Isotropic Wavelength Research Articles

Birefringence, isotropic points, and optical filters based on CuGaxAl1-xSe(S)2 crystals

A brief review on birefringence and optical filters based on CuGaxAl1-xS2 and CuGaxAl1-xSe2 crystals is given in this paper. Two types of isotropic points are analyzed; one of them refers to a case when the wavelength at which the ordinary and extraordinary refractive indices are equal is situated near the absorption edge, while in the second case the isotropic wavelength is in the region of crystal transparency. It is shown that both of them can be used for the development of band-pass and band-reject filters. The possibility of adjusting the spectral position of an isotropic point by changing the chemical composition of a solid solution is illustrated, and the applicability of CuGaxAl1-xS2 and CuGaxAl1-xSe2 for adjusting the isotropic point to most of Ar+, Kr+, He–Ne, and He–Cd gas lasers lines, as well as to semiconductor lasers, is revealed

Revisiting polarimetry near the isotropic point of an optically active, non-enantiomorphous, molecular crystal.

Accurate polarimetric measurements of the optical activity of crystals along low symmetry directions are facilitated by isotropic points, frequencies where dispersion curves of eigenrays cross and the linear birefringence disappears. We report here the optical properties and structure of achiral, uniaxial (point group D2d ) potassium trihydrogen di-(cis-4-cyclohexene-1,2-dicarboxylate) dihydrate, whose isotropic point was previously detected (S. A. Kim, C. Grieswatch, H. Küppers, Zeit. Krist. 1993; 208:219-222) and exploited for a singular measurement of optical activity normal to the optic axis. The crystal structure associated with the aforementioned study was never published. We report it here, confirming the space group assignment I 4¯c2, along with the frequency dependence of the fundamental optical properties and the constitutive tensors by fitting optical dispersion relations to measured Mueller matrix spectra. k-Space maps of circular birefringence and of the Mueller matrix near the isotropic wavelength are measured and simulated. The signs of optical rotation are correlated with the absolute crystallographic directions.

Anisotropic Short-Wavelength Friedel Oscillation with Adjustable Envelope in Graphene* *Supported by the National Basic Research Program of China (973 Program) under Grant No. 2013CB934001, the State Key Laboratory of Software Development Environment under Grant No. SKLSDE-2013ZX-28

We investigate theoretically the intervalley charge density oscillation and the screened ionic potential in graphene caused by the intervalley scattering. We demonstrate that the contribution from the intervalley scattering is comparable with that from the intravalley scattering, and oscillation rather than decaying dominates at a large distance away from the external impurity. We show that the intervalley oscillation is strongly anisotropic because of the inequivalency between neighboring valleys. The anisotropic oscillation consists of an anisotropic short-wavelength oscillation with an anisotropic fixed wavelength and an isotropic long-wavelength envelop with an isotropic wavelength modulated by doping, making an adjustable-widthed wave-packet propagation. One weakens the screening and gets anisotropic short-wavelength oscillation by introducing short-range mechanism in graphene and graphene-like materials.

Birefringence in quantum wells of heterostructures In0.68Al0.1Ga0.13As/In0.42Al0.22Ga0.24As/InP

The polarization dependences of reflection and wavelength modulated reflection spectra of quantum wells In0.68Al0.1Ga0.13As/In0.42Al0.22Ga0.24As were investigated. Spectral dependences of refractive indices, extinction coefficients, real and imaginary parts of dielectric constants of quantum wells structures for different polarizations were calculated by the Kramers–Kronig analysis. A phenomenon of birefringence and an interference of polarized light waves in quantum wells were researched. The isotropic wavelength λ0=1.246μm was found out. Interference spectra changes the density of fringes and refractive indices (Δni, ΔnKK=nР,Р–nS,S) intersect zero axis at energy of isotropic wavelength (0.955eV).

Birefringence of Cu2ZnSiSe4 single crystals

In Cu2ZnSiSe4 single crystals an interference spectra of the Fabry–Perot type due to observed birefringence were studied and spectral dependence of the refractive index n|| (E||c) and n⊥ (E⊥c), which intersect at isotropic wavelength λ0=622nm (300K) and 605nm (10K) were defined. The spectral dependence of Δn=n⊥–n|| in the short-wave and long-wave side of the wavelength λ0 was determined. It was found that when λ>λ0 Δn is negative and when λ<λ0 Δn is positive. Wavelength λ0 shifts as the temperature decreases toward shorter wavelengths. Phases of the n|| and n⊥ light waves in λ>λ0 and λ<λ0 are different. At a wavelength of 538nm (10K) Δn has a maximum. Absorption coefficient in the fringes varies up to 102–103 times.

Interference of birefractive waves in CdGa2S4 crystals

In СdGа2S4 crystals the Fabry–Perot and birefringence interference spectra were investigated. Spectral dependences of refraction indexes for ordinary (no) and extraordinary (ne) light waves are defined. The spectral dependence Δn=ne−no from the short and long-wavelength parts of isotropic wavelength λ0=485.7nm (300K) is determined. It is established that at λ>λ0 Δn is positive and at λ<λ0 Δn is negative. Wavelength λ0=485.7nm shifts with decreasing temperature to short-wavelengths. The phase difference of ordinary and extraordinary light waves for λ>λ0 and λ<λ0 was determined. The band in reflection spectra observed at the isotropic wavelength has a small halfwidth (∽3–5Å). Another isotropic wavelength was found in the short-wavelength region (433nm) for crystals obtained by iodine transport method.

The band structure of birefractive CdGa2S4 crystals

In this paper, we report on the spectral dependence of Δn=no−ne for CdGa2S4 single crystals for shorter and longer wavelengths than the isotropic wavelength λ0=485.7nm (300K). It was established that Δn is positive at λ>λ0 and it is negative in the spectral range λ<λ0. The isotropic wavelength λ0 exhibits blue spectral shift with temperature decreasing. The ground and excited states of three excitonic series A, B and C with binding energies of 53meV, 52meV and 46meV, respectively, were found out at 10K. The effective masses of electrons for k=0 were derived from the calculation of excitonic spectra: mc∥(Е∥с)=0.21m0 and mc⊥(Е⊥с)=0.19m0. The holes masses are equal to 0.59m0 and 0.71m0 for Е∥с and Е⊥с, respectively. The value of valence bands splitting, V1−V2, by crystalline field equals 24meV, and V2−V3 splitting due to the spin–orbital interaction equals to 130meV. The optical functions n, k, ε1 and ε2 for Е⊥с and Е∥с polarizations were calculated by means of Kramers–Kronig analyses in the energy interval 3–6eV. The evidenced features are discussed taking into account the results of new theoretical calculations of CdGa2S4 band structure.

Birefringence of CuGaxAl1 − xSe2 crystals

Excitonic reflection spectra of CuAlSe2 crystals were studied at 10 K. The spectral dependences of refractive indicesno andne were calculated. The isotropic wavelength (the point of intersection ofno andne) in the excitonic region was determined. The spectral dependence ofΔn = no − ne was studied on the long-wavelength and short-wavelength sides of the isotropic wavelength ofCuGaxAl1 − xSe2 crystals. Up to 15 narrow transmission (absorption) lines were revealed in the interferencespectra of thin crystals, which can be used for the development of comb filters.

CdGa2S4-MGa2S4 (M = Zn and Mg) systems

A phase microdiagram of the system CdS-Ga2S3 in the CdGa2S3 homogeneous region was constructed, and the composition corresponding to congruent melting was determined (Cd0.498Ga1.004S2.004). A fragment of the CdGa2S4-ZnGa2S4 phase diagram was studied, and the solid solution composition suitable for growing homogeneous single crystals with respect to the isotropic wavelength λ0 within 492.7–532 nm was determined. A CdGa2S4-MgGa2S4 phase diagram was constructed, and the isotropic wavelength was studied as a function of magnesium prevalence in cadmium thiogallate. Magnesium and zinc codoping produces gyrotropic single crystals suitable for manufacturing optical elements of filtering devices with λ0 within 492.7–880 nm.

Photochemical color switching behavior of induced cholesteric liquid crystals for polarizer free liquid crystalline devices

Photoresponsive cholesteric (Ch) liquid crystals (LCs) showing reflection of visible light were prepared by the addition of chiral molecules and azobenzene molecules in a host nematic LC. The Ch–isotropic phase transition temperature and wavelength of reflected light were controlled by varying the enantiomer excess. Switching between reflection and transparent states was achieved by ultraviolet irradiation. In addition, the switching time was also explored by means of time-resolved measurements.

A criterion for Applying Chalcopyrite Semiconductors to Optical Line Elimination Filters

The optical line elimination filter consisting of a birefringent, optically active crystal and two polarizers has a narrow band at the wavelength of the accidental optical isotropy of crystal. Chalcopyrite semiconductors applicable to this filter are predicted from the contribution of anisotropy of transition intensity to birefringence. CuAlS2, AgAlS2, AgAlSe2, AgAlTe2 and AgInS2 are thought to satisfy the condition of the filter, i.e. the accidental optical isotropy. Experimental results show that CuAlS2 had the optical isotropic wavelength at 378 nm. The dependence of center wavelength of AgInxGa1-xSe2 filter on x is estimated from the band parameters. The center wavelength of AgInxGa1-xSe2 filter (x=0.5) is shown experimentally to be adjusted to the emission line of Nd:YAG laser.

Spectral range of operation of coupled-mode optical filters

The spectral range of operation of optical coupled-mode filters (CMF) is investigated. The CMF principle is based upon the stress-induced energy transfer between orthogonal polarized optical modes, which occurs in birefringent materials at the isotropic wavelength where phase-matching is ensured. A nonexhaustive list of birefringent materials which exhibit an isotropic wavelength is drawn up. The whole visible spectral range is covered continuously by using mixed graded-band-gap hexagonal crystals such as CdS-CdSe and ZnS-CdS. This range is extended up to 2.1 μm by using piezobirefringent cubic compounds. Materials with other crystalline structures enable the extension of this spectral range down to 1200 Å. The optical activity of chalcopyrite-structure semiconductors can advantageously replace stress-induced mode coupling.

Characteristics of coupled-mode optical filters

We provide a quantitative picture of the characteristics of a CdS coupled-mode band-pass optical filter. The operating principle of these coupled mode filters (CMF) is the energy exchange between the ordinary and extraordinary modes when the sample, inserted between two crossed polarizers ∥ and ⊥C, is strained by uniaxial stress at 45° from the C axis. The filter operates at the isotropic wavelength λi where ne=n0 ensures phase matching. The transmission rate and the selectivity are investigated as functions of the stress magnitude T and the sample thickness e. The best results are obtained by adjusting T so that the coherence length Lc=2e. For a 1-mm-thick crystal, the half-height passband is ∼4 Å, while the transmission rate is ∼70% at 77 °K and 20% at 300 °K. The large field of view, in practice ∼45°, constitutes a significant advantage over interference filters. We compare the CMF with zero crossing birefringent filters (ZCBF). Around λi, the characteristics of both filter types are rather similar, but the single transmission window of CMF gives the latter a distinct advantage over ZCBF. Transmission control by variation of applied stress is an added advantage of CMF.

Macroscopic analysis of electromagnetic waves coupling in CdS

The uniaxial stress-induced coupling of the optical ordinary and extraordinary modes in CdS is quantitatively investigated by analyzing, at room temperature, the transmitted light spectrum of a sample placed between two polarizers crossed along the natural neutral lines. Experiments are performed in the wavelength range 5200–7000 Å under application of a calibrated (&amp;lt;500 bar) uniaxial stress directed at 45° with respect to the C axis. At the near gap isotropic wavelength 5229 Å the phase matching allows energy transfer. It is shown that the high selectivity of the process is due to the fast rotation of the induced neutral lines. Due to the stress-induced splitting, the actual birefringence exhibits a sharp but nonzero minimum, at the isotropic wavelength, which is a linear increasing function of the pressure. Its linear pressure coefficient is measured, 6.3×10−6 bar−1, and this allows a direct calculation of the corresponding elasto-optic coefficient p44 = 9.9×10−2. The frequency dispersion of p44 is investigated near the fundamental absorption edge where it exhibits a broad maximum and a sharp structure. The main characteristics of a coupled mode band-pass optical filter are calculated and briefly discussed. Finally the residual internal strain field in most ’’as-grown’’ crystals is estimated as 1.5×10−5.

Graded-composition semiconductors as tunable narrow-band optical filters

Tunable narrow-band optical filters over a wide spectral range are constructed. These use the efficient coupling of orthogonally polarized optical waves near the isotropic wavelength in graded-composition mixed crystals of CdS1−x Sex and Zn1−x Cdx S. In one crystal of CdS1−x Sex, x varied continuously between 0.04 and 0.29 and the central wavelength λi of the corresponding tunable filter from 5130 to 5700 Å. In one sample of Zn1−xCdx S, x varied from 0.41 at one end to 0.70 at the other and λi shifted from 4272 to 4584 Å. In both cases the transmission spectra were narrow peaks with peak transmission of 10–20% and widths at half-maximum of approximately 10 Å.