1. Introduction

In optics, varifocal microlens arrays operating in the infrared (IR) are an interesting and important subject of research, and their potential applications can be extended to various fields, such as IR wavefront sensing and correction, functioned optical devices, or IR microbeam interconnection and switching [1,2]. However, the manufacturing process of conventional refractive zoom lens is complicated, and the bulky optical components must be assembled with tight alignment tolerances to decrease the system aberration. Fabrication and assembly become more difficult when further miniaturization or even integration of optical components is needed. Optical systems can generally be miniaturized using diffractive optical elements (DOEs). However, DOEs are much more wavelength-dependent than refractive optical elements [3]. Materials with tunable birefringence, such as typical liquid crystals (LCs), offer an alternative approach to integrated microlens chips with a tunable focal length [4–12]. However, owing to the limited number of available IR materials, most current applications based on LC microlens arrays (LCMLAs), such as typical light-field cameras [13–16] and wavefront measurement and correction sensors [17,18], operate in the visible region. To date, zoom microlens arrays operating in a wide IR range are less often reported.

To realize a broadband IR microlens, many materials have been investigated. A special LC material with high birefringence is isothiocyanato-based nematic LCs (NCSNLCs), which are suitable for the IR band, in particular the near-IR (1–3 μm, NIR) and mid-wave IR (3–5 μm, MIR). NCSNLCs have relatively high birefringence (∆n = ~0.253) at a wavelength of ~14 μm, which is an important performance metric for IR-LCMLAs [19,20]. Zinc selenide (ZnSe) generally exhibits low absorption of less than ~30% in a wide range from the visible to the far IR (typical wavelength range: 0.6–14 μm), so it should be an effective material for IR applications [14].

Optically transparent conductors are also an essential component of advanced optical or optoelectronic devices. Research and application have revealed that graphene has several impressive features, including IR transparency, low resistivity, chemical inertness, and excellent conductivity [21–23, 37]. Recently, large monolayer graphenes suitable for transparent conducting electrodes in optoelectronic devices were successfully fabricated using various methods [24,25]. Furthermore, graphene offers directional assembly of ordered nanomaterials (e.g., LC molecules) via van der Waals interactions at the interface, which improves the functionalities of ordered nanomaterials [26]. In addition to these intrinsic properties of graphene, LC devices based on graphene electrodes have been studied [27–30]. The main advantage of LC devices based on graphene electrodes is that the traditional alignment layer coating and mechanical rubbing processes can be eliminated. LC devices without an alignment layer will not suffer from the effects of LC alignment operation, such as typical microbeam diffraction crosstalk and relatively strong anchoring of LC molecules distributed around alignment grooves; thus, graphene is an excellent electrode material for IR LC devices.

With the recent rapid development of micro-optics techniques, typical refractive and diffractive microlenses with a fixed focus have been proposed and used. Zhang et al. presented IR microlenses for Shack–Hartmann sensors by combining high-speed single-point diamond milling and precision compression molding processes [1]. Karlsson and Nikolajeff designed an arrayed spherical microlens with a diameter of 90 μm using diamond materials to improve the transmittance [31]. Diffractive microlenses combined with InSb focal-plane optoelectronic arrays have been proposed to enhance the quantum efficiency and thus reduce the crosstalk [32]. However, the manufacturing strategy is still complicated, so the device cost is relatively high. Current metasurfaces present another way to realize a lens effect by constructing special surface micro-/nanostructures [33–35]. Polarization-insensitive and micron-thick microlenses based on plasmonic metasurfaces with a dense arrangement of subwavelength resonators were proposed by Arbabi et al. [33]. In the same year, Arbabi et al. used MIR dielectric metasurface flat lenses to collimate the output microbeams of single-mode quantum cascade lasers [34]. However, metasurfaces are inherently sensitive to the wavelength of incident beams, so they have a relatively large chromatic aberration. The focal length of the designs mentioned above is not tunable.

In this study, we demonstrate an electrically tunable LCMLA operating in a wide IR region from ~0.9 to ~11 μm. The LCMLA is based on a type of NCSNLC material sandwiched between a graphene electrode and an aluminum electrode without any traditional pre-alignment layer, which eliminates microbeam diffraction crosstalk and also obviously decreases anchoring of LC molecules distributed around alignment grooves [41,42]. The focus is tuned using a spatially nonuniform electric field in conjunction with an IR LC thin film. The desired graded-index lens effect is realized by coupling an aluminum microhole-array electrode and a planar graphene electrode. The LC molecules are efficiently reoriented by varying the RMS value of the signal voltage applied over the coupled electrodes, which produces a corresponding variation in the focal length. The IR-LCMLA acts only as a phase retarder without any driving signal or need for the RMS value of the signal voltage to be less than a threshold (typically 1.6 V_rms) under our current experimental conditions. We theoretically and experimentally demonstrate the transmittance and focusing performance of the developed devices in a broad IR region using a commercial optical microscope (Hyperion 3000, Bruker) attached to a Fourier-transform IR (FTIR) spectrometer (Vertex 80, Bruker). The experimental results indicate potential applications of the IR-LCMLA for advanced IR sensors using an arrayed microflux shaped and adjusted by the IR-LCMLA, IR microbeam interconnection and switching, and adaptive imaging based on IR wavefront measurement and correction.

2. Infrared liquid-crystal microlens array

A schematic of the IR-LCMLA is shown in Fig. 1. The IR LC microlens device is fabricated using LC materials customized for use in a wide IR region. The microcavity is composed of an aluminum-coated ZnSe substrate and a graphene-coated ZnSe substrate. Both ZnSe substrates are 1 mm thick, and one substrate is precoated with a conductive 150-nm-thick aluminum layer. The other ZnSe substrate is precoated with monolayer graphene on Cu foil by polymethyl methacrylate wet transfer (SixCarbon Tech. Co. Ltd.) [14]. The aluminum-coated ZnSe substrate is attached directly to the graphene-coated substrate without pre-alignment layers. The depth of the microcavity, which is determined by the diameter of glass spacers mixed in the adhesive, is 20 μm. The microcavity is continuously infiltrated with an IR LC material (Nematic Mixture HB76800, Xi'an Modern Chemistry Research Institute). HB76800 is a combination of fluorinated isothiocyanato-tolane and phenyl-tolane LC derivatives. It shows a relatively high birefringence (∆n = 0.43) and a relatively large dielectric anisotropy (∆ε = 18.6) at a wavelength of 589 nm, and the birefringence reaches 0.35 at a wavelength of 1 μm [19,20]. The clearing point (T_c) is 126.5°C. The physical properties of HB76800 are listed in Table 1. As shown in Fig. 1(a), the graphene serves as a planar electrode. Aluminum microholes with a diameter of 128 μm and a center-to-center spacing of 160 μm are etched by traditional ultraviolet photolithography and wet etching, so the aluminum serves as a microhole-patterned electrode. The aperture ratio of each microhole-patterned electrode is ~80%. Owing to the relatively strong IR diffraction effect, the actual filling factor of a single microlens, which is generally ~10% higher than the aperture ratio above, can reach ~0.88. A 1 kHz AC square wave, which can reduce ion migration [36], is used to drive the IR-LCMLA. A fabricated IR-LCMLA is shown in Fig. 1(b).

 

Fig. 1 (a) Schematic of our IR-LCMLA prototype. IR beams irradiating the IR-LCMLA interact with the LC material, which is controlled by an external voltage signal. Because LC materials are anisotropic, the micro-optical device exhibits a distinct polarization dependence. When an electric field is applied across the LC cell, the orientation of LC molecules distributed from the center to the edge of each microhole changes remarkably, resulting in tunable focusing and thus forming corresponding micron-scale focal spots on the focal plane. Without any voltage signal or the RMS value of the signal voltage being held below a threshold, the IR-LCMLA will act only as a phase retarder. (b) Appearance of the IR-LCMLA.

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Table 1. Physical Properties of HB76800 at T = 20°C^a

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It is important to note that the LC molecules' orientation in most LC devices depends on the pre-rubbing direction of the top and bottom polyimide (PI) alignment layers. Figure 2(a) and 2(b) schematically show the difference in LC alignment on rubbed PI and graphene, respectively. Figure 2(a) shows a schematic of LC alignment on the substrate with a rubbed PI layer. According to our previous research, the spacing and depth of the rubbing grooves on PI layers are ~0.75 μm and ~50 nm, respectively [18]. The two-dimensional (2D) intensity profile of a single microlens of the IR-LCMLA with an antiparallel rubbed PI alignment layer in the absence of an external voltage signal is shown in Fig. 2(d). Owing to the alignment anchoring and periodical refractive index change, the alignment grooves, which are completely filled with LC molecules, will exhibit strong microbeam crosstalk at each PI groove. To enhance the IR transmittance, the IR-LCMLA fabricated in this study has no pre-alignment layers. In addition, the diffraction crosstalk in the IR regime caused by interaction of the PI alignment grooves and LC molecules with incident microbeams is eliminated [29].

 

Fig. 2 (a) Schematic of LC alignment on substrate with rubbed PI layer. (b) Directional alignment of LC molecule axes with the graphene zigzag lattice direction. (c) Cross-sectional view of the equivalent refractive index profile in a single microlens with or without PI alignment layer. (d) 2D intensity profile of a single microlens of IR-LCMLA with antiparallel rubbed PI alignment layer at 0 V_rms. (e) 2D intensity profile of a single microlens of IR-LCMLA used in this study without alignment layer at 0 V_rms. (d) and (e) are obtained using the experimental setup in Fig. 3 (central wavelength: 980 nm) with the same colorbar (Range: 0-1).

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The directional alignment of LC molecules on the graphene electrode is illustrated in Fig. 2(b). The blue dotted lines represent specific lattice directions of graphene at 120° intervals. The alignment direction of LC molecules on graphene tends to parallel one of the three zigzag lattice directions, and LC molecules with the same orientation form a domain [27,28,38,39]. A large graphene electrode has many domains. The alignment direction of LC molecules in each domain is different, as demonstrated by the 2D intensity profile of a single microlens of the IR-LCMLA without an alignment layer at a voltage of 0 V_rms [Fig. 2(e)].

Figure 2(c) shows a cross-sectional view of the equivalent refractive index profiles (the black dashed lines) of a single microlens of the IR-LCMLA with two different configurations. Owing to relatively strong anchoring based on preshaped trenches in the PI layers of the LC cell, almost all the LC molecules are parallel to the rubbing direction of the PI layers. Compared with the IR-LCMLA without alignment layers, the anchoring energy of the LC cell associated with alignment will be high. Thus, the effective refractive index of the LC materials distributed in each microlens of the IR-LCMLA with alignment layers will differ slightly from that of the IR-LCMLA without alignment micro-/nanostructure under the same driving voltage signal. The difference in anchoring energy between the two configurations will lead to a relatively large variance of the effective refractive index of the LC materials sandwiched between the top and bottom electrodes. A roughly equivalent refractive index profile corresponding to the IR-LCMLA without PI alignment indicates that the convergence of IR microbeams will be greater.

In the design, the LC molecules will be reoriented along the nonhomogeneous electric field by the graded-index lens effect. According to Fermat's theorem, the focal length of the IR-LCMLA can be characterized by the relation

3. Results and discussion

To characterize the typical optical properties of the IR-LCMLA in a wide IR region, a measurement system is constructed, as illustrated in Fig. 3.

 

Fig. 3 Experimental setup for measuring the optical properties of our fabricated IR-LCMLA.

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As shown, the distance between the microlenses and the focal plane of the microscope is d. The key performance parameters, including the point spread function (PSF) and the focal length of the IR-LCMLA, are measured using a 40 × objective with a numerical aperture (NA) of 0.65 and a standard beam-profiling camera (WinCamD, DataRay, Inc.). Collimated beams with a central wavelength of 980 nm from a laser source (Changchun New Industries Optoelectronics Tech. Co. Ltd) are normally incident on the sample. The transmittance, PSF, and focal length of the IR-LCMLA illuminated by IR beams (2.5–11 μm) are also measured using the following instruments. Transmittance spectra are acquired by a 15 × Cassegrain objective (0.4 NA) on a commercial optical microscope (Hyperion 3000, Bruker) attached to an FTIR spectrometer (Vertex 80, Bruker). IR light waves are used to illuminate the sample from the bottom of the microscope. The objective collects the transmitted and absorbed light waves. Several images are collected using the same optical microscope with the same objective. Measurements in a broad IR band are performed at different locations of the IR-LCMLA, and the massive test data are processed by the OPUS software.

3.1 Focus variation of IR-LCMLA

To demonstrate the focusing performance of the IR-LCMLA, the relationship between the focal length and the driving signal voltage is illustrated in Fig. 4. A laser with a central wavelength of 980 nm is used as a light source. The IR-LCMLA acts only as a phase retarder without any driving signal or need for the RMS value of the signal voltage to be less than a threshold (typically 1.6 V_rms) under our current experimental conditions. When a voltage signal with the desired RMS value is applied to the IR-LCMLA, LC molecules will be reoriented according to the nonuniform electric field generated between the electrodes. As shown, the deflection degree of the LC molecules also depends on the variation of the electric field strength in the wavelength range tested. Thus, the focal length of the IR-LCMLA can be changed effectively by varying the applied voltage signal. Between ~2.0 and ~8.0 V_rms, the focus-to-voltage curve exhibits a declining trend. As shown in Fig. 4, when the driving signal voltage is more than ~2.0 V_rms but less than ~6.0 V_rms, the focal length decreases rapidly with increasing RMS voltage. When the experimental value exceeds ~6.0 V_rms, the focal length tends to be constant as the signal voltage increases from ~6.0 to ~8.0 V_rms.

 

Fig. 4 Relationship between the focal length and RMS voltage of the IR-LCMLA illuminated by an NIR laser with a central wavelength of 980 nm. The insets show the corresponding focal spots with the 2D and 3D intensity distribution at ~3.0 V_rms.

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When the driving voltage exceeds ~8.0 V_rms, the beam converging behavior of the IR-LCMLA gradually disappears, which can be attributed to a saturation effect in which the effective rotation of the LC molecules is already complete, causing the gradient refractive index distribution to disappear at relatively high driving voltages. Both the 2D intensity profiles and PSF of a single focal spot of the IR-LCMLA are acquired by a standard beam-profiling camera (insets in Fig. 4) and indicate that the NIR rays are already highly focused on the focal plane of the device at ~3.0 V_rms and that the focal length is ~1.74 mm.

3.2 Transmittance of IR-LCMLA

To remove the microbeam crosstalk between adjacent microlenses, we measure only the beam distribution and transmittance spectra of a single LC microlens with an aperture of ~128 μm, as shown in Fig. 5(a). Figure 5(b) shows the typical initial three-dimensional (3D) profile of microbeams passing through the microlens at ~6.58 V_rms when the microscope is focused on the microlens (d = 0). The color indicates the transmittance level in the IR band of ~2.5 to ~3.3 μm. The full transmittance spectra of the IR-LCMLA are shown in Fig. 5(c). The black and orange solid lines are the transmittance spectra obtained along the optical axis and at an edge point of the microlens, respectively. An obvious difference, ∆T = 15%, can be observed between the curves at short wavelengths, as indicated by red dotted lines in Fig. 5(c). A similar situation can also be observed at wavelengths of ~3.53 to ~4.20 μm. The green dotted line is the observed threshold line, which indicates the mean value of ∆T for the entire measured wavelength range; thus, the microlens will exhibit relatively strong absorption when the transmittance curve is below it. Owing to the uneven distribution of LCs around the impurity in the center of the microlens, the effective refractive index around the impurity is disturbed. Thus, a black line appears at the center.

 

Fig. 5 Fabricated microhole structure and transmittance spectra. (a) Microscopic image of a single microlens with an aperture of ~128 μm. (b) Initial 3D beam intensity distribution of the microlens at 6.58 V_rms (d = 0). (c) FTIR transmittance spectra measured along the optical axis (black) and at an edge point of the measured microlens (orange). The scale bars of the wavelength are not equal. (d) Transmittance spectra of each substrate (Al–ZnSe, graphene–ZnSe) at wavelengths of 2.5 to 15 μm.

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As shown, the transmittance of the IR-LCMLA remains substantially the same from ~2.5 to ~11.0 μm except in some narrow subbands. Because the measured transmittance of the ZnSe substrate at normal incidence is essentially unchanged, the remarkable absorption can be attributed to the LC materials in specific narrow subbands, which are indicated by blue dotted vertical lines. At wavelengths exceeding 5 μm, the transmittance curve acquired along the central optical axis demonstrates a dramatic fluctuation compared to that at wavelengths below than 5 μm. This can also be attributed to the LC materials, which exhibit different absorption rates in a wide IR wavelength region. Note that the scale bars of the wavelength in Fig. 5(c) are not equal because of the coordinate conversion between the wavenumber and the wavelength. If the absorption bands in the transmittance spectra are ignored, the transmittance of the microlens along the central optical axis is less than 25%. Figure 5(d) shows the transmittance spectra of the aluminum-coated and graphene-coated ZnSe substrates at wavelengths of 2.5 to 15 μm. The transmittance of the unetched aluminum-coated ZnSe substrate is between 10% and 15%. The transmittance of the graphene electrode is close to 70%, which also proves its excellent broad-spectrum characteristics. From the structure of the IR-LCMLA shown in Fig. 1, the incident beams propagating along the optical axis of the microlens will be absorbed by each layer. Therefore, the IR transmittance of the microlens regions is between 20% and 40%. The measured value is consistent with the theoretically calculated value.

3.3 Focusing performance of IR-LCMLA

The optical performance of the IR-LCMLA is characterized using the setup illustrated in Fig. 3. A single microlens is illuminated from the back by IR light waves (wavelength range: ~2.5 to ~11 μm) emitted by an FTIR spectrometer. The measured 2D intensity profiles at different focusing distances in two IR bands are shown in Fig. 6(a) and 6(d). Figure 6(b) and 6(e) show 3D intensity profiles in the same wavelength region and at the same focusing distance. The 2D and 3D intensity profiles demonstrate the desired convergence of the LC microlens in the two IR bands, as indicated by red dotted lines and shaded areas in Fig. 6(c) and 6(f).

 

Fig. 6 IR microbeam convergence by only one microlens in the IR-LCMLA in two different IR wavelength regions. (a) and (d) 2D intensity profiles of the microlens with an aperture of ~128 μm at a signal voltage of ~6.53 V_rms. (b) and (e) 3D intensity profiles corresponding to the 2D images. (c) and (f) Transmittance spectra measured when the microscope is focused on the focal plane of the microlens. Shaded areas indicate the integrated band for shaping the 2D and 3D intensity profiles of the processed microbeams.

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To effectively evaluate the focusing performance of the IR-LCMLA in a broad IR region, both wavelength regions (shaded areas) are selected. The distance between the microlenses and the focal plane of the microscope is d, as shown in Fig. 3. The detailed convergence characteristics at wavelengths of ~2.5 to ~3.3 μm are shown in Fig. 6(a). When the microscope is first focused on the microhole pattern of the microlens (d = 0), we see a circular microbeam distributed uniformly over the entire microhole. As d increases to ~68 μm, the beams passing through the microlens gradually converge. The final focus spot is obtained when d is ~156 μm. The intensity profiles acquired along the optical axis of the microlens exhibit a small circular focus spot with a very sharp PSF at a signal voltage of ~6.52 V_rms, indicating that the incident IR rays are already highly focused. Note that the current focal length of ∼1.156 mm is the sum of the substrate thickness and the distance d.

To confirm that the IR-LCMLA is suitable for use in a wide IR wavelength range, the beam convergence behavior of the same microlens operating in the range of ~3.53 to ~4.20 μm is also illustrated in Fig. 6(d) to 6(f). As demonstrated, the birefringence of the IR LC materials decreases as the wavelength increases [18]. According to Eq. (1), the focal length of the IR-LCMLA should increase as the wavelength of the incident beams is increased. Thus, the focusing spot size will increase slightly at longer wavelengths when the microscope is focused at the distance shown in Fig. 6(d). Figure 6(c) and 6(f) show the transmittance spectra when the microscope is focused on the focal plane of the microlens. The shaded area indicates the integrated band of the microlens used to form the corresponding 2D and 3D intensity distributions [last column of Fig. 6(a), 6(b), 6(d), and 6(e)]. The red dotted lines in Fig. 6(c) and 6(f) indicate the average transmittance of the peripheral and central points of the IR-LCMLA, respectively. The green dotted lines are the threshold lines, which indicate the mean value of ∆T corresponding to the integrated wavelength range. The average transmittance along the optical axis of the microlens is already increased by more than 25% when the microscope is focused on the focal plane of the microlens. The different values, 18% and 16%, in the two measured bands also indicate that the incident microbeams are already focused so as to shape desired focal spots.

Although the proposed IR-LCMLA demonstrates an electrically tunable focal length in a wide IR wavelength region, ideal IR devices for practical applications will also require optimized microfabrication technologies to remarkably reduce the aberration and thus greatly enhance the IR transmittance. Enhancing the robustness of the IR-LCMLA at room temperature is also essential to commercial applications.

4. Summary

We demonstrated an IR optoelectronic device with a tunable focal length determined by the driving voltage signal in a wide IR regime of ~0.9 to ~11 μm. The graded-index lens effect is achieved using a spatially nonuniform electric field generated by a microhole-patterned electrode in conjunction with a high-birefringence IR LC film. IR microbeams are effectively focused by adjusting an external signal voltage applied to the IR-LCMLA, which exhibits electrically driven tunable microlenses. The proposed framework makes the IR-LCMLA an attractive candidate for adaptive IR imaging sensors and microsystem and advanced IR interconnections.