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A colour microprint containing one or more holograms

Technology Title

Holographic optical element and method of forming thereof

Technology Overview

Conventional optical security devices provide authentication by manipulating a specific  property of light to produce a distinctive optical signature. For instance, microscopic  colour prints modulate the amplitude, whereas holograms typically modulate the phase  of light. However, their relatively simple structure and behaviour is easily imitated. We  designed a pixel that overlays a structural colour element onto a phase plate to control  both the phase and amplitude of light, and arrayed these pixels into monolithic prints that exhibit complex behaviour. Our fabricated prints appear as colour images under  white light, while projecting up to three different holograms under red, green, or blue  laser illumination. These holographic colour prints are readily verified but challenging  to emulate and can provide enhanced security in anti-counterfeiting applications. As  the prints encode information only in the surface relief of a single polymeric material,  nanoscale 3D printing of customised masters may enable their mass-manufacture by  nanoimprint lithography. 

 

This technology solves the limitation of simultaneously control the phase and  amplitude of the light in conventional optical security devices. The next generation  display or optical anticounterfeiting companies are the potential purchasers of these  holographic colour prints. The IP realizes a physical unclonable function device with a  multichannel anticounterfeiting optical element.

 

 

 

Figure 1. Structure and function of the holographic colour print, a multi-layered  structure in which colour filters are stacked on top of holograms on a substrate. Under  white light transmission illumination, a colour image is seen due to the arrangement of the colour filters. With laser illumination, each wavelength selects a different multiplexed holographic projection. 

 

 

Figure 2. Color print and R, G, B holographic projections with one fabrication  hologram. The optical element is formed of multilevel phase blocks with different  nanopillars on the top (Scanning electron microscope image, false colored for the corresponding wavelength). Each unit is made of one phase block with the size of 3×3  µm2 and 3×3 nanopillars atop. Under the white light illumination, one color print can be  observed in the transmission mode (captured with microscope). Under laser illumination (449, 527, and 638 nm), three different holographic projections with corresponding colors can be captured on the screen. 

 

 

Technology Specifications
  • The optical device is fabricated via two-photon polymerization lithography. The  femtosecond laser is 780 nm with 90 fs pulse duration at 80 MHz repetition rate,  and is focused via a 63×, NA 1.4 immersive objective. The laser power incident  on the entrance aperture of the objective lens was controlled by an acousto – optic modulator.
  • The material for element fabrication is a photoresist IP-Dip, while for further  replication and nanoimprinting, other curable photoresists can be applied.
  • The design is based on a modified Gerchberg-Saxton algorithm for far field  diffraction in transmission mode with multiple channel information encrypted.
  • The area to be patterned is split into a quare grid of 120×120 µm2 write fields  based on the maximum undistorted field of view of the microscope objective.
  • The original images for holographic projection can be colorful and grayscale,  and the image for color print is colorful. 
  • The number N of image with pixels N×N can be several hundreds to thousands. • The size of each phase block representing one pixel in the image is 3×3 µm2 in  the xy-plane. 
  • A standard glass substrate is made of fused silica, 25 mm2 with a thickness of  0.7 mm, while the optical element is possible to be fabricated on any other  transparent substrate with a flat surface and an area larger than the printed  size. 
  • Polymerized IP-Dip photoresist with a refractive index of 1.54 to 1.58 across  the visible spectrum, and the block thickness determined for 2π phase  modulation was 0.79 µm, 0.95 µm, and 1.17 µm at the design wavelengths of  449, 527, and 638 nm. To span the required or desired range of thicknesses and avoid unwanted shifts in filter colour at thicknesses below 0.6 µm, a  thickness range of 0.6 um to 1.8 um is used.
  • With a strict lower limit of 100 nm on the thickness step size, the number of  phase levels used in the final prints is rounded down to 7, 9, and 11,  respectively for the three wavelengths. 
  • For phase blocks, line scan mode is used with the hatching pitch 250 nm and  slice thickness 700 nm, at femtosecond laser power of 21.0 mW for the first  raster scan and 16.8 mW for the second raster scan. The laser scan speed is  8000 µm/s. 
  • For pillars, pulse mode is used with exposure time 0.02 – 0.04 ms, laser power  33.3 – 46.4 mW, and slice thickness 0.69 – 1.01 µm. 
  • The pitch of the pillar arrays is 1.0 µm. The pillar height and diameter are varied  in the ranges 0.5 µm to 2.7 µm and 310 nm to 390 nm, respectively. For design  wavelengths of 449, 527, and 638 nm, the pillar heights are 0.7 µm, 2.6 µm,  and 1.9 µm, respectively. The pillar diameters are 380 nm, 390 nm, 390 nm, respectively.
  • The slice thickness is adjusted to match the (dose-dependent) axial elongation  of the point spread function of the laser spot while maintaining a vertical overlap  of approximately 30 % (300 nm to 430 nm depending on the size of the laser  spot in the vertical direction). 
  • For various example embodiments, the dimensions of nanopillars: height may  be in a range from 0.5 to 3.0 µm; and diameter may be in a range from 100 to  500 nm. The dimensions of nanoblocks: thickness may be in a range from 50  nm to 2000 nm; lateral size (e.g., width and length) may be in a range from 1 to  10 µm. 
  • To wash away the excess unexposed liquid photoresist, development should  be carried out by immersion of the sample in polyethylene glycol methyl ether  acetate (PGMEA) for 5 minutes and then isopropyl alcohol (IPA) for 3 minutes,  followed by transfer into nonafluorobutyl methylether (NFBME) as a low surface  tension solvent for the final drying step. 
  • For color print observation, the sample is backlit by halogen lamp illumination  and measured in transmission through a 5x / 0.15 NA objective lens. The  transmittance spectra are measured in a narrow cone of acceptance angles  using an objective with a numerical aperture of 0.15 (a half – angle of 8.6°). The illumination numerical aperture may be limited to below 0.4 (a beam angle of  up to ± 23°).
  • The transmittance spectra are averaged over blocks of thicknesses 0.6, 1.0, 1.4 and 1.8 µm, and the transmittance values further are averaged over a  narrow bandwidth of 4 nm centred at the wavelengths 449 nm (blue), 527 nm (green), and 638 nm (red), as well as a broadband spectral range of 450-650  nm (white). 
  • The overall transmission efficiency for a given channel may be the product of  the area fraction occupied by the channel and the weighted average of the transmittance of the colour filters on that channel, with an upper bound of 33 % for the case of equal area fractions in an RGB hologram. 
  • Holographic photographs can be taken at different projection distances d, with  the sample placed near a white sheet of paper (d = 20 cm) and far from a white  wall (d = 135 cm). The projection at d = 20 cm is much less apparent at d = 135  cm, at which the projection has expanded from 2 cm to 12.5 cm, the higher  orders have faded, and the unwanted projection is now over 70 cm away from  the main projection. 
  • The holographic projection is essentially angle-insensitive between -5° and 10°  and suffers a slight decrease in brightness at -10° and 15° with little loss in  quality. The projection is faintly visible at –15° and +15° and disappears as the  angle is increased farther. A slight asymmetry in the usable range of illumination  angles might be due to a small average tilt in the pillars of 2° to 3° relative to  the normal, possibly introduced during the drying step of the development  process.
  • The projection is bright and clear in the presence of strong background light in  the room despite using only 2 mW of laser power to illuminate the sample. The  laser is expected to illuminate the sample at 0° normal incidence under ideal  condition for the best projection quality, while larger angle is still acceptable.

 

Sector

Singapore: optical anti-counterfeiting 

 

Market Opportunity

The global anti-counterfeiting market size is valued at USD 117.2 billion in 2021 and  is projected to reach USD 211.3 billion by 2026, around 12.5% each year. The market  is expected to witness high growth due to increasing focus of manufacturing on brand  protection to reduce counterfeiting.

 

Developing a low-cost, easy to fabricate but hard to forge method is always pursued  in the market. Gratings based optical images /holograms are the most employed one  currently as they fulfil the above requirements. While these traditional optical security  devices provide authentication simply by the grating effect, and their relatively simple  structure and behaviour is easily imitated. Our fabricated prints can provide four  channels of encrypted information, greatly enhancing the security. In addition, these 

holographic colour prints can be mass-produced with simple nanoimprinting  lithography.

 

 

Applications

The unique anti-counterfeiting properties can be employed for banknotes, credit card,  passport, and secretive documents etc., also can be used for the authentication label of food, medicine, luxuries, and other products.

 

 

Customer Benefits
  • Multi-Information: One simple device but with four channels of different  information inside for authentication. 
  • Simple-Authentication: The inspection method is remarkably simple: the  image of color print can be easily observed through the transmitted white light  or under a microscope/a smartphone with transmissive illumination. The three  holograms can be seen via RGB laser illumination. There is not specific  requirement for high power laser, thus laser pointer is enough. The illumination  angle is not restricted. 
  • Mass-Production: The structure is comprised of 2.5D micro-nanostructures,  and the surface relief fabricated by 3D printing is readily to be used as master  mold for mass-manufacture by nanoimprint lithography. 
  • Economic-Friendly: The material for the device is based on the low-refractive index materials, hence most of the transparent polymers, plastics, glasses can  be employed here for low-cost fabrication. In addition, biocompatible materials  like cellulose, chitosan can also be used for biomedical applications. The design  can also be transferred to other refractive index materials thus the devices can  be sealed to prevent damage and forging. 
  • Auto–Algorithm: Based on the vast test, the algorithm is easy to operate by  simple changing the color print images and projections images to any desired  ones. It can efficiently provide the design in several minutes. 

 

 

Technology Readiness Level

TRL5: Technology component and the basic technology subsystem validation in a  relevant environment. 

 

Ideal Collaboration Partner 

The partner is expected to help the innovation step into market stably while protecting  the IP rights and making sound business decisions. They can help identify new  business opportunities, conduct a landscape analysis and help focus investment efforts and maximize profits. They should also help defend against emerging threats to protect IP assets and remain competitive, continually monitoring the external  environment is essential. 

 

Collaboration Mode

We are happy to provide R&D collaboration, licensing, and IP acquisition as required  by the potential customers.

 

 

 

 

NAME OF TECHNOLOGY MANAGER:

 

Dr Ler Ser Yeng 

 

EMAIL: seryeng_ler@sutd.edu.sg

 

 

NAME OF PRINCIPAL INVESTIGATOR:

 

Associate Professor Joel Yang

 

EMAIL: joel_yang@sutd.edu.sg