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Abstract
Controlling the spectral and angular response of infrared (IR) radiation is a challenging task of paramount importance to various emerging photonic applications. Here, we overcome these problems by proposing and analyzing a new design of a tunable narrowband directional optical transmission filter. The presented thermally controlled multilayer filter leverages the temperature dependent phase change properties of vanadium dioxide (VO2) to enable efficient and reversible fast optical switching by using a pump-probe laser excitation setup. More specifically, transmission is blocked for high intensity probe lasers due to the VO2 metallic properties induced at elevated temperatures while at low probe laser intensities high transmission through the filter occurs only for a narrowband IR range confined to near normal incident angles. The proposed multilayer composite dielectric filter is expected to have applications in optical communications, where it can act as dual functional infrared filter and optical switch.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, the emerging area of tunable photonics has attracted increased attention as an efficient way to design new optical devices that can dynamically adapt their properties to a variety of diverse applications. A particular phase change material commonly used in tunable photonic applications is vanadium dioxide (VO2). It undergoes a reversible phase transition from insulator to metal as its temperature is increased by switching phases at a critical temperature (∼340K) [1], which can be controlled and tuned via doping with tungsten [2]. Thus, VO2 enables efficient thermal control over the optical properties of photonic structures that is incorporated into. Many works were focused on using VO2 to thermally switch between states of high and low reflection, transmission, or absorption. As an example, tunable reflection in the visible spectrum was demonstrated by using VO2 as a defect layer in a Bragg filter design [3]. Enhanced tunability of absorption/emissivity in the mid-infrared (IR) band was demonstrated using a multilayer design composed of alternating layers of VO2 and metals [4]. Another design utilized the refractive index change of VO2 to create a tunable optical response in a thin film structure [5]. A VO2 filled double cavity was designed for operation in the near IR that could act as band-pass filter or optical switch [6]. More recently, switchable transmission in the mid-IR range was demonstrated using a thin VO2 layer in a multilayer filter design [7]. However, most of the previously works demonstrate switching and reconfigurability only by varying the bulk temperature of the obtained device and not by dynamically changing the incident laser power in a pump-probe laser experimental configuration setup. The latter approach will be much faster in terms of switching operation since laser-induced heating can occur in extremely fast time scales [8].
Another important aspect of light propagation that the works above do not consider is filtering the incident angle of light that can be narrowed to a small range leading to directional transmission response. For full control of light, it is desirable for optical filters to not only control the spectral response but also the spatial directionality of incident light. In the past, angular filtering of light was demonstrated using a multilayer structure surrounded by a fluid serving as broadband impedance matching to the surrounding material [9]. This multilayer filter design featured efficient and broadband transmission restricted to only angles near 60°, however the design only worked for transverse-magnetic (TM) polarized light which limits its thermal emission applications. A narrowband polarization independent filter that confines the transmission of light to only near normal incidence has also been demonstrated in recent years [10]. While both of these designs perform well in terms of simultaneous spectral and angular filtering, they have no mechanisms to control or modulate their behavior which will be very beneficial for tunable photonic filter applications.
As mentioned before, another interesting application for tunable filters is optical switching based on pump-probe laser experiments. The binary switching of VO2 between two phases makes it a good candidate for use in various switching scenarios. Optical switching using VO2 thin films [11,12] and fast switching using VO2 nanocrystals has been demonstrated for operation at near-IR frequencies [13]. Other designs have attempted to improve the modulation depth in the near-IR range, such as structures consisting of gold nanowires over a VO2 thin spacer layer that leverages the plasmonic modes of metallic VO2 to enhance tunability [14]. In the mid-IR range, another design used a two-dimensional (2D) array of aluminum nanoantennas over a thin VO2 layer that alternated between high reflection and absorption depending on the VO2 phase [15]. However, both these works require complex nanostructuring which makes their fabrication challenging and impractical.
Beyond VO2, several alternative methods exist to create tunable filters and optical switches. Graphene is one common approach, where the Fermi energy of graphene can be tuned via electrostatic gating. This allows for control over the plasmonic resonances of graphene enabling tunable optical filters consisting of graphene layers combined with plasmonic gratings [16,17]. However, the use of graphene can result in additional fabrication complexity. Other techniques to create optical switches include controlling the position of liquid droplets [18], or the use of liquid crystals [19], Dirac semimetals [20], and topological semimetals based on antimonene [21]. A notable advantage of VO2 over the other approaches is that the extreme difference between its metallic and insulating phase enable very high modulation depths [15] that can occur in rapid time scales in case the phase transition is induced by probe lasers.
In this work, we present the design of a new optically switchable narrowband directional transmission filter based on a simple multilayer dielectric structure. We utilize the thermally dependent optical properties of VO2 to introduce control in the directional filtering response. When the VO2 layers are in the insulating phase, the structure exhibits very narrowband transmission of light at incident angles close to normal. In the metallic phase, the VO2 layers prevent transmission through the structure and turn the device to a mirror or absorber of mid-IR radiation when high probe laser intensities are utilized. The optical response of the design is accurately analyzed using the transfer matrix method and the dynamic temperature dependence is characterized using the VO2 Bruggeman effective medium theory. Transient modeling under pump-probe laser illumination is carried out to demonstrate the applicability of the presented filter to fast optical switching at the mid-IR range. The presented directional and rapid switching response is unique among other relevant optical filters and provides an additional degree of control compared to other designs. The proposed optical filter is envisioned to have a plethora of photonic applications, such as infrared filtering and optical switching.
2. Narrowband directional filter analysis
The schematic of the proposed multilayer directional transmission filter is shown in Fig. 1(a). The design consists of five unit cells of alternating layers of VO2 and calcium fluoride (CaF2). The thickness of each VO2 layer is 450nm while each CaF2 layer is 2.55µm thick. Both VO2 and CaF2 materials are modelled using their frequency dispersive dielectric constants obtained from relevant experimental data [22–24]. In the insulating state, the VO2 acts as a high refractive index dielectric. When combined with the low index CaF2 material, a special Bragg mirror effect occurs that works for both polarizations, creating a band of high reflection at wavelengths above 7.7µm. At the edge of this reflection band, a transmission resonance occurs, and a highly directional response is obtained. In a typical Bragg reflector design, the layer thicknesses are chosen to be ¼ the center wavelength of the stop band. Using this as a starting point, the thicknesses of the layers were optimized until the highly narrowband directional transmission results were obtained. The multilayer structure can be feasibly fabricated using thin film deposition techniques. Growth of VO2 thin films on the order of 100 nm thick have been demonstrated using RF magnetron sputtering [25] and thicker high quality VO2 films have recently been demonstrated using the vapor transport method [26]. Likewise, CaF2 films of micron thickness have been deposited using resistive evaporation [27]. Alternating these deposition techniques can realize the filter’s multilayer structure.
Fig. 1. (a) Schematic of the multilayer directional transmission filter. (b) Transmittance as a function of wavelength for normal incidence illumination. Only the primary resonance is shown. There is substantial transmission change between insulating and metallic VO2 phases. (c)-(d) Transmittance as a function of wavelength and incident angle for (c) TM and (d) TE polarization. A broader wavelength range containing higher order resonances is shown in these figures. (e) Directional transmittance pattern as a function of angle for fixed 7.6µm operation wavelength.
The computed transmittance versus wavelength is shown in Fig. 1(b) at the primary resonance of the structure. The full width at half maximum (FWHM) for this resonance is found to be ∼240 nm which further demonstrates that the presented response is very narrowband. The transmittance spectra as a function of incidence angles are shown in Fig. 1(c) for TM polarization and Fig. 1(d) for transverse-electric (TE) polarization when VO2 is in the insulating phase. A broader wavelength range is shown that includes multiple higher order resonances. These results prove that the presented filter can work for both polarizations, i.e., incoherent light illumination, consisting ideal property for thermal imaging and sensing. In the metallic VO2 phase, the transmission is zero for all wavelengths and incident angles and the filter exhibits mirror-like response for near-normal incidence, while higher angles of incidence show broadband absorption. More information on the reflectance and absorptance in the metallic state is available in Supplement 1. Figure 1(e) demonstrates the directional response of the filter when VO2 is in the insulating phase, where it is confirmed that the optical transmission is limited to only a small range of angles near normal incidence. Interestingly, the narrowband directional response is maintained for a reduced number of material layers without a drastic drop in performance, as discussed in more details in Supplement 1.
The transmittance spectra shown in Figs. 1(b)-(e) were obtained analytically using the transfer matrix method. Each layer was modelled as a transmission line and the ABCD parameters for each layer were obtained using [28]:
The transmission coefficient (S21) through the structure can be found by converting the ABCD parameters of Eq. (2) into the S-parameters [28]:
where the free space impedance for oblique incident waves is ${Z_0} = {\eta _0}/\cos (\theta )$ for TE polarization and ${Z_0} = {\eta _0}\cos (\theta )$ for TM polarization. The transmittance through the structure is then found as |S21|2.From the results in Figs. 1(b)-(e), it can be derived that the transmission through the filter is very narrowband at ∼7.6µm and is restricted to within ∼15° from normal incidence. This small transmission band leads to narrow directional spatial filtering obtained at the edge of the multilayer structure’s stopband. The highest transmission occurs closest to the stopband of the filter, though the fringes at lower IR wavelengths can also be used for similar performance filtering. The operating wavelength and fringe wavelengths can be adapted to different frequency ranges by simple redesigning the layer thicknesses. Interestingly, the presented filter design is polarization independent, thus the filter can be used with randomly polarized incoherent light sources, e.g., mid-IR thermal emission.
The directionality was further verified using finite element method 2D full wave simulations. An out of plane line current was used to model a point source located above the filter at a wavelength of 7.6µm. Again, the VO2 and CaF2 materials were modeled using their frequency dependent dielectric constants [22–24]. The induced field distributions through the filter for the VO2 insulating and metallic phases are shown in Figs. 2(a) and 2(b), respectively. Directional radiation is only present for the VO2 insulating phase (Figs. 2(a)) while the transmission is terminated when the VO2 is metallic (Figs. 2(b)). These results further prove the unique directionality and tunability of the filter design.
Fig. 2. Field distributions through the presented directional filter using a point source when VO2 is in the (a) insulating and (b) metallic phase, respectively.
The transmission of the presented filter does not change abruptly during the VO2 phase transition process, but it has a dynamic temperature dependent response that follows the change of the VO2 dielectric constant. More specifically, the VO2 phase transition occurs at the critical temperature $({{T_C} \simeq 340K} )$. Interestingly, the VO2 phase is in transition mode at temperatures very close to critical, and its complex dielectric constant can be accurately computed by using the Bruggeman effective medium theory [29]:
Fig. 3. (a) Dynamic transmission response versus temperature at normal incidence illumination. The temperature ranges from insulating to metallic VO2 phase and a clear transition area occurs before the VO2 critical temperature TC. (b) Angular transmittance as a function of temperature. Both results are plotted at a fixed wavelength of 7.6µm.
The switching behavior of the optical filter as the temperature is increased is not abrupt but it has a dynamic transition range which is clearly illustrated in Fig. 3(a). At lower temperatures than TC, when VO2 is in the insulating phase, transmission through the structure is greater than 80%, similar to Fig. 1(b). At higher temperatures compared to TC, VO2 transitions to the metallic phase and transmission drops to zero. However, the directionality of the filter is maintained during the insulating and transition phases, as depicted in Fig. 3(b). The relative abrupt but continuous transition between phases can enable fast optical transmission switching and modulation when the temperature rapidly changes. This type of response is ideal for efficient infrared optical communications, where the selective angular response of the filter can be used to eliminate noise and erroneous stray signals coming from oblique incident angles. Furthermore, the VO2 critical temperature can be further reduced via doping with tungsten, thus the presented optical filter can be made tunable and even operate at room temperature.
3. Optical transmission switching
Transient simulations based on pump-probe laser excitations are performed to demonstrate the ability of the directional filter to operate as a fast optical transmission switch. The heating process in the simulations is induced by the pump laser excitations, where two different lasers operating at distinct wavelengths and input intensities are used. A detailed explanation of the transient simulations is available in Supplement 1. Comsol Multiphysics is used to simulate the thermal and optical behavior of the filter. The pump and probe lasers are modelled as plane waves which are launched towards the multilayer structure. Although typical laser pulses have a gaussian beam spatial profile, their spatial extend usually covers a large area. Hence, we focus our simulations near the maximum of the spatial gaussian beam distribution and approximate the impinging lasers as plane waves. The temperature of the structure is initialized to room temperature (293 K) and the Comsol’s electromagnetic heating interface is used to determine the heating of the structure according to the following equations:
Fig. 4. (a)-(b) Temperature and (c)-(d) transmittance of the directional filter as a function of time for the (a), (c) Nd:YVO4 and (b), (d) Th doped fiber pump laser, respectively. The transient results are presented for different pump laser intensities. (Inset in (d)) A simple schematic of the simulation setup showing the pump/probe scheme.
The first choice of pump laser is a Thulium (Tm) doped fiber laser operating at a wavelength of 1940nm with intensities of 5, 7.5, and 10mW/cm2 [30]. The second pump choice is an yttrium vanadate (Nd:YVO4) laser operating at 532nm with slightly lower intensities of 2.5, 5, and 7.5mW/cm2 [31]. The chosen pump lasers are commercially available continuous wave (CW) lasers, but their excitation is chosen to last for 5ms to allow the structure to cool and demonstrate its transmission switching behavior during the different phases. The filter is always illuminated by a normal incident low power laser source acting as probe with fixed wavelength (7.6µm) in the mid-IR range that does not cause additional heating. The computed induced temperature along the filter as a function of time for the two pump lasers is demonstrated in Figs. 4(a) and 4(b), respectively. The filter is initially held at room temperature (293K). Heating is practically uniform throughout the structure, and the computed temperature represents the average temperature through the entire filter geometry. The induced temperature exceeds the critical temperature (purple dashed line in Figs. 4(a) and 4(b)) associated to the VO2 phase transition, as was described in Fig. 3. This phase transition leads to a transient and relatively abrupt switching in the transmittance of the filter for each probe laser which is depicted in Figs. 4(c) and 4(d).
The process of transmission switching takes less than a millisecond with both currently used pump lasers. More specifically, it is derived from Fig. 4 that the insulating to metallic phase change occurs at approximately 635µs and 361µs for the fiber laser at 7.5 and 10 mW/cm2 intensities, respectively. The transition from the metallic back to the insulating state takes 240µs and 368µs for the same laser with similar intensities. In the case of the yttrium vanadate laser, the insulator-metal transition occurs at 335µs and 186µs while the metal-insulator transition happens at 176µs and 335µs for intensities of 5 and 7.5mW/cm2 respectively. We define the insulator-metal transition time as the time it takes for the transmission to drop to zero after the start of the pump laser pulse. The metal-insulator transition time is defined as the time it takes for the transmission to increase back to maximum after the pump laser pulse is removed. The yttrium vanadate laser has faster switching performance mainly due to the stronger absorption of insulating VO2 in the visible emitting wavelength of this pump laser. The filter’s switching process can become even faster, approaching few micro- or pico-seconds, if less layers (see Supplement 1 [32]) or ultrafast pulsed lasers are used, respectively, resulting to a more rapid heating process [33–35]. The results shown here are very fast compared to other relevant schemes, such as liquid-based switching, which had a ∼200 ms response time [18], or liquid crystal designs that had switching times on the order of milliseconds [19].
4. Conclusions
We presented the design of a new thermally controlled directional narrowband optical transmission filter. The use of VO2 enables tunable and fast switching response between high and zero transmission. The transmission through the filter is limited to narrow bands in the mid-IR range and only light near normal incidence is allowed to pass through the filter. The filter works for both polarizations and, as a result, can operate with incoherent light sources, such as thermal radiation. The proposed optical filter will have applications in infrared optical communications, where its switching capability and directional selective transmission can lead to a reconfigurable and tunable response [36].
Funding
Office of Naval Research (N00014-19-1-2384); Nebraska Space Grant Consortium; National Science Foundation (OIA-2044049).
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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