High-contrast nonlinear spiral phase contrast imaging via four-wave mixing in atomic medium.

Nonlinear imaging takes advantage of the localized nature of the interaction to achieve high spatial resolution, optical sectioning, and deeper penetration in tissue. However, nonlinear contrast (other than fluorescence or harmonic generation) is generally difficult to measure because it is overwhelmed by the large background of detected illumination light. Especially challenging to measure is the nonlinear refractive index – accessing this quantity would allow the extension of widely employed phase microscopy methods to the nonlinear regime. We have developed a technique to suppress the background in these types of measurements by using femtosecond pulse shaping to encode nonlinear interactions in background-free regions of the frequency spectrum. Using this individual pulse shaping based technique we have been able to measure self-phase modulation (SPM) in highly scattering environments, such as biological tissue, with very modest power levels. Using our measurement technique we have demonstrated strong intrinsic SPM signatures of glutamate-induced neuronal activity in hippocampal brain slices. We have also extended this measurement method to cross-phase modulation, the two-color analogue to SPM. The two-color approach dramatically improves the measurement sensitivity by reducing undesired background and associated noise. We will describe the nonlinear phase contrast measurement technique and report on its application for imaging neuronal activity.

Nonlinear imaging takes advantage of the localized nature of the interaction to achieve high spatial resolution, optical sectioning, and deeper penetration in tissue. However, nonlinear contrast (other than fluorescence or harmonic generation) is generally difficult to measure because it is overwhelmed by the large background of detected illumination light. Especially challenging to measure is the nonlinear refractive index - accessing this quantity would allow the extension of widely employed phase microscopy methods to the nonlinear regime. We have developed a technique to suppress the background in these types of measurements by using femtosecond pulse shaping to encode nonlinear interactions in background-free regions of the frequency spectrum. Using this individual pulse shaping based technique we have been able to measure self-phase modulation (SPM) in highly scattering environments, such as biological tissue, with very modest power levels. Using our measurement technique we have demonstrated strong intrinsic SPM signatures of glutamate-induced neuronal activity in hippocampal brain slices. We have also extended this measurement method to cross-phase modulation, the two-color analogue to SPM. The two-color approach dramatically improves the measurement sensitivity by reducing undesired background and associated noise. We will describe the nonlinear phase contrast measurement technique and report on its application for imaging neuronal activity.

Generation of quadripartite entanglement from a hybrid scheme with a four-wave mixing process and linear beam splitters

We propose a scheme to transfer the spatial intensity as well as phase information encoded initially in the spatial profile of a weak probe field onto a newly generated Stokes field in a nonlinear process of four-wave mixing (FWM). The FWM process is explored in a gaseous medium consisting of atoms modeled as a three-level system in the $\mathrm{\ensuremath{\Lambda}}$ configuration. We found that different orders of Hermite-Gaussian and Laguerre-Gaussian modes carried by a probe beam are effectively transferred to a Stokes beam. Interestingly, the transferred intensity of the Stokes beam is a clone of the probe beam, whereas the phase profile is conjugate to the probe. The phase distribution of the transmitted modes at the exit of the medium is explored by superimposing them on a copropagating plane wave. Moreover, the parametric amplification due to the nonlinear process of FWM prevents any information loss due to linear absorption and permits us to work at high optical depths. We found a structural similarity between transferred images of about $99%$, which provides clear evidence for the successful transfer of information in the FWM process.

Graded-index (GRIN) fibers are often used for making high-power Raman amplifiers. We employ numerical and semi-analytical techniques to model such amplifiers and include not only the signal’s amplification and pump’s depletion but also various nonlinear interactions between the signal and pump beams and the self-imaging effects within the GRIN fiber. We solve the coupled nonlinear equations of the pump and signal beams numerically. We also employ the variational technique to obtain simpler equations that can be solved much faster than the full model and still agree with it in most cases of practical interest. We discuss the evolution dynamics of the pump and signal beams, along with a novel process of energy exchange between the two beams because of self-imaging inside the GRIN fiber. The dependence of the signal’s amplification on various input parameters is analyzed in detail to optimize the device’s design and enhance the signal’s amplification for a given pump power and fiber length. Based on our analysis, we establish a resonant condition for the maximum energy transfer from the pump to the signal being amplified. We further show that the periodic self-imaging of the pump and signal beams inside a GRIN fiber leads to higher output powers compared to step-index fibers.

Nonlinear metasurfaces have experienced rapid growth recently due to their potential in various applications, including infrared imaging and spectroscopy. However, due to the low conversion efficiencies of metasurfaces, several strategies have been adopted to enhance their performances, including employing resonances at signal or nonlinear emission wavelengths. This strategy results in a narrow operational band of the nonlinear metasurfaces, which has bottlenecked many applications, including nonlinear holography, image encoding, and nonlinear metalenses. Here, we overcome this issue by introducing a new nonlinear imaging platform utilizing a pump beam to enhance signal conversion through four-wave mixing (FWM), whereby the metasurface is resonant at the pump wavelength rather than the signal or nonlinear emissions. As a result, we demonstrate broadband nonlinear imaging for arbitrary objects using metasurfaces. A silicon disk-on-slab metasurface is introduced with an excitable guided-mode resonance at the pump wavelength. This enabled direct conversion of a broad IR image ranging from >1000 to 4000 nm into visible. Importantly, adopting FWM substantially reduces the dependence on high-power signal inputs or resonant features at the signal beam of nonlinear imaging by utilizing the quadratic relationship between the pump beam intensity and the signal conversion efficiency. Our results, therefore, unlock the potential for broadband infrared imaging capabilities with metasurfaces, making a promising advancement for next-generation all-optical infrared imaging techniques with chip-scale photonic devices.

Optical vortices (OVs) are singular optical beams with a spiral phase dislocation in their wavefront. When two pump beams at the frequencies ω 1 and ω 2 interact in a Kerr-type nonlinear medium, the degenerate four-wave mixing (FWM) process results in the generation of new distinct sum and difference frequency components at the output. Importantly, the FWM process is expected to preserve the topological charge of the OVs, and thus it can be employed for the generation of white-light vortices, in contrast to the vortex propagation in a Raman nonlinear medium [1]. However, as the FWM process is accompanied by noticeable nonlinear instabilities due to self- and cross-phase modulation, the phase information in the newly generated frequency components may be destroyed [2]. Therefore, it is important to reveal the possible regimes where the FWM process will dominate nonlinear instabilities, leading to the stable generation of white-light optical vortices.

We demonstrate that two-beam interferometry is applicable to phase detection of nonlinear susceptibility of biological samples in the stimulated parametric emission (SPE) microscopy, which is one of the coherent nonlinear optical microscopy techniques. We experimentally observed stained mouse kidney cells and successfully confirmed nonlinear phase shift associated with an electronic resonance at stained nuclei. In addition, phase blurring was found to be almost completely eliminated in a nonlinear differential interferometric contrast image, which can be produced through a simple post-processing procedure.

Acoustic Angiography is a novel ultrasound imaging technology for visualizing intravascular microbubble contrast agents. It uses a prototype dual-frequency transducer to transmit ultrasound pulses at a low frequency near microbubble resonance, and confocally receives higher frequency signal from the broadband microbubble response. By utilizing two widely separated frequencies (transmit at 4 MHz, receive at 30 MHz), it is possible to form a high-resolution image of the vascular structure without any tissue background. Both Acoustic Angiography and standard amplitude-modulation, nonlinear contrast perfusion imaging were used to compare the vascularity of xenograft tumors composed of two tumor cell sub-types with known differences in vascularity. In both image types, tumors were manually segmented in three dimensions and the percent of pixels containing contrast signal was computed. Acoustic Angiography imaging proved to be more sensitive to the vascular differences than standard nonlinear perfusion imaging. For a population of 16 animals, the sensitivity of Acoustic Angiography was 0.73, whereas the sensitivity of non-linear perfusion imaging was 0.13, given a significance level of 0.05. Additionally, two-sided t-tests showed a statistically significant difference between tumor types for Acoustic Angiography images (p = 0.0025), but not for standard nonlinear contrast imaging (p = 0.39).

We demonstrate nonlinear phase contrast imaging in highly scattering media using rapid femtosecond pulse shaping of mode-locked laser pulses. We will also discuss potential applications of this technique for intrinsic functional neuronal imaging.

Brillouin induced four-wave mixing can be used to phase conjugate weak signals with reflection coefficients of up to 106.1 This high reflectivity results from an absolute instability that produces exponential temporal growth in both the signal and phase-conjugate beams. This occurs when the pump beams have exceeded a certain critical intensity.2 Theoretical predictions of critical intensity values have been found to agree well with experimental results.3 The effects of varying the signal frequency can also be predicted by theory. We report on experiments designed to test these predictions. The pump and signal beams were produced from two separately injection seeded Nd:YAG Q-switched lasers, with pulse durations of 40 ns. The relative frequencies of the lasers were precisely controlled by feedback. The four-wave mixing medium was carbon disulphide. The second pump beam was produced by reflecting the first from an SBS cell. Different liquids in this cell were used to vary the phase mismatch. We measured the signal amplification vs signal frequency for three different phase mismatch values. The results were found to agree well with theory. We carried out a heteorodyne measurement of the phase conjugate frequency, and found that it remained constant as the signal frequency was varied; this is also predicted by theory.

Previously we have shown that a suitable design of the excitation envelope to generate a self-demodulation (SD) signal at the subharmonic (SH) frequency can enhance the SH emission of ultrasound contrast agent (UCA) more than 20 dB at 10 MHz transmit frequency. The SH emission from the UCA is of interest since it is produced only by the UCA and is free of artifacts produced in harmonic imaging modes. In this study we used such optimized excitation combined with conventional nonlinear contrast detection methods such as pulse inversion (PI), amplitude modulation (AM) and a combination on those (AMPI) in an array-based micro-ultrasound system (Vevo 2100) for in vitro and in vivo validation of real time SH imaging at high frequencies. For better axial resolution, short bursts (6-cycle) and for better separation of the SH peak in the frequency domain, long bursts (20-cycle) with rectangular envelope were transmitted with the following pulse sequences: B-mode, PI, AM and AMPI. For the in vitro study, tissue-mimicking material was prepared. Identical rectangular region of interest at the same depth, in the tissue-mimicking material and UCA (MicroMarker), were selected for contrast to tissue ratio (CTR) calculations. For the in vivo validation the chicken embryos were scanned after administering the UCA. In vitro, the best CTR was obtained with PI sequence filtered at the SH frequency with 26 dB enhancement compared with the fundamental B-mode image. Then, performance of SH imaging with PI sequence was validated in vivo in chicken embryo model. This study suggests that SH imaging is feasible with short transmit bursts (6-cycle) and low acoustic pressure (~200 kPa) at high frequencies (>15 MHz) benefiting from the stimulation effect of the S-D signal on the SH emission of UCA.

Detection of high-order nonlinear components issued from microbubbles has emerged as a sensitive method for contrast agent imaging. Nevertheless, the detection of these high-frequency components, including the third, fourth, and fifth harmonics, remains challenging because of the lack of transducer sensitivity and bandwidth. In this context, we propose a new design of imaging transducer based on a simple fabrication process for high-frequency nonlinear imaging. The transducer is composed of two elements: the outer low-frequency (LF) element was centered at 4 MHz and used in transmit mode, whereas the inner high-frequency (HF) element centered at 14 MHz was used in receive mode. The center element was pad-printed using a lead zirconate titanate (PZT) paste. The outer element was molded using a commercial PZT, and curved porous unpoled PZT was used as backing. Each piezoelectric element was characterized to determine the electromechanical performance with thickness coupling factor around 45%. After the assembly of the two transducer elements, hydrophone measurements (electroacoustic responses and radiation patterns) were carried out and demonstrated a large bandwidth (70% at -3 dB) of the HF transducer. Finally, the transducer was evaluated for contrast agent imaging using contrast agent microbubbles. The results showed that harmonic components (up to the sixth harmonic) of the microbubbles were successfully detected. Moreover, images from a flow phantom were acquired and demonstrated the potential of the transducer for high-frequency nonlinear contrast imaging.

The dynamical evolution of both signal and pump beams are traced by numerically solving the coupled-wave equation for a photorefractive two-wave mixing system. The direct simulations show that, when the intensity ratio of the pump beam to the signal beam is large enough, the pump beam presents a common decaying behaviour without modulational instability (MI), while the signal beam can evolve into a quasistable spatial soliton within a regime in which the pump beam is depleted slightly. The larger the ratio is, the longer the regime is. Such quasistable solitons can overcome the initial perturbations and numerical noises in the course of propagation, perform several cycles of slow oscillation in intensity and width, and persist over tens of diffraction lengths. From physical viewpoints, these solitons actually exist as completely rigorous physical objects. If the ratio is quite small, the pump beam is apt to show MI, during which the signal beam experiences strong expansion and shrinking in width and a drastic oscillation in intensity, or completely breaks up. The simulations using actual experimental parameters demonstrate that the observation of an effectively stable soliton is quite possible in the proposed system.

Wavefront manipulations have enabled wide applications across many interdisciplinary fields ranging from optics and microwaves to acoustics. However, the realizations of such functional surfaces heavily rely on micro/nanofabrication to define the structured surfaces, which are fixed and only work within a limited spectrum. To address these issues, previous attempts combining tunable materials like liquid crystal or phase-change ones onto the metasurfaces have permitted extra tunability and working spectra, however, these additional layers bring in inevitable loss and complicate the fabrication. Here we demonstrate a fabrication-free tunable flat slab using a nonlinear four-wave mixing process. By wavefront-shaping the pump onto the flat slab, we can successfully tune the effective nonlinear refraction angle of the emitting FWM beams according to the phase-matching condition. In this manner, a focusing and a defocusing nonlinear of FWM beam through the flat slab have been demonstrated with a converging and a diverging pump wavefronts, respectively. Furthermore, a beam steering scheme over a 20° angle has been realized through a non-degenerate four-wave mixing process by introducing a second pump. These features open up a door to manipulating light propagation in an all-optical manner, paving the way to more functional and tunable flat slab devices in the applications of imaging and all-optical information.