Ultrafast spin-lasers.

Lasers have both ubiquitous applications and roles as model systems in which non-equilibrium and cooperative phenomena can be elucidated1. The introduction of novel concepts in laser operation thus has potential to lead to both new applications and fundamental insights2. Spintronics3, in which both the spin and the charge of the electron are used, has led to the development of spin-lasers, in which charge-carrier spin and photon spin are exploited. Here we show experimentally that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 gigahertz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation of the resultant spin-laser relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both of which are commonly viewed as detrimental in spintronics3 and conventional lasers4. Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication.

Femtosecond pulse generation from external cavity diode laser based on self-mode-locking.

In this Letter, we report optical pulse generation from a single-section diode gain chip, employed in an external cavity geometry based on the self-mode-locking regime. The gain chip emits light at 1550 nm wavelength range. The external cavity is operated at various repetition rates, ranging from 1 to 2.5 GHz. An optical pulse width of approximately 650 fs is obtained by fitting a Lorentzian distribution. A low RF spectral width of 78.875 kHz is measured corresponding to a low pulse-to-pulse RMS timing jitter of 1.273 ps. This system paves the way towards ultra-compact, cost-effective, and chirp-compensated femtosecond laser pulse sources with adjustable repetition rates.

Digital holography for spatially resolved analysis of the semiconductor optical response.

We present spatially resolved measurements of the below-band-gap carrier-induced absorption and concurrent phase change in a semiconductor with the help of transmission digital holography. The application is demonstrated for a bulk GaAs sample, while the holograms are recorded with a conventional CMOS sensor. We show that the phase information enables spatially resolved monitoring of excess carrier distributions. Based on that, we discuss a phase-based approach for separation of carrier and heat related effects in the semiconductor optical response.

Confocal laser scanning holographic microscopy of buried structures.

In this paper, we present a confocal laser scanning holographic microscope for the investigation of buried structures. The multimodal system combines high diffraction limited resolution and high signal-to-noise-ratio with the ability of phase acquisition. The amplitude and phase imaging capabilities of the system are shown on a test target. For the investigation of buried integrated semiconductor structures, we expand our system with an optical beam induced current modality that provides additional structure-sensitive contrast. We demonstrate the performance of the multimodal system by imaging the buried structures of a microcontroller through the silicon backside of its housing in reflection geometry.

Two-photon polymerization with diode lasers emitting ultrashort pulses with high repetition rate.

In this Letter, we investigate the resolution of two-photon polymerization (2PP) with an amplified mode-locked external cavity diode laser with adjustable pulse length and a high repetition rate. The experimental results are analyzed with a newly developed 2PP model. Even with low pulse peak intensity, the produced structural dimensions are comparable to those generated by traditional 2PP laser sources. Thus, we show that a compact monolithic picosecond laser diode without amplification and with a repetition rate in the GHz regime can also be applied for 2PP. These results show the high application potential of compact mode-locked diode lasers for low-cost and compact 2PP systems.

Monitoring of photochemically induced changes in phase-modulating samples with digital holographic microscopy.

This paper analyzes the performance of single-shot digital holographic microscopy for rapid characterization of static step-index structures in transparent polymer materials and for online monitoring of the photoinduced polymerization dynamics. The experiments are performed with a modified Mach-Zehnder transmission digital holographic microscope of high stability (phase accuracy of 0.69°) and of high magnification (of ≈90×). Use of near-infrared illumination allows both nondestructive examination of the manufactured samples and monitoring of optically induced processes in a photosensitive material concurrently with its excitation. The accuracy of the method for a precise sample's topography evaluation is studied on an example of microchannel sets fabricated via two-photon polymerization and is supported by reference measurements with an atomic force microscope. The applicability of the approach for dynamic measurements is proved via online monitoring of the refractive index evolution in a photoresin layer illuminated with a focused laser beam at 405 nm. High correlation between the experimental results and a kinetics model for the photopolymerization process is achieved.

High-resolution 3D light fluence mapping for heterogeneous scattering media by localized sampling.

We demonstrate an innovative concept for three-dimensional optical fluence mapping in heterogeneous highly scattering media as, e.g., biomedical tissues. We propose to use the relative light extinction analysis principle together with a miniaturized collection fiber in a direct fluence measurement setup as a method to obtain the spatially resolved light intensity distribution under transversally inhomogeneous light propagation conditions and provide local characterization of the transport medium. System performance is validated in two extreme conditions: an optically thin scattering medium and an absorption-dominated light transport. Both extremes demonstrate good agreement to theoretical expectations. Finally, we successfully prove the ability of the system to deliver high-resolution fluence maps through a model study of the light distribution induced in a scattering medium by a vertical diode laser stack with individual bars pitched only 500 µm apart.

Femtosecond semiconductor laser system with resonator-internal dispersion adaptation.

We present a femtosecond laser diode system that is capable of autonomously adjusting itself to compensate for the external dispersion in an arbitrary application. The laser system contains a spatial light modulator inside the cavity which is controlled by an evolutionary algorithm in order to allow for phase and amplitude shaping of the laser emission. The cavity-internal dispersion control is shown to be much more efficient than an external control with a pulse shaper.

Self-optimizing femtosecond semiconductor laser.

A self-optimizing approach to intra-cavity spectral shaping of external cavity mode-locked semiconductor lasers using edge-emitting multi-section diodes is presented. An evolutionary algorithm generates spectrally resolved phase- and amplitude masks that lead to the utilization of a large part of the net gain spectrum for mode-locked operation. Using these masks as a spectral amplitude and phase filter, a bandwidth of the optical intensity spectrum of 3.7 THz is achieved and Fourier-limited pulses of 216 fs duration are generated after further external compression.

Mode-locked semiconductor laser system with intracavity spatial light modulator for linear and nonlinear dispersion management.

We analyze the influence of second and third order intracavity dispersion on a passively mode-locked diode laser by introducing a spatial light modulator (SLM) into the external cavity. The dispersion is optimized for chirped pulses with highest possible spectral bandwidth that can be externally compressed to the sub picosecond range. We demonstrate that the highest spectral bandwidth is achieved for a combination of second and third order dispersion. With subsequent external compression pulses with a duration of 437 fs are generated.

Axial scanning in confocal microscopy employing adaptive lenses (CAL).

In this paper we analyze the capability of adaptive lenses to replace mechanical axial scanning in confocal microscopy. The adaptive approach promises to achieve high scan rates in a rather simple implementation. This may open up new applications in biomedical imaging or surface analysis in micro- and nanoelectronics, where currently the axial scan rates and the flexibility at the scan process are the limiting factors. The results show that fast and adaptive axial scanning is possible using electrically tunable lenses but the performance degrades during the scan. This is due to defocus and spherical aberrations introduced to the system by tuning of the adaptive lens. These detune the observation plane away from the best focus which strongly deteriorates the axial resolution by a factor of ~2.4. Introducing balancing aberrations allows addressing these influences. The presented approach is based on the employment of a second adaptive lens, located in the detection path. It enables shifting the observation plane back to the best focus position and thus creating axial scans with homogeneous axial resolution. We present simulated and experimental proof-of-principle results.

Multiwavelength phase unwrapping and aberration correction using depth filtered digital holography.

In this Letter, we present a new approach to processing data from a standard spectral domain optical coherence tomography (OCT) system using depth filtered digital holography (DFDH). Intensity-based OCT processing has an axial resolution of the order of a few micrometers. When the phase information is used to obtain optical path length differences, subwavelength accuracy can be achieved, but this limits the resolvable step heights to half of the wavelength of the system. Thus there is a metrology gap between phase- and intensity-based methods. Our concept addresses this metrology gap by combining DFHD with multiwavelength phase unwrapping. Additionally, the measurements are corrected for aberrations. Here, we present proof of concept measurements of a structured semiconductor sample.

Depth-filtered digital holography.

We introduce depth-filtered digital holography (DFDH) as a method for quantitative tomographic phase imaging of buried layers in multilayer samples. The procedure is based on the acquisition of multiple holograms for different wavelengths. Analyzing the intensity over wavelength pixel wise and using an inverse Fourier transform leads to a depth-profile of the multilayered sample. Applying a windowed Fourier transform with a narrow window, we choose a depth-of interest (DOI) which is used to synthesize filtered interference patterns that just contain information of this limited depth. We use the angular spectrum method to introduce an additional spatial filtering and to reconstruct the corresponding holograms. After a short theoretical framework we show experimental proof-of-principle results for the method.

Investigation of Laser-Induced Periodic Surface Structures Using Synthetic Optical Holography.

In this paper, the investigation of laser-induced periodic surface structures (LIPSSs) on a polycrystalline diamond substrate using synthetic optical holography (SOH) is demonstrated. While many techniques for LIPSS detection operate with sample contact and/or require preparation or processing of the sample, this novel technique operates entirely non-invasively without any processing of or contact with the LIPSS sample at all. The setup provides holographic amplitude and phase images of the investigated sample with confocally enhanced and diffraction-limited lateral resolution, as well as three-dimensional surface topography images of the periodic structures via phase reconstruction with one single-layer scan only.

Photorefractive two-wave mixing for image amplification in digital holography.

We use photorefractive two-wave mixing for coherent amplification of the object beam in digital holographic recording. Both amplitude and phase reconstruction benefit from the prior amplification as they have an increased SNR. We experimentally verify that the amplification process does not affect the phase of the wavefield. This allows for digital holographic phase analysis after amplification. As the grating formation in photorefractive crystals is just driven by coherent light, the crystal works as a coherence gate. Thus the proposed combination allows for applying digital holography for imaging through scattering media, after the image bearing light is coherence gated and filtered out of scattered background. We show experimental proof-of principle results.

High-throughput characterization of film thickness in thin film materials libraries by digital holographic microscopy.

A high-throughput characterization technique based on digital holography for mapping film thickness in thin-film materials libraries was developed. Digital holographic microscopy is used for fully automatic measurements of the thickness of patterned films with nanometer resolution. The method has several significant advantages over conventional stylus profilometry: it is contactless and fast, substrate bending is compensated, and the experimental setup is simple. Patterned films prepared by different combinatorial thin-film approaches were characterized to investigate and demonstrate this method. The results show that this technique is valuable for the quick, reliable and high-throughput determination of the film thickness distribution in combinatorial materials research. Importantly, it can also be applied to thin films that have been structured by shadow masking.

Applying machine learning to optical coherence tomography images for automated tissue classification in brain metastases.

PURPOSE: A precise resection of the entire tumor tissue during surgery for brain metastases is essential to reduce local recurrence. Conventional intraoperative imaging techniques all have limitations in detecting tumor remnants. Therefore, there is a need for innovative new imaging methods such as optical coherence tomography (OCT). The purpose of this study is to discriminate brain metastases from healthy brain tissue in an ex vivo setting by applying texture analysis and machine learning algorithms for tissue classification to OCT images. METHODS: Tumor and healthy tissue samples were collected during resection of brain metastases. Samples were imaged using OCT. Texture features were extracted from B-scans. Then, a machine learning algorithm using principal component analysis (PCA) and support vector machines (SVM) was applied to the OCT scans for classification. As a gold standard, an experienced pathologist examined the tissue samples histologically and determined the percentage of vital tumor, necrosis and healthy tissue of each sample. A total of 14.336 B-scans from 14 tissue samples were included in the classification analysis. RESULTS: We were able to discriminate vital tumor from healthy brain tissue with an accuracy of 95.75%. By comparing necrotic tissue and healthy tissue, a classification accuracy of 99.10% was obtained. A generalized classification between brain metastases (vital tumor and necrosis) and healthy tissue was achieved with an accuracy of 96.83%. CONCLUSIONS: An automated classification of brain metastases and healthy brain tissue is feasible using OCT imaging, extracted texture features and machine learning with PCA and SVM. The established approach can prospectively provide the surgeon with additional information about the tissue, thus optimizing the extent of tumor resection and minimizing the risk of local recurrences.

Lensless digital holographic microscopy as an efficient method to monitor enzymatic plastic degradation.

A big challenge of the 21st century is to cope with the huge amounts of plastic waste on Earth. Especially the oceans are heavily polluted with plastics. To counteract this issue, biological (enzymatic) plastic decomposition is increasingly gaining attention. Recently it was shown that polyethylene terephthalate (PET) can be degraded in a saltwater-based environment using bacterial PETase produced by a marine diatom. At moderate temperatures, plastic biodegradation is slow and requires sensitive methods for detection, at least at initial stages. However, conventional methods for verifying the plastic degradation are either complex, expensive, time-consuming or they interfere with the degradation process. Here, we adapt lensless digital holographic microscopy (LDHM) as a new application for efficiently monitoring enzymatic degradation of a PET glycol copolymer (PETG). LDHM is a cost-effective, compact and sensitive optical method. We demonstrate enzymatic PETG degradation over a time course of 43 days employing numerical analysis of LDHM images.

Prostate Cancer Foundation Hormone-Sensitive Prostate Cancer Biomarker Working Group Meeting Summary.

Androgen deprivation therapy remains the backbone therapy for the treatment of metastatic hormone-sensitive prostate cancer (mHSPC). In recent years, several treatments, including docetaxel, abiraterone + prednisone, enzalutamide, and apalutamide, have each been shown to demonstrate survival benefit when used upfront along with androgen deprivation therapy. However, treatment selection for an individual patient remains a challenge. There is no high level clinical evidence for treatment selection among these choices based on biological drivers of clinical disease. In August 2020, the Prostate Cancer Foundation convened a working group to meet and discuss biomarkers for hormone-sensitive prostate cancer, the proceedings of which are summarized here. This meeting covered the state of clinical and biological evidence for systemic therapies in the mHSPC space, with emphasis on charting a course for the generation, interrogation, and clinical implementation of biomarkers for treatment selection.

Femtosecond passively modelocked diode laser with intracavity dispersion management.

We report on the generation of ultrashort pulses by dispersion management of a passively modelocked external cavity diode laser. Pulse widths down to 200 fs are obtained at 830 nm emission wavelength. We use intracavity dispersion management to increase the spectral bandwidth and compress the strongly chirped pulses externally with a grating compressor.