Robust and high-contrast multifocal focusing method based on amplitude-type spatial light modulators

Background Introduction:

Optical scattering is a widespread physical phenomenon in nature, and light scattering is due to the complexity and spatio-temporal inhomogeneity of light propagation paths in media, e.g., structural disorder and inhomogeneity are widely present in complex photonic media such as biological samples and white paints. Light is inevitably scattered multiple times as it passes through the disordered medium, but the interference information of the light is retained. The study of interference of scattered light is of great significance, for example, Anderson localisation, coherent backscattering (CBS), and random laser phenomena have been discovered by studying the interference of scattered light. Notably, it has been shown that active control of multiple scattered light can be achieved with the help of wavefront shaping (WFS) techniques through real-time feedback or measurements of the transmission matrix. Scattered light is manipulated by loading some specific phase modes on the wavefront of the incident light, which provides the ability to reveal fundamental physical properties of complex scattering processes, such as opening/closing of intrinsic channels, energy enhancement inside the scattering medium, and transverse localisation of transmission eigenchannels. In addition, the control of scattered light has very important applications in optical imaging, optical communication, nonlinear optics, and biomedicine.

Spatial light modulator working principle:

The display of the amplitude-type spatial light modulator TSLM023-A is a twisted nematic panel (TN panel), which is a liquid crystal operating mode with a spinning effect that can change the direction of light polarisation. By applying electricity to the liquid crystal molecules to change the deflection angle of the liquid crystal, the strength of the spinning effect is adjusted, and the amplitude modulation can be achieved by combining with the polarisation device. This process of birefringence effect and spin effect coexist, the increase of liquid crystal thickness can make the birefringence effect weakened, to achieve pure amplitude modulation. When the thickness of the liquid crystal box is large enough, and the liquid crystal molecule tilt angle is low, only the phase modulation, no amplitude modulation; in the liquid crystal molecule tilt angle is large, there will be amplitude modulation, at this time the amplitude and phase modulation at the same time, the liquid crystal tilt angle is loaded on the liquid crystal molecule at both ends of the pixel voltage decision, so the liquid crystal pixel voltage range determines the liquid crystal device working in the amplitude modulation or phase modulation area. So, by changing the loaded image using TSLM023-A some phase modulation can also be achieved.

The main research work of this article:

The experimental setup used in the article is shown in Fig. 1, where the expanded beam of 532 nm light is passed through an amplitude-based spatial light modulator (CSCS TSLM023-A) to achieve pure phase control of the wavefront using holography. M pixels are used on the SLM to control the individual phases of the wavefront, and the size of a single pixel is 26 um, corresponding to the total number of controllable points of the transmitted light is the same as the number of pixels, i.e., the size of the target area corresponds to the number of SLM pixels, and the pure phase control is achieved by aligning the measured pattern with the high latitude vector of the target pattern.

Fig. 1 Experimental setup. In the dashed box, the scattering sample is placed on white paper with black bars, indicating that the sample strongly scatters light.

Fig. 2. Simulation results. Three-focusing pattern with time-reversal WFS (a) and that with feedback WFS (c), which correspond to the first simulation result in (b) and (d),

respectively. The peak-to-background ratio (𝜂)of the constructed three-focusing pattern in ten different simulations with the time-reversal WFS (b) and feedback WFS (d).

Fig. 3. Experimental results. (a) The measured intensity pattern of the transmitted light within the target area before WFS. (b) The convergence curve of feedback WFS. (c) After WFS, the three focuses are constructed at the predefined positions. (d) The peak-to-background ratio (𝜂) of the constructed three-focusing pattern in 10 different experiments. (e)The peak-to-background ratio changes with the number of focuses.

The real-time feedback WFS system proposed in the article not only successfully constructs multiple focusing points in the diffraction limit at a predetermined location, but also significantly suppresses the random perturbations to the focusing spot induced by the background field, which is expected to realise the multiple focusing points of scattered light, which in turn can be applied in the directions of quantum interference, optical imaging, optical manipulation, and the interaction between light and matter.

The parameter specifications of the CSCS transmitted spatial light modulator TSLM023-A used in this experiment are as follows:

Written at the end:

Transmissive spatial light modulators have a wide range of applications in the field of optics and optoelectronics, covering a variety of aspects such as optical imaging, optical communication, optical information processing, optical sensing, optical information storage, etc., which provide important support and impetus to the development and innovation of optoelectronic technology.