[Experimental investigation of the safety of terahertz radiation in corneal hydration assessment]. / Eksperimental'noe obosnovanie bezopasnosti primeneniya teragertsovogo izlucheniya dlya analiza izmenenii gidratatsii rogovitsy.

Application of terahertz (THz) radiation in novel non-invasive biomedical technologies has recently received considerable attention. However, experimental data about the safety of exposure to THz radiation for biological objects (including eye structures in vivo) are limited. To our knowledge, the safety of THz reflectometry (frequency range of 0.30-0.40 THz) has not been closely examined in an animal model with subsequent morphological assessment of corneal tissues. PURPOSE: To assess the safety of pulsed THz radiation with various parameters (time, power, and frequency) for the cornea in a rabbit model. MATERIAL AND METHODS: The sample for the current study consisted of 18 Chinchilla rabbits (18 eyes). Corneal imaging and epithelial cell density before and after the exposure were evaluated using confocal laser scanning microscopy (CLSM). The histological study for objective assessment of the cornea state (day 1 and day 14) was performed after experiment termination. RESULTS: Single and multiple exposures of laser radiation at a frequency below 0.1 THz and power density below 30 nW/cm2 do not cause visible structural changes in any layers of the rabbit cornea. The results obtained in the long-term period showed insignificant reversible morphological changes only within the epithelium. CONCLUSION: The described parameters of terahertz and subterahertz radiation can be considered safe for assessing changes in corneal epithelium hydration level using non-invasive methods based on THz reflectometry.

[Current methods of tear film stability assessment]. / Sovremennye metody issledovaniia stabil'nosti sleznoi plenki.

Dry eye syndrome (DES) is a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles. The clinical manifestations of DES can be highly variable, so its diagnosing is typically based on a combination of symptoms, signs, and clinical tests. Majority of the techniques involved in the diagnosis of this disease has a large degree of subjectivity. The present article reviews current dry eye diagnostic tests and presents a new tear film stability assessment technique that may potentially become the objective method for establishing DES diagnosis.

Fibonacci-like photonic structure for femtosecond pulse compression.

The compression of femtosecond laser pulses by linear quasiperiodic and periodic photonic multilayer structures is studied both experimentally and theoretically. We compare the compression performance of a Fibonacci and a periodic structure with similar total thickness and the same number of layers, and find the performance to be higher in the Fibonacci case, as predicted by numerical simulation. This compression enhancement takes place due to the larger group velocity dispersion at a defect resonance of the transmission spectrum of the Fibonacci structure. We demonstrate that the Fibonacci structure with the thickness of only 2.8 microm can compress a phase-modulated laser pulse by up to 30%. The possibility for compression of laser pulses with different characteristics in a single multilayer is explored. The operation of the compressor in the reflection regime has been modeled, and we show numerically that the reflected laser pulse is subjected to real compression: not only does its duration decrease but also its amplitude rises.

Enhancement of sum frequency generation near the photonic band gap edge under the quasiphase matching conditions.

We analyze theoretically and study experimentally the mechanisms of enhancement of the sum frequency and second harmonic generation in a finite one-dimensional photonic band gap structure with second order nonlinearity under Bragg diffraction conditions. It is shown that, near the photonic band gap edge, the efficiency of conversion in sum frequency and second harmonic generation processes can be significantly enhanced if two conditions are fulfilled simultaneously: grating assisted phase matching, or quasiphase matching, and an increase of the electromagnetic field density at the fundamental frequencies near the photonic band edges. The role of each mechanism is discussed.

Compression of femtosecond laser pulses in thin one-dimensional photonic crystals.

We demonstrate experimentally the effect of compression of femtosecond laser pulses in thin (a few micrometers) one-dimensional photonic crystal. We show that the compression effect is reasonably described by the linear dispersion properties of the photonic crystal itself and the quadratic dispersion approximation cannot be efficiently used for the description of interaction of the femtosecond laser pulses with the thin photonic crystal. For given parameters of the femtosecond pulse it leads to the existence of the optimal dimension of the photonic crystal from the point of view of the compression efficiency. Due to the wide spectral width of the femtosecond laser pulses the high-order dispersion effects play an important role in pulse propagation in photonic crystals and as a result the pulse compression occurs for both positive and negative signs of chirp of the incoming femtosecond pulses.

Enhancement of second-harmonic generation with femtosecond laser pulses near the photonic band edge for different polarizations of incident light.

We present experimental evidence of enhancement of second-harmonic generation as a result of an increase of the fundamental-field energy density within a multilayer structure near the photonic band edge.