Fabrication And Characteristic Analysis Of Holographic Subwavelength Anti-reflective Gratings

Subwavelength structures refer to the periodic structures whose sizes are smaller than the light wavelengths.When the periods of the subwavelength structures are smaller or much smaller than light wavelengths,traditional diffraction phenomenon vanishes and only the zeroth order diffraction exists.The reflectivity and transmittance of such structures can be changed by changing the periods and etching depths of the structures,thus subwavelength gratings can be used as anti-reflective surfaces.Since the anti-reflection effects of TE and TM polarization states are different for one-dimensional(1D) gratings,in practical applications 1D gratings are inconvenient.In order to realize good antireflection effect and to achieve the similar effects for two states of polarization,two-dimensional(2D) cross gratings with the periods smaller than visible wavelengths and with sinusoidal shape were designed and fabricated in the work.The measured result shows that the gratings have enhanced the transmittance at visible light wave band,like gradient-index anti-reflective coatings. The experimental result is in good agreement with the theoretical analysis.2D hexagonal lattice structures can be regarded as consisting of multi-group gratings with different orientations.The anti-reflective film with 2D hexagonal lattice structure was fabricated using embossing technology.The measured result shows that the film has enhanced the transmittance at infrared wave band,in good agreement with the theoretical analysis.The main contents of the dissertation are as follows.1.On the basis of the effective medium theory(EMT),the anti-reflection property of holographically fabricated sinusoidal shaped gratings with filling factor changing with depth is analyzed in detail.A sinusoidal shaped grating can be regarded as being composed of many thin rectangular gratings with different filling factors.The filling factor increases gradually with grating depth so that the effective refractive index also increases gradually with depth,forming a kind of anti-reflective layer with gradient refractive index. 2.Cross gratings with periods of 345 nm,290 nm,255 nm and 235 nm were designed and fabricated using holographic double-exposure method,with the two coherent beams incident to the recording medium symmetrically.The measured result shows that the grating has enhanced the transmittance at visible wave band.The measured transmittance profiles also show that the smaller grating period and the deeper grating depth,the higher transmittance at visible wave band can be achieved. Cross grating with 255 nm period and 260 nm depth provides the best effect,with an average transmittance enhancement of 20%in visible wave band.3.The analysis of 2D hexagonal lattice structure consisting of multi-group gratings with different orientations is implemented.The 2D hexagonal structure with 1.25 nm lattice constant was fabricated in photoresist with holographic three-beam interference method.Taking the pattemed photoresist as the master and using embossing technology,the relief structure was replicated to plastic film.The measured result shows that the film has enhanced the transmittance at infrared wave band.In 1μm to 2μm band,12%average decrease of reflectivity has been achieved.