CN102147539B - Method for generating one-dimensional periodic structure light field based on pure-phase type liquid crystal spatial light modulator - Google Patents

Abstract

The invention discloses a method for generating a one-dimensional periodic structure light field based on polarization modulation of a single light beam. An experimental device is shown in a figure of an abstract. A pure-phase type liquid crystal spatial light modulator 3 adjusts an incident ray polarized light, so that a one-dimensional circular polarization sinusoidal periodically-distributed light field can be generated. The light field passes through a polarizing film 4, so that a one-dimensional strength sinusoidal periodically-distributed light field can be generated. Compared with the conventional method for generating the one-dimensional strength sinusoidal periodically-distributed light fields by coherent superposition of two light beams, the method provided by the invention has the advantages that: the experimental device is easy; the generated periodic structure light fields are stable; and a computer is easy to control, and parameters are easy to adjust.

Description

A kind of one dimension periodic structure light field production method based on pure phase type LCD space light modulator

Technical field

The present invention relates to a kind of light field production method of one dimension periodic structure.Be characterized in utilizing the polarization state of the direct modulated beam of light of pure phase type spatial light modulator to distribute, produce the light field that intensity and circular polarization one dimension cycle change.Transport kinetic measurement and the microstructure photoetching field of manufacturing has significant application value at semiconductor.

Background technology

One dimension periodic structure light field science and technology field in modern times is with a wide range of applications.Traditional production method is to use the relevant superposition of the linearly polarized light beam of the non-parallel propagation of two bundles, produce the light field of one dimension periodic structure in the crossover zone, and the cycle is by the angle decision of two-beam.The light superposition of two bundle polarization parallel can produce the light field that the light intensity one dimension cycle changes.This cycle light field electronics in the photoetching making of one dimension sinusoidal pattern grating, semiconductor is bipolar to be transported and is applied aspect the kinetic measurement.Two bundle polarized orthogonals or vertical light superposition can produce the light field of circular polarization or the variation of polarization state one dimension cycle.The electron spin in semiconductor of this cycle light field transports and is applied aspect the kinetic measurement.Yet the shortcoming that this twin-beam intersects the superposition production method is significantly, needs two-beam, has increased the complicacy of light path.In addition, the periodic structure of generation receives the optical path difference fluctuation between the two-beam easily and floats.Simultaneously,, also need accurately to regulate the light path of two-beam, make it almost equal, because only tens microns of path-length errors of allowing for the femtosecond pulse bundle.These shortcomings have limited the widespread use that the relevant superposition of this twin-beam produces one dimension periodic structure light field method, have increased the complicacy and the experimental implementation difficulty of equipment.Thereby, invent new, simple one dimension cycle light field production method and be very important.

Summary of the invention

The present invention has developed a kind of simple, method of producing one dimension periodic structure light field based on pure phase type LCD space light modulator modulation single beam.The principle of production method is as shown in Figure 1.Produce the contrast of one dimension periodic structure light field method with the relevant superposition of the twin-beam of present report, new method of the present invention has following four advantages.The first, experimental provision is simple in structure; Because only need a branch of light.The second, the one dimension periodic structure light field of generation is stable; Because the one dimension periodic structure that does not exist the light path shake of two-beam in the relevant superposition of twin-beam to cause floats.The 3rd, do not need the light path match and regulate.Even to femto-second laser pulse, do not relate to light path match and regulate problem, because use single beam yet.The 4th, computer control changes the parameter of cycle light field easily.

Optical principle of the present invention is as shown in Figure 1.Its key feature is to introduce pure phase type liquid crystal spatial light modulator 3.As everyone knows, liquid crystal molecule is arranged in the same way in the pure phase type LCD space light modulator (PPLCSLM), and liquid crystal molecule has birefringence effect.If normal incidence is passed the light of PPLCSLM, the polarization direction of its ordinary and extraordinary ray component is respectively along X, the Y direction.The refractive index n of extraordinary ray _eChange with applying voltage, and the refractive index n of ordinary light ₀Do not change with applying voltage.So when on a certain space cell of PPLCSLM or pixel, applying voltage V, the extraordinary of the light through this pixel and the phase difference between the ordinary light component are:

$\mathrm{\φ}(x,y,V)=\frac{2\mathrm{\π}}{\mathrm{\λ}}[n_e(x,y,V)-n_0]d---\left(1\right)$

X in the formula, y are the position coordinates of pixel unit, and V also is coordinate x for the voltage that applies on this pixel, the function of y, can clearly be written as in case of necessity V (x, y), λ is an optical wavelength, d is the thickness of liquid crystal layer.

As shown in Figure 1, when the polarization direction of the polarizer 2 becomes miter angle with the Y axle, and the amplitude of the linearly polarized light of its transmission is when being A, after this linearly polarized light sees through PPLCSLM, respective coordinates be (x, the transmission light field E of pixel y) can be expressed as:

$E(x,y,V)=\left(\begin{array}{c}{E}_x\\ E_y\end{array}\right)=\frac{A}{\sqrt{2}}\left(\begin{array}{c}1\\ e^{\mathrm{i\φ}(x,y,V)}\end{array}\right)---\left(2\right)$

E in the formula _x, E _yThe X that representes light field respectively, the Y component.

Following formula is described a polarization state arbitrarily.Through control apply voltage V (x, y), make phase difference (x, y V) equal 0 respectively, pi/2, π and 3 pi/2s then can be distinguished and obtain linear polarization accordingly, right-hand circular polarization revolves the linear polarization and the left circularly polarized light that turn 90 degrees.Obviously, (x, y V) change 2 π to phase, and these polarization states repeat to occur.So (x, y), (x, y V) change with the coordinate x cycle, then can produce the light field that the polarization state one dimension cycle changes to make φ to control the voltage V that applies of different pixels.

Description of drawings

Fig. 1 one dimension periodic structure light field generation device schematic diagram

Among Fig. 1,1 is incident beam; 2 is polaroid, is used for converting incident beam 1 into linearly polarized light; As 1 being linearly polarized light, then should its polarization direction be adjusted to consistent with 2 polarization direction so that light energy losses is minimum; 2 bends represent to allow the linear polarization of passing through, and become miter angle with the Y axle; 3 is pure phase type LCD space light modulator, by computer control; 4 is polaroid, and its bend is represented the polarization direction, becomes miter angle with the Y axle; Frame of broken lines representes that 4 can increase and decrease according to producing one dimension periodic structure light field needs of different nature.

Embodiment

According to above-mentioned principle, the present invention practical implementation two instances, produce one dimension intensity sinusoidal period respectively and distribute and one dimension circular polarization sinusoidal period distribution light field.

Instance one one dimension circular polarization sinusoidal period distribution light field produces

Experiment device schematic diagram is as shown in Figure 1, but polaroid 4 needs to remove.The voltage of going up on each row pixel through control PPLCSLM changed with the x cycle, just can produce the light field that distributes along X axle circular polarization sinusoidal period.

Suppose will to be the light field that the one dimension circular polarization sinusoidal period of Λ pixel width distributes in the generation cycle, and the phase differential of corresponding equation (1) to be the voltage that applies of 0 and 2 π be respectively V ₀And V _{2 π}, then, in PPLCSLM, apply following voltage on each pixel column when through computing machine

$V(x,y)=V_0+\frac{V_0-V_{2\mathrm{\π}}}{\mathrm{\Λ}}\mathrm{RMod}\left(\frac{x}{\mathrm{\Λ}}\right)---\left(3\right)$

The time, the phase differential that each pixel column produces is:

$\mathrm{\φ}(x,y,V)=\frac{2\mathrm{\π}}{\mathrm{\Λ}}\mathrm{RMod}\left(\frac{x}{\mathrm{\Λ}}\right)---\left(4\right)$

Function R Mod () expression two integral divides are got remainder in the formula.The voltage adjustable extent of introducing this function and be the birefringent phase difference of considering liquid crystal is limited, so, only use the phase differential of 0-2 π scope in this instance; X=0,1 ..., M-1 is the row index of PPLCSLM array; M is the columns of cell array among the PPLCSLM.

See through the light field of PPLCSLM and described by equation (2) this moment.The circular polarization P of this polarization state is:

P(x，y)＝sin(φ(x，y，V)) (5)

Because phase difference is provided by equation (4), be the linear function of x coordinate; So equation (5) shows that the circular polarization of light field changes with the x sinusoidal period, has promptly produced the light field that one dimension circular polarization sinusoidal period distributes.The electron spin in semiconductor of such light field transports has significant application value in the kinetic test.Its vitalizing semiconductor can produce transient state spin grating.And in the present experiment, be to use the linearly polarized light superposition of two bundle polarized orthogonals to produce such light field, cause experimental provision complicated.

Instance two one dimension intensity sinusoidal period distribution light fields produce

Experiment device schematic diagram is as shown in Figure 1.Need now to keep polaroid 4, and its polarization direction becomes miter angle with the Y axle.The light field that sees through spatial light modulator 3 is described by equation (2).This light field can be expressed as after seeing through 4:

E _P(x，y)＝E _x(x，y)sin45+E _y(x，y)cos45 (6)

EP is for seeing through the light field of polaroid 4 in the formula; Ex, Ey represent the x of incident field respectively, the y component, and they are provided by equation (2).

Each component substitution (6) formula in the equation (2) is got:

$E_P(x,y)=\frac{A}{\sqrt{2}}[\sin 45+e^{\mathrm{i\φ}(x,y,V)}\cos 45]=\frac{A}{2}[1+e^{\mathrm{i\φ}(x,y,V)}]---\left(7\right)$

Light intensity is:

$I_P(x,y)={\left|E_P(x,y)\right|}^2=\frac{I_0}{2}[1+\cos (\mathrm{\φ}(x,y,V)]---\left(8\right)$

I in the formula ₀=A ²

When PPLCSLM went up each pixel column and applies voltage and determined by equation (3), the phase differential of each pixel was shown in equation (4).At this moment, (x, y V) are the linear function of x coordinate to phase difference; So equation (8) shows that the intensity of light field changes with the x sinusoidal period, has promptly produced the light field that one dimension intensity sinusoidal period distributes.Such light field bipolar transporting of electronics in the photoetching making of sinusoidal period grating and semiconductor all has significant application value in the kinetic measurement.And in the present experiment test, be to use the linearly polarized light of two bundle polarization parallel to intersect the such light field of superposition generation, cause experimental provision complicated.

Claims (2)

1. an one dimension periodic structure light field production method is characterized in that using pure phase type LCD space light modulator modulation single beam to replace the relevant superposition of traditional twin-beam, produces one dimension periodic structure light field stable, easiness in handling; Linearly polarized light beam sees through pure phase type LCD space light modulator; When its plane of polarization becomes miter angle with the ordinary polarization direction of liquid crystal birefringence; And when the LCD space light modulator pixel column applied the said voltage of formula (1), then can produce the light field that one dimension circular polarization sinusoidal period changes;

$V(x,y)=V_0+\frac{V_0-V_{2\mathrm{\π}}}{\mathrm{\Λ}}\mathrm{RMod}\left(\frac{x}{\mathrm{\Λ}}\right)---\left(1\right)$

V in the formula ₀And V _{2 π}Correspond respectively to the voltage that is applied when the LCD space light modulator pixel produces 0 and 2 π phase differential; Λ is the number of picture elements in the cycle of one dimension cycle light field; X, y are the position coordinates of pixel unit; Function R Mod () representes two integral divides, gets remainder.

2. the one dimension periodic structure light field production method described in the claim 1, it is characterized in that output light field again through a polarization direction with after liquid crystal ordinary light polarization direction becomes the polaroid of miter angle, can produce the light field of one dimension light intensity sinusoidal period variation.