Spatial Light Modulator

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A Spatial Light Modulator (SLM) is an electrically programmable device that modulates light according to a fixed spatial (pixel) pattern. SLM are typically used to control incident light in amplitude, phase, or the combination of both.

Product Introduction

A Spatial Light Modulator (SLM) is an electrically programmable device that modulates light according to a fixed spatial (pixel) pattern. SLMs have an expanding role in several optical areas where light control on a pixel-by-pixel basis is critical for optimum system performance. SLMs are typically used to control incident light in amplitude, phase, or the combination of both.

SLM Device Construction

Several parameters help define SLM characteristics. Pixel pitch is defined as the center-to-center spacing between adjacent pixels. Interpixel gap describes the edge-to-edge spacing between adjacent pixels.

Polarized light enters the device from the top, passes through the cover glass, transparent electrode and liquid crystal layer, is reflected off the aluminum pixel electrodes, and returns on the same path. Drive signals travel through the pins on the bottom of the pin-grid array package, through the bond wires, and into the silicon die circuitry. The voltage induced on each electrode (pixel) produces an electric field between that electrode and the transparent electrode on the cover glass. This field produces a change in

the optical properties of the LC layer. Because each pixel is independently controlled, a phase pattern may be generated by loading different voltages onto each pixel.

Why choose our Reflective SLMs ?

High Voltage Backplanes = Fastest Response Times

Our SLMs use custom backplanes, and proprietary drive schemes to achieve response times down to 1 ms (wavelength dependent). Most other liquid crystal spatial light modulators utilize display backplanes built with standard Nematic liquid crystal, limiting response time to >30 ms.

Highest Phase Stability Commercially Available –Our backplanes are custom designed to allow high refresh rates (up to 6 kHz), and direct analog drive schemes. Refreshing the voltage at the pixel at rates far surpassing the response time of the liquid crystal ensures high temporal phase stability. Further, use of direct analog drive schemes, as opposed to digital dithering, reduces optical flicker as low as 0.1% (0.001 π radians). Low Inter-pixel Cross Talk - Our backplanes are custom designed to offer high voltage at the pixel (5 – 12 V), and a large pixel pitch. Further, our SLMs are built with Meadowlark Optics proprietary liquid crystal which minimizes the required thickness of the LC layer in the SLM. By maximizing the ratio of pixel pitch to LC thickness we are able to offer SLMs with minimal inter-pixel effects.

Broad Wavelength Capabilities - Meadowlark Optics is the only SLM supplier capable of offering SLMs designed for use from UV (>365 nm) up to the LWIR (8 - 12 µm). Analog is Better - All Meadowlark SLMs have been designed for phase modulation. Unlike many display LCoS backplanes which require a pulse width modulation (PWM) scheme, Meadowlark backplanes utilize analog voltages at each pixel. This results in a very stable phase response over time.

High Bit Depth Controllers - Meadowlark offers 8, 12, and 16-bit controllers to provide the most linear resolvable phase levels commercially available (up to 500). Fast transfer speeds from the computer to the SLM are offered up to 2 kHz.

Overview

Polarized light enters the device from the top, passes through the cover glass, transparent electrode and liquid crystal layer, is reflected off the aluminum pixel electrodes, and returns on the same path. Drive signals travel through. There are 2 types of special light modulators: reflective analog SLMs and transmissive SLMs.

Reflective Analog SLMs: All of our liquid crystal on silicon (LCoS) backplanes incorporate analog data addressing with high refresh rates to provide the lowest phase ripple SLMs available. User’s can select standard or high speed liquid crystal for optimal performance. Liquid cooling systems are available to remove heat via the back of the SLM chip in order to maximize optical power handling capabilities:

Transmissive SLMs: All of our liquid crystal on glass (LCoG) SLMs enable simple optical systems when low pixel counts are sufficient. Users can select single-mask or configurations for phase or amplitude modulation, or a dual-mask configuration for combined phase and amplitude modulation.

1. Reflective Analog Spatial Light Modulators (SLMs)

1.1 Small 512 x 512 Reflective Spatial Light Modulators, – Entry Level – Educational – Economical

Our legacy SLM is now available as our E-Series model. It is ideally suited for labs with a limited budget or researchers who do not require the high speed features of our premium SLMs, yet still demand high performance. This entry-level SLM is affordably priced without sacrificing quality.

Our  Liquid Crystal on Silicon (LCoS) Spatial Light Modulators (SLMs) are uniquely designed for pure phase applications and incorporate analog data addressing with high refresh rates. This combination provides user’s with the fastest response times and highest phase stabilities commercially available. We offer both transmissive and reflective SLMs in either one or two dimensions. Phase-only SLMs can also be used for amplitude-only or a combination of both.

The 512 x 512 SLM is good for applications requiring high speed, with synchronization / triggering capabilities. The optional dielectric mirror coating provides users with 100% fill factor, which increases optical efficiency.

Features:

Specifications:

Wavelength (nm)

Wavefront distortion

Liquid crystal response time

[standard / high efficiency] (ms)

AR coatings [Ravg<1%] (nm)

 

 

Model E512/PDM512

Model HSP512/HSPDM512

Model ODP512/ODPDM512

 

405

λ/5

25 / 33.3

N/A

3 / 4

400 - 850

532

λ/7

33.3 / 45

7 / 10

3.5 / 4.5

400 - 850

635

λ/8

33.3 / 45

12 / 16.7

4 / 5

400 - 850

785

λ/10

55.5 / 80

17.2 / 22.2

4.5 / 5.5

600 - 1300

1064

λ/10

66.7 / 100

10 / 16.7

5 / 6

600 - 1300

1550

λ/12

100 / 130

20 / 28.5

6 / 7

850 - 1650

* Diffraction efficiency of silicon backplane. Performance varies as a function of wavelength and pixel value.

Controller Models

Model

PCIe 8-bit

PCIe 16-bit

DVI 16-bit

Controller phase levels

256 / 8-bits

65536 / 16-bits

65536 / 16-bits

CPU to controller transfer time (computer dependent)

0.6ms

2.1ms

16.7ms

OverDrive Plus (ODP) for Ultra-High Speed Operation

The use of ODP has shown reductions of the liquid crystal response times by a factor of up to 8x through use of the transient nematic effect, phase wrapping, and regional calibrations. The base technology is the transient nematic effect, utilizing intermediate transition voltages beyond the target voltage needed to achieve the desired phase value. The second technology development is the use of phase wrapping, which is based on the cyclical nature of light wherein adding or subtracting 2π from any phase value in a hologram results in an equivalent hologram. Often times it is faster to switch from ϕ1 → ϕ2 ± 2π instead of switching from ϕ1 → ϕ2. ODP automatically implements the faster of the two transitions, based on the calibration data. The third technology development is the utilization of regional calibrations of an SLM. Because most optical applications require precision on the order of a fraction of a wavelength, nearly all SLMs will have some inherent phase errors across the aperture that may impact the performance of the optical system. OverDrive Plus utilizes the phase modulation capabilities of the SLM to calibrate these errors out of the reflected wave, while also utilizing the regional calibrations when determining the length of time required for the transient nematic effect on a pixel by pixel basis.

OverDrive Plus for Ultra-High Speed Modulations

Low Phase Ripple - Our Spatial Light Modulators are known for having the highest phase stability on the market. Our backplanes are custom designed with high refresh rates and direct analog drive schemes resulting in phase ripple less than 1% - 3% (depending on SLM model). Phase ripple is quantified by measuring the 1st order ripple as compared to the mean intensity while writing a repeating linear phase ramp to the SLM.

1st order intensity when writing a phase ramp to the SLM

High Power Capability - our Spatial Light Modulators have been tested for compatibility with high power pulsed and CW lasers. In the measurements below, the optical response of the 512 x 512 pixel SLM was measured as the incident power was incremented up to a peak power density of 112 MW/cm2 . Thermal effects resulted in a reversible reduction in modulation depth, however no permanent damage was observed.

512x512 SLM tested at 1064nm

* Average power of 1W to 16W with a repetition rate of 1MHz, pulse width of 600fs, and 5.5mm beam diameter results in a peak power density of up to 112MW/cm^2, without dielectric mirror coating or active cooling

1.2 Large 512x512 Reflective Spatial Light Modulators

This high voltage, large pixel SLM is optimized for high power applications requiring faster response times. The analog, high fill factor, high refresh rate backplane provides better optical efficiency and high temporal stability. Large pixels reduce pixel-to-pixel crosstalk.

Our Liquid Crystal on Silicon (LCoS) Spatial Light Modulators (SLMs) are uniquely designed for pure phase applications and incorporate analog data addressing with high refresh rates. This combination provides user’s with the fastest response times and highest phase stabilities commercially available. We offers both transmissive and reflective SLMs in either one or two dimensions. Phase-only SLMs can also be used for amplitude-only or a combination of both.

The Large 512x512 SLM is good for applications requiring high speed, with synchronization / triggering capabilities. The large active area is also good for high laser power density applications.

Features:

Specifications

Wavelength (nm)

Wavefront distortion

Liquid crystal response time

[standard / high efficiency] (ms)

AR coatings [Ravg<1%] (nm)

 

 

Model P512L/PDM512L

Model HSP512L/HSPDM512L

 

405

λ/5

3.0 / 4.5

N/A

400 - 850

532

λ/5

4.0 / 6.0

1.2 / 2.0

400 - 850

635

λ/6

4.5 / 7.0

1.7 / 3.0

400 - 850

785

λ/7

7.5 / 12.0

2.5 / 4.0

600 - 1300

1064

λ/10

10.0 / 15.0

3.3 / 5.0

600 - 1300

1550

λ/12

15.0 / 25.0

4.2 / 6.5

850 - 1650

*Silicon backplane, performance varies as a function of mirror coating, wavelength and pixel value

Large 512x512 Controller Models

Model

PCIe 8-bit

DVI 16-bit

Computer to controller resolution

256 / 8-bits

65536 / 16-bits

Controller to SLM resolution

65536 / 16-bits

65536 / 16-bits

CPU to controller transfer time (computer dependent)

1.4ms

16.7ms

Ultra-High Speed Operation – The Large 512 x 512 SLM was designed to minimize the liquid crystal response time without the need for the OverDrive Plus approach developed for the Small 512 x 512 SLM. This native high-speed performance results in liquid crystal response times as fast as 1.2 ms, with computer-to-controller transfers speeds as fast as 1.4 ms, the combination provides a

continuous 2π phase stroke at throughputs exceeding 700 Hz.

Two Controllers – PCIe or DVI.  The high-speed performance is best achieved using a PCIe controller offering 8-bits per pixel image transfers from the computer to the controller. A hardware-based lookup table (LUT) converts the 8-bits to 16-bits prior to the analog conversion for the SLM chip. The result is a linear 2π phase stroke with a phase resolution of λ/256 at frame rates up to 714 Hz. The PCIe controller also offers both input and output triggering capabilities to ease synchronization with other equipment.

A DVI controller with 16-bits of analog voltage resolution from the computer to the SLM is also available. With this controller the SLM can easily obtain more than 1000 linear resolvable phase levels. This λ/1000 phase resolution can be maintained over a broad wavelength range by tuning the look-up-tables / calibrations for the incident wavelength. The frame rate is dependent upon the graphics card used (typically 60 – 200 Hz).

Low Phase Ripple – Our Spatial Light Modulators are known for having the highest phase stability on the market. Our backplanes are custom designed with high refresh rates and direct analog drive schemes resulting in phase ripple less than 1% - 3% (depending on SLM model). Phase ripple is quantified by measuring the 1st order ripple as compared to the mean intensity while writing a repeating linear phase ramp to the SLM.

High Power Capability – Our Spatial Light Modulators have been tested for compatibility with high power pulsed and CW lasers. In the chart below, the optical response of the Large 512 x 512 pixel SLM was measured as the incident power was incremented up to a peak power density of 527 MW/cm2 , and an average power density of 518 W/cm2 . A liquid cooling system is also available to offset any thermal effects

Peak power density up to 527 MW/cm2 , average power density up to 518 W/cm2 , with a repetition rate of 123 Hz, pulse width of 8 ns, and 0.227 mm beam diameter, without dielectric mirror coating or active cooling. Damage occurred when the Ratio of Ein to Eout started climbing.

1.3 1920x1152 Reflective Spatial Light Modulators – New!

This SLM offers large format, high fill factor (high optical efficiency), high-speed (as fast as 1.4 ms), low phase ripple (.2 – 3%), high optical power handling (up to 15 GW/cm2 peak power density), and high refresh rate. This analog, high voltage backplane produces very stable phase patterns, coupled with fast liquid crystal response times.

 Our Liquid Crystal on Silicon (LCoS) Spatial Light Modulators (SLMs) are uniquely designed for pure phase applications and incorporate analog data addressing with high refresh rates. This combination provides user’s with the fastest response times and highest phase stabilities commercially available. We offer both transmissive and reflective SLMs in either one- or two dimensions. Phase-only SLMs can also be used for amplitude-only or a combination of both.

The 1920x1152 SLM is good for applications requiring high speed, high diffraction efficiency, low phase ripple and high power lasers.

Features:

Specifications:

Wavelength (nm)

Wavefront distortion

Liquid crystal response time

[standard / high speed] (ms)

AR coatings [Ravg<1%] (nm)

 

 

Model P1920

Model HSP1920

 

405

λ/3

6

N/A

400 - 850

532

λ/5

9

1.4

400 - 850

635

λ/6

12

1.8

400 - 850

785

λ/7

19

2.5

600 - 1300

1064

λ/10

25

3.3

600 - 1300

1550

λ/12

33

5.0

850 - 1650

*Silicon backplane, performance varies as a function of wavelength.

High Phase Stability - Our SLMs are known for having the highest phase stability on the market. Our backplanes are custom designed with high refresh rates and direct analog drive schemes resulting in phase ripple as low as 0.2% (0.002 π radians) for standard speed, and as low as 2% (0.02 π radians) for high-speed. Phase ripple is quantified by measuring the 1st order ripple as compared to the mean intensity while writing a repeating linear phase ramp to the SLM.

High Power Capability- Our Spatial Light Modulators have been tested for compatibility with high power pulsed and CW lasers. In the graph below, the optical response of the 1920x1152 pixel SLM was measured as the incident power was incremented up to 15 GW/cm2 peak power or 204 W/cm2 average power. A liquid cooling system is available to reduce thermal effects. Optional water cooling system maintains consistent temperature and phase stoke when using high power lasers.

1.4 1x12,288 Linear Reflective Spatial Light Modulators

The only high resolution linear array on a silicon backplane available on the market. The high refresh rate analog backplane provides excellent temporal stability. Our production process results in 100% fill factor, giving high optical efficiency.

Features

Applications

Specifications


Wavelength (nm)

Liquid crystal response time (ms), Model P1920

AR coatings [Ravg<1%] (nm)

405

N/A

N/A

532

4.5

400 - 850

635

5

400 - 850

785

8.5

600 - 1300

1064

15

600 - 1300

1550

30

850 - 1650

 

 

Model P12m288—λ(nm)--PT

Array size

19.66x19.66nm

Design wavelength (nominal)

523 – 1550nm (Specify wavelength, λ in nm when ordering

Diffraction efficiency (zero-order)

80 – 95% (max)

Duty cycle

Up to 100%

Eternal window¹

Broadband AR coated for Ravg<1.25%

(450-865nm, 600-1300nm, 850-1650nm)

Fill factor

100%

Format

1 x 12288 (12288 active pixels)

Mode

Reflective

Steering angle

± 4-7°

Modulation

Controllable index of refraction

Phase levels (resolvable)

500 linear levels (min) for 2 π phase stroke

Phase stroke (double-pass

Typically 2π at user specified laser line (up to 6π available

Pixel pitch

1.6 μm

Reflected wavefront distortion (rms)²

λ/3

Liquid crystal response time³

5-30 ms

Above specs are subject to change without notice. Please contact us for additional updates

  1. Custom AR coating options are available, including V-type for optimum optical efficiency at a single laser wavelength
  2. At nominal wavelength
  3. Phase stroke, temperature and wavelength dependent

1.5 E Series Reflective Spatial Light Modulators

The new E-series 512x512 liquid crystal on silicon (LCoS) SLM is ideally suited for labs with a limited budget or researchers who do not require the high speed features of our premium SLMs, yet still demand high performance. This entry level SLM is affordably priced without sacrificing quality.

Optically Flat: All the SLMs, including the E-Series, are designed and fabricated to be optically flat. Native flatness can be as low as λ/8. Using the SLMs wavefront correction capabilities, the compensated flatness can be better than λ /12 with simple Zernike polynomials, or flatter than λ/50 with regional calibration methods.

High Phase Stability: The E512 is designed with a backplane refresh rate of 6 kHz, and a direct analog drive scheme which provides unsurpassed phase stability.  By refreshing each pixel at rates far surpassing the response time of the liquid crystal, we are able to offer a SLM with phase ripple as low as 0.20%.

16-bit DVI Controller: 16-bit images can be transferred across the DVI interface at a rate supported by the graphics card used (60–200Hz).  With 16-bits of analog voltage resolution, the SLM can be used to easily obtain more than 1000 linear resolvable phase levels. This λ/1000 phase resolution can be maintained over a broad wavelength range by tuning the look-up-tables / calibrations for your incident wavelength.

KEY FEATURES

Entry-level

 

Entry level

E512-λ-DVI

High efficiency

-PDM512

-HSPDM512

-ODPDM512

High Speed

-HSP512L

-HSP512

-HSPDM512

-ODP512

-ODPDM512

High resolution

-P1920

Pixel format

512x512

512x512

512x512

1920x1152

Pixel pitch (um)

15

15

15 or 25

9.2

Wavelength (nm)

405

532

635

1064

1550

405-1550

488-1550

405-1550

Liquid crystal response time (ms)

25.0

33.3

33.3

66.7

100

4-130

1.2-28.5

6-33

Zero order diffraction efficiency (%)

Up to 61

Up to 95

Up to 95

Up to 84

Phase stroke

≥ 3π radians

≥ 3π radians

≥ 3π radians

≥ 3π radians

Controller

DVI

DVI, PCIe 8-bit, PCIe 16-bit

PCIe 8-bit

HDMI

Array size (mm²)

7.68x7.68

7.68x7.68

7.68x7.68 or

12.8x12.8

17.6x10.7

Fill factor (%)

83.4

96 - 100

83.4 - 100

96

2. Transmissive Spatial Light Modulators

All of our liquid crystal on glass (LCoG) SLMs enable simple optical systems when low pixel counts are sufficient. Users can select single-mask or configurations for phase or amplitude modulation, or a dual-mask configuration for combined phase and amplitude modulation.

2.1 HEX-127 Spatial Light Modulator

Our two dimensional SLMs are designed for adaptive optics applications. A two dimensional array of Liquid Crystal Variable Retarders acts as a real time programmable phase mask for wavefront correction of a linear polarized source. Unwanted aberration effects are removed by introducing the opposite phase shift through the Hex SLM. The most common applications involve high-resolution imaging where viewing through an aberrant medium is unavoidable. Examples include astronomical imaging with ground-based telescopes and medical imaging through bodily fluids. High-energy laser users also benefit from active phase compensation for beam profile correction.

2.2 1x128 Linear Array Spatial Light Modulator

The linear SLM has a linear pixel array geometry. This system can be used to alter the temporal profile of femtosecond light pulses via computer control. Applications requiring these short pulses include analysis and quantum control of chemical events, optical communication and biomedical imaging. This linear SLM offers high fill factor, good transmitted wavefront distortion, and options for single or dual-plane for modulating phase, amplitude, or both simultaneously. These SLMs find use in other applications including Hadamard spectroscopy, optical data storage and wavefront compensation.

Pixel format

Response time

Pixel pitch

Efficiency

Fill factor

Active area (mm)

1x128

35 – 70 ms

100 um

85 – 92%

98.0%

12.80 x 5.00

Hex

1  mm

》90%

 

93.1

12.00Ø

2.3 Spatial Light Modulator Controller

Our spatial light modulator controller allows for independent voltage control of up to 128 liquid crystal cells or pixels. The SLM Controller connects via USB cable to a Windows™ based computer. Supplied software allows for convenient setting of inpidual pixel retardance and for the programming of retardance profiles across a pixelated device. Custom software can be written using the included LabVIEW™ Virtual Instrument Library to allow for integration into custom applications.

Key Features

Optical head specifications

Retarder material

Nematic liquid crystal

Substrate material

Optically quality synthetic fused silica

Center wavelength

450-1800nm (specify)

Modulation range

Phase (min) amplitude

1λ optical path difference 0-100%

Retardance uniformity

<2%rms variation over clear aperture

Transmitted wavefront distortion

≤ λ/4 (P-V @ 633)

[≤ λ/10 (RMS @ 633)]

Surface quality

40-20 scratch-dig

Beam deviation

< 2 arc min

Transmittance

> 90% (without polarizers)

Reflectance (per surface)

≤ 0.5% at nominal incidence

Dimension

7.00 x 2.96 x 0.74 in

Recommended safe operating limit

500W/cm², CW

300mJ/cm², 10ns, 532nm

Temperature range

10 - 45 °C

 

Controller specifications

Output voltage

2kHz ac square wave digitally adjustable

0-10 Vrms

Voltage resolution

2.44mV (12 bit)

Computer interface

USB

Power requirements

100 – 240VAC @ 47-63Hz, 1A

Dimensions

9.50 x 6.25 x 1.50 in

Weight

2 lbs.

Note that the D31258 in included with the purchase of the SLM system

 

Ordering information

Name

Pixel geometry

Version

Part number

1 x 128

98 μm x 4 mm linear

Phase

SSP – 128P - λ

Amplitude

SSP – 128A - λ

Hexagonal 127

1 mm across flat

Phase

SSP – 127P - λ

Amplitude

SSP – 127A - λ

Please specify your operating wavelength λ in nm when ordering. Custom SLM sizes and formats are available

 

Optional polarizers

Type

Wavelength range (nm)

Part number

Visible

450 - 700

SDP – VIS

Near infrared 1

775 – 890

SDP – IR1

3. Optics Kit

Includes optics & mounts for simple phase or amplitude experiments. Available pre-aligned and ready to use over 405 - 1550 nm. Available with optional camera and laser.

Spend your time on important research rather than designing an optical system for your SLM.  The SLM Optics Kit provides you with a set of optics and cage-mount components enabling the user to start research with the SLM system immediately.  The kit includes a Half-Wave Retarder, a pair of Linear Polarizers, lenses, and all necessary mount hardware, including a custom adapter plate to quickly align the SLM system to the optics in an off-axis configuration.  Optional items are also available including a laser, beam expander optics, and a camera.  This approach provides optimum efficiency with minimal design effort.

Optics Kit includes:

4. 1-Photon SLM Microscopy Kit

The 1-Photon SLM Microscopy Kit is a scan-less SLM-based epi-fluorescence upright microscope that enables three dimensional calcium imaging and/or photoactivation of neurons in brain slices. The microscope can be used to excite and monitor activity of neuronal ensembles, enabling studies of neuronal circuit activity both in vitro and in vivo. Add-on to existing microscope or use as stand-alone microscope.

KEY FEATURES

 

5. Optical Tweezers Cube

Our cube provides researchers with a portable, stand-alone, optical tweezers system just one cubic foot in size. This compact instrument allows a user to optically trap and thus physically manipulate hundreds of microscopic objects in three dimensions (3D) using computer control to set and move each optical trap independently.

Optical trapping can be used to manipulate objects ranging in size from 10’s of nanometers to 10’s of microns and objects with a variety of material characteristics. Trapping examples include cellular organisms, dielectric spheres, metallic spheres, metallic nanoshells, carbon nanotubes, air bubbles, and even water droplets in air.

One application of the CUBE includes biological research. This tool enables measurements of cell properties and controlled studies of how cells interact with foreign objects. Another application example is trapping metallic objects and carbon nanotubes for engineering materials with unique thermal and electrical properties.

KEY FEATURES

Application Notes: Spatial Light Modulators

3D Mapping of Neural Circuits In Vivo Opens the Window on Neurological Disease

Modifications with SLMs to existing two-photon microscopes can provide noninvasive probes deep within the cortex.

Despite extensive research, brain function and neurological diseases are poorly understood. Complexities arise from the quantity of neurons in the brain and from the densely interconnected networks of intermixed cell types. Tools neuroscientists have traditionally relied upon include the patch clamp, which probes electrical activity of a single neuron, and fMRI, which images activity in volumes containing millions of neurons.

These approaches target two vastly different scales. However, it is possible that the brain functions through firing patterns in neural circuits and that neurological disease is the result of alterations to the physical structure of circuits or circuit dynamics. These circuits exist at an intermediate scale that neither patch clamp nor fMRI can readily address. In order to give neuroscientists a range of tools to study brain function, there is a need for methods that noninvasively probe the underlying microcircuitry in the brain with single-cell resolution.

Figure 1. By manipulating the wavefront of a single incident beam, the spatial light modulator (SLM) can be used to superimpose lens and grating functions with weighting functions to redirect light to arbitrary locations to simultaneously create hundreds of focal points within a 3D volume. Courtesy of Meadowlark Optics.

Over the last decade, calcium imaging and photoactivation have emerged as solutions to this problem, providing all-optical means to monitor and manipulate circuit activity1. Calcium imaging uses calcium indicators that bind with calcium to alter the fluorescence characteristics of neurons. When a neuron fires, there is an uptake of calcium into the cell body. If the firing neuron is illuminated with an excitation source during the firing event, then the fluorescence emission increases, generating an optical response that corresponds to electrical activity.

Complementary to calcium imaging is photoactivation, which can use photosensitive proteins (optogenetics) or opto-chemical (caged) compounds to manipulate firing patterns either by causing neurons to fire or by silencing neurons. This combination of calcium imaging and photoactivation offers a means for neuroscientists to record the spatiotemporal dynamics of activity and map physical structure of circuits with single-cell resolution. However, without advanced microscopes for neuroscience, the benefits of calcium imaging and photoactivation cannot be realized.

Confocal microscopes have become a core technology for biology, but have fundamental limitations that hinder their use for neuroscience. The first is slow temporal resolution from raster scanning a laser through the sample to build an image pixel by pixel. Without the ability to parallelize excitation to arbitrary locations within a 3D volume, it is impossible to monitor firing patterns of multiple cells simultaneously. This is critical for mapping connectivity of neural circuits and understanding circuit dynamics.

The second limitation is two-dimensional imaging, which is inappropriate for studies of neural circuits. This restricts studies to a small subset of the neurons and limits the scope of the circuits that neuroscientists are trying to map and understand. The third limitation is confocal microscopy’s coupling of one-photon excitation with a pinhole to block out-of-focus fluorescence emission. This results in low signal from trivially low depths in strongly scattering and absorbing samples, such as neurons within the cortex.

Two-photon microscopy provides submicron lateral and axial excitation confinement without requiring a pinhole, and the longer wavelength simultaneously minimizes scattering. When coupled with spatial light modulators (SLMs), two-photon microscopes are capable of parallelized excitation for photoactivation and volumetric imaging. SLMs can come in a variety of forms, including micromirror arrays and liquid-crystal (LC)-on-silicon modulators.

In a two-photon microscope, the micromirror array is imaged to the sample so that pixels turned on reflect light to neurons for excitation, and pixels turned off reflect light to a block. This allows a simple method to illuminate cell bodies. Micromirror arrays also offer response times on the order of 20 kHz, far surpassing the current response time requirements of neuroscience. However, because the micromirror array is an amplitude modulator as opposed to a phase modulator, it is not possible to generate lens functions for probing activity in a 3D volume or to actively redirect light from pixels that are turned off to desired focal point locations in the sample.

These limitations are overcome through use of LC-SLMs in microscopes. The SLM acts as a programmable lens manipulating the wavefront of the excitation source. In its simplest form, the SLM can be used as a programmable prism, redirecting light to a single focal point with a lateral shift. By adding prism functions together, the SLM can be used to create multiple focal points within a 2D plane. Furthermore, by adding weighting functions and lens functions, the SLM can redirect light to hundreds of focal points with a programmable intensity in a 3D volume (Figure 1).

In two-photon microscopes, LC-SLMs enable multisite 3D scanless excitation for photoactivation2,3,4,5,6, as well as high-speed volumetric imaging to record a volume of circuit activity7. This combination provides neuroscientists with a toolbox for in vivo studies deep within the cortex to better understand the physical structure of neural circuits, the relationship of firing patterns, external stimuli and the resulting behavior, and how these processes are altered in the presence of neurological disease.

3D photoactivation

Traditional two-photon microscopes contain galvanometer-scanning mirrors used to raster scan the laser focus through the sample. The mirrors are conjugate to the back focal plane of the objective. The SLM is added to the system through an additional relay prior to the galvanometer scanning mirrors (Figure 2). The addition of the SLM and two lenses transforms the function of the microscope so that it can deliver light to any location in the field of view and simultaneously excite multiple 3D sites and use a fast camera to capture their responses.

Figure 2. Optical layout of a two-photon microscope with an SLM to enable 3D photoactivation (a). Traditional scanning is used to map the locations of neurons in the sample (b, top). After the cell bodies have been found, specific cells in the field of view can be targeted using the SLM (b, middle). As the cells are excited, the response of the cell bodies can be recorded to map connectivity and record circuit dynamics (b, bottom). Courtesy of A. Packer, L. Russell, H. Dalgleish and M. Hausser, University College London.

In a typical experiment, the galvanometer mirrors raster scan the sample to find the location of cell bodies in the field of view. Holograms then are generated to modulate the wavefront of the source to illuminate inpidual neurons. This can be used to photoactivate specific cells to replicate firing patterns that have been identified or to manipulate firing patterns that have been observed. Following photoactivation, the response of the surrounding cells can be monitored to understand the impact of photoactivation on the response of the circuit.

When designing the microscope, there are several key criteria that should be considered. The resolution of the SLM determines the number of locations where light can be directed in the sample. The resolution and pixel pitch together determine the dimensions of the volume within the sample that the SLM can excite9. Ideally, the SLM will have a small pixel pitch with high resolution so it can steer to wide angles without under-filling the objective and sacrificing the lateral and axial excitation confinement.

The temporal phase stability of the SLM also is important to ensure reliable excitation. This is particularly important when piding the light among many neurons and operating near the minimum threshold for excitation. Finally, the response time of the SLM will have significant impact on replicating the spatiotemporal dynamics, which can occur at rates up to 1 kHz110,11.

Volumetric imaging

The ability to manipulate firing patterns is critical to understanding circuit activity, but equally important is the ability to record the response of surrounding neurons at the highest possible frame rate. Traditional two-photon imaging systems build an image volume by mechanically scanning the objective and collecting 2D images (Figure 3). The time required to image the volume can be on the scale of minutes, which is sufficient for static samples. In neuroscience, the dwell time requirement coupled with indicators with limited brightness results in the inability of traditional two-photon imaging to monitor action potentials in complete neural circuits. This opens up the possibility of misinterpretation of action potentials because of the interaction of localized excitation with animal movement.

Figure 3. Comparison of Gaussian and Bessel imaging of a mouse dendritic spine (left). Scanning of a Gaussian focus coupled with dwell time requirements for fluorescence excitation leaves a small portion of the sample illuminated and an increased likelihood of activity occurring without fluorescence excitation. Bessel imaging monitors a volume at the same rate of 2D imaging with Gaussian illumination. The “Bessel module” easily integrates with existing 2P microscopes without software changes, enabling easy adaptation of existing microscopes and significantly enhanced capability (middle). A demonstration of the Bessel module used for imaging inhibitory neurons in a mouse. With Gaussian imaging, a series of 2D scans are required to build the 3D projection, but the Bessel module enables imaging the entire volume without axial scanning (right). Courtesy of Na Ji, Janelia Research Campus.

One solution for high-speed volumetric imaging, presented by Na Ji, group leader at the Janelia Research Campus of the Howard Hughes Medical Institute, uses a Bessel focus-scanning technology (BEST) that samples activity in a volume with hundreds of microns in each dimension in the equivalent time that a Gaussian two-photon microscope images a 2D plane6.

The module for 3D imaging is simple and widely compatible with existing microscopes, consisting of an SLM, a static amplitude mask and three lenses (Figure 3). The lenses relay the image of the SLM to the sample. The amplitude mask is a static patterned mirror that selectively transmits the first diffracted order. The optional flip mirrors at the entrance and exit of the module allow optical addition or removal of volumetric imaging so that structures can be imaged with traditional Gaussian illumination if desired. The use of SLMs here allows flexible generation of Bessel foci of varying lateral sizes, axial lengths and axial intensity distribution, permitting users to optimize BEST for specific samples.

Ji has demonstrated the approach for enabling discoveries for neurobiology by imaging the calcium dynamics of volumes of neurons and synapses in fruit flies, zebrafish larvae, mice and ferrets in vivo. Calcium signals in objects as small as dendritic spines could be resolved at video rates. High-speed volumetric imaging is a critical advancement for microscopes adapted specifically to the needs of the neuroscience community.

The combination of SLMs, two-photon microscopy, calcium imaging and photoactivation is leading to advanced tools for neuroscientists to monitor and manipulate the activity of neural circuits in the brain. The methods require minor modifications to existing microscopes, allowing researchers to inexpensively and readily adapt existing tools to support 3D photoactivation with high-speed volumetric imaging. This significantly enhances capabilities of microscopes, providing a complete tool enabling studies of neural circuits, expanding the field of view, the depth and the temporal limits at which neuroscientists can monitor and manipulate circuit activity.

 Polarizser,  Waveplate Retarder,  Liquid Crystal Device,  Spatial Light ModulatorApplications of SLM

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