Design of an enhanced surface plasmon polaritons (SPPs)–based nanostructure for the purpose of beam shaping is discussed. An indentation with depth-tuned grooves is presented to realize the beam shaping and extraordinary transmission. The nanostructure is directly fabricated using focused ion beam (FIB) milling on an Ag thin film coated on quartz with a thickness of 200 nm. A large measurement error is found during geometrical characterization of the nanostructures by use of an atomic force microscope (AFM) working in tapping mode. Apex wearing and 34 deg full cone angle of the probe generate the measurement errors during the characterization of nanostructures with a feature size of 200 nm and below. To solve this problem, an FIB trimmed AFM probe is employed in the geometrical characterization. The results show that the error is improved greatly using the trimmed probe. The desired excitation of the SPPs is derived using an optical fiber coupled CCD spectrometer after the modified geometrical characterization. The designed structure can be used as an optical probe for future inspection and detection use.
We have established a new type of Hartmann-Shack (H-S) wavefront sensor which based on a micro-grating array. A H-S wavefront sensor is frequently used in an Adaptive Optics (AO) system to detect the aberrations for the system. Besides this situation, the H-S wavefront sensor is also adopted to measure those static or dynamic aberrations which exist in other kind of systems. In any one of these H-S wavefront sensors, a lenslet array seems so indispensable. But, it is difficult for a lenslet array to coincide with the photoelectric detector, some times a relay system will be necessary. And the variance of the focal length of the lenslet array will bring error to the measurement. To conquer these deficiencies, a new type of H-S wavefront sensor based on a micro-grating array other than a lenslet array has been created recently. This new kind of H-S wavefront sensor substitutes the lenslet array for a micro-grating array and a lens. On this new H-S wavefront sensor, some excellent experimental results have been obtained.
Metallic nano-slits film is proposed to implement beam manipulation, such as focusing, deflecting and imaging etc. The principle of this novel nano metric device, termed as plasmonic nano lens, is based on the different phase retardation of light when transmitted through a metallic film with arrayed nano-slits, which have constant depth but variant widths. The slits transport electro-magnetic energy in the form of surface plasmon polaritons (SPPs) in nanometric waveguides and provide desired phase retardations of beam manipulating with variant phase propagation constant. Numerical simulations of illustrative examples are performed through finite-difference time-domain (FDTD) method and show its validity as a lens and other potential photonic devices. In addition, extraordinary optical transmission of SPPs through sub-wavelength metallic slits is observed in the simulation and implies higher efficiency than usual binary devices featured with transparent and opaque regions.
In this paper, we discussed the beam focusing of light emerged from a subwavlength metallic slit surrounded by a set of grooves with constant space and width but variant depth at the exit side surface. Based on the numerical model presented by L. Martin-Moreno, F. J. Garcia-Vidal etc. (published in PRL 167401), we attempted to optimize grooves depth to obtain general beam manipulation, such as beam focusing. This attempt did not prove successful for many cases with variant focal length in our optimization practice, although some specific results display agreeable beam focusing with elongated focal depth. Further numerical computation shows that the excited electromagnetic field intensity around groove openings has a strong dependence on the groove depth, but the phase only vary with a maximum change value of π by tuning the groove depth. This property restricts greatly the modulation of electromagnetic field by just changing each groove depth. More geometrical parameters, including groove space and width, are recommended for optimization in the design of nano metallic groove and slit structures for specific beam manipulation.
In this paper, we investigated the electric field profiles and phase distribution at the metal interfaces of the structure, and then analyzed their dependence on the groove depth and distance between slit and grooves though finite-difference time-domain (FDTD) simulation. Calculated results show that variant groove depth generates phase difference periodically, which indicates the existence of standing wave in the groove. The results also show that the phase of the emission at each groove is proportional to the distance travelled by the surface wave in one period. Based on these facts, a simple process of the transmission model is illuminated.
It is reported in this paper a method to fabricate three dimensional continuous micro-optical structures using half tone masks and proximity printing. The frequency modulation coded half tone masks are employed to obtain the predetermined exposure distribution. Several components like Cylinder microlens arrays, phase correctors are realized in photoresists.
It is presented in this paper an improved simulated annealing algorithm with the ability of increasing sampling resolution and generating grating pattern with smoothed borders to design two-dimensional (2-D) non-separable Damann grating. Numerical simulations are performed for the lateral fabrication error accompanied in the lithography procedure and the measured experimental results of the fabricated grating agree with the simulation.
To collimate effectively the beam emitted from the stacked laser diode in which the lasing surfaces of the diode bars are not located in a plane, a new type of fast-axis collimator, refractive cylindrical microlens array with tunable focal lengths, is presented in this paper. Each lens of the array has the same diameter of 300μm but different focal lengths, ranged from 430μm to 540μm. By means of the mask moving lithography and replication technology, the microlens array was successfully fabricated. The measured fast-axis divergence of the stacked laser diode beam after the collimator was 25mrad, about half of the one (40mrad) for the microlens array with common focal lengths of 400μm.
Based on the focus principle of the zone plate and the encoding method of the computer-generated hologram, a zone plate applied in OCT to increase the focal depth is proposed. The phase distribution of this kind of zone plate is constructed by superimposing a shifted elliptical phase to the phase function of a common sine zone plate, which will confine the spot size of the focused beam within a particular range along axis and thus extend the focal depth. The numerical simulation shows that the focal segment of the zone plate meet the requirement of the commonly used OCT imaging system in the two aspects of focal depth and transverse resolution. Because the zone plate has many advantages such as flexible focal length and depth design, simple structure for fabrication, low cost and convenient micromation and integration, it is a potential long focal depth component for optical coherence tomography.
A new alternative coding method is proposed for the fabrication of circular diffractive optical elements with half tone masks, in which a series of rings with variant widths are used to produce desired gray distribution. Analytical analysis of this coding method is given and illustrated with numerical simulation examples.