 90°, theory predicts [12, 14, 15] that the narrowband spike in the transmittance spectrum for co-handed incident circularly polarized radiation in Fig. 2 is replaced by an ultranarrowband hole in the transmittance spectrum for cross-handed incident circularly polarized radiation. However, that phenomenon has not been experimentally verified because even weak dissipation in the chiral STF is inimical to its existence [18], although a new scheme has been recently formulated for experimental verification [19].

3. Device fabrication

In order to make both wideband-rejection filters and reflection-hole filters, all chiral STFs were deposited on 25 mm × 76 mm pre-cleaned borosilicate glass microscope slides (Fisherfinest premium microscope slides) as substrates. Each substrate was attached with kapton tape to a 75-mm-diameter stainless-steel substrate holder. This assembly was then placed in a vacuum chamber which was pumped to a base pressure of ∼ 2 × 10⁻⁶ Torr prior to deposition. The chalcogenide glass with nominal composition Ge₂₈Sb₁₂Se₆₀ (commercially available from Schott North America) was then thermally evaporated from a dimpled tungsten boat (R. D. Mathis P/N S9E-.010W) located 14 cm below the substrate. The deposition rate and the total thickness of the growing chiral STF were monitored in situ by a quartz crystal monitor (QCM). The deposition rate was held constant during the growth of the chiral STF by adjusting the current passing through the tungsten boat and monitoring the rate indicated by the QCM. Currents of magnitude between 192 and 199 A were required to maintain the desired deposition rates. The duration of the depositions ranged from 138 to 168 min, depending on the total thickness 4NΩ of the chiral STF.

The motion of the substrate was controlled by a pair of computer-controlled stepper motors. The first motor controls the angle χ_v between the substrate plane (the xy plane) and the average direction of the incident vapor flux. This angle, fixed at 15° for all samples fabricated for this research, is crucial to the control of ɛ_a,b,c and χ [20, 21]. The second stepper motor controls the rotation of the substrate about the central axis perpendicular to the substrate plane, this axis being designated as the z axis. Coordination between the deposition rate and the rotation motor allows for the variation of the morphology of the chiral STF, when the angle χ_v is fixed.

A technique called serial bi-deposition (SBD), originally devised to produce highly anisotropic columnar thin films [22, 23], was adapted to produce chiral STFs with high contrast between ɛ_c and ɛ_d [24]. The higher the contrast, the smaller the value of N needed to produce a satisfactory wideband filter. In the SBD process, deposition occurs as a sequence of discrete sub-depositions [24, 25].

In order to produce chiral STFs, the following SBD methodology was adopted: The fabrication begins with a sub-deposition for just 2 s on the stationary substrate, followed by quick rotation in 0.5 s of the substrate by 183° about the z axis during which minimal deposition occurs, followed by another sub-deposition for 2 s on the stationary substrate, and so on. The two-step cycle was repeated 120 times to produce one structural period, of thickness 2Ω, of the chiral STF.

In order to fabricate a wideband-rejection filter, the cycle was repeated 240N times. For the fabrication of a narrowband reflection-hole filter, the SBD process was carried out for 120N cycles, the substrate was quickly rotated a quarter turn about the z axis, and the SBD process was resumed for another 120N cycles.

The structural period of the chiral STF can be altered by adjusting the duration of the sub-depositions while maintaining a constant deposition rate, or conversely by altering the deposition rate while maintaining the sub-deposition duration. We chose to maintain the deposition rate while the sub-deposition duration was altered to produce the desired structural period. With a deposition rate of 1.15 nm s⁻¹, we fabricated chiral STFs with Ω = 300 nm in test runs.

4. Structural and optical characterization

Let us now present data on the following two devices fabricated for this paper:

For both devices, we fixed h = +1 and N = 8. The structural periods were chosen based on spectral measurements of reflectance and transmittance of columnar thin films made of chalcogenide glass, whereby the value of the composite refractive index

 

Fig. 3 Cross-sectional SEM images of (a) WRF and (b) RHF. The arrow indicates the location of the central 90°-twist defect in RHF.

Download Full Size | PDF

The morphology of each device was verified as follows: During the fabrication of each device, a silicon wafer was left next to the substrate in order to have a nearly identical chiral STF grown on it. This witness sample was cleaved in liquid nitrogen and then mounted in a Hitachi S-3500N secondary-emission scanning electron microscope (SEM) to obtain a clear cross-sectional view of the morphology. Cross-sectional SEM images of WRF and RHF presented in Figs. 3(a) and (b), respectively, show the helical shape of each column. The arrow in Fig. 3(b) identifies the central 90°-twist defect. These SEM images leave no doubt that both devices are structurally chiral and therefore will discriminate between circularly polarized radiation of different handedness in their respective circular Bragg regimes (Chap. 10 of [7]), because theoretical predictions have already been verified experimentally in the visible regime [11, 13]. What remains is to verify that the fabricated devices have enough structural periods (i.e., 2N is large enough) to exhibit well-developed circular Bragg phenomenon for functioning adequately in the near-infrared regime [6].

Both devices were characterized by measuring the transmittances T_RR, T_LL, T_LR, and T_RL for normal incidence as functions of the free-space wavelength λ₀ on a Perkin-Elmer Lambda 950 UV/Vis/NIR spectrophotometer with the standard detector module installed. This is a double-beam, double-monochromator-ratio recording system with prealigned tungsten-halogen and deuterium lamps as sources. The wavelength reproducibility is 0.02 nm in the near-infrared regime. Combinations of an adjustable linear polarizer and a Fresnel rhomb retarder were mounted on either side of the device (either WRF or RHF) between the source and detector ports of the spectrophotometer.

The circular Bragg regime of WRF is almost fully developed, with a center wavelength of 1225 nm and a full-width-at-half-maximum bandwidth of ∼ 80 nm in the plot of T_RR in Fig. 4. Whereas T_LL < 10% for about 25 nm on either side of the center wavelength, T_RR > 58% over the same wavelength regime, signifying acceptable performance as a wideband-rejection filter. Increasing N somewhat could enhance the performance, and the proper selection of χ_v would also assist in that goal. Furthermore, the ratio of the center wavelength (1225 nm) to the structural period (700 nm) equals 1.75, which lies on the lower side of the center of the estimated range (1.67 to 2) of n_Br.

 

Fig. 4 Measured transmittances T_RR, T_LL, T_LR, and T_RL of the device labeled as WRF (Ω = 350 nm) as functions of the free-space wavelength λ₀. The device was fabricated of chalcogenide glass with nominal composition Ge₂₈Sb₁₂Se₆₀ and its helical columns have 2N = 16 complete turns. Data were acquired at 2-nm λ₀-intervals.

Download Full Size | PDF

The center wavelength of the T_RR spike of RHF is 1445 nm in Fig. 5, whereas the structural period is 800 nm. The ratio of the former to the latter is 1.81, quite in the center of the estimated range of n_Br. The full-width-at-half-maximum bandwidth of the spike in the plot of T_RR is 18 nm, with a peak at 52%. Calculations show that the peak value could continue to rise with N, reach a maximum and then begin falling down [12, 14, 19].

 

Fig. 5 Same as Fig. 4, except for the device labeled as RHF (Ω = 400 nm).

Download Full Size | PDF

Were the chalcogenide glass endowed with nondispersive properties in the chosen spectral regime, the T_RR spike of RHF would have been centered at λ₀ = 1225 × (800/700) = 1400 nm. The fact that it is actually centered at λ₀ = 1445 nm indicates that the chiral STF is optically denser at wavelengths around 1450 nm than around 1230 nm. A similar feature is evident in Fig. 3 of [8], though either at somewhat shorter wavelengths or smaller χ_v. Dispersion is always accompanied by dissipation—by virtue of the principle of causality [26]—which would depress the peak of T_RR in the spike.

5. Concluding remarks

We have shown experimentally that chiral STFs with quite a small number (16) of structural periods and made of chalcogenide glass can function adequately as wideband-rejection filters for circularly polarized radiation in the IR-A spectral regime. We also successfully fabricated a reflection-hole filter for operation near the short-wavelength edge of the IR-B spectral regime. Both types of filters are relevant to spectroscopic measurements of optical activity [27, 28]. Improvements in performance can be effected by the incorporation of impedance-matching layers [29].

Coverage of the entire IR-B regime would require chiral STFs of up to 1600-nm structural period without possibly some increase in the number of structural periods, which was not feasible with our experimental vacuum chamber but should be easily possible in the larger vacuum chambers commonly employed in the thin-film industry. Post-deposition wet etching [30] could be used to tune the spectral characteristics of chiral STFs [31].

The foregoing developments strongly suggest that many different types of optical devices—such as polarizers, rugate filters, and waveplates—for use in the IR-A and IR-B spectral regimes can be made of chalcogenide glass with STF technology (Chap. 10 of [7]). As this technology employs thermal evaporation, a workhorse of the thin-film industry [32], the shortage of optical devices in the 1000-to-3000-nm wavelength range can be considerably alleviated. Furthermore, the photochromic properties of chalcogenide glass also promise active tunability of these devices, and their porosity suggests [33, 34] their ability to function as infrared sensors of fluids. The emerging technology of interactive video, where both visible and infrared signals are used to track and image the users through transparent displays, could also benefit from such optical filters.