Continuously Tunable and Bandwidth Variable Optical Notch/Band-Pass Filters Over 500 nm Spectral Range Using Cholesteric Liquid Crystals

Abstract

We report continuously tunable and bandwidth variable optical notch and bandpass filters created by combining four left-and right-handed circular cholesteric liquid crystal cells without extra optical components. Filter performance was greatly improved by introducing an anti-reflection layer on the filter device. The filter comprised cholesteric liquid crystal wedge cells with continuous pitch gradient. The band wavelength position was spatially tuned from 470 nm to 1000 nm. The notch filters are polarization-independent in the spectral ranges. The band pass filters can be designed to be polarization-independent or polarization-dependent via cell alignment and the bandwidth can be reversibly controlled from the original bandwidth (60∼18 nm). These tunable filter strategies could make a paved road to wavelength tunable spectroscopic instruments applications in the VIS and near infrared response spectral range.

Multifunctional Optical Device with a Continuous Tunability over 500 nm Spectral Range Using Polymerized Cholesteric Liquid Crystals

“…After forming a continuous spatial pitch gradient in the wedge cell over 500 nm spectral range (from ~450 nm to ~950 nm), the CLC structure was polymerized by UV. By fabricating the device with a polymerizable CLC, we could solve the unstable problem of pitch gradient in the general CLC structure shown in our previous study (the second paper mentioned earlier) [17]. The strategy of forming a continuous spatial pitch gradient in the wedge CLC cell can increase the wavelength tuning range by five times compared to the results of our previous study (the first paper mentioned earlier) [16].…”

“…For an unpolarized incident light, a CLC cell with a right-handed helix reflects right-circularly polarized light and transmits left-circularly polarized light in a photonic bandgap wavelength range. Therefore, for a light with the same handedness as the CLC, selective (Bragg) reflection occurs in a wavelength range n o × p < λ < n e × p with a photonic bandgap (PBG) (|n e − n o |× p), where p (pitch) is the length for one full rotation of the director around the helix axis (it could be controlled by the ratio of chiral molecules to the nematic Polymers 2021, 13, 3720 3 of 16 liquid crystal) and n o and n e are ordinary and extraordinary refractive indices of nematic molecules, respectively [15][16][17][18][19][20]. We can obtain left-and right-circularly polarized light in the selective band of a CLC cell.…”

“…Thus, the photonic bandgap (PBG) wavelength position can move when ambient temperature changes, thus requiring the selection of liquid crystals not affected by temperature changes. In the second paper [17], CLC devices with spatial pitch gradients by combining four CLC cells showed continuously tunable and bandwidth-variable optical notch and bandpass filters. The band wavelength position could be spatially tuned from 470 nm to 1000 nm, and the bandwidth could be reversibly controlled from the original bandwidth (from 60 nm to 18 nm).…”

“…In particular, spiral nanostructures of CLCs can be used as laser resonators due to their selective reflective properties, thus drawing keen attention from laboratories and academia [5][6][7][8][13][14][15]. Recently, the applicability of CLCs as another wavelength tuning and multifunction optical component other than lasers or displays has been reported [16,17]. Not long ago, we developed optical filter devices using CLCs, reported their optical versatility function, and showed their applicability as optical devices in two papers [16,17].…”

“…Recently, the applicability of CLCs as another wavelength tuning and multifunction optical component other than lasers or displays has been reported [16,17]. Not long ago, we developed optical filter devices using CLCs, reported their optical versatility function, and showed their applicability as optical devices in two papers [16,17]. In the first paper [16], CLC cell devices showed the function of a continuously wavelength-tunable optical notch filter over a 100 nm spectral range by rotating them.…”