Bio-inspired tunable optics and photonics: bridging the gap between nature and technology

Figures & data

Figure 1. Overview of tunable optics for biological/bio-inspired systems. Light-manipulation in tunable optics: (a) focus adjustment and (b) active camouflage. Light-adaptation in tunable optics: (c) light intensity adjustment and (d) sensitivity enhancement.

Figure 2. Active light-manipulation in biological systems. (a) Schematic of the focus adjustment in biological systems by modulating lens shape and lens position. (b) Lens shape variations in (i) Mammals^[74] and (ii) Birds.^[75] (c) Focus adjustment through lens movement in i) Fish,^[76] ii) Amphibians,^[76] and (iii) Cephalopods.^[78] d) Illustration of active camouflage mechanisms, including adjustments in surface morphology, nanocrystal density, and liquid penetration. (e) Photographs of active camouflage through surface morphology alteration in iridophores.^[79] (f) Images of material density tuning in (i) nano crystal^[80] and (ii) liquids.^[81]

Figure 3. Active light-adaptation in biological systems. (a) Schematic of light intensity adjustment by tuning pupil area and retinal structure. (b) Images of pupil area variations in (i) chambered eyes^[83] and (ii) compound eyes.^[84] (c) Light intensity adjustment with (i) tunable tapetum^[86] and (ii) tunable retinal pigment.^[82] (d) Illustration of contrast sensitivity enhancement via eye rotation and pupil shape variations. (e) Enhancement in the polarization detection through three-dimensional eye rotation.^[93] (f) Variations in pupil shape exhibiting (i) vertical pupils^[92] and (ii) W-shaped pupils.^[89]

Figure 4. Mechanisms and applications of bio-inspired light-manipulation system. (a) Deformation of lens shape from (i) fluidic flow control^[96] or (ii) electrical vias with electrowetting material^[97] or volume control.^[104] (b) Shifting position of (i) retina to focal spot^[107] or (ii) freeform lens horizontally.^[108] (c) Demonstration of a single device integrating both tunable lens and tunable sensor.^[69] (d) Miniaturized tunable camera capable of small-size integration.^[107] (e) Endoscope suitable for use in extremely restricted spaces.^[108] (f) Electronical actuation for tunable camouflage. (i) Reversible electrodeposition of Ag shell covering Au core for plasmonic tuning.^[112] Inset shows strong resonance at the bottom edge of the structure. (ii) Wrinkled surface actuated both mechanically/electrically to adjust transmittance at Visible/IR range.^[113] (g) Thermal actuation for tunable camouflage (i) Tunable plasmonic system coupled by cavity.^[115] Inset shows changing length of the cavity with the phase of VO₂. (ii) Thermochromic liquid crystal (TLC) exhibiting tunable coloration in response to heat generated by patterned Ag nanowire.^[116] Inset shows temperature dependency of coloration. (h) Demonstrated chameleon mockup with plasmonic camouflage cell.^[112] (i) Transparency of actuated wrinkled surface for both visible and IR range.^[113] (j) IR encoded picture only visible at low temperatures depending on hole size at single pixel.^[115]

Figure 5. Mechanisms and applications of bio-inspired light-adaptive system. (a) Artificial iris with extrinsic control system. (i) LCE with external heater^[117] or (ii) PDLC with external bias voltage source.^[118] (b) Intrinsic response to light of polymer used to self-controlled artificial iris. (i) photochromic^[120] or (ii) liquid crystal elastomer (LCE) doped with photothermal.^[121] (c) Real-time distance sensor in automobile using artificial iris.^[124] (d) Multi-functional wearable glasses.^[125] Combined with external UV sensor, extrinsic self-controlling system is demonstrated. (e) (i) W-shaped pupil balancing light under uneven (i.e., upper dominant) illumination, (ii) selective polarization sensing to opt redundant information out and (iii) High resolution at region of interest (ROI).^[90] (f) Light-concentrating structure of the inhabiting at dim circumstance.^[129] (g) Blackbox camera showcased in daytime illumination with W-shaped pupil.^[90] (h) Night-vision system utilizing light-accumulating cup. Inset shows sensitivity improvement with the structure.^[129] (i) Microscope inspired by eye of scallop and Schmidt telescope. Inset is pictures of pollen pellet imaged in air.^[130]

Table 1. The performance and applications of the tunable optics introduced in Section 3.

Parts (Tunability)	Working principle	Tunability	Response time	Application	Note	Ref
Focus Adjustment (Focal length)	Fluid	16 mm–55 mm	N/A	Adaptive imager	Magnification function	[96]
	Volume control (electrowetting)	3.8 mm–22.3 mm	680 ms	Tunable lens module		[97]
	Volume control (zipping)	Variable to dimension	260 ms	Tunable lens module		[104]
	Detector shifting	2.95 mm–4.72 mm	N/A	Miniaturized camera		[107]
	Alvarez lens shifting	5.5 mm–7.5 mm	110 ms	Endoscope		[108]
Active camouflage (Operating band)	Electrodeposition	Visible	<1 s	Real-time camouflage	Plasmonic display	[112]
	Electromechanical (Wrinkled surface)	Visible to IR	570 ms	Concealment	–	[113]
	PCM	IR	<1 s	IR encoding	–	[115]
	TLC	Visible to IR	<0.6 s	Customized camouflage	–	[116]
Intensity adjustment (Iris change)	Direct heating	Diameter 2.7 mm–3.8 mm	Opening 7 s Closing 3 s	Miniaturized photographic optics	–	[117]
	PDLC	Diameter 1.5 mm–8 mm	N/A	Robotic vision	–	[118]
	Photochromic	Absorption in visible 50%–65%	30 s	Ocular prosthesis	Intensity dependence	[120]
	Photothermal	Transmission power 10%–70%	5 s–30 s	Micro-robotics	Intensity dependence	[121]

Table 2. The summary and applications of the contrast sensitivity enhancement optics.

Parts	Working principle	Target Condition	Inspiration	Application	Note	Ref
Contrast sensitivity enhancement	Pupil shape (W-shape)	Uneven illumination	Cuttlefish	Light balancing	–	[90]
	Micro-cup structure	Dim condition	Elephantnose fish	Night-vision	–	[129]
	Multi-immersion	Microscopic condition	Scallop	Microscope	–	[130]

Figure 6. Current challenges and perspectives for bio-inspired tunable optics. Current challenges: (a) auto-tuning for tunable devices,^[115,133] (b) fast response for tunable devices,^[121,148] (c) small optical aberration of tunable optics,^[155,158] (d) high resolution of tunable image sensors,^[69,107,159] and (e) compact optical devices.^[163,166] Perspective innovations and applications: (f) adaptive camouflage to the background,^[116] (g) self-driving automobiles with tuning the f-number,^[124] (h) bio-medical optics with magnification capabilities,^[129,178] and (i) compact mobile devices for focus tuning.^[164]

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