Many research teams have begun pursuing optical micromachining technology in recent years due to its associated noncontact and fast speed characteristics. However, the focal spot sizes and the depth of focus (DOF) strongly influenced the design requirements of the micromachining system. The focal spot size determines the minimum features can be fabricated, which is inversely proportional to the DOF. That is, smaller focal spot size led to shorter DOF. However, the DOF of the emitted visible or near-infrared light beam is typically limited to tens of nanometers for traditional optic system. The disadvantages of using nanosecond laser for micromachining such as burrs formation and surface roughness were found to further influence the accuracy of machined surfaces. To alleviate all of the above-mentioned problems, sub-wavelength annular aperture (SAA) illuminated with 780 nm femtosecond laser were integrated to develop the new laser micromachining system presented in this paper. We first optimized the parameters for high transmittance associated with the SAA structure for the 780 nm femtosecond laser used by adopting the finite difference time domain simulations method. A lateral microscope was modified from a traditional microscope to facilitate the measurement of the emitted light beam optical energy distribution. To verify the newly developed system performance the femtosecond laser was used to illuminate the SAA fabricated on the metallic film to produce the Bessel light beam so as to perform micromachining and process on silicon, PCB board and glass. Experimental results were found to match the original system design goals reasonably well.
Extraordinary light transmission effect on a metal surface, also known as surface plasmon resonance, has been widely discussed in recent years. Extending from this line of research, surface plasmons generated by subwavelength annular apertures (SAA) on metallic film has been identified to have the ability to create sub-wavelength Bessel-like beams. It has also been found that this type of Bessel beam can be used to produce high-aspect ratio microstructures when adopted in laser micromachining. However, the drawback is that the Bessel beams produced by the SAA structure is often characterized as having a low transmission efficiency and high side lobes. In order to improve these shortcomings, an improved SAA-like structure is proposed in this paper. A new photon-sieve replaces the annular aperture by an array of holes which can lower the side lobes of the emitted Bessel beams. More specifically, the original ring-shaped holes are now replaced by a series of smaller holes arranged in a circular shape to mimic a ring. We show by FDTD (Finite-Difference Time-Domain) simulation that a glass substrate removed from this newly created SAA-like structure can increase transmission efficiency by 27.5%. Considering the absorption of the glass substrate is only in the range of 4%-5%, the additional efficiency can actually be attributed to the surface plasmon effect involved in the symmetric nano-structures. Our simulation results were verified by experimental results. The high-aspect ratio microstructures fabricated are also be detailed.
Even though light field technology was first invented more than 50 years ago, it did not gain popularity due to the limitation imposed by the computation technology. With the rapid advancement of computer technology over the last decade, the limitation has been uplifted and the light field technology quickly returns to the spotlight of the research stage. In this paper, PBRT (Physical Based Rendering Tracing) was introduced to overcome the limitation of using traditional optical simulation approach to study the light field camera technology. More specifically, traditional optical simulation approach can only present light energy distribution but typically lack the capability to present the pictures in realistic scenes. By using PBRT, which was developed to create virtual scenes, 4D light field information was obtained to conduct initial data analysis and calculation. This PBRT approach was also used to explore the light field data calculation potential in creating realistic photos. Furthermore, we integrated the optical experimental measurement results with PBRT in order to place the real measurement results into the virtually created scenes. In other words, our approach provided us with a way to establish a link of virtual scene with the real measurement results. Several images developed based on the above-mentioned approaches were analyzed and discussed to verify the <i>pros and cons</i> of the newly developed PBRT based light field camera technology. It will be shown that this newly developed light field camera approach can circumvent the loss of spatial resolution associated with adopting a micro-lens array in front of the image sensors. Detailed operational constraint, performance metrics, computation resources needed, etc. associated with this newly developed light field camera technique were presented in detail.
Out of the many wavefront sensing techniques, Shack Hartmann wavefront sensor remains the most
popular and the most versatile. Its optical configuration utilized a micro-lens array to measure the
directivity of the light beam associated with each micro-lens. In this design, smaller size of micro-lens
leads to angular resolution improvement. However, smaller size micro-lens typically is associated to
shorter depth of focus, which makes it difficult to focus on sensor array properly. In addition, the size
of micro-lens array is limited by the diffraction limit. In today’s technology, micro-lens with
dimensions in size of a few hundred of microns is possible. This dimension posts the limitation of the
angular resolution possible for Shack Hartmann wavefront sensor. To alleviate the compromise
between the angular resolution and the depth of focus, a sub-wavelength annular aperture (SAA)
structure was developed to generate Bessel light beams. That is, the SAA performs similar functions as
that of the micro lens array in traditional wave front sensors. It is shown that this design maintains a
sub-wavelength focusing capability while achieves tens of micron depth of focus in the far-field region,
which leads to an improved wavefront sensor. Both simulation and experimental results are detailed.
Adopting optical technique to pursue micromachining must make a compromise between the focal spot sizes the depth
of focus. The focal spot size determines the minimum features can be fabricated. On the other hand, the depth of
focus influences the ease of alignment in positioning the fabrication light beam. A typical approach to bypass the
diffraction limit is to adopt the near-field approach, which has spot size in the range of the optical fiber tip. However,
the depth of focus of the emitted light beam will be limited to tens of nanometers in most cases, which posts a difficult
challenge to control the distance between the optical fiber tip and the sample to be machined optically. More
specifically, problems remained in this machining approach, which include issues such as residue induced by laser
ablation tends to deposit near the optical fiber tip and leads to loss of coupling efficiency. We proposed a method based
on illuminating femtosecond laser through a sub-wavelength annular aperture on metallic film so as to produce Bessel
light beam of sub-wavelength while maintaining large depth of focus first. To further advance the ease of use in one
such system, producing sub-wavelength annular aperture on a single mode optical fiber head with sub-wavelength
focusing ability is detailed. It is shown that this method can be applied in material machining with an emphasis to
produce high aspect ratio structure. Simulations and experimental results are presented in this paper.
The lens spatial resolution and depth of focus depends on the numerical aperture and the incident light beam wavelength. Traditionally, smaller focal spot size also means short depth of focus, which may hinder the system integration advancement. We investigate the possibility to break free the limitation associated with small focal size and short depth of focus. It was discovered by T. W. Ebbesen that periodic aperture on metallic film may improve the transmission and confine the divergent angle of the emitting light beams beyond the prediction of the Bethe’s theory. We developed single sub-wavelength annular aperture (SAA) first and then used tapered hollow micro-tube to mimic the function of SAA so as to generate Bessel beam in the far-field region. High aspect ratio microstructures were fabricated using the above-mentioned techniques. In addition, coupling annular aperture and periodic grating to further advance the high aspect ratio fabrication technique was also explored. Firstly, to maximize the transmission in specific wavelength, the pitch of grating was modulated. More specifically, we manipulated the periodic grating to search the maximum transmission efficiency in specific waveband. Secondly, in order to improve the depth of focus, we changed the grating slit width to induce proper phase delay. These studies were all verified by FDTD simulations and some experimental results.
We found that the tip size of a tapered hollow micro-tube can affect the properties of an emitted Bessel light beam. In addition, the incident light polarization state was also found to influence the characteristics of the emitted light beam. From a lithography viewpoint, we looked at the correlation between an emitted Bessel light beam from a subwavelength annular aperture on metallic film and from a tapered hollow micro-tube. Intensity profiles were analyzed using finitedifference time-domain (FDTD) simulations and lithography experiments were undertaken. The approaches used to couple the incident light into the hollow micro-tube and to the subwavelength annular aperture are discussed. Effects from the waveguide mode were studied. Our results showed that the tube thickness of the tapered hollow micro-tube tip is an important factor in generating the Bessel light beam wavelength. We show that lithography can be used with a through-silicon-via (TSV) process in a far-field region while maintaining a near diffraction-limit spot size. Our tapered hollow tube design is useful for applications such as optical lithography, super resolution optical detection, and fabrication of high aspect ratio structures.