Generation of sub-MV/cm terahertz fields with large-size Cherenkov-type optical-to-terahertz converters.

It is known that a structure comprising a tens of microns thick, and ∼1 × 1 cm2 in size, layer of LiNbO3 attached to a Si prism can serve as an efficient Cherenkov-type converter of tens of microjoules-energy femtosecond laser pulses to broadband terahertz radiation. Here we experimentally demonstrate scaling up the terahertz energy and field strength by extending the width of the converter to several centimeters, expanding appropriately the pump laser beam, and increasing the pump pulse energy to hundreds of microjoules. In particular, chirped Ti:sapphire laser pulses of 450 fs duration and 600 µJ energy were converted to 1.2 µJ terahertz pulses, and 0.5 MV/cm peak terahertz field was obtained when pumping by unchirped laser pulses of 60 fs duration and 200 µJ energy.

Scalable optical-to-terahertz converter with a prism-coupled plane-parallel lithium niobate plate.

A nonlinear optical converter of femtosecond laser pulses to terahertz radiation, which combines the tilted-pulse-front pumping and prism coupling techniques, is proposed and experimentally tested. In contrast to the conventional tilted-pulse-front scheme with a prism-shaped LiNbO3 crystal, the converter consists of a plane-parallel LiNbO3 plate sandwiched between two dielectric prisms. One prism is used to couple the pump beam into the LiNbO3 plate, another prism couples the generated terahertz radiation out of the plate. The proposed scheme enables scaling to large-diameter LiNbO3 wafers and large-aperture high-energy pump laser beams resulting in generation of strong terahertz fields. In a proof-of-principle experiment with a 1-mm thick, small size (1 × 2 cm2) LiNbO3 plate pumped by a mJ-class laser, the conversion efficiency comparable to that of the conventional scheme (∼0.1%) was demonstrated.

Terahertz beam spot size measurements by a CCD camera.

We present the experimental data on the direct measurements of spatial distribution of the terahertz (THz) pulse intensity profile using a commercial silicon-based charge-coupled device (CCD) camera in the spectral range from 1-3 THz. A method to measure the dimensions of a high-intensity THz radiation beam in the focal plane using the CCD camera is proposed and experimentally verified.

Generalized analysis of terahertz generation by tilted-pulse-front excitation in a LiNbO3 prism.

It is commonly adopted that the widely used method of terahertz generation by tilted-pulse-front pumping of a lithium niobate crystal is optimized when both the tilted front of the pump optical pulse and the image of the diffraction grating, which introduces the tilt, are parallel to the exit surface of the crystal. We consider more general situations. In particular, we show that a deviation of the grating image plane from parallel to the crystal exit surface leads to deterioration of the terahertz beam quality but does not significantly affect the terahertz energy. If the grating image plane and the crystal exit surface are mutual parallel, but they are nonparallel to the pulse front, the terahertz beam quality remains unaffected and the terahertz energy can be unexpectedly higher (about two times for pump pulses of a ~400 fs duration) than in the standard scheme. We propose a modified design of the tilted-pulse-front scheme based on our findings.

Decay of femtosecond laser-induced plasma filaments in air, nitrogen, and argon for atmospheric and subatmospheric pressures.

The temporal evolution of a plasma channel at the trail of a self-guided femtosecond laser pulse was studied experimentally and theoretically in air, nitrogen (with an admixture of ∼3% O_{2}), and argon in a wide range of gas pressures (from 2 to 760 Torr). Measurements by means of transverse optical interferometry and pulsed terahertz scattering techniques showed that plasma density in air and nitrogen at atmospheric pressure reduces by an order of magnitude within 3-4 ns and that the decay rate decreases with decreasing pressure. The argon plasma did not decay within several nanoseconds for pressures of 50-760 Torr. We extended our theoretical model previously applied for atmospheric pressure air plasma to explain the plasma decay in the gases under study and to show that allowance for plasma channel expansion affects plasma decay at low pressures.

Two-color laser-plasma generation of terahertz radiation using a frequency-tunable half harmonic of a femtosecond pulse.

We investigate for the first time, both experimentally and theoretically, low-frequency terahertz (THz) emission from the ambient air ionized by a two-color femtosecond laser pulse containing, besides the fundamental-frequency main field, a weak additional field tunable near the frequency of the half harmonic. By controlling the mutual polarization and the powers of the main and additional fields, we determine the dependences of the THz power and polarization on the parameters of the two-color pulse. We also discover the resonantlike dependence of the THz yield on the frequency detuning of the additional field. The analytical formulas obtained using the model of the free-electron residual current density give an excellent agreement with the experimental results.

Influence of cubic nonlinearity on accuracy of polarization transformation by means of a quarter-wave plate.

We consider the propagation of powerful laser radiation in an anisotropic medium with natural birefringence and cubic nonlinearity. By the example of a quarter-wave plate, we show theoretically and experimentally that, under the simultaneous influence of linear birefringence and nonlinearity, the accuracy of polarization transformation decreases in proportion to squared В-integral.

Highly efficient optical-to-terahertz conversion in a sandwich structure with LiNbO3 core.

We demonstrate experimentally the efficiency of a recently proposed scheme of terahertz generation based on Cherenkov emission from ultrashort laser pulses in a sandwich structure. The structure has a thin nonlinear core covered with a prism of low terahertz absorption. Using an 8 mm long Si-LiNbO(3)-BK7 structure with a 50 microm thick LiNbO(3) core, we converted 40 microJ, 50 fs Ti:sapphire laser pulses into terahertz pulses of approximately 3 THz bandwidth with a record efficiency of over 0.1%.

Nonlinear absorption of intense femtosecond laser radiation in air.

A new mechanism of nonlinear absorption of intense femtosecond laser radiation in air in the intensity range I = 10(11)-10(12) W/cm(2) when the ionization is not important yet is experimentally observed and investigated. This absorption is much greater than for nanosecond pulses. A model of the nonlinear absorption based on the rotational excitation of molecules by linearly polarized ultrashort pulses through the interaction of an induced dipole moment with an electric field is developed. The observed nonlinear absorption of intense femtosecond laser radiation can play an important role in the process of propagation of such radiation in the atmosphere.

[Detection of TT DNA virus (TTV) in blood donors and patients with hepatitis of unclear etiology]. / Vyiavlenie DNk virusa TT (TTV) u donorov krovi i bol'nykh gepatitom neiasnoi étiologii.

A new human infective agent: TT virus (TTV) has been recently identified. The polymerase chain reaction detected TTV DNA in the sera of 5 (31.3%) out of 16 children with acute hepatitis, 5 (17.2%) out of 29 children and 3 (14.3%) out of 21 adults with liver diseases of unknown etiology, and 18 (13.2%) out of 136 free-of-charge blood donors. These results indicate a high prevalence of TTV infection in Russia and absence of an obvious correlation between this infection and nonA, nonB, nonC hepatitis in examined patients. Phylogenetic analysis of amplified fragments of viral DNA from 3 patients selected at random showed that the isolated strains belong to subtype 1a, most prevalent in the world.

[The clinical picture, course and outcome of viral hepatitis A in children with chronic HBV infection]. / Klinika, techenie i iskhody virusnogo gepatita A u detei s khronicheskoi HBV-infektsiei.

The clinical picture, course and outcomes of hepatitis A developing as a superinfection in children with chronic hepatitis B virus (HBV) infection, hospitalized in connection with acute viral hepatitis, were determined on the basis of the dynamic clinico-laboratory investigation and identification of viral hepatitides A, B and delta by the method of enzyme immunoassay. In all children hepatitis B virus surface antigen (HBsAg), i.e. the serological sign of HBV infection, and anti-HAV IgG, the serological sign of hepatitis A virus (HAV) infection, were present in the blood. 11 out of 12 children had never before been examined for diseases of the liver. Only in one child chronic HBsAg carriership had been detected 14 months before this hospitalization. Considering the data of epidemiological history and clinical observations, as well as the results of serological survey, these children were found to have hepatitis A superimposed on chronic HBV infection. As revealed in the present investigation, in cases of HAV superinfection observed simultaneously with chronic hepatitis B viral hepatitis A was manifested by typical clinico-laboratory symptoms.

[Prevalence of hepatitis B in the focus of HIV infection]. / Rasprostranennost' virusnogo gepatita B v ochage infektsii, vyzvannoi virusom immunodefitsita cheloveka.

Highly sensitive up-to-date research methods (enzyme immunoassay) for HbsAg and antibodies against HBsAg and HBcAg were employed to examine 565 adults and children living in the Kalmuck++ ASSR (Elista and adjacent regions), where in 1988-1989 cases of HIV-infection were recorded. Markers of HBV-infection were detected with a high enough frequency in the control group of children (15.2%). The prevalence of the infection in the adult population was at an intermediate level (38.9%). The high infectivity with HBV-infection was recorded among children who had been hospitalized before (for different diseases including chronic ones), among women who had undergone laboratory studies because of the diagnosis of HIV-infection in their children; among adult patients suffering from tuberculosis of morons from a closed boarding school. The highest frequency of HBV-infection marker demonstration was found in HIV-infected children (70%). The data obtained point to the necessity of vaccination against viral hepatitis B in risk groups (children and adults) and in the newborn as well as of raising the efficacy of preventing measures against infections transmitted via blood.

[The comparative characteristics of treating herpes virus diseases of the oral mucosa with antiviral preparations]. / Sravnitel'naia kharakteristika lecheniia virus-gerpeticheskikh zabolevanii slizistoi obolochki polosti rta protivovirusnymi preparatami.

The efficacy of human leukocytic interferon (ointment), 20000 IU/g, in the treatment of viral diseases of the buccal mucosa was studied in 152 patients. The results were compared with those achieved with the commercial antiviral agent 0.5% oxolin ointment. The results evidence a manifest antiviral effect of the interferon ointment, recommending it for practical dentistry.

[The detection of hepatitis B virus DNA in the blood serum of children with chronic HBsAg-positive hepatitis]. / Vyiavlenie DNK virusa gepatita B v syvorotke krovi u detei s khronicheskim HBsAg-pozitivnym gepatitom.

The presence of HBV DNA in the blood serum of 50 children with HBsAg-positive chronic hepatitis (36 with chronic hepatitis B, 14 with chronic hepatitis delta) was determined by molecular hybridization method. No significant differences were observed between the frequency of HBV DNA detection and clinicomorphological type of chronic HBsAg-positive hepatitis. In chronic HBV infection, HBV DNA was shown to be detected more frequently than in chronic hepatitis delta. A correlation was confirmed between the frequency of HBV DNA detection and demonstration of HBeAg in the blood. With the increase in the duration of the disease and age of the patients the HBV DNA detection rate was found to diminish.

[Current etiological structure of acute viral hepatitis in children]. / Etiologicheskaia struktura ostrykh virusnykh gepatitov u detei na sovremennom étape.

The etiological structure of viral hepatitides (VH) was defined in 345 children admitted to the clinic during one year with a diagnosis of acute viral hepatitis. It was established on the basis of clinico-laboratory studies and identification of serological markers of viral hepatitides A, B and delta. It has been demonstrated that in the structure of viral hepatitides in children living in Moscow, VHA accounts for 83.1%, acute VHB for 11.1%, HBV and HDV coinfection for 2.6%, and VH non A non B for 1.7%. In children of the first year of life, acute hepatitis was etiologically related to HBV-infection or to HBV and HDV coinfection. In a negligible part of the patients, VHA was diagnosed in the presence of chronic HBV-infection (1.2%) or exacerbation of chronic hepatitis B (0.3%). These data should be taken into consideration in planning and carrying out diagnostic, treatment and prophylactic measures aimed at solving VH problem in childhood.

[The treatment of herpetic stomatitis in children with ointments containing alpha- and gamma-interferons]. / Lechenie gerpeticheskogo stomatita u detei maziami, al'fa- i gamma-interfon.

Clinical efficacies of ointments containing alpha- and gamma-interferon and characterized by antiviral activity (2.0 x 10(4) IU) were studied. Antiviral effect of these ointments used in the treatment of 168 children suffering from herpetic stomatitis was assessed from the clinical and immunologic viewpoints. Measurements of secretory IgG carried out over the course of therapy evidenced positive changes in local immunity.

An experience with application of human leucocyte interferon for cervical cancer treatment.

Interferon was injected intracervically as the first stage of combination treatment in 125 cases of histologically verified squamous Ca in situ and invasive cervical carcinoma. Colpomicroscopic, cytological and histological examination was carried out before and after treatment. The pronounced effect of interferon as an antitumor agent was established. Post-treatment histological examination failed to detect any tumor cells in 36 out of 125 patients (28.8%). Widening the scope of indications for preservative operations in patients with preinvasive and microinvasive cervical carcinoma after preliminary interferon therapy was suggested.

[Biosynthesis and main biological properties of human gamma interferon]. / Poluchenie i osnovnye biologicheskie svoistva gamma-interferona cheloveka.

Conditions for and regularities of gamma-interferon production were elucidated. High interferon inducing activity of the known inductors such as concanavalin A, phytohemagglutinin and lentil lectin was asserted. It was shown that aqueous and alcoholic extracts of legumes seeds were also able to induce interferon synthesis in cultures of human peripheral blood leukocytes. By physicochemical properties such as stability to acids and liability to heating and by antigenic properties, interferon produced under such conditions was of type 2 (gamma). The maximum yield of interferon was observed in stationary and roller leukocyte cultures at a density of 2.10(6)-4.10(6) cells/ml on commercial rich media (IMDM and Eagle medium). Dynamics of gamma-interferon biosynthesis was determined. The peak of interferon production after the leukocyte induction with plant lectins was recorded in 72-96 hours.