KEYWORDS: Light sources and illumination, Diffuse optical tomography, Biological imaging, 3D modeling, Time of flight imaging, Point spread functions, Mathematical optimization, Data hiding, Stereoscopy, Optical transmission
Imaging through highly diffusive media is challenging because of the extensive spreading of light propagation in both time and space. The most advanced technique utilizes an expensive time-of-flight imaging system. Here, we present a simple and efficient approach for computational diffuse optical tomography. A typical CMOS camera captures the transmission light through an object buried between two thick diffusive media. Multiple illumination points provide more information, allowing reconstruction of the hidden object at a higher quality than computational time-of-flight diffuse optical tomography. The demonstration shows a low-cost diffuse optical tomography with high accuracy and low computational complexity.
Applying super-resolution imaging techniques to recover object behind scattering media can obtain more information. Our previous work stochastic optical scattering localization imaging breaks the diffraction-limit via object blinking. Here, we proposed a more practical method using speckle fluctuation to achieve super-resolution imaging. The speckle fluctuation in the illumination part cause object fluctuating, resulting in a series of speckle fluctuation frames in the camera. By analyzing the high order cumulants of deconvolution frames, not only the noise artifacts are suppressed but also resolution enhanced by a factor of square root of N , where N is the cumulant order.
Optical barcodes have demonstrated a great potential in multiplexed bioassays and cell tracking for their distinctive spectral fingerprints. The vast majority of optical barcodes were designed to identify a specific target by fluorescence emission spectra, without being able to characterize dynamic changes in response to analytes through time. To overcome these limitations, the concept of the bioresponsive dynamic photonic barcode was proposed by exploiting interfacial energy transfer between a microdroplet cavity and binding molecules. Whispering-gallery modes resulting from cavity-enhanced energy transfer were therefore converted into photonic barcodes to identify binding activities, in which more than trillions of distinctive barcodes could be generated by a single droplet. Dynamic spectral barcoding was achieved by a significant improvement in terms of signal-to-noise ratio upon binding to target molecules. Theoretical studies and experiments were conducted to elucidate the effect of different cavity sizes and analyte concentrations. Time-resolved fluorescence lifetime was implemented to investigate the role of radiative and non-radiative energy transfer. Finally, microdroplet photonic barcodes were employed in biodetection to exhibit great potential in fulfilling biomedical applications.
Here we report X- and gamma-ray scintillation properties from solution-processable perovskite (SPP) halide single crystals and quantum dots. We tune the properties of single crystals by replacing the cation, changing the organic ammonium cation spacer, and varying the halide anion. For quantum dots, we tune the properties by changing the halide ion composition while we also try to replace the lead with the bismuth. Finally, we summarize the advantages and disadvantages of SPP single crystals and quantum dots to pave the way the research for the new high light yield scintillators.
Scattering media scramble light paths, create seemingly random speckle patterns and hinder even our simple visualization of objects. Here, we demonstrate stochastic optical scattering localization imaging (SOSLI) to achieve super-resolution non-invasively through not only static, but also dynamic scattering media (up to 80% decorrelation). A camera captures multiple speckle patterns created by stochastic emitters in the object. Then our computational approach can retrieve a super-resolution image of hidden objects, surpassing the diffraction limit by factor of five, while posing no fundamental-limit in achieving higher spatial-resolution. Our demonstration paves the way to non-invasively visualize various biological samples with unprecedented levels of detail.
Due to the large exciton binding energy, two-dimensional perovskite has demonstrated the potential as high-performance while low-cost scintillator. In our experiment, first we systemically investigate the effect of Li-ion dopant in phenethylammonium lead bromide, (PEA)2PbBr4 perovskite crystals under soft X-ray radiation of 15 keV. Successful inclusion of Li at four doping concentrations was confirmed by X-ray photoelectron spectroscopy. Li doping causes no substantial change in the crystal structure judging from the X-ray diffraction pattern but induces stronger emission tail as observed in the temperature-dependent X-ray luminescence (XL). Upon higher Li concentration, the emissions become broader due to possible Li trap emission as indicated by increasing traps induced by more Li in the X-ray thermoluminescence spectra. The behavior of negative thermal quenching is found in the XL and it can yield a benefit such as the possible light yield improvement in the X-ray imaging application. After the soft X-ray characterizations, we further explore our crystals in gamma-ray detection. In the gamma-ray pulse height measurement, relatively broad peaks can be resolved with the light yield of about 10,000 photons/MeV at 662 keV. The result from alpha particle pulse height measurement also indicates that we could even utilize our crystals in alpha particle detection at 5.8 MeV. Based on this feature and Li-ion capability as dopants, our crystals promise a good performance in thermal neutron detection. Finally, we can realize a versatile radiation detector that works in broad range of energy from soft to high energy radiation.
We present our recent research results on luminescence of halide perovskites under various excitation sources: photoexcitation, electrical current and high-energy radiation. Our photonics crystal can reduce the emission rate of perovskite film by preventing the photoluminescence coupling to the film, but enhance the out-coupling efficiency by 23.5 times. Our solution-grown single-crystal perovskite hetero-structure was successful with different halide compositions. A pristine single-crystal light emitting device was demonstrated with excellent protection and encapsulation from material synthesis to device characterization. Lastly, with engineered perovskite materials we demonstrate a multifunctional scintillator for high-energy radiation from X-rays, gamma rays to thermal neutrons.
KEYWORDS: Speckle pattern, Scattering media, 3D image processing, Scattering, Point spread functions, 3D displays, 3D vision, Cameras, Deconvolution, 3D-TOF imaging
In this paper, we present a single-shot multiview imaging technique for three-dimensional objects by utilizing the nature randomness of scattering media. The uncorrelated point spread functions of different scattering regions help to de-multiplex multiple elemental images at different viewing angles by deconvolution from only a single speckle pattern. Our demonstration shows that not only stereo capture with large disparity, but also, up to 7 viewing angles of a 3D object can be reconstructed with just a single shot, if corresponding PSFs are premeasured once. The elemental images are consistent with 3D object projection and images taken by multi-shot imaging.
Here we present two techniques, which have advantages in the perovskite single crystal devices. First, we demonstrate modulation-doped layer growth and double heterostructure using a millimeter-sized hybrid halide perovskite crystal as a substrate. We show that previously known limiting factor of halide ion inter-diffusion can be constrained to few microns by (1) using low halide composition gradient and (2) adjusting solution concentrations just above the critical super saturation. In the solvo-thermal growth process, our layer growth time could be conveniently extended as necessary to grow a uniform layer, with only ~5 µm inter-diffusion region. This is a significant improvement compared to few seconds dipping time previously reported for a rapid ion exchange process without any layer growth. The growth of CH3NH3PbBr3 layer on top of CH3NH3Pb(Br0.85Cl0.15)3 bulk substrate is studied for different growth times to obtain up to 30µm layer thickness. Ion diffusion profile, layer thickness and crystallographic orientation have been characterized by cross sectional characterization using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Electron Back-Scattering Diffraction (EBSD) . With this advancement, we are able to grow two consecutive perovskite layers to create a double heterostructure for the first time. Second, we demonstrate an as-grown milliliter-sized perovskite bulk crystal light emitting device. This device can be easily lighten up at low voltage (6-20 V) and at slightly low temperatures than room temperature (160-230 K). We are aiming to integrate both technologies with further optimization to produce efficient, pure-color perovskite light emitting devices for entire visible spectrum with low-cost and simple infrastructure.
As an optical gain medium, colloidal quantum dots (CQDs) are suffering from band-edge state degeneracy which demands multiple-exciton to achieve population inversion. However, fast non-radiative Auger recombination in the multiple-exciton CQDs increases the lasing threshold and limits the gain lifetime. Here, by embedding the quasi type-II CQDs (CdSe/CdS/ZnS core/shell/shell) into the Sawyer−Tower circuit to apply a potential that is experienced as an electric field by the CQDs, we have reached and showed tunable amplified spontaneous emission (ASE) threshold in a long-sought practical device where the CQDs sandwiched between two dielectric layers to retain their high quantum efficiency as in parent solution (quantum yield of > 70%). Singly-charged CQDs help building up population inversion due to pre-existing electrons while strongly enhanced Auger recombination in multiple-charged CQDs stymies the optical amplification. The approach allows us to fine-tune and achieve the optimal charging level to utilize the advantages of singly charged CQDs and avoid the adverse effect of doubly charged CQDs.
In addition to experimentally demonstrating threshold tunability, we also developed a kinetic equation model to systematically analyze the electric field dependent ASE threshold. The kinetic model not only confirms our experimental results but also presents to be a reliable tool for accessing the requirements of charging level to achieve nearly zero-threshold trion gain in CQDs. The implications, then, to potential applications of our robust and environment-undependable tuning method are broad, from controlling exciton recombination dynamics to continuous wave (CW) or possibly electrically pumped CQD lasers.
As widely known, random diffraction effect due to refractive index inhomogeneity is considered as an annoying factor for propagating light through scattering media. Here, instead of overcoming the random diffraction, we utilize the natural randomness of strongly scattering media and their sensitivity to inhomogeneity of refractive index to develop a novel optical sensor. Unlike various sensing technologies with trade-off among complexity, sensitivity and stability, here we demonstrate a very simple sensing technique which uses scattering media to achieve super sensitivity, speedy response time and possibly high stability. In our sensing principle, a lasing beam passing through a turbid medium creates a speckle pattern on a camera due to interference of random refracted light off the scattering media. Light is scattered multiple times at multiple interfaces between ground glass and the surrounding environment creating a speckle pattern which is sensitive to the environment’s refractive index. The correlation of speckle patterns indicates the change of refractive index around the scattering medium. Simply placing the rough surface of ground glass in contact with sensing solutions, we are able to measure glucose, or sodium chloride concentration with sensitivity in the order of micro grams per liter. More interestingly, the sensitivity of the proposed approach could be improved simply by adding more scattering surfaces in contact with the target medium. Therefore, our simple technique could be very useful for prominent applications in refractive index sensing such as measuring solution concentration, distinguishing different gases, detecting pressure change and so on.
Systematic investigation of temperature-dependence on thermal quenching of X-ray luminescence (XL) from various single perovskite crystals was carried out. In the family of methylammonium lead halide perovskites (MAPbX3, MA = methylammonium, X= Cl, Br or I), the quenching temperature of XL decreases from Cl to I. According to our analysis, such behavior is strongly affected by their corresponding decrease of thermal activation energy ▵Eq from 53 ± 3 to 6 ± 1 meV. Different concentrations of Bi3+-doped MAPbBr3 are also prepared and both four-point probe measurement and X-ray thermoluminescence (TL) confirms the successful doping. When we dope MAPbBr3 with Bi3+, Γ0/Γv increases to 78 ± 18 for crystal with Bi/Pb ratio of 1/10 in precursor solution.
Inner diffraction phenomenon is known as the major obstacle of light transmission through scattering media such as ground glasses, skin or biological tissue. Recently, the most effective and convenient solution is wave-front shaping technique which modulates the field profile of incident light by using a spatial light modulator (SLM). For practical and advanced biomedical applications, requirement of speedy response, high accuracy and large energy delivery are necessary. In our previous work, we presented a wave-front shaping technique and utilized optical memory effect for swiftly drawing various 2 dimensional (2D) shapes or contours through a scattering medium without any mechanical movement. However, with process of scanning angle phase profile and shifting phase pattern on SLM, the accuracy and beam energy utilization are still very much restricted. Here, we present an exceeding improvement from previous technique by establishing optical conjugate planes between SLM and scattering medium surface, which is also known as 4F system. With only one phase profile for creating a focus spot behind scattering medium, we are able to swiftly move focus spot in 3D space or draw any 3D contours through turbid medium without scanning or shifting process. The new approach allows us to deliver laser energy through a scattering medium to any spot within 3D memory effect space with very fast response, high accuracy and more importantly, fully utilized laser beam energy. Our approach demonstrates a practical method to control light through scattering media for prominent applications such as opto-genetic excitation, minimal invasive laser surgery and other related fields.
Propagation of light through scattering media such as ground glass or biological tissue limits the quality and intensity of focusing point. Wave front shaping technique which uses spatial light modulator (SLM) devices to reshape the field profile of incoming light, is considered as one of the most effective and convenient methods. Advanced biomedical or manufacturing applications require drawing various contours or shapes quickly and precisely. However, creating each shape behind the scattering medium needs different phase profiles, which are time consuming to optimize or measure. Here, we demonstrate a technique to draw various shapes or contours behind the scattering medium by swiftly moving the focus point without any mechanical movements. Our technique relies on the existence of speckle correlation property in scattering media, also known as optical memory effect. In our procedure, we first modulate the phase-only SLM to create the focus point on the other side of scattering medium. Then, we digitally shift the preoptimized phase profile on the SLM and ramp it to tilt the beam accordingly. Now, the incoming beam with identical phase profile shines on the same scattering region at a tilted angle to regenerate the focus point at the desired position due to memory effect. Moreover, with linear combination of different field patterns, we can generate a single phase profile on SLM to produce two, three or more focus points simultaneously on the other side of a turbid medium. Our method could provide a useful tool for prominent applications such as opto-genetic excitation, minimally invasive laser surgery and other related fields.
Large field of view multispectral imaging through scattering medium is a fundamental quest in optics community. It has gained special attention from researchers in recent years for its wide range of potential applications. However, the main bottlenecks of the current imaging systems are the requirements on specific illumination, poor image quality and limited field of view. In this work, we demonstrated a single-shot high-resolution colour-imaging through scattering media using a monochromatic camera. This novel imaging technique is enabled by the spatial, spectral decorrelation property and the optical memory effect of the scattering media. Moreover the use of deconvolution image processing further annihilate above-mentioned drawbacks arise due iterative refocusing, scanning or phase retrieval procedures.
A position-multiplexing technique with ultra-broadband illumination is proposed to enhance the information security of an incoherent optical cryptosystem. The simplified optical encryption system only contains one diffuser acting as the random phase mask (RPM). Light coming from a plaintext passes through this RPM and generates the corresponding ciphertext on a camera. The proposed system effectively reduces problems of misalignment and coherent noise that are found in the coherent illumination. Here, the use of ultra-broadband illumination has the advantage of making a strong scattering and complex ciphertext by reducing the speckle contrast. Reduction of the ciphertext size further increases the strength of the ciphering. The unique spatial keys are utilized for the individual decryption as the plaintext locates at different spatial positions, and a complete decrypted image could be concatenated with high fidelity. Benefiting from the ultra-broadband illumination and position-multiplexing, the information of interest is scrambled together by a truly random method in a small ciphertext. Only the authorized user can decrypt this information with the correct keys. Therefore, a high performance security for an optical cryptosystem could be achieved.
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