Four-Wave Mixing Microscopy (FWM): Imaging Nanostructures

Four-Wave Mixing (FWM) microscopy is an imaging technique that utilizes nonlinear optical processes to generate contrast in biological samples. It enables label-free, high-resolution imaging of molecular vibrations and structural details, essential for biomedical research and understanding complex biological systems.

3D Image from Four Wave Mixing Microscopy

Overview

In the realm of advanced microscopy techniques, Four-Wave Mixing (FWM) microscopy has emerged as a powerful tool for investigating nanostructures and examining into the intricacies of material properties at the molecular level. This cutting-edge imaging technique utilizes the principles of nonlinear optics to provide high-resolution, label-free imaging with applications ranging from biological studies to material science. In this comprehensive article by Academic Block, we will explore the fundamental concepts of Four-Wave Mixing microscopy, its applications, and its potential impact on various scientific fields.

Understanding the Basics of Four-Wave Mixing

Four-Wave Mixing is a nonlinear optical process that involves the interaction of four laser beams to generate a new signal. To grasp the basics of FWM microscopy, it's crucial to understand the fundamental principles of nonlinear optics and the underlying physics of the Four-Wave Mixing process.

Nonlinear Optics:

Nonlinear optics deals with the interaction of intense laser light with materials, leading to optical effects that are not proportional to the incident light's intensity. In traditional linear optics, light-matter interactions are characterized by linear relationships. However, at high intensities, nonlinear effects become prominent, giving rise to phenomena such as FWM.

Four-Wave Mixing Process:

The FWM process involves four laser beams interacting within a nonlinear medium. These beams, usually denoted as pump (P), probe (Pr), and two conjugate waves (C1 and C2), generate a new signal at the difference frequency between the pump and probe beams. The resulting signal carries information about the sample's properties, allowing for high-resolution imaging.

Key Components of Four-Wave Mixing Microscopy

To implement FWM microscopy, several key components are required, each playing a crucial role in achieving high-quality imaging and extracting valuable information from the sample.

Nonlinear Medium:

The choice of the nonlinear medium is critical in FWM microscopy. Commonly used materials include photonic crystals, semiconductor quantum dots, and organic molecules. The nonlinear response of these materials enhances the efficiency of the Four-Wave Mixing process.

Ultrafast Lasers:

FWM microscopy relies on ultrafast laser sources to generate short pulses of light. These lasers provide the necessary intensity for inducing the nonlinear effects. Ti:sapphire lasers and optical parametric oscillators are often employed for their ultrafast pulse durations.

Detection System:

A sensitive and high-resolution detection system is essential for capturing the FWM signal. Photodetectors, such as photomultiplier tubes or avalanche photodiodes, are commonly used to detect the generated signal. The detected signal is then processed to reconstruct the image.

Applications of Four-Wave Mixing Microscopy:

The versatility of FWM microscopy makes it applicable across various scientific disciplines. Let's explore some of the key applications that showcase the potential of this advanced imaging technique.

Biomedical Imaging: In the field of biomedical research, FWM microscopy offers label-free imaging of biological samples with high spatial resolution. This is particularly advantageous for studying live cells and tissues without the need for exogenous contrast agents. FWM microscopy has been utilized in visualizing cellular structures, membrane dynamics, and studying molecular processes within living organisms.
Materials Science: FWM microscopy plays a crucial role in materials science, enabling researchers to investigate the properties of nanomaterials and nanostructures. The technique provides insights into the structural and optical characteristics of materials at the nanoscale, aiding the development of novel materials with tailored properties.
Quantum Information Processing: In the realm of quantum information processing, FWM microscopy contributes to the characterization of quantum systems and the study of quantum coherence. The ability to probe quantum states at the nanoscale opens new possibilities for advancing quantum technologies and understanding the behavior of quantum systems.
Environmental Sensing: FWM microscopy has shown promise in environmental sensing applications. By studying the interaction of light with environmental samples, researchers can gain insights into pollutant levels, chemical composition, and other environmental parameters. This can have implications for monitoring and managing environmental resources.

Mathematical equations behind the Four-Wave Mixing Microscopy

The mathematical description of Four-Wave Mixing (FWM) microscopy involves principles from nonlinear optics. To understand the equations behind FWM, it's essential to grasp the nonlinear interaction of multiple laser beams within a medium. The following equations provide a simplified overview of the mathematical foundations of FWM microscopy:

Nonlinear Polarization (P_nl):

The nonlinear polarization, resulting from the interaction of the electric field (E) with the nonlinear medium, can be expressed as a power series expansion:

P_nl = ε₀ (χ⁽¹⁾⋅E + χ⁽²⁾⋅E² + χ⁽³⁾⋅E³ + …) ;

Here, ε₀ is the vacuum permittivity, and χ⁽¹⁾, χ⁽²⁾, χ⁽³⁾,… are the linear, second-order, third-order, etc., nonlinear susceptibilities of the medium.

Four-Wave Mixing Polarization (P_FWM):

FWM involves the interaction of four waves (pump, probe, and two conjugate waves). The resulting polarization due to FWM can be expressed as:

P_FWM = χ_FWM⁽³⁾⋅ E_P⋅ E_pr⋅ E_C1⋅ E_C2 ;

Here, E_P, E_pr, E_C1, E_C2 are the electric field amplitudes of the pump, probe, and conjugate waves, and χ_FWM⁽³⁾ is the third-order nonlinear susceptibility specific to the FWM process.

Electric Field Amplitudes:

The electric field amplitudes for the interacting waves can be written as:

E_P(t) = E_P0 ⋅ e^−iωPt ;

E_pr(t) = E_pr0 ⋅ e^−iωPrt ;

E_C1(t) = E_C10⋅e^−iωC1t ;

E_C2(t) = E_C20⋅e^−iωC2t ;

Where ω_P, ω_Pr, ω_C1, ω_C2 are the angular frequencies, and E_P0, E_pr0, E_C10, E_C20 are the electric field amplitudes of the pump, probe, and conjugate waves.

Frequency Relationships:

FWM involves specific frequency relationships among the interacting waves:

ω_P = ω_Pr − ω_C1 − ω_C2 ;

This frequency relationship ensures that the energy conservation condition is satisfied in the FWM process.

Intensity of the FWM Signal:

The intensity (I) of the FWM signal, which is proportional to the square of the electric field amplitude, can be expressed as:

I_FWM ∝ ∣E_FWM∣² = ∣χ_FWM⁽³⁾∣²⋅ ∣E_P∣²⋅ ∣E_pr∣²⋅ ∣E_C1∣²⋅ ∣E_C2∣² ;

The intensity provides information about the generated FWM signal strength, which is crucial for imaging and analysis.

It's important to note that these equations provide a simplified representation, and the actual mathematics involved can become more complex depending on factors such as the specific characteristics of the nonlinear medium and the experimental setup

Challenges and Future Directions

While Four-Wave Mixing microscopy has demonstrated remarkable capabilities, there are still challenges and areas for improvement that researchers are actively addressing. Some of the current challenges include:

Sensitivity and Signal-to-Noise Ratio: Enhancing the sensitivity of FWM microscopy and improving the signal-to-noise ratio are ongoing challenges. Researchers are exploring new techniques and technologies to optimize the detection system and increase the efficiency of signal collection.
Imaging Speed: The imaging speed of FWM microscopy is a critical factor, especially for dynamic biological processes. Efforts are underway to develop faster scanning techniques and improve the temporal resolution of FWM microscopy to capture rapid changes in samples.
Integration with Other Imaging Modalities: Integrating FWM microscopy with other imaging modalities, such as fluorescence microscopy or Raman spectroscopy, can provide complementary information and enhance the overall imaging capabilities. Researchers are working on developing multimodal imaging systems for a more comprehensive analysis of samples.
Biocompatibility: In the context of biomedical applications, ensuring the biocompatibility of the FWM microscopy setup is essential. Researchers are exploring ways to minimize the impact of laser light on living tissues and cells to enable longer imaging sessions without compromising cell viability.

The future directions of FWM microscopy involve addressing these challenges and expanding its capabilities. Some promising developments include:

Multimodal Imaging Platforms: Advancements in multimodal imaging platforms will likely enable researchers to combine FWM microscopy with other imaging techniques, providing a more comprehensive understanding of complex samples.
Improved Nonlinear Materials: Research is ongoing to discover and develop new nonlinear materials that can enhance the efficiency of the Four-Wave Mixing process, leading to improved imaging quality and sensitivity.
Real-Time Imaging: Efforts are being made to achieve real-time imaging capabilities with FWM microscopy. This is particularly important for studying dynamic processes in live cells and tissues, opening up new possibilities for biological research.

Final Words

In this article by Academic Block, we have seen that, the Four-Wave Mixing microscopy represents a paradigm shift in the world of advanced imaging techniques, offering researchers a powerful tool to explore the nanoscale world with unprecedented precision. From unraveling the mysteries of biological structures to advancing materials science and quantum technologies, the applications of FWM microscopy are diverse and promising. As researchers continue to overcome challenges and push the boundaries of this technology, the future holds exciting possibilities for unlocking new realms of knowledge and understanding in various scientific disciplines. Please provide your comments below, it will help us in improving this article. Thanks for reading!

This Article will answer your questions like:

+ What is Four-Wave Mixing (FWM) microscopy? >

Four-Wave Mixing (FWM) microscopy is a nonlinear optical imaging technique that leverages the interaction of multiple laser beams to generate new wavelengths through a nonlinear process. In FWM microscopy, typically three incident laser beams interact within a sample, generating a fourth beam that carries information about the sample's properties at high resolution. This technique is particularly useful in biomedical imaging due to its ability to provide label-free, high-resolution images of biological samples, capturing details such as chemical composition and structural integrity with minimal photodamage. FWM microscopy's nonlinear nature and coherence properties make it a powerful tool for studying complex biological systems and materials.

+ How does Four-Wave Mixing microscopy utilize nonlinear optical processes? >

Four-Wave Mixing (FWM) microscopy utilizes nonlinear optical processes by exploiting the interaction of multiple laser beams at different frequencies within a sample. These beams mix nonlinearly, generating new wavelengths that are not present in the incident beams. This nonlinear interaction allows FWM microscopy to achieve high spatial resolution beyond the diffraction limit of traditional microscopy techniques. By carefully controlling the phase matching conditions and laser parameters, FWM microscopy can selectively probe specific chemical bonds and structures in samples, providing detailed information about their composition and morphology. This makes it a valuable tool in materials science and biological imaging for studying complex samples in their native state.

+ What are the key principles behind Four-Wave Mixing microscopy? >

The key principles behind Four-Wave Mixing (FWM) microscopy include phase matching, coherence control, and nonlinear optical processes. Phase matching ensures that the generated FWM signal is maximized by aligning the wave vectors of the interacting laser beams. Coherence control ensures that the beams interact coherently, enhancing the signal-to-noise ratio and image quality. Nonlinear optical processes involve the generation of new wavelengths through the nonlinear susceptibility of the sample, which depends on its molecular structure and optical properties. By harnessing these principles, FWM microscopy achieves high-resolution, label-free imaging of biological and materials samples, offering insights into their chemical composition, structural organization, and dynamics at the nanoscale level.

+ What is the 4 zone mix theory? >

The 4 zone mix theory refers to a model used in optical communications to understand signal processing in nonlinear media. It divides the interaction region into four distinct zones, each representing different nonlinear behaviors of light waves. This model helps in predicting the outcomes of interactions such as four-wave mixing and self-phase modulation, thus optimizing system designs for efficient signal transmission and minimizing interference and distortion in optical fibers.

+ How does FWM microscopy achieve high spatial resolution and contrast? >

FWM microscopy achieves high spatial resolution and contrast by exploiting the nonlinear interaction of laser beams within a sample. Unlike linear imaging techniques, FWM generates signals only where the incident beams overlap coherently and interact nonlinearly, typically at the focal point of the microscope. This enables FWM microscopy to resolve features well below the diffraction limit of traditional microscopy, revealing fine structural details and chemical composition with exceptional clarity. Additionally, FWM microscopy benefits from high signal-to-noise ratios due to phase matching and coherence control, enhancing contrast and enabling sensitive detection of weak signals. These capabilities make FWM microscopy a powerful tool for studying biological processes, materials science, and nanotechnology applications.

+ What types of samples and materials are suitable for FWM microscopy? >

FWM microscopy is suitable for a wide range of samples and materials, particularly those with nonlinear optical properties. Biological samples such as cells, tissues, and biomolecules can be imaged effectively using FWM microscopy without the need for fluorescent labels, preserving their natural state. Materials with specific vibrational modes or nonlinear susceptibilities, such as crystals, polymers, and nanostructures, are also well-suited for FWM microscopy. These materials exhibit distinct spectral features that can be probed using FWM to gain insights into their composition, structure, and dynamics at the micro- and nanoscale levels. The label-free nature and high spatial resolution of FWM microscopy make it a versatile technique for both research and industrial applications in various fields of science and engineering.

+ What is the Kerr effect and four-wave mixing? >

The Kerr effect is a nonlinear optical phenomenon where the refractive index of a medium changes in response to an applied electric field. This effect is significant in four-wave mixing (FWM), where it facilitates the interaction of multiple light waves within a nonlinear medium. By altering the refractive index, the Kerr effect enhances the efficiency of FWM processes, enabling advanced applications in telecommunications, including wavelength division multiplexing and optical signal processing.

+ How does FWM microscopy compare to other nonlinear optical imaging techniques? >

FWM microscopy offers distinct advantages compared to other nonlinear optical imaging techniques such as Second Harmonic Generation (SHG) and Coherent Anti-Stokes Raman Scattering (CARS). Unlike SHG, which only probes non-centrosymmetric structures, and CARS, which relies on Raman scattering and suffers from non-resonant background signals, FWM microscopy provides label-free imaging based on coherent anti-Stokes signals. This allows FWM to selectively probe specific vibrational modes and chemical bonds in samples, offering higher sensitivity and spatial resolution. Moreover, FWM microscopy can operate in both transmission and reflection modes, making it versatile for various sample types and experimental setups. These characteristics position FWM microscopy as a powerful tool for studying complex biological systems and materials at the molecular level.

+ What are the advantages of using FWM microscopy in biomedical imaging? >

FWM microscopy offers several advantages in biomedical imaging, primarily due to its label-free nature and high spatial resolution. By utilizing the intrinsic optical properties of biological samples, FWM can provide detailed images of cells, tissues, and biomolecules without the need for exogenous labels or stains, preserving their natural state and minimizing phototoxicity. This enables researchers to study dynamic biological processes in real time with minimal interference. Additionally, FWM microscopy's high spatial resolution allows for the visualization of subcellular structures and molecular interactions at the nanoscale, providing insights into disease mechanisms and therapeutic responses. These capabilities make FWM microscopy a valuable tool for advancing biomedical research, diagnostics, and drug development.

+ What is degenerate four-wave mixing and non-degenerate four-wave mixing with an example? >

Degenerate four-wave mixing (DFWM) occurs when all four waves involved have the same frequency, resulting in the generation of new frequencies due to nonlinear interactions. Conversely, non-degenerate four-wave mixing involves waves of different frequencies. For example, in DFWM, a pump wave and two signal waves can create a new wave at a frequency equal to the difference between the two signals. This process is crucial for applications in optical signal processing and wavelength conversion.

+ What are the main components of a Four-Wave Mixing microscopy setup? >

The main components of a Four-Wave Mixing (FWM) microscopy setup include tunable lasers, typically operating in the near-infrared range to minimize sample photodamage and maximize penetration depth. These lasers are synchronized and focused onto the sample through a microscope objective, which collects the generated FWM signals. A detection system, often based on photodiodes or CCD cameras, captures the emitted signals and converts them into images or spectra. Phase matching optics, such as prisms or gratings, are used to ensure optimal alignment of the laser beams' wave vectors, maximizing FWM signal generation. Additionally, specialized software is employed for data acquisition, processing, and analysis, facilitating quantitative measurements of sample properties such as chemical composition and structural integrity. These components work synergistically to enable high-resolution, label-free imaging in FWM microscopy experiments.

+ How does phase matching affect the efficiency and quality of FWM microscopy? >

Phase matching in Four-Wave Mixing (FWM) microscopy is critical for optimizing the efficiency and quality of signal generation. It ensures that the wave vectors of the incident laser beams satisfy momentum conservation laws within the sample, maximizing the nonlinear interaction efficiency. Proper phase matching minimizes energy losses and enhances the coherence of the generated FWM signal, resulting in higher signal-to-noise ratios and improved imaging resolution. Phase matching can be achieved through various techniques, including spatial and temporal phase matching methods, depending on the experimental setup and sample properties. By optimizing phase matching conditions, FWM microscopy can selectively probe specific vibrational modes and chemical bonds in samples, providing detailed spectroscopic information crucial for understanding biological processes and materials science applications.

+ What are the limitations and challenges of Four-Wave Mixing microscopy? >

Four-Wave Mixing (FWM) microscopy faces several limitations and challenges, including the complexity of experimental setups and the requirement for precise phase matching conditions. Achieving and maintaining phase matching across a wide range of sample types and conditions can be technically demanding, affecting signal quality and imaging resolution. FWM microscopy is also sensitive to sample motion and vibrations, which can degrade image quality and require robust stabilization methods. Moreover, while FWM offers high spatial resolution, it may struggle with depth penetration in thick or highly scattering samples, limiting its applicability in certain biological and materials imaging scenarios. Addressing these challenges requires ongoing advancements in laser technology, optical design, and image processing algorithms to fully exploit FWM microscopy's potential in research and industrial applications.

+ How is data from FWM microscopy processed and analyzed? >

Data from Four-Wave Mixing (FWM) microscopy is processed and analyzed using specialized software tools designed for spectral and spatial data extraction. Initially, raw data from photodiodes or CCD cameras capture the intensity and wavelength information of the generated FWM signals. This data undergoes background subtraction to remove noise and non-resonant contributions, followed by spectral analysis to identify specific vibrational modes and chemical signatures within the sample. Spatially resolved images are reconstructed using algorithms that correlate signal intensity with sample features, providing detailed maps of chemical composition and structural organization. Quantitative measurements such as peak intensities and linewidths are extracted to characterize sample properties, enabling researchers to study dynamic processes and interactions at the molecular level. Advanced data processing techniques continue to evolve, enhancing the utility of FWM microscopy in biomedical and materials science research.

+ What recent advancements have been made in Four-Wave Mixing microscopy technology? >

Recent advancements in Four-Wave Mixing (FWM) microscopy technology have focused on improving imaging speed, sensitivity, and versatility for diverse applications. Enhanced laser sources with broader tunability and higher power output enable faster data acquisition and improved signal-to-noise ratios, enhancing image quality and reducing experimental time. Novel phase matching techniques, including adaptive optics and nonlinear crystals, have extended FWM microscopy's applicability to challenging sample types and conditions, such as thick tissues and live cells. Integration of machine learning algorithms for data analysis allows for real-time processing and automated feature identification, accelerating scientific discovery and biomedical diagnostics. These technological advancements continue to push the boundaries of FWM microscopy, making it a powerful tool for unraveling complex biological processes and advancing materials science research.

Hardware and software required for Four-Wave Mixing Microscopy

Hardware Components

Laser Source: Ultrafast laser sources are crucial for generating short pulses of light, which are essential for inducing nonlinear optical effects. Titanium:sapphire lasers and optical parametric oscillators (OPOs) are commonly used.
Nonlinear Medium: The choice of nonlinear medium is critical. Materials with a high nonlinear susceptibility, such as photonic crystals, semiconductor quantum dots, or organic molecules, are often employed.
Beam Splitters and Optics: Optical components such as beam splitters, mirrors, lenses, and other beam shaping elements are used to control and manipulate the laser beams for the FWM process.
Detection System: High-sensitivity photodetectors, such as photomultiplier tubes (PMTs) or avalanche photodiodes (APDs), are used to detect the FWM signal. The detection system should be capable of capturing weak signals with high signal-to-noise ratio.
Scanning System: In microscopy applications, a scanning system is often required to obtain images of the sample. This may involve galvanometric mirrors, piezoelectric stages, or other mechanisms for precise beam positioning.
Sample Chamber: A chamber or platform for holding the sample is necessary. For biological applications, this may involve a microscope slide or a cell culture dish. In material science, a sample stage compatible with various sample formats is needed.
Data Acquisition System: A system for acquiring and digitizing the signals from the detectors. This may include analog-to-digital converters (ADCs) and associated electronics.
Control Electronics: Electronics for controlling the laser source, scanning system, and other components. This includes timing electronics to synchronize laser pulses and data acquisition.

Software Components

Data Acquisition Software: Software for controlling the data acquisition system, including settings for signal processing, gain adjustments, and data storage.
Image Acquisition and Analysis Software: For microscopy applications, software is needed for controlling the scanning system, acquiring images, and performing basic analysis tasks. This software may include features for stitching together images, adjusting contrast, and extracting quantitative information.
Numerical Modeling and Simulation Software: Researchers often use numerical modeling and simulation software to predict and analyze the expected outcomes of FWM experiments. This helps in optimizing experimental parameters and understanding the underlying physics.
Data Processing and Analysis Tools: Software tools for processing and analyzing FWM data. This may involve techniques such as Fourier transformation for spectral analysis, image processing algorithms for enhancing contrast, and statistical analysis tools for quantifying results.
Custom Scripting and Programming Environments: Depending on the complexity of the experiments, researchers may use custom scripts or programming environments (e.g., Python, MATLAB) for advanced data analysis and automation of experimental control.
Visualization Software: Tools for visualizing and presenting FWM data. This can include 2D and 3D visualization software for rendering reconstructed images and visual representation of spectral data.

Facts on Four-Wave Mixing Microscopy

Nonlinear Optical Process: FWM microscopy is based on a nonlinear optical process where four laser beams interact within a nonlinear medium to generate a new signal. This process allows for imaging with enhanced spatial resolution compared to traditional linear optical techniques.

Generation of New Frequencies: In FWM, the interaction of the four laser beams leads to the generation of a new signal at the difference frequency between the pump and probe beams. This newly generated signal carries information about the sample’s properties.

Label-Free Imaging: FWM microscopy enables label-free imaging, eliminating the need for exogenous contrast agents. This is particularly advantageous in biological applications, allowing researchers to study live cells and tissues without introducing foreign substances.

Applications in Biomedical Research: FWM microscopy has found applications in biomedical research for imaging cellular structures, membrane dynamics, and molecular processes. It has been used to visualize neurons, study myelination patterns in the brain, and explore disease-related changes in tissues.

Materials Science and Nanomaterials Characterization: FWM microscopy plays a crucial role in materials science for characterizing nanomaterials and nanostructures. It provides insights into the structural and optical properties of materials at the nanoscale, aiding in the development of novel materials.

Quantum Information Processing: In the field of quantum optics, FWM microscopy contributes to the study of quantum systems and coherence. It has been used to characterize quantum dots and other quantum systems, with implications for quantum information processing.

Environmental Sensing: FWM microscopy has shown promise in environmental sensing applications. By studying the interaction of light with environmental samples, it can provide information about pollutant levels, chemical composition, and other environmental parameters.

Ultrafast Laser Sources: FWM microscopy relies on ultrafast laser sources to generate short pulses of light. Titanium:sapphire lasers and optical parametric oscillators are commonly used for their ultrafast pulse durations.

Challenges and Ongoing Research: Challenges in FWM microscopy include enhancing sensitivity, improving signal-to-noise ratio, and optimizing imaging speed. Ongoing research focuses on addressing these challenges and expanding the capabilities of the technique.

Multimodal Imaging Possibilities: FWM microscopy can be integrated with other imaging modalities, such as fluorescence microscopy or Raman spectroscopy, to provide multimodal imaging. This enables researchers to obtain complementary information about samples.

Academic References on Four-Wave Mixing Microscopy

Wang, Y., Lin, C. Y., Nikolaenko, A., Raghunathan, V., & Potma, E. O. (2011). Four-wave mixing microscopy of nanostructures. Advances in optics and photonics, 3(1), 1-52.
Min, W., Lu, S., Rueckel, M., Holtom, G. R., & Xie, X. S. (2009). Near-degenerate four-wave-mixing microscopy. Nano Letters, 9(6), 2423-2426.
Kim, H., Bryant, G. W., & Stranick, S. J. (2012). Superresolution four-wave mixing microscopy. Optics express, 20(6), 6042-6051.
Kim, H., Sheps, T., Collins, P. G., & Potma, E. O. (2009). Nonlinear optical imaging of individual carbon nanotubes with four-wave-mixing microscopy. Nano letters, 9(8), 2991-2995.
Jakubczyk, T., Delmonte, V., Koperski, M., Nogajewski, K., Faugeras, C., Langbein, W., … & Kasprzak, J. (2016). Radiatively limited dephasing and exciton dynamics in MoSe2 monolayers revealed with four-wave mixing microscopy. Nano letters, 16(9), 5333-5339.
Kravtsov, V., Ulbricht, R., Atkin, J. M., & Raschke, M. B. (2016). Plasmonic nanofocused four-wave mixing for femtosecond near-field imaging. Nature nanotechnology, 11(5), 459-464.
Lefrancois, S., Fu, D., Holtom, G. R., Kong, L., Wadsworth, W. J., Schneider, P., … & Wise, F. W. (2012). Fiber four-wave mixing source for coherent anti-Stokes Raman scattering microscopy. Optics letters, 37(10), 1652-1654.
Garrett, N., Whiteman, M., & Moger, J. (2011). Imaging the uptake of gold nanoshells in live cells using plasmon resonance enhanced four wave mixing microscopy. Optics express, 19(18), 17563-17574.
Kiefer, J., & Ewart, P. (2011). Laser diagnostics and minor species detection in combustion using resonant four-wave mixing. Progress in Energy and Combustion Science, 37(5), 525-564.
Palomba, S., Zhang, S., Park, Y., Bartal, G., Yin, X., & Zhang, X. (2012). Optical negative refraction by four-wave mixing in thin metallic nanostructures. Nature materials, 11(1), 34-38.

What is Four Wave Mixing Microscopy (FWM)?