Cong Chen

Extreme ultra-low lasing threshold of full-polymeric fundamental microdisk printed with room-temperature atmospheric ink-jet technique

We experimentally demonstrated an extreme ultra-low lasing threshold from full-polymeric fundamental microdisk cavities fabricated by a novel fabrication method, the ink-jet printing method, which is much simpler and easier than previous methods such as lithography. The ink-jet printing method provides additive, room-temperature atmospheric, rapid fabrication with only two steps: (i) stacking cladding pedestal and waveguiding disk spots using the ink-jet technique, and (ii) partial etching of the cladding pedestal envelope. Two kinds of low-viscosity polymers successfully formed microdisks with high surface homogeneity, and one of the polymers doped with LDS798 dye yielded whispering-gallery-mode lasing. The fundamental disks exhibited an extremely ultra-low lasing threshold of 0.33 μJ/mm2 at a wavelength of 817.3 nm. To the best of our knowledge, this lasing threshold is the lowest threshold obtained among both organic and inorganic fundamental microdisk cavity lasers with a highly confined structure.

Formation and evolution of ripples on ion-irradiated semiconductor surfaces

We have examined the formation and evolution of ripples on focused-ion-beam (FIB) irradiated compound semiconductor surfaces. Using initially normal-incidence Ga+ FIB irradiation of InSb, we tuned the local beam incidence angle (θeff) by varying the pitch and/or dwell time. For single-pass FIB irradiation, increasing θeff induces morphological evolution from pits and islands to ripples to featureless surfaces. Multiple-pass FIB irradiation of the rippled surfaces at a fixed θeff leads to island formation on the ripple crests, followed by nanorod (NR) growth. This ripple-NR transition provides an alternative approach for achieving dense arrays of NRs.

Nanodot formation induced by femtosecond laser irradiation

The femtosecond laser generation of ZnSe nanoscale features on ZnSe surfaces was studied. Irradiation with multiple exposures produces 10–100 nm agglomerations of nanocrystalline ZnSe while retaining the original single crystal structure of the underlying material. The structure of these nanodots was verified using a combination of scanning transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. The nanodots continue to grow hours after irradiation through a combination of bulk and surface diffusion. We suggest that in nanodot formation the result of ultrafast laser induced point defect formation is more than an order of magnitude below the ZnSe ultrafast melt threshold fluence. This unique mechanism of point defect injection will be discussed.

On-chip, high-sensitivity temperature sensors based on dye-doped solid-state polymer microring lasers

We developed a chip-scale temperature sensor with a high sensitivity of 228.6 pm/°C based on a rhodamine 6G (R6G)-doped SU-8 whispering gallery mode microring laser. The optical mode was largely distributed in a polymer core layer with a 30 μm height that provided detection sensitivity, and the chemically robust fused-silica microring resonator host platform guaranteed its versatility for investigating different functional polymer materials with different refractive indices. As a proof of concept, a dye-doped hyperbranched polymer (TZ-001) microring laser-based temperature sensor was simultaneously developed on the same host wafer and characterized using a free-space optics measurement setup. Compared to TZ-001, the SU-8 polymer microring laser had a lower lasing threshold and a better photostability. The R6G-doped SU-8 polymer microring laser demonstrated greater adaptability as a high-performance temperature-sensing element. In addition to the sensitivity, the temperature resolutions for the laser-based sensors were also estimated to be 0.13 °C and 0.35 °C, respectively. The rapid and simple implementation of micrometer-sized temperature sensors that operate in the range of 31 – 43 °C enables their potential application in thermometry.We developed a chip-scale temperature sensor with a high sensitivity of 228.6 pm/°C based on a rhodamine 6G (R6G)-doped SU-8 whispering gallery mode microring laser. The optical mode was largely distributed in a polymer core layer with a 30 μm height that provided detection sensitivity, and the chemically robust fused-silica microring resonator host platform guaranteed its versatility for investigating different functional polymer materials with different refractive indices. As a proof of concept, a dye-doped hyperbranched polymer (TZ-001) microring laser-based temperature sensor was simultaneously developed on the same host wafer and characterized using a free-space optics measurement setup. Compared to TZ-001, the SU-8 polymer microring laser had a lower lasing threshold and a better photostability. The R6G-doped SU-8 polymer microring laser demonstrated greater adaptability as a high-performance temperature-sensing element. In addition to the sensitivity, the temperature resolutions for the laser-based...

0.6 – 1.6 THz band spectroscopy of organic thermally activated delayed fluorescence materials

Newly developed thermally activated delayed fluorescence (TADF) materials are attractive for application in efficient displays. Five TADF materials, including PXZ-TRZ and four carbazolyl dicyanobenzene (CDCB) derivatives of 4CzTPN, 4CzTPN – Ph, 2CzPN, and 4CzIPN, were investigated using terahertz spectroscopy in the 0.60 – 1.60 THz range. While PXZ-TRZ was almost transparent, the carbazolyl dicyanobenzene (CDCB) derivatives, especially 4CzIPN, exhibited intrinsic absorption features. Comparing these results with density functional theorem calculations, each absorption feature was clarified to originate from the intramolecular motions of the carbazole units.

Low-threshold lasing from organic and polymeric microdisk printed by room temperature atmosphere ink-jet technique

Novel microdisk optical 3D structure fabrication was demonstrated in a room temperature and atmospheric condition with tens-seconds processing duration. This additive manufacturing scheme of polymeric microdisk was based on the ink-jet technique with hyperbranched polymer TZ-001. And whispering-gallery mode (WGM) lasing with a low threshold was confirmed by doping LDS798 or Rhodamine 590 dyes. The diameters both 75 µm were archived. The WGM lasing at around 800 nm wavelength by the LDS798, and at around 600 nm by the Rhodamine 590. Low lasing threshold about 2~3 µJ/mm2 were confirmed. And shift of peak wavelength was observed on the Rhodamine590:TZ-001 disk.

Demonstration of versatile whispering-gallery micro-lasers for remote refractive index sensing

We developed chip-scale remote refractive index sensors based on Rhodamine 6G (R6G)-doped polymer micro-ring lasers. The chemical, temperature, and mechanical sturdiness of the fused-silica host guaranteed a flexible deployment of dye-doped polymers for refractive index sensing. The introduction of the dye as gain medium demonstrated the feasibility of remote sensing based on the free-space optics measurement setup. Compared to the R6G-doped TZ-001, the lasing behavior of R6G-doped SU-8 polymer micro-ring laser under an aqueous environment had a narrower spectrum linewidth, producing the minimum detectable refractive index change of 4 × 10-4 RIU. The maximum bulk refractive index sensitivity (BRIS) of 75 nm/RIU was obtained for SU-8 laser-based refractive index sensors. The economical, rapid, and simple realization of polymeric micro-scale whispering-gallery-mode (WGM) laser-based refractive index sensors will further expand pathways of static and dynamic remote environmental, chemical, biological, and bio-chemical sensing.

Effects of edge inclination angles on whispering-gallery modes in printable wedge microdisk lasers

The ink-jet technique was developed to print the wedge polymer microdisk lasers. The characterization of these lasers was implemented using a free-space optics measurement setup. It was found that disks of larger edge inclination angles have a larger free spectral range (FSR) and a lower resonance wavelength difference between the fundamental transverse electric (TE) and transverse magnetic (TM) whispering-gallery modes (WGMs). This behavior was also confirmed with simulations based on the modified Oxborrows model with perfectly matched layers (PMLs), which was adopted to accurately calculate the eigenfrequencies, electric field distributions, and quality parameters of modes in the axisymmetric microdisk resonators. Combined with the nearly equivalent quality factor (Q-factor) and finesse factor (F-factor) variations, the correlations between the TE and left adjacent TM modes were theoretically demonstrated. When the edge inclination angle is varied, the distinguishable mode distribution facilitates the precise estimation of a resonance wavelength shift. Therefore, the flexible and efficient nature of wedge polymer microdisk lasers extends their potential applications in precision sensing technology.

Investigation of protein adsorption for biosensors based on ink-jet printed active microdisk resonator

Whispering Gallery Modes (WGMs) resonator based biosensors have received much attention due to the advantages of small volume, high sensitivity and label-free. Such sensors can be assigned into passive resonator sensor [1] and active resonator sensor. Different with passive resonator sensor, active resonator based sensor [2] is potential because there are (i) no need an expensive tuneable laser as source; (ii) no need a waveguide or a tapered fiber to couple evanescent field; (iii) no need a nanopositioner for adjusting coupling distance. However, as one of active resonators drawbacks, the limited lifetime of lasing determined by organic dye should be considered. In order to solve this problem, many researchers use renewable gain material to reproduce active resonators. Such renewable devices usually based on a liquid system. For solid-state sensing system, another scheme is to use a disposable resonator. To achieve such a resonator, a low-cost, on-demand, rapid fabrication method is required. Based on this consideration, we demonstrated a printable active microdisk resonator for protein adsorption sensing by using ink-jet printing fabrication method [3].

Water-like-refractive-index microdisk cavity by the ink-jet printing method

Microdisk cavities are a kind of whispering gallery mode (WGM) resonators, which are continually studied owing to their high quality factor (Q factor) and small mode volume. These microdisks were previously fabricated using a subtractive method [1, 2] such as photolithography method. These subtractive methods are suitable for the mass-produced industrial structure due to an ability of the mass fabrication in large areas. On the other hand, we recently developed an ink-jet printing method as a novel, simple fabrication process for a full-polymeric fine microdisk, which is very superior at producing low waste materials, requires little energy consumption, and is suitable for on-site and on-demand fabrication because it is an additive method [3]. Since this method provides a completely organic microdisk platform, it is highly compatible with the biosensor. However, since the refractive index of used hyper-branched polymers (HBP) (n = 1.45–1.89) is a relatively high among organic materials, it is not preferable to use previous HBPs in a biosensor using a water-based liquid sample (n ∼ 1.33) because of the short interaction length owing to reducing the evanescent part. In this study, we experimentally succeeded fabrication of microdisks with water-like refractive index by the ink-jet printing method. Additionally, the low-refractive-index microdisks doped with Rhodamine6G dye successfully achieved WGM lasing. This low-refractive-index microdisk cavity by ink-jet printing method is the first demonstration under atmospheric pressure and RT among inorganic microdisks.