arXiv subjects

Eric O. Potma

Publications and source records attributed to Eric O. Potma.

At least 19 recordsLinked to original sources

arXiv2026

Nanometric voids as optical antennas for rewritable momentum-engineered photonics in silicon

Optical antennas are widely used to localize electromagnetic fields far below the diffraction limit, enabling enhanced light-matter interactions across nanophotonics. Yet the regime in which optical confinement approaches the electronic de Broglie wavelength in a solid - where the photon momentum distribution broadens sufficiently to relax optical selection rules - remains largely unexplored. Here we show that nanometric voids embedded within crystalline silicon act as such optical antennas, dramatically altering the optical response of an indirect semiconductor without the introduction of any foreign material. Using an electrically induced melt-quench process, we generate nanometric voids throughout bulk silicon, confirmed by high-resolution electron microscopy, diffraction analysis, Fourier-filtered lattice reconstruction, elemental mapping, and supported by optical and vibrational spectroscopies. The void-containing silicon exhibits intense broadband photo- and electroluminescence spectrally indistinguishable from that produced by metallic or semiconductor nanoconfiners of similar dimensions, establishing that dielectric discontinuity, not confiner composition, governs the observed momentum-assisted optical transitions. The luminescence can be repeatedly written, erased, and rewritten through alternating electrical conditioning and optical recrystallization. These findings establish nanometric voids as a previously unexplored platform for extreme optical confinement and demonstrate that photonic functionality can be embedded and reconfigured directly within bulk silicon.

physics.optics↗

arXiv2026

Broadband Photo- and Electroluminescence from Bulk Silicon via Strong Photonic Confinement

Silicon indirect bandgap fundamentally limits its ability to emit light, hindering the development of silicon-based light sources. Here, we explore a conceptually new solution to this long-standing challenge. We demonstrate ultrabroadband photo- and electroluminescence from bulk silicon, enabled by a radiative pathway mediated by momentum-expanded photonic states that bypass phonon-assisted transitions. This mechanism, previously demonstrated using metallic nanoparticles as photon confiners, is here realized in an all-silicon system using embedded sub-1.5 nm silicon nanoparticles. Since such ultrasmall particles possess negligible intrinsic emission efficiency, we instead demonstrate that they act as photonic confiners, enabling radiative recombination in the surrounding bulk material. The agreement with prior metal-based systems confirms that confinement size, rather than material composition, governs the activation of radiative transitions in a momentum-forbidden system. The emission spans the visible to near-infrared spectral range, with electroluminescence in an undoped semiconductor device visible under ambient conditions and a quantum efficiency estimated as ~0.2%. These findings establish a new route to efficient light emission in silicon and reveal a hybrid light-matter regime in which extreme photonic confinement reshapes the electronic transition landscape.

physics.optics↗

arXiv2026

Video-rate mid-infrared imaging in the molecular fingerprint region via nanosecond non-degenerate two-photon absorption

Non-degenerate two-photon absorption (NTA) offers an attractive route for wide-field mid-infrared (MIR) imaging by mapping long wavelength information into the spectral detection windows of mature near-infrared detector technologies. However, existing NTA implementations rely almost exclusively on complex, large-footprint femtosecond laser systems, severely limiting practicality and scalability. Here, we demonstrate an NTA imaging platform that replaces the ultrafast laser with a compact nanosecond mid-IR source coupled to a high-definition indium gallium arsenide (InGaAs) camera. Operating in the nanosecond regime removes stringent temporal-overlap requirements, dramatically simplifying system architecture while preserving high nonlinear sensitivity. Using this approach, we achieve chemically selective, wide-field imaging deep into the mid-IR molecular fingerprint region and demonstrate, for the first time, video-rate NTA imaging in this spectrally rich regime. By combining relaxed alignment constraints, compact excitation, and high-speed fingerprint-region imaging, this work establishes nanosecond NTA as a practical and scalable foundation for next-generation mid-IR chemical imaging.

physics.optics↗

arXiv2025

Overcoming the indirect bandgap: efficient silicon emission via momentum-expanded photonic states

Silicon's inherently indirect bandgap severely limits its radiative efficiency, posing a fundamental challenge to the development of practical silicon-based light sources. While strategies such as nanoscale confinement of electrons and holes (quantum dots), Mie resonators, and hybrid plasmonic structures have improved emission, they typically require complex fabrication workflows. Here, we demonstrate a conceptually distinct and scalable approach to enable light emission from a bulk silicon wafer by decorating its surface with gold or copper nanoparticles. Remarkably, the effect is nearly identical for Au and Cu, with particle size emerging as the dominant factor. We show that strong luminescence from the bulk wafer emerges only when the nanoparticle diameter is below 2 nm. We attribute this effect to the formation of spatially confined photonic states with broadened momentum distributions, which must enable diagonal, phonon-independent optical transitions that bypass the limitations imposed by silicon's indirect bandgap. This mechanism yields broadband emission across the visible and near-infrared spectrum, with quantum efficiencies comparable to direct bandgap semiconductors, representing a 10^5-fold increase in integrated spectral intensity. This discovery challenges the conventional understanding of silicon's optical constraints and opens a practical pathway toward high-performance silicon-based optical and optoelectronic components.

physics.optics↗

arXiv2025

Nonlinear optical microscopy with an obscuration-free, freeform reflective objective

Nonlinear optical (NLO) imaging platforms traditionally rely on refractive microscope objectives, which suffer from chromatic aberrations and temporal dispersion of pulsed excitation light. These issues degrade spatial imaging properties and signal brightness. Furthermore, the limited transmission range of refractive materials restricts NLO imaging, especially for applications requiring short- to mid-wave infrared excitation. While reflective microscope objectives offer an achromatic solution and broader transmission range (from visible to mid-infrared), conventional Schwarzschild designs have a central obscuration, which limits transmission throughput, imparts diffraction effects into the images, and, more generally, hinders the adoption of reflective optics in NLO microscopy. We overcome these obscuration-based limitations by developing a novel, reflective microscope objective using freeform mirrors in a non-coaxial geometry. This innovative, obscuration-free design boasts a 0.65 numerical aperture (NA), near diffraction-limited imaging performance, and offers significantly improved transmission with wider fields-of-view. We demonstrate its utility by integrating it into a standard laser-scanning microscope and performing NLO microscopy across a wide range of excitation wavelengths. Our freeform microscope objective outperforms standard reflective designs, providing an achromatic, dispersion-free alternative to refractive lenses for NLO imaging.

physics.optics↗

arXiv2024

Two-photon absorption in silicon using real density matrix approach

Two-photon absorption in indirect gap semiconductors is an frequently encountered, but not well-understood phenomenon. To address this, the Real Density Matrix Approach is applied to describe two-photon absorption in silicon through the excitonic response to the interacting fields. This approach produces an analytical expression for the dispersion of the two-photon absorption coefficient for indirect-gap materials, and can be used to explain trends in reported experimental data for bulk silicon both old and new with minimal fitting.

physics.optics↗

arXiv2024

Photon momentum enabled light absorption in bulk silicon

Photons do not carry sufficient momentum to induce indirect optical transitions in semiconducting materials such as silicon, necessitating the assistance of lattice phonons to conserve momentum. Compared to direct bandgap semiconductors, this renders silicon a less attractive material for a wide variety of optoelectronic applications. In this work, we introduce an alternative strategy to fulfill the momentum-matching requirement in indirect optical transitions. We demonstrate that when confined to scales below ~3 nm, photons acquire sufficient momentum to allow electronic transitions at the band edge of Si without the assistance of a phonon. Confined photons allow simultaneous energy and momentum conservation in two-body photon-electron scattering; in effect, converting silicon into a direct bandgap semiconductor. We show that this less-explored concept of light-matter interaction leads to a marked increase in the absorptivity of Si from the UV to the near-IR. The strategy provides opportunities for more efficient use of indirect semiconductors in photovoltaics, energy conversion, light detection and emission.

physics.optics↗

arXiv2023

Electronic Raman scattering in silicon glass

The nature of enhanced photoemission in disordered and amorphous solids is an intriguing open question. A point in case is light emission in silicon, which occurs when the material is porous or nanostructured, but the effect is absent in the bulk crystalline phase, a phenomenon that is still not fully understood. In this work, we study structural photoemission in a heterogeneous cross-linked silicon glass, a material that represents an intermediate state between the amorphous and crystalline phases, characterized by a narrow distribution of structure sizes. This model system shows a clear dependence of photoemission on size and disorder across a broad range of energies. While phonon-assisted indirect optical transitions are insufficient to describe observable emissions, our experiments suggest these can be understood through electronic Raman scattering instead. This phenomenon, not commonly observed in crystalline semiconductors, is driven by structural disorder. We attribute photoemission in this disordered system to the presence of an excess electron density of states within the forbidden gap (Urbach bridge), where electrons occupy trapped states. Transitions from gap states to the conduction band are facilitated through electron-photon momentum matching, which resembles Compton scattering, but observed for visible light and driven by the enhanced momentum of a photon confined within the nanostructured domains. We interpret the light emission in structured silicon glass as resulting from electronic Raman scattering. These findings emphasize the role of photon momentum in the optical response of solids that display disorder at the nanoscale.

physics.optics↗

arXiv2023

Light-controlled multi-phase structuring of perovskite crystal enabled by thermoplasmonic metasurface

Halide perovskites belong to an important family of semiconducting materials with unique electronic properties that enable a myriad of applications, especially in photovoltaics and optoelectronics. Their optical properties, including photoluminescence quantum yield, are affected and notably enhanced at crystal imperfections where the symmetry is broken and the density of states increases. These lattice distortions can be introduced through structural phase transitions, allowing charge gradients to appear near the interfaces between phase structures. In this work, we demonstrate controlled multi-phase structuring in a single perovskite crystal. The concept uses cesium lead bromine (CsPbBr3) placed on a thermoplasmonic TiN/Si metasurface and enables single, double and triple phase structures to form on demand above the room temperature. This approach opens up application horizons of dynamically controlled heterostructures with distinctive electronic and enhanced optical properties.

physics.optics↗

arXiv2022

Spectral imaging at high-definition and high speed in the mid-infrared

Spectral imaging in the mid-infrared (MIR) range provides simultaneous morphological and chemical information of a wide variety of samples. However, current MIR technologies struggle to produce high-definition images over a broad spectral range at acquisition rates that are compatible with real-time processes. We present a novel spectral imaging technique based on non-degenerate two-photon absorption of temporally chirped optical MIR pulses. This new approach avoids complex image processing or reconstruction and enables high-speed acquisition of spectral data cubes [xyw] at high pixel density in under a second.

physics.optics↗

arXiv2022

Force Detection of Electromagnetic Beam Chirality at the Nanoscale

Many nanophotonic applications require precise control and characterization of electromagnetic field properties at the nanoscale. The chiral properties of the field are among its key characteristics, yet measurement of optical chirality at dimensions beyond the diffraction limit has proven difficult. Here we theoretically show that the chiral properties of light can be characterized down to the nanometer scale by means of force detection. To measure the chiral properties of a beam of given handedness at the nanoscale, we determine the photo-induced force exerted on a sharp tip, which is illuminated first by the beam of interest and second by an auxiliary beam of opposite handedness, in a sequential manner. We show that the difference between the force measurements is directly proportional to the chiral properties of the beam of interest. In particular, the gradient force difference $Δ\langle F_{grad, z}\rangle$ is found to have exclusive correspondence to the time-averaged helicity density of the incident light, whereas the differential scattering force provides information about the spin angular momentum density of light. We further characterize and quantify the helicity-dependent $Δ\langle F_{grad, z}\rangle$ using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range.

physics.optics↗

arXiv2021

Photoinduced Magnetic Force Microscopy: Enabling Direct and Exclusive Detection of Optical Magnetism

Modern optical nano-elements pursue ever-smaller sizes and individualized functionalities. Those elements that can efficiently manipulate the magnetic field of light boast promising future applications with a great challenge: the magnetic near field is irretrievable from conventional optical far-field characterization. Here we propose photoinduced magnetic force microscopy to directly and exclusively sense the magnetic field of light at the nanoscale. The proposed instrument exploits a magnetic nanoprobe with exclusive magnetic excitation under structured light illumination. The magnetic nanoprobe detects the photoinduced magnetic force, which is defined as the dipolar Lorentz force exerted on the photoinduced magnetic dipole in the nanoprobe. Since the resulting magnetic force is proportional to the incident magnetic field, the measured force reveals the magnetic near-field distribution at the nanoscale. The proposed instrument represents a fundamental step towards comprehensive electric and magnetic near-field detection and/or manipulation in single nano-element optical devices.

physics.optics↗

arXiv2021

High-speed 2D and 3D mid-IR imaging with an InGaAs camera

Recent work on mid-infrared (MIR) detection through the process of non-degenerate two-photon absorption (NTA) in semiconducting materials has shown that wide-field MIR imaging can be achieved with standard Si cameras. While this approach enables MIR imaging at high pixel densities, the low nonlinear absorption coefficient of Si prevents fast NTA-based imaging at lower illumination doses. Here we overcome this limitation by using InGaAs as the photosensor. Taking advantage of the much higher nonlinear absorption coefficient of this direct bandgap semiconductor, we demonstrate high-speed MIR imaging up to 500 fps with under 1 ms exposure per frame, enabling 2D or 3D mapping without pre- or post-processing of the image.

physics.optics↗

arXiv2021

Rapid chemically selective 3D imaging in the mid-infrared with a Si-based camera

The emerging technique of mid-infrared optical coherence tomography (MIR-OCT) takes advantage of the reduced scattering of MIR light in various materials and devices, enabling tomographic imaging at deeper penetration depths. Because of challenges in MIR detection technology, the image acquisition time is however significantly longer than for tomographic imaging methods in the visible/near-infrared. Here we demonstrate an alternative approach to MIR tomography with high-speed imaging capabilities. Through femtosecond non-degenerate two-photon absorption of MIR light in a conventional Si-based CCD camera, we achieve wide-field, high-definition tomographic imaging with chemical selectivity of structured materials and biological samples in mere seconds.

physics.optics↗

arXiv2020

Simple all-optical method for in situ detection of ultralow amounts of ammonia

As a key precursor for nitrogenous compounds and fertilizer, ammonia affects our lives in numerous ways. Rapid and sensitive detection of ammonia is essential, both in environmental monitoring and in process control for industrial production. Here we report a novel and nonperturbative method that allows rapid detection of ammonia at detection levels of only a few thousand molecules, based on the non-contact, all-optical detection of surface-enhanced Raman signals. We show that this simple and affordable approach enables ammonia probing at selected regions of interest with high spatial resolution, making in situ and operando observations possible.

physics.app-ph↗

arXiv2020

Infrared chemical imaging through nondegenerate two-photon absorption in silicon-based cameras

Chemical imaging based on mid-infrared (MIR) spectroscopic contrast is an important technique with a myriad of applications, including biomedical imaging and environmental monitoring. Current MIR cameras, however, lack in performance and are much less affordable compared to mature Si-based devices, which operate in the visible and near-infrared. Here we demonstrate fast MIR chemical imaging through non-degenerate two-photon absorption (NTA) in a standard Si-based charge-coupled device (CCD). We show that wide-field MIR images can be obtained at 100 ms exposure times using picosecond pulse energies of only a few fJ per pixel through NTA directly on the CCD chip. Because this on-chip approach does not rely on phase-matching, it is alignment-free and does not necessitate complex post-processing of the images. We emphasize the utility of this technique through chemically selective MIR imaging of polymers and biological samples, including MIR videos of moving targets, physical processes and live nematodes.

physics.ins-det↗

arXiv2018

Nanoscale spectroscopic studies of two different physical origins of the tip-enhanced force: dipole and thermal

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can con-tribute to the measured photo-induced force simultaneously. Of particular interest are the instantaneous force be-tween the induced dipoles in the tip and in the sample and the force related to thermal heating of the junction. A key difference between these two force mechanisms is their spectral behaviors. The magnitude of the thermal response follows a dissipative Lorentzian lineshape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Be-cause the two interactions are sometimes comparable in magnitude, the origin of the nanoscale chemical selectivity in the recently developed photo-induced force microscopy (PiFM) is often unclear. Here, we demonstrate theoretically and experimentally how light absorption followed by nanoscale thermal expansion generates a photo-induced force in PiFM. Furthermore, we explain how this thermal force can be distinguished from the induced dipole force by tuning the relaxation time of samples. Our analysis presented here helps the interpretation of nanoscale chemical measure-ments of heterogeneous materials and sheds light on the nature of light-matter coupling in van der Waals materials.

physics.optics↗

arXiv2017

Effect of Scatterering on Coherent Anti-Stokes Raman Scattering (CARS) signals

We develop a computational framework to examine the factors responsible for scattering-induced distortions of coherent anti-Stokes Raman scattering (CARS) signals in turbid samples. We apply the Huygens-Fresnel Wave-based Electric Field Superposition (HF-WEFS) method combined with the radiating dipole approximation to compute the effects of scattering-induced distortions of focal excitation fields on the far-field CARS signal. We analyze the effect of spherical scatterers, placed in the vicinity of the focal volume, on the CARS signal emitted by different objects (2μm diameter solid sphere, 2μm diameter myelin cylinder and 2μm diameter myelin tube). We find that distortions in the CARS signals arise not only from attenuation of the focal field but also from scattering-induced changes in the spatial phase that modifies the angular distribution of the CARS emission. Our simulations further show that CARS signal attenuation can be minimized by using a high numerical aperture condenser. Moreover, unlike the CARS intensity image, CARS images formed by taking the ratio of CARS signals obtained using x- and y-polarized input fields is relatively insensitive to the effects of spherical scatterers. Our computational framework provide a mechanistic approach to characterizing scattering-induced distortions in coherent imaging of turbid media and may inspire bottom-up approaches for adaptive optical methods for image correction.

physics.comp-ph↗