ABSTRACT
The flexible organic amine cations on the interfaces of two-dimensional (2D) hybrid organic-inorganic perovskite nanosheets could form relaxed structures, which would lead to exotic optoelectronic properties but are hard to understand. Here, the unusual interfacial relaxation of nanosheets exfoliated from an orthorhombic 2D lead halide perovskite, [(C6H5CH2NH3)2]PbCl4, is interrogated via ultrafast second-harmonic generation (SHG) spectroscopy. The in-plane SHG intensity anisotropy of these nanosheets is found to decrease with reducing layer thickness. Combined first-principles calculations and Monte Carlo simulations reveal that the induced second-order polarization arises primarily from the (C6H5CH2NH3)+ cations; and these organic amine cations form significantly reorganized conformations with decreasing nanosheet thickness due to weakened van der Waals interactions. Because the orientations of organic components at the interface determine their electric properties and specifically the dipolar susceptibility, the resulting structure leads to striking changes in the SHG properties.
ABSTRACT
The extreme ultraviolet (EUV) emission characteristics from Sn plasma for lithography produced by a pulse discharge CO2 laser was investigated under different conditions. Extreme ultraviolet spectral measurements were made throughout the wavelength region of 6.5 nm to 16.8 nm using a grazing incidence flat-field spectrograph coupled with an X-ray charge-coupled device camera for detection of time-integrated spectra. The dependence of spectral properties of the EUV emission on pulse duration, incidence pulse energy, and buffer gas pressure was investigated. The results show that the peak of EUV spectra was located at 13.5 nm. The intensity of EUV emission increased with increasing laser energy ranging from 30 mJ to 600 mJ in a nonlinear manner with saturation effect. The critical energy of incident pulse laser for the generation of EUV emission is near 30 mJ in our experiment. The highest conversion efficiency of 1.2% in producing 13.5 nm EUV light with 0.27 nm bandwidth was achieved at pump energy of 425 mJ. The EUV spectra from a plate target produced by laser pulse with full width at half maximum range from 50 ns to 120 ns were recorded and negligible differences in their spectral features noticed even though higher spectral intensity was observed by shorter pulse duration. The 2% in-band EUV intensity with 52 ns pulse duration was 1.6 times higher than that with 120 ns pulse duration due to the increase in laser intensity. It was also found that the detected EUV spectral intensity rapidly decreased with increasing buffer air pressure, and the EUV emission could be totally absorbed at the pressure of 200 Pa, while weak EUV emission could be still detected at the buffer He gas pressure of 7 x 10(4) Pa. The experimental results showed that the absorption coefficient of 13.5 nm light at air buffer gas pressure of 100 Pa was 3.0 m(-1), while the absorption coefficient was 0.96 m(-1) at the same He buffer gas pressure.
ABSTRACT
AIM: To experimentally compare the external sclerostomy produced using a femtosecond laser with that made by a surgical knife and to evaluate the healing patterns, efficacy and technical advantages of femtosecond laser sclerostomy. METHODS: In a prospective randomized, controlled, masked-observer study, 10 pigmented rabbits underwent external sclerostomy with a femtosecond laser in the right eye; 10 additional rabbits underwent sclerostomy with a surgical superblade in the right eye. Clinical characteristics, which included bleb morphology and intraocular pressure, were recorded for 1 month after surgery. Six additional rabbits underwent external femtosecond laser sclerostomy in the right eye and mechanical sclerostomy in the left eye and were killed at day 14 after surgery. Histologic staining, immunohistochemistry and scanning electron microscopy were subsequently performed to assess the morphology of the filtering fistula. The titanium-sapphire femtosecond laboratory laser was operating at a repetition rate of 1 kHz, 0.4 mJ pulse energy, a central wavelength of 800nm and a pulse duration of 50 femtoseconds. Mann-Whitney and Kaplan-Meier tests were used for statistical analysis. RESULTS: Successful complete sclerostomy was achieved in each laser-treated eye which was hit only once by the laser. The laser treated time was approximately 15s-16s. In the laser-treated group (n=16), 2 eyes (12%) developed mild hyphema at the site of entry and 8 eyes (50%) showed transient edema in the corneal periphery adjacent to the laser impact zone. The differences between the groups in duration of function blebs and pressure reduction were statistically significant (P=0.025 and 0.016, respectively). The success rate of the laser-treated group was significantly higher than the knife group (P=0.005). Histologically, the subconjunctival connective tissue was loosely arranged with partially patent sclerostomy in the laser-treated eyes at postoperative day 14. This contrasted with the completely scarred sclerostomy tract in the knife group. The mean numbers of fibroblasts and new vessels as well as the amount of new collagen deposition at bleb site were significantly decreased in the laser group (P=0.045, 0.013 and 0.036, respectively). CONCLUSION: The current study demonstrates that external femtosecond laser sclerostomy may offer a safe and effective alternative for the minimally invasive surgical management of glaucoma.
ABSTRACT
OBJECTIVE: To explore the feasibility, efficacy and safety of performing ab externo sclerostomy with a femtosecond laser in rabbits with chronic ocular hypertension. METHOD: The chronic ocular hypertension model was induced by injecting α-chymotrypsin into posterior chamber. Twenty rabbits with chronic ocular hypertension were randomly divided into experiment and control groups, each group consisting of 10 rabbits. An ab externo sclerostomy using a femtosecond laser was performed in the right eyes in the experiment group. The right eyes in control group were unoperated. The laser was a pulsed titanium-sapphire laser, operating at a repetition rate of 1000 per second, 0.4 mJ pulse energy, a central wavelength of 800 nm and a pulse duration of 50 femtoseconds. The survival of filtration blebs, clinical manifestation and intraocular pressure (IOP) were observed for 1 month after surgery. Animals were killed on days 3, 7, 14 and 30 post-operatively. Hematoxylin and eosin (HE) staining and scanning electron microscopy (SEM) were performed to evaluate the histopathologic changes in filtering tracts. The differences in IOP between the experiment and control groups were analyzed by repeated measures analysis of variance. RESULTS: A 2 mm × 1 mm clear full-thickness scleral incision was created in each eye in the experiment group which was hit only once by the laser. The laser treated time was approximately 15 - 16 s. There was a significant difference (F = 117.46, 39.96, 15.17, 11.62, 15.31, 11.10; P < 0.01). IOP between experiment and control groups at post-operatively day 1, 3, 7, 14, 21, 30. No serious intra- or post-operative complications happened in the treated eyes except for anterior chamber hemorrhage in 2 eyes triggered by laser injury to iris root. A conjunctival bleb was successfully formed in all treated eyes at 1 day after surgery and lasted from 14 to 21 days. Histopathology showed that the perforating scleral incisions created by femtosecond laser were sharply defined, with almost no collateral damage to the surrounding tissue. The tissue repair response in the filtering tracts was characterized by mild hyperplasia of fibroblasts and the loose deposition of a small amount of new collagen fibers. CONCLUSION: The current study demonstrates that ab externo femtosecond laser sclerostomy is a feasible, safe and effective option for the treatment of glaucoma.
