The overall goal of this experimental procedure is to demonstrate the fabrication process and subdiffraction imaging of the two-dimensional hyperlens device. This novel super-resolution imaging technique has the advantages of real-time imaging and simple implementation to conventional optics. This method can help answer key question in the field of super-resolution imaging such as imaging living cell and dynamic nanoparticle below the fraction limit.
The hyperlens is a special spherical lens with multilayer structure having a flat hyperbolic dispersion that supports magnification of high frequency information and resolution of similar optics in the far field in real time. The main advantage of the spherical hyperlens is that it can magnify two-dimensional information at visible frequencies. A spherical hyperlens can also be easily integrated to conventional microscopy without additional complex system.
Demonstrating the procedure will be Dasol Lee and Inki Kim who are graduate students of my laboratory. To begin, spin coat the quartz wafer with a positive photoresist at 2, 000 rpm and bake for 60 seconds at 90 degrees Celsius. Then, use a dicing machine to cut the wafer with photoresist into small pieces 20 by 20 square millimeters in size.
Blow the pieces using a compressed nitrogen gun to remove any particulates resulting from the cutting step. Next, place the cut wafer into an ultrasonic bath of deionized water for five minutes at 45 degrees Celsius. Remove the photoresist layer using an ultrasonic bath of acetone for five minutes at 45 degrees Celsius.
Then, clean the substrate by placing it into an isopropyl alcohol ultrasonic bath for five minutes at 45 degrees Celsius. Dry the substrate with a compressed nitrogen gun. To etch the mask pattern, first load the clean quartz substrates into a high vacuum electron beam evaporation system.
Deposit the chromium layer with a deposition rate of two angstroms per second. Press the vent button to vent the chamber. Mount a sample on the Focused Ion Beam or FIB holder using conducting copper tape.
Then, load the FIB holder into the FIB chamber. Close the chamber door and press the pump button to evacuate the chamber. Select Beam On under the beam control tab and set the ion beam current and acceleration voltage for FIB mode.
Turn on the ion beam system. Select Beam On under the beam control tab to turn on the electron beam and focus the image with low magnification using software. Then, set the working distance to four millimeters under the navigation tab in scanning electron microscope mode.
Set the tilt angle of the holder to 52 degrees and take the SEM images at different magnifications before hole array mask pattern fabrication. Under the patterning tab, choose the patterning region and make a 50 nanometer hole array on the chromium layer. After finishing, turn off the electron beam and ion beam systems and cool them down.
Pres the vent button to vent the chamber with nitrogen gas. Then, take the holder out of the chamber. Next, put the patterned substrate into one to 10 buffered oxide etchant for five minutes.
Put the patterned substrate into deionized water to clean the buffered oxide etchant. Then, dry the sample with compressed nitrogen gas. Put the patterned substrate into chromium etchant to remove the chromium mask layer.
Finally, put the patterned substrate into deionized water for five minutes to clean it. Press the vent button of the electron beam evaporation system and wait until the vent is over. Then, load the patterned substrate into a high vacuum electron beam evaporation system after the vent.
Close the chamber door and evacuate the chamber by pressing the pump button. Deposit the silver layer with a growth rate of one angstrom per second and deposit a 15 nanometer thick silver layer. After the deposition of the silver layer, cool down the substrate for five minutes.
Change the pocket of the electron beam evaporation system by choosing another crucible and deposit the titanium oxide layer with a growth rate of one angstrom per second. Then, deposit a 15 nanometer thick titanium oxide layer. After the deposition of the titanium oxide layer, cool down the substrate for five minutes.
Repeat the deposition steps for tens of cycles to deposit a multilayer of silver and titanium oxide. Change the pocket of the electron beam evaporation system and deposit the chromium layer at a thickness of 50 nanometers. After the deposition of a chromium layer, turn off the electron beam evaporation system.
Press the vent button and vent the chamber by introducing nitrogen gas. After the vent, open the chamber door and take the mount holder out of the chamber. Strip off the fabricated hyperlens device.
Then, close the chamber door and evacuate the chamber by pressing the pump button. Mount the hyperlens deposited with chromium into the FIB milling system and pattern a nano-sized structure per the manufacturer's instructions. Next, place a conventional transmission-type optical microscope on the optical table.
Connect a white light source to the microscope illumination path using an adapter. Place an optical bandpass filter centered at 410 nanometers. Select a high magnification oil immersion objective lens and use a high quality CCD camera to obtain the images.
Place a drop of immersion oil on the objective lens. Finally, place a hyperlens on the sample stage and capture the images. Shown here is a hyperlens composed of silver and titanium oxide multilayers deposited alternately.
The cross-sectional image shows that the multilayer of silver and titanium oxide thin film is deposited with uniform thickness on the hemispherical quartz substrate. A hyperlens consisting of silver and titanium oxide has a great performance at the wavelength of 410 nanometers because the dispersion relation of the stacked multilayers has a hyperbolic dispersion curve as shown here. High spatial wave vector components can propagate along the radial direction of the hyperlens.
The small features having high frequency components which cannot be captured by conventional optics can propagate to the far field through the hyperlens as calculated by finite element simulation. After the fabrication, the hyperlens can be integrated to the conventional microscope system as shown in this simple schematic of the hyperlens imaging system. The hyperlens is placed on the objective lens.
For demonstration of the hyperlens, an artificial pattern is inscribed on the inner surface of the hyperlens. The results show the images captured through the hyperlens. The gap sizes are from 160 nanometers to 180 nanometers in each case.
The subdiffraction limited features are resolved and the super-resolving power of the hyperlens can be confirmed. The development of the hyperlens paved the way for super-resolution imaging technique to explore nano-sized biomolecule machinery and inorganic nanoparticle. After watching this video, you may have a good understanding of how to fabricate a high quality hyperlens and setup for your own super-resolution imaging system.
We expect the hyperlens technique will be improved in practicality by adopting a scalable and reproducible fabrication method. The hyperlens will allow scientists to observe biophysical dynamics occurring in nano scale in real time and to work as the next generation super-resolution imaging in various applications such as biology, medical science, material science, and nanotechnology.
