Method of designing thickness of coating film and semiconductor photonic device

Abstract

A coating film is provided on an end surface of a semiconductor photonic element including an active layer through which light propagates. The coating film has a two-layer structure including a first layer film and a second layer film arranged in a stacked relation. The thicknesses of the first and second layer films are determined so that the value of the amplitude reflectivity of the coating film is equal to an imaginary number.

1 . A method of designing the thickness of a coating film including a plurality of layers and provided on an end surface of a semiconductor photonic element including an active layer through which light propagates, said method comprising the steps of: 
 (a) selecting an imaginary number as a value of an amplitude reflectivity of said coating film; and    (b) determining the thickness of each of said plurality of layers of said coating film so that the value of said amplitude reflectivity of said coating film is equal to said imaginary number selected in said step (a).    
     
     
         2 . The method according to  claim 1 , wherein 
 the thickness of each of said plurality of layers of said coating film is determined in said step (b) so that a center wavelength of a wavelength band for which a power reflectivity of said coating film falls within a predetermined range when the wavelength of light propagating through said active layer is hypothetically varied is equal to a design value of said wavelength.    
     
     
         3 . A semiconductor photonic device comprising: 
 a semiconductor photonic element including an active layer through which light propagates; and    a coating film including a plurality of layers and provided on an end surface of said semiconductor photonic element,    said coating film having an amplitude reflectivity taking on a value set at an imaginary value.    
     
     
         4 . The semiconductor photonic device according to  claim 3 , wherein 
 the width of a first wavelength band for which a power reflectivity of said coating film falls within a predetermined range when the wavelength of light propagating through said active layer is hypothetically varied is greater than the width of a second wavelength band for which a power reflectivity of a single layer film obtained when said single layer film is provided on said end surface of said active layer falls within said predetermined range when said wavelength is hypothetically varied, said single layer film having a refractive index satisfying        R   t =(( n   c   −n   f   2 )/( n   c   +n   f   2 )) 2      where n f  is the refractive index of said single layer film, n c  is an effective refractive index of said semiconductor photonic element, and R t  is a median value of said predetermined range, said single layer film having a thickness 5/(4n f ) times the center wavelength of said first wavelength band.    
     
     
         5 . The semiconductor photonic device according to  claim 3 , wherein 
 the value obtained by dividing the width of a wavelength band for which a power reflectivity of said coating film falls within a predetermined range when the wavelength of light propagating through said active layer is hypothetically varied by the center wavelength of said wavelength band is greater than 0.06.    
     
     
         6 . The semiconductor photonic device according to  claim 4 , wherein 
 the sum of the products of the thicknesses and refractive indices of the respective layers of said coating film is greater than 3λ c /4 where λ c  is said center wavelength of said first wavelength band.    
     
     
         7 . The semiconductor photonic device according to  claim 5 , wherein 
 the sum of the products of the thicknesses and refractive indices of the respective layers of said coating film is greater than 3λ c /4 where λ c  is said center wavelength of said wavelength band.    
     
     
         8 . The semiconductor photonic device according to  claim 4 , wherein 
 said predetermined range is ±2% from the median value thereof.    
     
     
         9 . The semiconductor photonic device according to  claim 5 , wherein 
 said predetermined range is ±2% from the median value thereof.    
     
     
         10 . The semiconductor photonic device according to  claim 3 , wherein 
 a power reflectivity of said coating film is less than a power reflectivity at said end surface defined by the effective refractive index of said semiconductor photonic element and a refractive index in a free space contacting said end surface when said coating film is absent.    
     
     
         11 . The semiconductor photonic device according to  claim 3 , wherein 
 said coating film includes two material layers, and the number of layers of said coating film is selected from the group consisting of six, seven, eight and nine.    
     
     
         12 . The semiconductor photonic device according to  claim 3 , wherein 
 said coating film includes three material layers, and the number of layers of said coating film is selected from the group consisting of seven, eight and ten.    
     
     
         13 . The semiconductor photonic device according to  claim 3 , wherein 
 said coating film includes at least two selected from the group consisting of a silicon oxide layer, a tantalum oxide layer, an alumina layer and an aluminum nitride layer.