Jones Calculus in Phase-Only Liquid Crystal Spatial Light Modulators
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Appendix Jones Calculus in Phase-Only Liquid Crystal Spatial Light Modulators In Jones calculus, the polarization state of light is represented by a Jones vector, Ex EEE ⇒ (A.1) E y where Ex and Ey are normalized complex amplitude components along x- and y-axes, respectively. An optical element is represented by a Jones matrix, TT11 12 TTT ⇒ , (A.2) TT21 22 and light that impinges on it undergoes the transformation (out) (in) Ex 11 12 ETT x E(out) = TEE=TE (in) ⇒ = . (A.3) (out) TT (in) E y 21 22 E y A phase-only liquid crystal spatial light modulator (SLM) is a two-dimensional array of independently addressable uniaxial birefringent pixels. The pixels are characterized by a common ordinary refractive index, no, along one axis and individually voltage- controllable extraordinary refractive index, ne(V), along an orthogonal axis. To under- stand the various operation modes of an SLM, consider the optical setup shown in Fig. A.1. The SLM orientation is specified by the angle θ between the extraordinary axis and the y-axis. The Jones matrix for an SLM oriented with its extraordinary axis parallel to the y-axis (θ=0) is 2π expj dno 0 (0) λ TSLM = . (A.4) 2π 0 exp dne () x , y λ 306 Appendix: Jones Calculus in Phase-Only Liquid Crystal Spatial Light Modulators where d is the thickness of the SLM cell and the index modulation ne ( x, y) is controlled by the applied pixel voltages V= V( x, y) . To study the output polarization states, it is convenient to write the unrotated Jones matrix as 1 0 2π TTTSLM = expj dno , λ 0 expδ ()x , y 2π where δ ()x, y= d n() x, y− n describes a spatially varying phase retardation. λ e o Each SLM pixel acts as a programmable phase retarder, enabling various polarization states to be encoded on an incident light. Using this alternate expression with the rotation matrix, Rθ, we can apply a rotational transformation to get the new SLM Jones matrix for an arbitrary θ ()θ -1-1-1 (0) TRTRSLM= θ SLM θ 2 2 cosθ+ sin θ expj() δ sin θ cos θ− 1 + expj() δ . (A.5) = exp jφ ()o 2 2 sinθ cos θ− 1 + expj() δ sin θ+ cos θ expj() δ Note that the uniform phase offset, φ0 =(2 π λ )dno , which does not affect the polariza- tion, is commonly neglected in Jones calculus. polarization axis y x SLM axis θθθ Input light Polarizer SLM FigFigFig A.1 Optical setup for realizing different SLM operation modes. A linearly polarized (along the y-axis) light is incident on a spatial light modulator (SLM). The director axis of the liquid crystal molecules in the SLM is oriented at an angle, θ, with the incident polarization. A.1 Spatial Phase Modulation Consider a vertically polarized input light ()in 0 E = , (A.6) 1 A.2 Spatial Polarization Modulation 307 incident on an SLM oriented at θ=0. Using the SLM Jones matrix in Eq. (A.4), the output after SLM is a new vector EEE (out) given by 0 ()out (0)(0)(0) ()in ETE=SLM = exp jφ ()x , y . (A.7) 1 This is a vertically polarized output that contains a spatial phase modulation, 2π φ ()x, y= dne () x, y . (A.8) λ Thus, the SLM operates as a spatial phase-only modulator when its extraordinary axis is aligned with the incident polarization. A.2 Spatial Polarization Modulation o For an SLM oriented at θ = 45 , the rotationally transformed TSLM in Eq. (A.4) becomes 1+ expjδ 1 − exp j δ −1 1 ( ) ( ) RTR()()θSLM θ= exp()j φo . (A.9) 2 1− exp()()jδ 1 + exp j δ Using the same vertically polarized input EEE(in), the output field EEE(out) is − δ (out ) (0) 0 1 1 exp( j ) EET= TETSLM = exp()jφo 1 2 1+ exp()jδ (A.10) sinδ x , y /2 2φo + δ− π ()() = exp j . 2 jcos()δ ()x , y /2 The uniform phase offset, which is not relevant to the polarization state, can be neglected and we can describe the output polarization simply as sin(δ ( x , y) /2) EEE ()out = (A.11) jcos()δ ()x , y /2 In spatial amplitude modulation, an output polarizer isolates either the vertical or horizontal component of Eq. (A.11). Spatial polarization modulation utilizes the full output with both components. Spatially programmed polarization is achieved by controlling the spatial phase retardation of the SLM. Output polarization vectors for some values of δ are illustrated in Table A.1. Other δ -values yield elliptic polarization states with major axis aligned either along the horizontal or the vertical. 308 Appendix: Jones Calculus in Phase-Only Liquid Crystal Spatial Light Modulators Table A.A.A.1A.111 Polarization vector in Eq. (A.11) for various retardation, δ. δ ( x, y) E ()out 0 0 j ; 1 1 1 π ; /2 2 j 1 π ; 0 1 1 ; 3π/2 2 − j 0 − j ; 2π 1 Table A.2 A graphical representation of the output polarization ellipticity and orientation as a function of the phase shift implemented in corresponding regions of SLM-1 and SLM-2 A.3 Spatial Polarization Modulation with Arbitrary Axis 309 A.3 Spatial Polarization Modulation with Arbitrary Axis A polarizer and SLM can generate elliptically polarized light with major axis that can be vertical or horizontal. It is possible to generate an arbitrary state of polarization by taking this elliptically polarized light and rotating its major axis to a desired arbitrary orientation. This rotation can be realized using SLMs and two quarter wave plates, and the full polarization encoding system is schematically illustrated in Fig. A.2. Both SLMs are oriented at θ=45°. Elliptical generator (T1) Elliptical rotator (T2) 2D polarization Input light encoded light Polariser SLM-1 /4 SLM-2 /4 λ λ Fig. A.2Fig. A.2 An optical system for converting incident polarized light into an arbitrary state of elliptically polarized light with the major axis of the elliptically polarized light rotated an arbitrary angle. The lines denote the extraordinary axis of the SLMs, the quarter wave plates (λ/4) and the polarization direction of the linear polarizer. To verify the axis rotation, let us write the transformation matrix for the two quarter wave plates and SLM: 1 0 cos(δ2 (x ,/2 y) ) − j sin(δ2 ( x ,/2 y) ) 1 0 T = rotator 0 − j 0 j − jsin()δ2 () x , y /2 cos()δ2 ()x , y /2 (A.12) cos()δ2 ()x , y /2 sin()δ ()x , y /2 = . −sin()δ ()x , y /2 cos()δ2 ()x , y /2 This result shows that TTTrotator is, indeed, a rotation matrix with a rotation angle of φ2(x,y)/2. The net transformation matrix for the optical system in Fig. A.2 is (π /4) RTδ2 /2 SLM cos()δ2 ()x ,/2 y sin()δ2 ()x ,/2 y cos()δ1 ()x ,/2 y− j sin()δ1 () x ,/2 y = − δ δ − δ δ sin()2 ()x , y /2 cos()2 ()x , y /2 j sin()1 () x , y /2 cos()1 ()x , y /2 cos()δ1 ()x , y /2cos()δ2 ()x , y /2− j sin()δ1 () x , y /2sin()δ2 ()x , y /2 = . δ δ δ δ jsin()1 () x , y /2sin()() x , y /2 cos()1 ()x , y /2cos()2 ()x , y /2 (A.13) 310 Appendix: Jones Calculus in Phase-Only Liquid Crystal Spatial Light Modulators Output polarization states from this arbitrary polarization encoding system are graphically illustrated in Table A.2. The phase retardations of SLM-1 and SLM-2 determine the type and direction of the output polarization. It is interesting to note that the direction of the elliptically polarized light changes from left handed to right handed when the phase modulation of SLM-1 is above π. This table shows that it is possible to generate an arbitrary state of elliptical polarization if both SLM-1 and SLM-2 can produce a phase modulation of at least 2π. Experimental demonstrations of arbitrary polarization encoding are available in Ref. [1]. Reference 1. R. L. Eriksen, P. C. Mogensen, and J. Glückstad, “Elliptical polarisation encoding in two dimensions using phase-only spatial light modulators,” Opt. Commun. 187187187,187 325-336 (2001). Index 3D optical trapping and manipulation, 176 Brownian motion, 200 4f system, 80, 106, 109, 183, 220, 225, 229, 233, chrome-on-glass mask, 256, 259 242, 244, 257, 264, 277, 285, 287, 288, circ-function, 181, 249, 286 289, 291, 293 colloidal constellations, 176 filtering, 157, 287, 288 colour-based sorting, 156 combined filter parameter, 2, 27, 30–32, 36–38, aberration, 1, 35, 176, 186, 233, 235, 255, 278, 40, 43–44, 46, 250–254, 257, 286, 288, 300 280 common-path interferometer (CPI), 1–4, 8, 14– Airy disc, 51 24, 27–32, 35–59, 61, 235, 248–255, 263, Airy function, 29 296, 300–304 annular intensity profile, 156, 159, 161 pair configuration, 48 aperture matching, 257 types, 2, 23, 300 Argand diagram, 250, 251–252, 255, 260–261, common-path interferometry, 1, 3, 8, 258 287–288 complex conjugate, 237, 274 array illumination, 220, 243 complex filter space, 37, 41, 58, 300 array illuminator, 221 plot, 37, 41, 58, 300 artefacts, 51 compression factor, 237, 243 Ashkin, Arthur, 151, 156, 167, 180, 203 compromise filter, 39 automation, 191, 196, 197, 212 computational axial and transverse stiffness, 209 cost, 58 axial control, 168, 171–172, 175, 211 load, 163 axial dependency of the trap stiffness, 194 overhead, 176, 238 axial dynamic range of manipulation, 209 computer-controlled polarization landscape, 174 axial force curve, 203, 207 computer-generated holography (CGH), 85–95, axial manipulation, 209–210 119, 123, 240 concurrent top- and side-view imaging, 178–179 bacteriorhodopsin, 218, 224–225 confinement stress, 165 binary coupling grating, 277, 280 constant intensity criterion, 250, 252 binary diffractive optical element, 234 contrast, see generalized phase contrast (GPC) binary phase mask, 288 phase contrast, reverse phase contrast (RPC) binary phase modulation, 247, 253, 263, 288 convolution, 51, 238 binary phase patterns counter-propagating beam trap, 168, 169 decrypting by amplitude, 296 C-parameter, 45 biologically