Periodic Reporting for period 1 - ET-SLM neurons (Electrothermal Spatial Light modulator for neuronal tissue imaging in depth)
Reporting period: 2022-07-01 to 2024-06-30
The development of new technics for controlling light propagation has always been the source of major innovations and discoveries in imaging. With their ability to control actively and spatially the light components, the spatial light modulators (SLMs) represent the ultimate goal in light shaping. They allowed substantial growth in the display sector thanks to recent innovations such as liquid crystal display (LCD), or the digital light processing (DLP) video projector. In the research field, phase-only SLMs are central tools in modern imaging domains ranging from astronomy to microscopy. Liquid crystal SLMs (LC-SLM) have become the essential tool for beam shaping in microscopy featuring high resolution (few µm) and high definition (several millions of pixels). While LC-SLMs provide outstanding achievement in superresolution, 3D localization, or optogenetics, their limited performances still remain a roadblock in many applications. Indeed, LC-SLMs are polarization sensitive, suffer from strong chromatic diffractive effects, and have an intrinsic response time of several ms.
To overcome these limitations, spatial light modulation using thermo-optic effects has been recently proposed. Such an approach relies on heating materials displaying strongly temperature-dependent refractive indices, which refractive index is highly sensitive to temperature increase.
To overcome these limitations, an Electrothermal Spatial Light Modulator (ET-SLM) called SmartLens has been developped. These SmartLenses are polarization-insensitive since they involve non-birefringent thermo-optical index modulations. As they operate in a refractive rather than diffractive regime, they are also relatively achromatic (unlike LC-SLMs) and can be used over a broad wavelength range. Despite its great promises, the SmartLens concept is still in its infancy, with several limitations that currently prevent several applications such as the response time, the number of actuators, or the sharpness of the modulation.
The first objective of the project relies in improving these performances. They are particularly desired in the domain of imaging through dynamic scattering mediums such as in vivo brain tissues. Besides being one of the biggest scientific challenges of our times, deciphering how the brain works constitutes a priority research line for the European Union. Among the different optical tools employed, the use of SLM has revolutionized the field of optogenetics by enabling parallel stimulation.
In this context, the second objective of the project aims to combine an ET-SLM with advanced microscopes systems and perform neuronal imaging. The last step of the project relies in pushing the capabilities of the ET-SLM in order to perform fast thermal wavefront applied in in imaging in complex media such as in brain depths.
To overcome these limitations, spatial light modulation using thermo-optic effects has been recently proposed. Such an approach relies on heating materials displaying strongly temperature-dependent refractive indices, which refractive index is highly sensitive to temperature increase.
To overcome these limitations, an Electrothermal Spatial Light Modulator (ET-SLM) called SmartLens has been developped. These SmartLenses are polarization-insensitive since they involve non-birefringent thermo-optical index modulations. As they operate in a refractive rather than diffractive regime, they are also relatively achromatic (unlike LC-SLMs) and can be used over a broad wavelength range. Despite its great promises, the SmartLens concept is still in its infancy, with several limitations that currently prevent several applications such as the response time, the number of actuators, or the sharpness of the modulation.
The first objective of the project relies in improving these performances. They are particularly desired in the domain of imaging through dynamic scattering mediums such as in vivo brain tissues. Besides being one of the biggest scientific challenges of our times, deciphering how the brain works constitutes a priority research line for the European Union. Among the different optical tools employed, the use of SLM has revolutionized the field of optogenetics by enabling parallel stimulation.
In this context, the second objective of the project aims to combine an ET-SLM with advanced microscopes systems and perform neuronal imaging. The last step of the project relies in pushing the capabilities of the ET-SLM in order to perform fast thermal wavefront applied in in imaging in complex media such as in brain depths.
During the project, an effort has been made on the engineering electro-thermal system adapted to the constraints of microscopy imaging and the alignment of proof-of-concept optical system. Numerical simulations were performed in order to better understand the electro-thermo-optical interaction and find the best architecture for thermal phase modulation. From these results, several modulators were manufactured and were used for three different applications.
The first application relied on the use of a photothermal modulator able to locally induce a constant phase modulation which was combined with a microscope to performed enhanced quantitative wavefront imaging for nano-objects characterization (Gentner et. al. ACS Nano 2024).
With the aim of having a single SmartLens architecture featuring several optical functions, a reconfigurable design capable of tuning the defocus and the spherical aberration of an incoming beam has been built. A proof of concept of aberration control in calibrated sample has been made. This is an important step for the objective of imaging in brain depth. An article is in preparation.
A last application involved the use of a 5x5 array SmartLens ET-SLM design. A fluorescent microscope was built, with the ET-SLM and a structured illumination approach in order to perform adaptive surface imaging of neurons. After a full characterization of the system, we could demonstrate the possibilities of the system in vivo by simultaneously imaging, in the zebrafish larval brain, the activity of neurons activity from different depths at a rate of 0.5 kHz and over a large field of view of 360x360 µm². This microscope offers a unique capability to detect/image microscopic objects disseminated in a 3D volume and more specifically to monitor neuronal activity in vivo at high imaging rate. An article is in preparation.
The first application relied on the use of a photothermal modulator able to locally induce a constant phase modulation which was combined with a microscope to performed enhanced quantitative wavefront imaging for nano-objects characterization (Gentner et. al. ACS Nano 2024).
With the aim of having a single SmartLens architecture featuring several optical functions, a reconfigurable design capable of tuning the defocus and the spherical aberration of an incoming beam has been built. A proof of concept of aberration control in calibrated sample has been made. This is an important step for the objective of imaging in brain depth. An article is in preparation.
A last application involved the use of a 5x5 array SmartLens ET-SLM design. A fluorescent microscope was built, with the ET-SLM and a structured illumination approach in order to perform adaptive surface imaging of neurons. After a full characterization of the system, we could demonstrate the possibilities of the system in vivo by simultaneously imaging, in the zebrafish larval brain, the activity of neurons activity from different depths at a rate of 0.5 kHz and over a large field of view of 360x360 µm². This microscope offers a unique capability to detect/image microscopic objects disseminated in a 3D volume and more specifically to monitor neuronal activity in vivo at high imaging rate. An article is in preparation.
We anticipate that the development of ET-SLM have the potential to disseminate broadly. We could monitor, with such tool, simultaneously the activity of zebrafish neurons over different depth at a framerate limited by the speed of the camera (0.5kHz). It is important to notice that this could not be performed with standard Liquid-Crystal SLM.
Besides, we could, as a preliminary result, combine the SmartLens reconfigurable system with an endoscope system and could adaptively image fluorophore through a fibre bundle. This leads the path toward high-speed 3D microscopy and will find extensive applications in neuroscience and in a more general point of view in life science. Indeed, numerous applications can be found with this approach such as in multiphoton microscopy and in endomicroscopy, tools that are necessary for imaging neuronal activity of brain animals.
In the longer term, the applications proposed here and related to scattering media imaging would have direct applications in tissue imaging and endoscopy. The latter will clearly be of use to the thousands of medical doctors using endoscopes every day.
Besides, we could, as a preliminary result, combine the SmartLens reconfigurable system with an endoscope system and could adaptively image fluorophore through a fibre bundle. This leads the path toward high-speed 3D microscopy and will find extensive applications in neuroscience and in a more general point of view in life science. Indeed, numerous applications can be found with this approach such as in multiphoton microscopy and in endomicroscopy, tools that are necessary for imaging neuronal activity of brain animals.
In the longer term, the applications proposed here and related to scattering media imaging would have direct applications in tissue imaging and endoscopy. The latter will clearly be of use to the thousands of medical doctors using endoscopes every day.