R.K. Garg

Multiphonon infrared absorption in silicon

Investigations have been carried out on silicon crystals, grown by float zone (FZ) and Czochralski (CZ) methods, of infrared absorption bands using a Fourier transform infrared spectrophotometer. Multiphonon bands are identified in the light of recent theoretical calculations based on the total energy of silicon crystal lattice. Theoretical results of Ihm et al.(1) and Yin and Cohen(2,3) are found to be in good agreement with the experimental observations of multiphonon infrared bands.

Far-infrared characteristics of multi-element interference filters using different grids

Abstract Theoretical expressions for multigrid interference filters consisting of different grids have been determined. These are reported for filters comprising of two-grid, three-grid and four-grid systems. Theoretical equations may be used to calculate transmission and reflection properties of interference filters made of either wire mesh or reciprocal grid type elements. Some combinations have been reported in which spikes in the pass band are reduced. Calculated transmission of a four-grid interference filter is compared with the available experimental data.

Five- and six-grid interference filters for far infrared

Abstract Theoretical equations describing the coefficients of transmission and reflection of five-and six-grid interference filters, consisting of identical grids, are reported. Transmission curves of these filters made from metallic meshes, are plotted for the non-diffraction region. It is shown that such filters have high transmission in the pass band and sharp cut off edges.

Far infrared transmission characteristics of multielement reciprocal grid interference filters

Abstract Transmission characteristics of multielement reciprocal grid interference filters are described. Conditions of maximum transmission for interference filters consisting of identical grids or reciprocal grids, have been obtained. Theoretical transmission of three-, four-, five and six-reciprocal grid interference filters, is calculated. It is shown that the sharpness of cut-off edge increases with the number of elements in the interference filters.

Interaction of oxygen and carbon impurities with the multiphonon infrared absorption bands of Silicon at low temperatures

Infrared absorptance measurements of polycrystalline silicon and single crystals of silicon having oxygen and carbon impurities were carried out at low temperatures in the wavelength range of multiphonon infrared absorption of silicon. Peak absorptance and frequencies were measured at temperatures from 10 to 130 K at intervals of 10 K. Absorptance-temperature curves were plotted for multiphonon bands and impurity bands. It has been observed that the three phonon band [2 TO (Γ) + TA (L)] exhibits a resonance-like behaviour with the anti-stretching mode of Si-O at low temperatures.

Electrically balanced pyroelectric radiometer

Abstract A TGS Pyroelcctric detector has been used for absolute measurements of low level optical radiation. The front electrode of the detector is used as: (i) a high efficiency radiation absorption film: (ii) an electrode for the pyroelectric condenser; and (iii) as an electric heater for producing heating pulses. Radiation is chopped by a 20% open mechanical chopper. The pyroelectric signal of the detector contains pulses due to radiation and electric heating. These two pulses are balanced by a null detection method in a lock-in amplifier. Thus electrical power is matched with radiation power for the absolute measurement of the latter. The operating frequency of the detector lies between the thermal relaxation frequency and the electrical cut-off frequency. Applications of this radiometer include absolute measurements of black body radiation and spectral radiation.

FTIR spectroscopic study for improving material preparation of high Tc superconductors

FTIR spectroscopic studies have been carried out to understand high Tc superconductor YBa2Cu3O7−δ during growth process. Characteristic IR absorption bands of the material are compared with those of the constitutent compounds to determine the presence of unused components and the completion of solid state reactions. IR spectroscopy has been found to be very convenient in such investigations.

Far infrared absorption studies of high Tc superconducting material YBa2Cu3O7−x

The high Tc superconducting material YBa2Cu3O7−x has been studied for its absorption in the FIR region. Measurements have been carried out using a Fourier Transform Infrared (FTIR) spectrophotometer at different temperatures above and below Tc. Observed bands are examined in the light of theoretical calculations based on different models. Certain IR absorption bands show screening effects in the measurements below Tc. Experimental values for observed IR bands are compared with the work of other authors and the bands are assigned to different IR active modes of vibration of the material.

Studies on the domain wall coupling in the pyroelectric detector-modulator

The infrared signal detected by a pyroelectric detector-modulator1 superimposes over the carrier frequencies, fed through an annular metallic ring formed near the front electrode of the pyroelectric detector. Electrodes of the detector and metallic ring are formed on the same chip of an unpoled single ferroelectric crystal. The coupling of the carrier frequency with the low frequency infrared signal has been demonstrated by various experiments and explained theoretically on the basis of electrical and mechanical solitary waves2 in the ferroelectric crystal. These solitary waves give rise to domain wall or electromechanical coupling. The electrical field of the carrier frequency causes growth of domains with polarisation aligned along carrier frequency field and the movement of the domain walls results in domain wall coupling. However, it is essential that the carrier frequency field should be greater than the threshold electric field of the ferroelectric crystal. In contrast to other modes of coupling, domain wall coupling is very efficient to modulate infrared signal through pyroelectric detector-modulator for remote transmission of the same.

Validity of reradiation theory in the diffraction region

Abstract Experimental studies on the optical properties of metallic meshes have been carried out with a view to ascertaining the causes of the high transmission differences ( ΔT ) between theoretical and experimental values in the diffraction region defined by λ g (1 + sin α ). It is observed that a part of the radiation power deviates from the optical path due to diffraction. The magnitude and wavelength of the transmission maxima vary from collimated to converging or diverging beams.