RU1805351C - Method of measuring light-induced dichroism and birefringence - Google Patents

Abstract

Usage: the invention relates to nonlinear optics and can be used to study materials exhibiting dichroism of enlightenment and birefringence when irradiated with polarized light. Essence: the method includes irradiating the test medium with exciting pulses of linearly polarized light, probing the excited region of the medium with test light pulses, recording the time dependence of the extracted polarization component of the test pulses and determining the parameters of dichroism and birefringence from the calculated expressions of 3 ill.

Description

The invention relates to nonlinear optics and can be used to study materials exhibiting dichroism of enlightenment and birefringence when irradiated with polarized light,

An object of the invention is to increase the measurement performance of light-induced dichroism and birefringence.

The goal is achieved in that in a method for measuring light-induced dichroism and birefringence, including irradiating the test medium with exciting pulses of linearly polarized light, probing the excited region of the medium with probe light pulses and recording the polarized characteristics of the probe light pulses transmitted through the material depending on the delay time of probe pulses , according to the invention, record the time dependence of the selected polarization component of the test them pulse, and the parameters dichroism and birefringence in the material e is determined in the following calculation expression:

TV7i exp {3 / 21tg 5-v

H $ 2

00

about eating

W

ate

where a

(S / Si) 0

(S / S1) ma) (S / Sl) mir

(S / Sl) minV0 / 1 in

wherein TC is the transmission of probe radiation to polarize it parallel to the polarization of the excitation;

TI — transmission of probe radiation to polarize it, perpendicular to the polarization of excitation;

H is the phase difference for the components of the polarization of the probe radiation parallel

and perpendicular to polarization of the excitation;

e is the angle of detuning of the analyzer from the position of the cross with the polarization of the probe radiation;

(S / Si) 0 is the ratio of the energy of the probe radiation in front of and behind the analyzer in the absence of excitation;

(S / Si) min is the ratio of the energy of the probe radiation in front of and behind the analyzer in a minimum kinetic dependence;

(S / Si) max is the ratio of the probe radiation energy in front of and behind the analyzer at the maximum kinetic dependence.

The increase in the measurement performance is due to the fact that in the proposed method there is no need to vary the angular position of the analyzer, provided that the polarization of the probe light pulses does not coincide with the direction of polarization of the exciting pulse and would not be perpendicular to this direction. Such a choice of the direction of polarization of the probe pulses makes it possible to reduce the measurement time with a corresponding calculation modification, since the dependence of the test light pulse parameters on the delay time is recorded only once. The essence of this possibility is that during the relaxation of light-induced dichroism and derefraction the polarization state of the probe pulse changes, i.e. assumes various positions relative to the analyzer, thereby performing an angle scan. After the dye solution is irradiated with a linearly polarized radiation pulse with an energy density sufficient to clear the solution (10 - 10 J / cm2), a bleaching dichroism appears in it, characterized by the transmittance transmittance Tii / Tj for the polarization components parallel and perpendicular to the polar excitation, to birefringence, characterized by the magnitude of the phase difference Φ between the same components. The values of Tc / Tx and H depend on the density of the excitation energy and the properties of the medium: the initial transmission of the solution T0, the degree of anisotropy of individual dye molecules, etc., and their relaxation can be quantitatively described using the expressions for the components of the electric field probe pulse:

-t / T

-t / gd

E || b x Etsvh / Goehr e. (Ј +). Еххых ЕГ / ГойхР е 1 / Г (| +) (1)

where A and B are complex numbers, t is the delay time of the probe pulse with respect to excitation, g is the electron relaxation time, and Td is the time of rotational diffusion of the dye molecules. The dichroism Tn / Td and the birefringence of the H solution are associated with the real and imaginary part of the quantity and at the initial moment (t 0) are equal, respectively

Tn / ti- I E || out / E || VC I2 / I Eieux / Ei / V eB

ft)

5

0

6

0

5

0

5

0

5

Ф - („, In (Ецвых / Е || Вх) - In (

3

 ImB

Figure 1 is a diagram of an apparatus for implementing the method; Fig. 2 is a polarization diagram of a test and exciting pulses and an analyzer transmission; Fig. 3 is a qualitative temporal dependence of the ratio of the measured S / Si signals.

A device for implementing the method comprises a splitting plate mounted in series on the optical axis

I, probe 2 of the probe wavelength, delay line 3, polarizer 4, splitting plate 5 ,. analyzer 6 and photodiode 7, On the path otre-. a rotary mirror 8, a wavelength transducer 9 of the exciting pulse and a half-wavelength phase plate 10 are placed from the pulse splitting plate 1, and a photodiode 11 is placed in the path of the probe pulse reflected from the splitting plate 5, and a probe pulse path is placed between the delay line 4 and the splitting plate 5 III crosses the path of the exciting pulse

II, and the test sample 12 is placed at their intersection.

The measurement method includes the following operations. A linearly polarized laser pulse 1, the duration of which is selected by at least an order of magnitude shorter than the time of restoration of the initial properties of the medium, i.e. shorter than the relaxation times of dichroism and birefringence, splitting plate 1, is divided into two parts. One part is used to form exciting II, and the other to probe III impulses. Using wavelength converters 2 and 9, the wavelengths of excitation and sounding of interest are selected. Converters 1 and 9 are used which do not increase the pulse duration. Test medium 12 is irradiated with an exciting

pulse II, which induces dichroism of enlightenment (or darkening) and birefringence in medium 12. The state of the medium 12 is analyzed by the emission of a probe pulse III after excitation with a time delay t varying from zero to several times of relaxation of dichroism and birefringence using an adjustable delay line 3.

The angle between the polarizations of the exciting and probe pulses is set to 54.7 ° using a half-wave phase plate 10. The value of this angle can be chosen arbitrarily (except for 0 and up to 90 °), however, the value of the angle 54.7 greatly simplifies the formulas for calculation of dichroism and birefringence. Under these conditions, the direction of polarization makes the polarization state of the probe pulse at the exit from medium 12 sensitive to induced dichroism and birefringence and simplifies the calculation ratios. The ratio of the probe pulse passing through medium 12 is analyzed by the time dependence of the intensity of the isolated polarization component. For this, an analyzer 6 is placed in the path of the probe pulse behind medium 12, the transmission plane of which is rotated with respect to. the direction of the cross with polarizer 4 in the direction of the plane of polarization of the exciting pulse by the angle d (see p. 8). The photodetector 7 registers the energy of the probe pulse passing through the analyzer 6, and the signal of the photodetector 11 is proportional to the energy of the probe pulse before it hits the analyzer 6 and. necessary to normalize the signal of the photodetector 7. The part of the probe pulse is cleaved to the photodetector 7 by a beam splitter plate 5, mounted at an angle close to 90 ° to the optical axis, so that the polarization state of the probe pulse is not disturbed.

For a more detailed disclosure of the essence of the method, we will carry out a detailed analysis of the occurring physical phenomena.

We introduce the angles characterizing the position of the planes of polarization of the exciting and probe pulses and the analyzer as shown in figure 2, where

Evkh, Evikh - states of the electric field of the probe pulse at the entrance and exit of the test sample;

Ei, BX, Ef. E „B1X ExB1X — parallel and perpendicular to the field of the exciting pulse are components of the probe pulse at the input and output of the test sample;

Q is the angle of rotation of the main axis of the ellipse of the probe pulse;

p is the angle between the fields of the exciting and probe pulses at the entrance to the sample; .6 is the detuning angle of analyzer 6 with respect to the state of cross with polarizer 4.

Then, at the entrance to the medium 12 for the components of the probe pulse, we have ECB EcosBxy E / x Esin in #. The field at the exit from the medium is described by mathematical expressions (1). The signal passed through analyzer 6 is proportional

S E7Xsm (V-S) () f- | E6 | 2T0exp (ReAe-t f «

x coscf.s, n (-6) exp (Be-t / fSi /) - rin s, ““ {{, -) “f (- | е: 4№-“) “. (3) The signal Si is proportional

MEG | EG1 - | B 1 | yTvv r (KeL - No.). ("Enter- |} - + 5ina4 exp (-ReBe-t -t 4l..

(4)

5

0

5

0

0

5

Taking into account that the value of B in dye solutions does not exceed unity and it can be approximately assumed that exp B is 1 + B and the angle p arctg / is chosen (the usual procedure for polarization measurements), for the signal ratio

S / VM- oefc- -t. V ,.

/ 1 „6

IS.-W-t

Wm ™) (5) Taking into account (2), the last dependence becomes

., „/, To enCTn / Tj

S / V (6,

No.

We-t№) 4

.f-YAG cpeoaSe),

(6)

where 1 / g 1 / r + 1 / rg For values of analyzer rotation through an angle of 0 5 5 Re B / f, the dependence of S / Si on the delay time has the characteristic form c (see Fig. 3) with a maximum immediately after excitation and a minimum at some delay. Since a similar curve was obtained experimentally with the medium under study, fitting it to expression (5) allows us to establish the values of ReB, lmB and 1 / g + 1 // gd. and therefore calculate TH / TJ and p. Such an approximation may not be carried out in full, but at some points. One method is provided below. It consists in using the signal values at the maximum of the (5/51) max curve and for the unexcited solution (S / Si) 0 (values at negative delays), taken with respect to the minimum signal (S / Si).

(5 / S,). (5 / 3,1,

a

(5 / 5.1)

mo

(5/

Iminv-i mimm

The corresponding S / le values (5) are:

ISb.) (6/5, tfn J J (SfS, 1n ,, (b-).

I6 | "

Moreover, the minimum value (S / Si) min is achieved at

gsh-s ",, | Ґ.

| S

f.

For a and b we get IBI a 1g

° CHO1

(8) The solution of system (8) with respect to ReB and

((e..t)

.mb. & S (45Trt), (9)

which solves the problem of the quantities W: T, IVexp 3 (Jtf

I.

Ґi (In the prototype, for measuring dichroism and birefringence, m - n measurements are required, where m is the number of analyzer positions (t 2) and p is the number of delay times at which measurements are made. In the proposed method, the number of measurements is reduced, since there is no need to vary analyzer position: it is equal to the number of delay times p. Thus, the gain in measurement performance is m times, where m 2.

An example of a specific implementation.

The frequency dependences of light-induced dichroism and birefringence of an ethanol solution of the rhodamine 6G dye were studied. The measurements were carried out using a picosecond phosphate glass laser spectrometer generating light pulses of duration (5i 2) ps.

A second harmonic generator was used as a transducer 9 of the exciting pulse wavelength, and a system consisting of a second harmonic generator, a parametric generator, was used as a probe 2 transducer 2

light and the second second harmonic generator, providing smooth tuning

wavelengths in the region of 0.4-0.8 microns. Before falling onto the cell 12 with the studied solution, the probe pulse passed through the delay line 3 and polarizer A. The polarization of the exciting pulse with the help of phase plate 10 was rotated so that it made an angle p: 54.7 ° s

polarization of the probe pulse. Behind the cell 12 in the path of the probe pulse, the installed analyzer 6 was oriented so that its transmission plane was rotated by an angle b: 4 ° in

the ratio of intersection with the polarizer 4. This angle corresponded to approximately half the maximum rotation of the main axis of the ellipse of polarization of the probe pulse (Fig. 2) and provided registration of the characteristic time dependence of the S / Si ratios (Fig. 3). The energy of the probe pulse was recorded until it fell on the analyzer 6 and behind it by photodiodes 7 and 11, respectively, by scanning using line 3

delays every 5 ps, the dependence of the S / Si ratio on the delay time was measured (Fig. 3). For each delay, the results of ten shots were averaged; According to the obtained time dependence

(Fig. H), the ratios (S / Si) 0, (S / Si) MHH and (5/51) max were determined, by which, using expressions (9), (8) and (2), they were determined magnitudes of induced dichroism and birefringence. By conducting a series of such measurements at sounding wavelengths of rhodamine 6G fluorescence absorption bands changed in the region of the absorption bands of ethanol every 4 nm, the frequency dependences of these parameters were determined.

It was found that light-induced dichroism has a maximum at the frequency of a purely electronic transition, and induced birefringence has a negative value in the region of the fluorescence band,

and positive, in the region of the absorption band with maxima near the maxima of the fluorescence and absorption bands, respectively.

Measurements were taken in a week,

Claims (1)

and by the method, moreover, as a prototype, it would take at least a month. SUMMARY OF THE INVENTION A method for measuring the parameters of light-induced dichroism and birefringence, including irradiating the test medium with exciting pulses of linearly polarized light, probing the excited region of the medium with probe light pulses, recording the polarization parameters transmitted through the medium and probe light pulses analyzer depending on the probe pulse delay time characterized in that, in order to increase the measurement productivity, a time dependence is recorded the extracted polarization component of the probe pulses, and the parameters of dichroism and birefringence in the material are determined from the following relationships: Wit-exp {3 / Ttgd- ll (+ ib-1)}  tg4 {fr +) (S / Si) o (S / Sl) min (S / Sl) nake (S / Si) min 0 5 0 In is the transmittance of the probe radiation for its polarization parallel to the polarization of the excitation; Tj, is the transmittance of the probe radiation to polarize it, perpendicular to the polarization of the excitation; H is the phase difference for the components of the polarization of the probe radiation parallel and perpendicular to the polarization of the excitation; e is the angle of detuning of the analyzer from the position of the cross with the polarization of the probe radiation; (S / Si) o is the ratio of the energy of the probe radiation in front of and behind the analyzer in the absence of excitation; (S / SI) MHH is the ratio of the energy of the probe radiation in front of and behind the analyzer in a minimum kinetic dependence; (S / Si) MaKc is the ratio of the probe radiation energy in front of and behind the analyzer at the maximum kinetic dependence. s / s ABOUT 5% gz