Advances in Information Security

The need for high-resolution MeV x-ray tomography to observe the three-dimensional structure of dense, large-sized objects is rapidly increasing for the non-destructive evaluation of critical additively manufactured parts, national security, and other applications. We report a demonstration of high-resolution MeV computed tomography of a dense, large object with a laser-driven x-ray source. A record detector-limited MeV radiograph resolution of <200µm as determined with the Bennett approximation of the point spread function was achieved by irradiating millimeter-thick tungsten targets with 300 TW femtosecond laser pulses at a 0.5 Hz repetition rate. A tungsten alloy step wedge spectrometer indicates that the peak of the x-ray emission is between 1 and 2 MeV, with an endpoint energy of 19 MeV. To illustrate the radiographic imaging capability of the system, a tomographic reconstruction of a nickel superalloy turbine blade (maximumρr=139g/cm2) with sub-millimeter resolution was performed using 2160 individual radiographs. The small x-ray source size opens the prospect of extremely high-resolution tomographs of large, dense objects. This laser-driven approach has major advantages for non-destructive evaluation.

On-chip nonlinear photonic conversion functions with wide and precise tunability, as well as high conversion efficiency, are highly desirable for a wide range of applications. Photonic crystal micro-ring resonators facilitate efficient nonlinear conversion and enable wavenumber-accurate selection of converted optical modes, but they do not support post-fabrication reconfiguration of these operational modes. Coupled-ring resonators, on the other hand, allow post-fabrication reconfiguration but suffer from ambiguity in mode selectivity. We propose a segmented photonic crystal micro-ring resonator featuring half-circumference gratings that decouples the locking between the grating Bragg reflection peak and micro-ring resonance frequencies. By introducing complementary thermo-optic controllers that allow differential tuning between the grating reflection peak and the micro-ring resonance, the device supports electrically reconfigurable wavenumber-accurate optical mode selectivity, experimentally demonstrated as a voltage-tunable, power-efficient optical parametric oscillator. The device demonstrates electric tuning of signal and idler frequencies both in a per-free spectral range stepwise manner and in a gap-free continuous manner, achieving a broad optical frequency tuning range of and a conversion efficiency of >5THz. This approach introduces design flexibility, as well as high and precise reconfigurability, to integrated nonlinear photonics, providing a pathway toward future high-performance on-chip nonlinear light sources.

The optical lever is a precision displacement sensor with broad applications. In principle, it can track the motion of a mechanical oscillator with added noise at the standard quantum limit (SQL); however, demonstrating this performance requires an oscillator with exceptionally high torque sensitivity or, equivalently, zero-point angular displacement spectral density. Here, we describe optical lever measurements on nanoribbons possessing torsion modes with torque sensitivities of and zero-point displacement spectral densities of . By compensating for aberrations and leveraging immunity to classical intensity noise, we realize angular displacement measurements with imprecisions 20 dB below the SQL and demonstrate feedback cooling, using a position-modulated laser beam as a torque actuator, from room temperature to Si3N4 phonons. Our study signals the potential for a new class of torsional quantum optomechanics.

Chip-scale broadband light sources in the mid-infrared spectral range are highly desirable for the development of compact spectroscopy and sensing instrumentation in the molecular fingerprint region. Recently, generation of sub-picosecond pulses with over 5 W of peak optical power was demonstrated from mode-locked mid-infrared quantum cascade lasers. Monolithic integration of these lasers with a waveguide for supercontinuum generation may dramatically broaden the emission spectrum of these devices. Here we report mid-infrared supercontinuum generation in dielectric waveguides with, to our knowledge, a record-low peak pump power of only approximately 6 W for a pump wavelength of 5 µm, which is required to double the spectral bandwidth of the pump pulse. This pump power level is more than an order of magnitude lower than the current state of the art. Our results pave the way towards the development of an electrically pumped chip-scale mid-infrared supercontinuum source based on a monolithic integration of a quantum cascade laser pump with passive waveguides on an InP platform.

We introduce a reflection-mode diffraction tomography technique that enables the simultaneous recovery of forward- and backward-scattering information for high-resolution 3D refractive index reconstruction. Our technique works by imaging a sample on a highly reflective substrate and employing a multiple-scattering model and a reconstruction algorithm. It combines the modified Born series as the forward model, Bloch and perfect electric conductor boundary conditions to handle oblique incidence and substrate reflections, and the adjoint method for efficient gradient computation in solving the inverse-scattering problem. We validate the technique through simulations and experiments, achieving accurate reconstructions in samples with high refractive index contrasts and complex geometries. Forward scattering captures smooth axial features, while backward scattering reveals complementary interfacial details. Experimental results on dual-layer resolution targets, 3D randomly distributed beads, phase structures obscured by highly scattering fibers, fixed breast cancer cells, and fixed C. elegans demonstrate its robustness and versatility. This technique holds promise for applications in semiconductor metrology and biomedical imaging.