Xianfan Xu

The widescale implementation of two-photon polymerization (2PP) is limited primarily due to the cost of high-power femtosecond laser despite the high throughput and superior resolution capability. To reduce the threshold power requirement, a method is developed combining the effect of single photon absorption from a low-cost 532 nm nanosecond fiber laser with the two-photon absorption from an 800 nm femtosecond laser. The effects of spatial and temporal overlap, relative beam sizes, and frequency synchronization are also investigated.

The goal of 3D printing is to realize complex 3D structures by locally adding material in small volume elements called voxels — in contrast to successively subtracting material by etching, milling or machining. This field started with optics-based proposals in the 1970s. Progress has required breakthroughs in physics, chemistry, materials science, laser science and engineering. This Review focuses on the physics underlying optics-based approaches, including interference lithography, tomographic volumetric additive manufacturing, stereolithography, continuous liquid-interface printing, light-sheet printing, parallelized spatiotemporal focusing and (multi-)focus scanning. Light–matter interactions that are discussed include one-photon, two-photon, multi-photon or cascaded nonlinear optical absorption processes for excitation and stimulated-emission depletion or excited-state absorption followed by reverse …

AbstractFocusing light to a highly confined spot with subwavelength size and high intensity has important applications in nanolithography, optical and magnetic data storage, spectroscopy, biosensing, molecular trapping, etc. However, light diffraction sets a fundamental limit on the far-field optical resolution on the order of wavelength, and it poses a critical challenge to the downscaling of nanoscale optical focusing. At the optical near-field, the light refraction also restricts the energy transmission through the subwavelength aperture with an extremely low efficiency that is proportional to (d/λ)4 [1], where d is the diameter of the aperture and λ is the wavelength of illumination light. As discussed in previous chapters, efficient light focusing can also be realized by taking advantage of its large wavevector of surface plasmon polaritons (SPPs). SPPs can be excited by incident light on some metallic interface and propagate …

Chirality arises from the asymmetry of materials, where two counterparts are the mirror image of each other. The interaction between circular-polarized light and quantum materials is enhanced in chiral space groups due to the structural chirality. Tellurium (Te) possesses the simplest chiral crystal structure, with Te atoms covalently bonded into a spiral atomic chain (left- or right-handed) with a periodicity of 3. Here, we investigate the tunable circular photoelectric responses in 2D Te field-effect transistors with different chirality, including the longitudinal circular photogalvanic effect induced by the radial spin texture (electron-spin polarization parallel to the electron momentum direction) and the circular photovoltaic effect induced by the chiral crystal structure (helical Te atomic chains). Our work demonstrates the controllable manipulation of the chirality degree of freedom in materials.

We use spatiotemporal focusing and imaging of ultrafast laser pulses to implement a projection two-photon polymerization process in a continuous fashion to fabricate complex 3D structures at a large print rate. Rapid fabrication of millimeter scale structures is achievable with this continuous, layer-by-layer projection two-photon process.

The lattice thermal conductivity (κ) of two ceramic materials, cerium dioxide (CeO 2) and magnesium oxide (MgO), is computed up to 1500 K using first principles and the phonon Boltzmann transport equation (PBTE) and compared to time-domain thermoreflectance (TDTR) measurements up to 800 K. Phonon renormalization and the four-phonon effect, along with high-temperature thermal expansion, are integrated in our ab initio molecular dynamics calculations. This is done by first relaxing structures and then fitting to a set of effective force constants employed in a temperature-dependent effective potential method. Both three-phonon and four-phonon scattering rates are computed based on these effective force constants. Our calculated thermal conductivities from the PBTE solver agree well with the literature and our TDTR measurements. Other predicted thermal properties including thermal expansion, frequency …

In recent years there have been notable efforts to make two-photon lithography more efficient and faster while avoiding trade-offs in resolution and print quality. The projection two-photon printing scheme achieves high-throughput while maintaining useful feature sizes. However, due to limitations of the temporal focusing process implemented, as well as photoresist kinetics, the resolution of this process does not yet reach that of single focus scanning two-photon lithography. This work explores the photoresist systems used for projection printing and the effect of additional optical beams for enhancing the printing capabilities of two-photon projection printing.

The spatiotemporal dynamics of energy carriers in silicon is studied under ultrafast laser excitation. A subpicosecond temporal resolution and precise, 10 nm spatial resolution system is used to study the spatiotemporal optical response of silicon. Two distinct sets of temporal information from a single experiment are obtained: the evolution of the peak amplitude and the spatial width of the optical reflectance trace. A modified two-temperature energy transport model incorporating the changes of carrier density under ultrafast laser excitation is used to describe the underlying carrier relaxation, carrier-phonon interaction, and energy diffusion process, and is integrated into an optical model to extract fundamental energy transport dynamics and properties. By tracking the evolution of the peak amplitude change and the spatial width with the high accuracy spatiotemporal pump probe system, the underlying relaxation rates …

As the multi-photon lithography (MPL) field continues to advance, it is necessary to improve understanding of the underlying kinetics. A comprehensive model of 3D nanolithography is developed, where both the excited state kinetics of initiation by multi-step absorption and the kinetics of the resulting polymerization are simulated. The different model parameters are studied by fitting the model to experimental results and simulating the determination of effective nonlinearity values of the MPL process. Finally, the application of this model to projection MPL is investigated.

It was with great sadness that the heat transfer community learned of the passing of Professor Raymond Viskanta on December 27, 2021. Raymond Viskanta was born in Marijampole, Lithuania in 1931. In 1944, his family escaped the advancing Russian-German front, fleeing to Germany. After a tenuous survival in Germany during the remainder of the war, they were moved to displaced persons camps in West Germany starting in 1945. In 1949, the family came to the United States, initially to work as farm laborers in Michigan. Raymond subsequently moved to Chicago in 1950, where he found employment as a factory worker during the day and attended high school classes in the evening.After receiving his high school degree in 1951, he began his higher education by taking night courses at a junior college, then enrolled full time at the University of Illinois-Navy Pier. He later transferred to the main campus where …

Hyperbolic material has attracted significant attentions due to its anisotropic hyperbolic dispersion which allows for propagation of hyperbolic phonon polaritons, especially the high-k modes within the Reststrahlen bands (RBs) in infrared (IR). Our study reveals that these high-k hyperbolic phonon polaritons also enabled radiative heat transfer inside hyperbolic materials, which has the potential to be utilized for thermal management. In this work, we quantitatively explore radiative transfer in hyperbolic materials. We first use Fourier Transfer Infrared microscopy (FTIR) to measure polarized reflection along in-plane principal axes to determine the permittivity (dielectric constants) along all principal axes. Then, a many-body radiative transport model is employed to compute an equivalent thermal conductivity by radiation, called radiative thermal conductivity. Our results show that radiative heat transfer in hyperbolic …

The systems for multiphoton 3D nanoprinting are rapidly increasing in print speed for larger throughput and scale, unfortunately without also improvement in resolution. Separately, the process of photoinhibition lithography has been demonstrated to enhance the resolution of multiphoton printing through the introduction of a secondary laser source. The photo-chemical dynamics and interactions for achieving photoinhibition in the various multiphoton photoinitiator systems are complex and still not well understood. Here, we examine the photoinhibition process of the common photoinitiator 7-diethylamino 3-thenoylcoumarin (DETC) with inhibition lasers near or at the multiphoton printing laser wavelength in typical low peak intensity, high repetition rate 3D nanoprinting processes. We demonstrate the clear inhibition of the polymerization process consistent with a triplet absorption deactivation mechanism for a DETC …

Several methods have recently been proposed for improving the processing speed of the popular two-photon polymerization process. One such method makes use of a spatiotemporal focusing technique to achieve a planar projection printing strategy. This works uses a projection two-photon polymerization process in a continuous fashion to fabricate complex 3D structures at a large print rate while maintaining smooth surface features. Fabrication of millimeter scale structures are achievable with this continuous, layer-by-layer projection two-photon lithography system.

Additive manufacturing of sophisticated 3D nanoscale objects commonly uses femtosecond lasers to photopolymerize a light‐sensitive resin using multi‐photon absorption. This nonlinear process provides high accuracy and flexibility in advanced 3D fabrication; however, it typically has limited throughput and high cost. This work makes use of a one‐photon‐based dosage‐nonlinearity to fabricate 3D nanostructures, demonstrating a cost‐effective method for 3D nanolithography using a low‐cost 405 nm continuous‐wave diode laser. This dosage‐nonlinearity is achieved by using controlled depletion of photo‐initiation species in an environment containing inhibiting species. By controlling multiple competing processes, the undesired dose accumulation outside the writing voxel is stopped, and thereby this method creates a confined writing voxel in space. A multiphysics model is developed to numerically study the …

Two-dimensional van der Waals ferroelectric semiconductors have attracted extensive research interest in both theoretical investigation and device applications due to their ferroelectricity and semiconducting nature. However, it is still not well understood how the ferroelectric phase is able to coexist with the semiconducting phase in this emerging material class. In this work, mm-scale continuous films of In 2 Se 3 with a thickness of∼ 3 nm were synthesized successfully by physical vapor deposition. Furthermore, we fabricated asymmetric ferroelectric semiconductor junctions (a-FSJs) from thick exfoliated and PVD-grown ultrathin In 2 Se 3 films. A high read current density of∼ 100 A/cm 2 and a distinction ratio of over 10 2 at V Read= 0.5 V are achieved in devices fabricated by a 3 nm-thick In 2 Se 3 film toward ultrahigh-density memory integration. Notably, the coercive voltage is constant, with In 2 Se 3 film thickness …

The role of a HfO 2 or ZrO 2 interlayer as a thermal bridge between a -Ga 2 O 3 channel and a sapphire substrate was investigated using a -Ga 2 O 3 nano-membrane FET as a test vehicle. A 35% less channel temperature increase was observed when a thin HfO 2 or ZrO 2 interlayer was inserted between the -Ga 2 O 3 channel and the sapphire substrate compared to devices without interlayers. Phonon density of states (PDOS) mismatch can explain the improvement of the thermal boundary conductance (TBC). In the acoustic region, the PDOS of HfO 2 or ZrO 2 has about a 700% larger overlap area with the PDOS of -Ga 2 O 3 compared to the PDOS of sapphire. This suggests that the insertion of a thermal bridge interlayer can provide a potential solution to the low thermal conductivity of -Ga 2 O 3 and the self-heating effect of -Ga 2 O 3 -based FETs.

A rapidly printing 3D nanostructures arrangement, comprising a first photonic source configured to provide photoinitiation energy to a polymer medium via a dynamic light spatial modulator to an excited state to initiate polymerization, a second photonic source configured to selectively provide inhibition energy to the polymerized medium to a depleted state to inhibit polymerization thereby generating a dead zone below a growth zone, the dead zone allows continuous 3D polymerization.

A mathematical model is developed to describe the photochemical processes in two-photon nanolithography, including two-step absorption leading to initiation and self-deactivation of the photoinitiator by laser irradiance, polymer chain propagation, termination, inhibition, and inhibitor and photoinitiator diffusion. This model is solved numerically to obtain the concentrations of the reaction species as a function of time and space as a laser beam is scanned through a volume of photoresist, from which a voxel size or linewidth is determined. The most impactful process parameters are determined by fitting the model to experimentally measured linewidths for a range of laser powers and scanning speeds, while also obtaining effective nonlinearities that are similar to previously measured values. The effects and sensitivities of the different process parameters are examined. It is shown that the photopolymerization …

Two-photon polymerization (TPP) is a prevalent 3D nanofabrication tool due to its superior resolution. However, the serial nature of the printing process often limits it applicability. Projection-based TPP in combination with spatiotemporal focusing using a digital micromirror device can be utilized to increase the scalability of the printing process while limiting the loss of resolution. A model is developed to simulate photopolymerization in the spatiotemporal focusing projection-based process. This model is used to better understand the 3D spatial and temporal interactions during the printing process and their effect on printing accuracy.

Inverse design has greatly expanded photonic devices for achieving optimized performance. However, the use of inverse design for plasmonic structures has been challenging due to local field concentrations that can lead to errors in gradient calculation when the continuum adjoint method is used. On the other hand, with the discrete adjoint method one can achieve the exact gradient. Historically the discrete version is exclusively used with a Finite Element model, and applying the Finite-Difference Time-Domain (FDTD) method in the inverse design of plasmonic structures is rarely attempted. Due to the popularity of using FDTD in simulating plasmonic structures, we develop the discrete adjoint method with FDTD and present a framework to carry out the inverse design of plasmonic structures using density-based topology optimization. We demonstrate the exactness of the gradient calculation for a plasmonic …