Digital holographic microscopy investigation of second harmonic generated at a glass/air interface

Recent Advances and Current Trends in Transmission Tomographic Diffraction Microscopy

Optical microscopy techniques are among the most used methods in biomedical sample characterization. In their more advanced realization, optical microscopes demonstrate resolution down to the nanometric scale. These methods rely on the use of fluorescent sample labeling in order to break the diffraction limit. However, fluorescent molecules' phototoxicity or photobleaching is not always compatible with the investigated samples. To overcome this limitation, quantitative phase imaging techniques have been proposed. Among these, holographic imaging has demonstrated its ability to image living microscopic samples without staining. However, for a 3D assessment of samples, tomographic acquisitions are needed. Tomographic Diffraction Microscopy (TDM) combines holographic acquisitions with tomographic reconstructions. Relying on a 3D synthetic aperture process, TDM allows for 3D quantitative measurements of the complex refractive index of the investigated sample. Since its initial proposition by Emil Wolf in 1969, the concept of TDM has found a lot of applications and has become one of the hot topics in biomedical imaging. This review focuses on recent achievements in TDM development. Current trends and perspectives of the technique are also discussed.

Phase-Modulated Degenerate Parametric Amplification Microscopy

Second-order nonlinear optical interactions, including second-harmonic generation (SHG) and sum-frequency generation (SFG), can reveal a wealth of information about chemical, electronic, and vibrational dynamics at the nanoscale. Here, we demonstrate a powerful and flexible new approach, called phase-modulated degenerate parametric amplification (DPA). The technique, which allows for facile retrieval of both the amplitude and phase of the second-order nonlinear optical response, has many advantages over conventional or heterodyne-detected SHG, including the flexibility to detect the signal at either the second harmonic or fundamental field wavelength. We demonstrate the capabilities of this approach by imaging multigrain flakes of single-layer MoS₂. We identify the absolute crystal orientation of each MoS₂ domain and resolve grain boundaries with high signal contrast and sub-diffraction-limited spatial resolution. This robust all-optical method can be used to characterize structure and dynamics in organic and inorganic systems, including biological tissue, soft materials, and metal and semiconductor nanostructures, and is particularly well-suited for imaging in media that are absorptive or highly scattering to visible and ultraviolet light.

Quantitative phase imaging of live cells with near on-axis digital holographic microscopy using constrained optimization approach

We demonstrate a single-shot near on-axis digital holographic microscope that uses a constrained optimization approach for retrieval of the complex object function in the hologram plane. The recovered complex object is back-propagated from the hologram plane to image plane using the Fresnel back-propagation algorithm. A numerical aberration compensation algorithm is employed for correcting the aberrations in the object beam. The reference beam angle is calculated automatically using the modulation property of Fourier transform without any additional recording. We demonstrate this approach using a United States Air Force (USAF) resolution target as an object on our digital holographic microscope. We also demonstrate this approach by recovering the quantitative phase images of live yeast cells, red blood cells and dynamics of live dividing yeast cells.

Hilbert reconstruction of phase-shifted second-harmonic holographic images

New techniques are presented that make phase-shifting holography viable for second-harmonic generation (SHG) holography with weak object fields. We developed an intrinsic phase shift calibration of SHG holograms, an algorithm that extracts the reference and object intensity directly from a set of phase-shifted holographic data, and a more robust phase-shifting holography reconstruction algorithm based on π-shifted hologram pairs that permits self-calibration of the phase shift and recovery of the complex field through a Hilbert transform.

Label-free second-harmonic phase imaging of biological specimen by digital holographic microscopy

We have previously developed a new way for nonscanning second-harmonic generation (SHG) microscopy [Opt. Lett. 34, 2450 (2009)]. Based on digital holography, this technique captures, in single-shot hologram acquisition, both the amplitude and the phase of a coherent SHG radiation, which makes possible second harmonic phase microscopy. In this work, we present holographic SHG phase microscopy of a label-free biological tissue and discuss its added value to SHG microscopy.

Imaging through turbid layers by scanning the phase conjugated second harmonic radiation from a nanoparticle

We demonstrate imaging through a turbid layer by using digital phase conjugation of the second harmonic field radiated from a beacon nanoparticle. We show that the phase-conjugated focus can be displaced from its initial position by illuminating the same region of the turbid layer with an angular offset. An image is obtained by scanning the phase-conjugated focus through the turbid layer in a region around the nanoparticle. We obtain a clear image of the target by measuring the light transmitted through it when scanning the focused beam.

Real time, nanometric 3D-tracking of nanoparticles made possible by second harmonic generation digital holographic microscopy

In the past decade, quantitative phase imaging gave a new dimension to optical microscopy, and the recent extension of digital holography techniques to nonlinear microscopy appears very promising, for the phase of nonlinear signal provides additional information, inaccessible to incoherent imaging schemes. In this work, we show that the position of second harmonic generation (SHG) emitters can be determined from their respective phase, at the nanometer scale, with single-shot off-axis digital holography, making possible real-time nanometric 3D-tracking of SHG emitters such as nanoparticles.

Digital phase conjugation of second harmonic radiation emitted by nanoparticles in turbid media

We demonstrate focusing coherent light on a nanoparticle through turbid media based on digital optical phase conjugation of second harmonic generation (SHG) field from the nanoparticle. A SHG active nanoparticle inside a turbid medium was excited at the fundamental frequency and emitted SHG field as a point source. The SHG emission was scattered by the turbid medium, and the scattered field was recorded by off-axis digital holography. A phase-conjugated beam was then generated by using a phase-only spatial light modulator and sent back through the turbid medium, which formed a nearly ideal focus on the nanoparticle.

Label-free second harmonic generation holographic microscopy of biological specimens

Second-order nonlinear holographic microscopy for high-speed, three-dimensional imaging is demonstrated. The use of harmonic generation allows image formation of endogenous features in biological samples such as muscle tissue. We have acquired holograms with acquisition times as short as 10 ms, limited by the switching speed of our shutter; frame rates of 100's of Hz are expected to be possible. The samples are imaged with a Yb:KGW femtosecond laser oscillator, whose 1027 nm wavelength is well suited to minimize absorption and scattering. The low average power of the oscillator prevents damage to the sample.