arXiv subjects

Demeng Feng

Publications and source records attributed to Demeng Feng.

7 recordsLinked to original sources

arXiv2025

Visible Imaging of Incoherent 1200-nm Light via Triplet--Triplet Annihilation Upconversion

Upconversion of low-energy photons to higher-energy photons provides an opportunity to surpass traditional limitations in fields such as 3D printing, photovoltaics, and photocatalysis. Triplet--triplet annihilation upconversion (TTA-UC) is particularly appealing for such applications as it can efficiently upconvert low-intensity, incoherent light. However, previously demonstrated thin-film TTA systems are simultaneously constrained by modest efficiencies and limited reach into the near infrared (NIR). Here, we design a single-layer thin-film bulk heterojunction that integrates PbS quantum dots (QDs) as tunable NIR absorbers within an organic semiconductor matrix of TES-ADT, achieving large anti-Stokes shifts up to 500 nm and high internal quantum efficiencies across the NIR-I and NIR-II windows (800-1200 nm). Through the incorporation of 5-tetracene carboxylic acid ligands on the PbS QD surface, the yield of sensitized triplets was boosted, as confirmed by transient absorption and time-resolved photoluminescence measurements. The resulting films demonstrated a 15-fold improvement in UC efficiency. Furthermore, we demonstrate visible imaging of incoherent 1200 nm light via thin-film TTA-UC at incident intensities at the imaging mask as low as 20 mWcm$^2$, marking a significant advance toward practical implementation of solid-state NIR-to-visible upconversion.

physics.optics↗

arXiv2024

All-passive upconversion of incoherent near-infrared light at intensities down to 10$^{-7}$ W/cm$^2$

Frequency upconversion, which converts low-energy photons into higher-energy ones, typically requires intense coherent illumination to drive nonlinear processes or the use of externally driven optoelectronic devices. Here, we demonstrate an upconversion system that converts low-intensity (down to ~10-7 W/cm$^2$) incoherent near-infrared (NIR) light into the visible, reaching intensities perceptible by the human eye, without the use of any external power input. Our upconverting element is enabled by the following ingredients: (1) photon upconversion via triplet-triplet annihilation in a bulk heterojunction of the organic semiconductors Y6 and rubrene; (2) plasmonic enhancement of absorption and field intensity in the heterojunction layer; (3) collection enhancement using a dichroic thin-film assembly. To enable high-resolution imaging, the upconverting element is inserted at an intermediate image plane of a dual-wavelength telescope system, which preserves the relative directionality of rays between the incident NIR light and output visible light. Our all-passive upconversion imaging system will enable NIR imaging and sensing in low-light environments under energy constraints.

physics.optics↗

arXiv2024

Preventing overfitting in infrared ellipsometry using temperature dependence: fused silica as a case study

Fitting oscillator models to variable-angle spectroscopic ellipsometry (VASE) data can lead to non-unique, unphysical results. We demonstrate using temperature-dependent trends to prevent overfitting and ensure model physicality. As a case study, we performed mid-infrared VASE measurements on fused silica (SiO2) of various grades, from room temperature to 600 {\deg}C. We fitted oscillator models independently at each temperature, and confirmed the model's physical validity by observing the expected monotonic trends in vibrational oscillator parameters. Using this technique, we generated a highly accurate dataset for the temperature-dependent complex refractive index of fused silica for modeling mid-infrared optical components such as thermal emitters.

physics.optics↗

arXiv2024

Self-referencing photothermal common-path interferometry to measure absorption of Si3N4 membranes for laser-light sails

Laser-light sails are a spacecraft concept wherein lightweight "sails" are propelled by high-intensity lasers. We investigated the near-infrared absorption of free-standing membranes of stoichiometric silicon nitride (Si$_3$N$_4$), a candidate sail material. To resolve the small but non-zero optical loss, we used photothermal common-path interferometry (PCI), for which we developed a self-referencing modality where a PCI measurement is performed twice: once on a bare membrane, and a second time with monolayer graphene deposited on the membrane. The graphene increases the absorption of the sample by orders of magnitude, such that it can be measured by ellipsometry, without significantly affecting the thermal properties. We measured the absorption coefficient of Si$_3$N$_4$ to be (1.5-3) $\times$ 10$^{-2}$ cm$^{-1}$ at 1064 nm, making it a suitable sail material for laser intensities as high as ~10 GW/m$^2$. By comparison, silicon-rich "low stress" SiN$_x$ (x~1), with a measured absorption coefficient of approximately 8 cm$^{-1}$, is unlikely to survive such high laser intensities. Our self-referencing technique enables testing of low-loss membranes of various materials for laser sails and other applications.

physics.optics↗

arXiv2023

The Effect of Dust and Hotspots on the Thermal Stability of Laser Sails

Laser sails propelled by gigawatt-scale ground-based laser arrays have the potential to reach relativistic speeds, traversing the solar system in hours and reaching nearby stars in years. Here, we describe the danger interplanetary dust poses to the survival of a laser sail during its acceleration phase. We show through multi-physics simulations how localized heating from a single optically absorbing dust particle on the sail can initiate a thermal-runaway process that rapidly spreads and destroys the entire sail. We explore potential mitigation strategies, including increasing the in-plane thermal conductivity of the sail to reduce the peak temperature at hotspots and isolating the absorptive regions of the sail which can burn away individually.

physics.space-ph↗

arXiv2021

Thermal runaway of silicon-based laser sails

Laser sail-based spacecraft -- where a powerful earth-based laser propels a lightweight outer-space vehicle -- have been recently proposed by the Breakthrough Starshot Initiative as a means of reaching relativistic speeds for interstellar spacetravel. The laser intensity at the sail required for this task is at least 1 GW$\cdot$m$^{-2}$ and, at such high intensities, thermal management of the sail becomes a significant challenge even when using materials with low absorption coefficients. Silicon has been proposed as one leading candidate material for the sail due to its low sub-bandgap absorption and high index of refraction, which allows for low-mass-density designs. However, here we show that the temperature-dependent bandgap of silicon combined with two-photon absorption processes can lead to thermal runaway for even the most optimistic viable assumptions of the material quality. From our calculations, we set bounds on the maximum laser intensities that can be used for a thermally stable, Si-based laser sail.

physics.optics↗

arXiv2021

Passive frequency conversion of ultraviolet images into the visible using perovskite nanocrystals

We demonstrate a passive down-conversion imaging system that converts broadband ultraviolet light to narrow-band green light while preserving the directionality of rays, and thus enabling direct down-conversion imaging. At the same time our system has high transparency in the visible, enabling superimposed visible and ultraviolet imaging. The frequency conversion is performed by a subwavelength-thickness transparent downconverter based on highly efficient CsPbBr3 nanocrystals incorporated into the focal plane of a simple telescope or relay-lens geometry. The resulting imaging performance of this down-conversion system approaches the diffraction limit. This demonstration sets the stage for the incorporation of other high-efficiency perovskite nanocrystal materials to enable passive multi-frequency conversion imaging systems.

physics.optics↗