Two-dimensional Spatial Light Modulator Research Articles

A full degree-of-freedom spatiotemporal light modulator

Harnessing the full complexity of optical fields requires the complete control of all degrees of freedom within a region of space and time—an open goal for present-day spatial light modulators, active metasurfaces and optical phased arrays. Here, we resolve this challenge with a programmable photonic crystal cavity array enabled by four key advances: (1) near-unity vertical coupling to high-finesse microcavities through inverse design; (2) scalable fabrication by optimized 300 mm full-wafer processing; (3) picometre-precision resonance alignment using automated, closed-loop ‘holographic trimming’; and (4) out-of-plane cavity control via a high-speed μLED array. Combining each, we demonstrate the near-complete spatiotemporal control of a 64 resonator, two-dimensional spatial light modulator with nanosecond- and femtojoule-order switching. Simultaneously operating wavelength-scale modes near the space–bandwidth and time–bandwidth limits, this work opens a new regime of programmability at the fundamental limits of multimode optical control. Panuski et al. demonstrate a programmable photonic crystal cavity array, enabling the spatiotemporal control of a 64 resonator, two-dimensional spatial light modulator with nanosecond- and femtojoule-order switching.

Vectorial sculpturing of spatiotemporal wavepackets

Spatiotemporal vectorial pulse shaping can produce ultrafast optical pulses that enable unprecedented coherent control for light–matter interactions. A traditional pulse shaper produces ultrafast pulses with scalar programmable waveforms for various quantum control applications. However, quantum systems are three dimensional in nature; thus, the interactions are inevitability vectorial. Existing polarization pulse shaping techniques, which are often difficult to align and cumbersome to handle, can only produce dynamic polarization modulation in the temporal domain. Through simply introducing a quarter-wave plate in a pulse shaper using a two-dimensional spatial light modulator, we show that wavepackets with much more sophisticated spatiotemporal vectorial structures such as spatiotemporal spin grating, spatiotemporal spin lattice, and spatiotemporally twisting polarization can be generated, significantly expanding our ability in coherently controlling light–matter interactions that may find broad applications.

Extended source of indistinguishable polarization-entangled photons over wide angles of emission

The generation of high-fidelity polarization-entangled photon pairs, to date, has been demonstrated on specific spatial modes or over relatively narrow apertures. We put forward and demonstrate an experimental scheme to extend the temporal and spatial indistinguishability of polarization-entangled photons over wide emission angles, which can be applied to cover the whole spontaneous parametric downconversion (SPDC) cone. Over such a wide angular extent, while the time-delay map is almost flat which renders the conventional compensation via a birefringent element an appropriate approach, the relative-phase map—verified as a quadratic function—necessitates a tunable compensation paradigm. Here, to do so, we employ a phase-only two-dimensional spatial light modulator (2D SLM) loaded by the complementary of the relative phase map to equalize the phase variations for one third of the noncollinear SPDC emission. After eliminating the temporal and spatial distinguishability over the 2D SLM area, a 97% polarization visibility is verified for the entangled photon pairs scattered widely across the SPDC cone.

Ultrafast optical control of multiple coherent phonons in silicon carbide using a pulse-shaping technique

We demonstrated ultrafast optical control of multiple coherent phonons in silicon carbide (SiC) using a pulse-shaping technique combined with sub-10-fs optical pulses and a two-dimensional spatial light modulator. Because our technique produces a pulse train with the desired number of pulses and time intervals, precise manipulation of the amplitudes of the zone-folded optical and acoustic phonon modes could be achieved simultaneously and was well reproduced by a model calculation assuming the two modes are excited independently by each pulse. These results show that our scheme can provide high controllability of multiple phonon modes in SiC.

Three-dimensional weight-accumulation algorithm for generating multiple excitation spots in fast optical stimulation

We report here an algorithm for calculating a hologram to be employed in a high-access speed microscope for observing sensory-driven synaptic activity across all inputs to single living neurons in an intact cerebral cortex. The system is based on holographic multi-beam generation using a two-dimensional phase-only spatial light modulator to excite multiple locations in three dimensions with a single hologram. The hologram was calculated with a three-dimensional weighted iterative Fourier transform method using the Ewald sphere restriction to increase the calculation speed. Our algorithm achieved good uniformity of three dimensionally generated excitation spots; the standard deviation of the spot intensities was reduced by a factor of two compared with a conventional algorithm.

Application of Waveguide/Free-Space Optics Hybrid to ROADM Device

As optical networks have evolved from point-to-point systems to ring or mesh networks, the optical devices that are needed to construct optical nodes have become more important and need to be more scalable. Hybridization of waveguide and free-space optics, or spatial and planar optical circuits (SPOCs), may provide the solutions for such needs. An SPOC platform is attractive because it can take advantage of both waveguide technology and free-space optics. Waveguide technology provides a high degree of integration of optical functionality for devices such as splitters and non-wavelength selective switches while free-space optics supplies a high degree of parallelism with two-dimensional spatial light modulators such as a liquid crystal on silicon (LCOS) devices. In this paper, we summarize the basics of SPOC technology and review its application to reconfigurable optical add-drop multiplexing (ROADM) devices. The key elements of a waveguide on an SPOC platform are an arrayed-waveguide grating and a spatial beam transformer. The latter functions as a microlens array and provides attractive features such as dense integration of switches. An LCOS device has numerous phase modulating pixels, enabling flexible manipulation of lightwaves. Using an SPOC platform, we constructed and demonstrated devices for ROADM applications including a wavelength filter, tunable optical dispersion compensators, and wavelength selective switches (WSSs). The WSSs range from an ultrahigh port count WSS to a single module wavelength cross connect.

Frequency-encoded multiplexed CARS microscopy by rapid pulse shaping

A new coherent anti-Stokes Raman spectroscopy (CARS) technique is reported for real-time detection and classification of several chemical constituents, utilizing a single detector and a single beam of shaped femtosecond pulses. The technique is based on rapidly switching between differently shaped pulses that either maximize or minimize the targeted vibrational lines excitation, thus creating temporally modulated ‘bright’ and ‘dark’ profiles in the total CARS signal that are measured by a single photomultiplier tube and demodulated by a multi-channel lock-in amplifier. Using a two-dimensional spatial light modulator displaying 24 different pulse shapes, we demonstrate pulse shaping at 80 kHz and chemically specific microscopy with pixel dwell times of less than 0.5 ms.

Ultrafast, large-field multiphoton microscopy based on an acousto-optic deflector and a spatial light modulator

We present an ultrafast, large-field multiphoton excitation fluorescence microscope with high lateral and axial resolutions based on a two-dimensional (2-D) acousto-optical deflector (AOD) scanner and spatial light modulator (SLM). When a phase-only SLM is used to shape the near-infrared light from a mode-locked titanium:sapphire laser into a multifocus array including the 0-order beam, a 136 μm × 136 μm field of view is achieved with a 60× objective using a 2-D AOD scanner without any mechanical scan element. The two-photon fluorescence image of a neuronal network that was obtained using this system demonstrates that our microscopy permits observation of dynamic biological events in a large field with high-temporal and -spatial resolution.

Note: Self-characterizing ultrafast pulse shaper for rapid pulse switching

We use a high-efficiency acousto-optic modulator at the input to a two-dimensional Fourier-domain pulse shaper to achieve built-in characterization of the shaped output pulses. The acousto-optic modulator directs the beam to different vertical positions on a two-dimensional spatial light modulator, each of which can contain a different pulse shape. The undiffracted portion of the light serves as a reference beam for characterizing the shaped pulse via spectral interferometry. Pulse switching rates of 100 kHz can be achieved, making the device especially useful for quantum-control spectroscopy.

Advanced matrix laser lithography for fabrication of photonic micro-structures

Direct laser beam writing techniques represent an interesting alternative to focused electron and ion beam writing approaches for the fabrication of photonic micro-structures. Although the resolution of laser beam writers is strongly limited by the Rayleigh diffraction limit, it is not always necessary to create features significantly smaller than the recording wavelength. In such cases, a laser exposure can be used, with all its advantages. In this paper a direct laser writing technique is presented one; which significantly improves the performance of commonly used direct laser writers by introducing an exposure of a large area within a single step. The elementary exposed area can contain a general micro-structure without any special requirements. In contrast to writing techniques based on a single spot focus, the writing beam can have the shape of a general aperiodic structure and exposes an area of about 10−2 square millimetre. This complicated exposure field is created using a demagnified projection of the exposed structure from a two-dimensional computer-driven spatial light modulator. The main advantages of the method compares to the single point exposure are a significantly improved exposure speed, a perfect alignment of features within a single exposed area, and the possibility of tuning the exposing field very precisely using continuous modulation of the signal on the spatial light modulator. The in-house-developed device is presented together with several realized samples.

In situ tomography of femtosecond optical beams with a holographic knife-edge

We present an in situ beam characterization technique to analyze femtosecond optical beams in a folded version of a 2f-2f setup. This technique makes use of a two-dimensional spatial light modulator (SLM) to holographically redirect radiation between different diffraction orders. This manipulation of light between diffraction orders is carried out locally within the beam. Because SLMs can withstand intensities of up to I ∼ 10(11) W/cm2, this makes them suitable for amplified femtosecond radiation. The flexibility of the SLM was demonstrated by producing a diverse assortment of "soft apertures" that are mechanically difficult or impossible to reproduce. We test our method by holographically knife-edging and tomographically reconstructing both continuous wave and broadband radiation in transverse optical modes.

Plasmonic all-optical bistable device based on nematic liquid crystal

Low-power all-optical bistable liquid crystal (LC) devices are reported in which surface plasmons propagate along a metallic grating. The threshold switching illumination was 1.4 mW/mm 2 in the device with the twisted-nematic LC cell. This is a lower value than for all-optical bistable devices reported previously. This low-power bistability device can be used in two-dimensional optical memories or spatial light modulators.

Analysis of fabricating arbitrary nanoscale patterns by LSPP direct writing lithography with two-dimensional metal hole-array

We propose to use two-dimensional metal hole-array (TMH) and spatial light modulator (SLM) technology to construct a parallel direct-writing system. SLM and movable platform are used to control the system for realizing multi-beam parallel scanning exposure to fabricate arbitrary patterns. In this system TMH is the key component which focuses the incident light beams into light spots in the photoresist by exciting local surface plasmon polariton (LSPP). Parameters of TMH are optimized to improve the focusing efficiency and transmission depth of the light spots. Theoretical analysis and numerical simulations show that the feature size and transmission depth can reach sub-80nm and 90nm, respectively, by an optimized TMH, which is feasible for parallel direct writing lithography.

Two-dimensional phase-only spatial light modulators for dynamic phase and amplitude pulse shaping

We consider a two-dimensional liquid crystal spatial light modulator (SLM) for femtosecond pulse shaping. Novel shaping schemes enabling a drastic speed increase (about four orders of magnitude as compared with conventional liquid-crystal SLM-based pulse shapers) and complex (phase and amplitude) femtosecond pulse shaping are discussed and experimentally demonstrated. In the first case, while a horizontal resolution of 1920 addressable pixels provided superior fidelity for generating complex waveforms, scanning across the vertical dimension (1080 pixels) has been used to facilitate an update rate in excess of 100 kHz. In the second case, we use the pixel count redundancy in the vertical direction and encode a spectrally-dependent diffraction grating for modulation of both spectral phases and amplitudes.

Spatiotemporal vector pulse shaping of femtosecond laser pulses with a multi-pass two-dimensional spatial light modulator

A novel non-interferometric vector pulse-shaping scheme is developed for femtosecond laser pulses using a two-dimensional spatial light modulator (2D-SLM). By utilizing spatiotemporal pulse shaping obtainable by the 2D-SLM, we demonstrate spatiotemporal vector pulse shaping for the first time.

Spatial light modulator of 648 pixels with liquid crystal transparent from ultraviolet to near-infrared and its chirp compensation application

We fabricated a 648 pixel, two-dimensional spatial light modulator (SLM) with a transmittance of more than 85% in the wavelength region from 260 to 1,100 nm. The phase modulation characteristics of the liquid-crystal SLM were clarified. Furthermore, the SLM enabled us to compensate for chirp of 270 fs UV pulses and chirp of 120 fs near-IR pulses so that the former and latter pulse durations, respectively, were shortened to 35 and 25 fs. This suggests that the SLM can be utilized, in the UV to near-IR region, for the generation of monocycle optical pulses, pulse shaping, and other applications.

Colloquium: Optimal control of high-harmonic generation

High-harmonic generation provides an attractive light source of coherent radiation in the extreme-ultraviolet (XUV) and soft-x-ray regions of the spectrum and allows for the production of single attosecond pulses or pulse trains. This Colloquium covers the control of high-harmonic spectra by temporal and spatial pulse shaping of the driving laser pulses and its implications on time-resolved XUV spectroscopy and attosecond pulse shaping. It summarizes important steps for extending existing pulse shaping techniques and control schemes from the near-infrared or visible part to shorter wavelengths. Using adaptive pulse shaping of the driving laser pulses, several groups have demonstrated control of the high-harmonic spectrum, including the author's work on the complete control over the XUV spectrum of high-order harmonics, generated in a gas-filled hollow fiber. It is possible to achieve both the enhancement and the suppression of single or several selected harmonic orders. These arbitrarily shaped soft-x-ray spectra will allow for important modifications of the resulting harmonic pulses in the temporal domain. This constitutes first steps towards direct attosecond pulse shaping in the soft-x-ray domain. Moreover, high-harmonic generation in a hollow-core fiber can be enhanced by coupling into a single fiber mode using a feedback-controlled adaptive two-dimensional spatial light modulator.

Phase and amplitude pulse shaping with two-dimensional phase-only spatial light modulators

We consider a programmable, phase, and amplitude femtosecond pulse shaper based on a two-dimensional (2D) reflective liquid-crystal (LC) spatial light modulator (SLM). A new zero-order pulse shaping scheme is introduced and compared to the first-order scheme, both theoretically and experimentally, using liquid crystal on silicon 2D SLM. While the spectral components of the pulse are spread across the horizontal dimension, we use the vertical direction for modulation of both spectral phases and amplitudes. It was found that while zero-order approach provided better light efficiency (67% versus 43%), the first-order scheme has superior dynamic range of amplitude modulation.

Optofluidic maskless lithography system for real-time synthesis of photopolymerized microstructures in microfluidic channels

The authors propose an optofluidic maskless lithography technique that can dynamically synthesize free-floating polymeric microstructures inside microfluidic channels by selectively polymerizing photocurable resin with high-speed two-dimensional spatial light modulators. The combination of programable optical projection and microfluidic devices allows one to precisely control the timing and location of the photopolymerization process for microstructure fabrication. Real-time generation of microparticles with various shapes, sizes, ordering, and material contents are experimentally demonstrated. Long polymeric structures of which size is not limited by the exposure field of view can also be fabricated.

Spatial optimization of filaments

We demonstrate the spatial homogenization of intense laser pulses by adaptive minimization of spatial chirp of the spectrally broadened output pulses of a filament. A liquid-crystal-based two-dimensional spatial light modulator is used to control the spatial phase of the driver pulse. An evolutionary algorithm finds the optimal spatial laser phase distribution that introduces minimal distortions during filamentation and enhances the beam quality of the output pulse. A homogeneous intensity distribution favours efficient temporal compression close to the bandwidth limit without the need for spatial filtering after the filament.