CN102183466A - Time resolution elliptical polarization spectrum measuring system - Google Patents

Abstract

The invention belongs to the technical field of optical and electronic devices, and in particular relates to a time-resolved ellipsometry measurement system. The measurement system includes lasers, beam splitters, mirrors, retarders, beam expanders, samples, polarizers, multi-channel analyzers, CCD detectors, computer systems, stepping motors and other components. The laser uses a tunable femtosecond ultrafast laser source. The pulsed light emitted by the light source is split into pump light and probe light by a beam splitter. The pump light is vertically incident on the sample after passing through the time delay device, two mirrors and the beam expander; the probe light is incident on the sample after passing through the beam expander and polarizer. After the light is reflected by the sample, it passes through the multi-channel analyzer and is detected by the CCD area array detector. The computer calculates the corresponding time-resolved optical constant spectrum based on the electrical signal received by the detector. The invention has broad application prospects in many fields such as physics, chemistry, biomedicine, and environmental science.

Description

A time-resolved spectroscopic ellipsometry system

technical field

The invention belongs to the technical field of optical and electronic devices, and in particular relates to a time-resolved ellipsometry measurement system.

Background technique

The interaction between light and matter involves many transient processes, such as electronic transitions in solids, fluorescence emission and internal transformation processes of biological and chemical materials, etc. By measuring the evolution of reflectivity, transmittance or polarization state over time after light passes through the material, the dynamic process inside the substance can be studied, and important information such as the state of motion of molecules or electrons in the substance, such as electronic energy levels and electronic state pairing can be obtained. . When light interacts with matter, the light field causes the polarization of molecules or electrons, resulting in the distribution of molecules or electrons and the evolution of their distribution function over time, so that the dielectric constant of the material ε (ω , t ) = ε ₀ (ω) + Δε(ω , t ) becomes a function of time, where ε ₀ (ω) is the dielectric constant of the substance without pump light irradiation, and Δε(ω , t ) is the change in the dielectric constant of the substance caused by pump light irradiation , and ε (ω , t ) is related to various optical constants of the material, such as refractive index, absorptivity, transmittance and reflectivity.

Spectroscopic ellipsometry is an effective and reliable spectroscopic measuring instrument for obtaining optical constants and film thicknesses of various materials. The properties of materials are analyzed to understand their physical nature, which leads to important applications of materials. At present, a variety of ellipsometric measurement modes have been developed, such as extinction mode and photometric mode. However, the current ellipsometry technology is limited to measuring the optical constants of materials in the steady state, and cannot measure the transient optical constants. To this end, the present invention combines femtosecond laser technology, multi-channel optical detection technology and polarization state parallel measurement mode ellipsometry technology to realize time-resolved ellipsometric measurement to obtain the transient optical constants of materials. This invention will help to deepen the understanding of the excitation and relaxation process of matter, and provide an effective means for studying the excitation dynamics and luminescence dynamics of atoms, molecules and condensed matter. Time-resolved polarization ellipsometry can be applied to many fields such as physics, chemistry, biomedicine, environmental science, etc. It has important basic research significance and broad application prospects.

Contents of the invention

The object of the present invention is to propose a time-resolved polarization ellipsometry system so as to obtain the transient optical constants of materials.

The time-resolved ellipsometric measurement system proposed by the present invention includes the following components connected in sequence: a laser, a beam splitter, a first reflector, a retarder, a second reflector, a third reflector, and a first beam expander Mirror, sample, second beam expander, polarizer, multi-channel analyzer, CCD detector, computer control system, stepping motor, etc. Wherein, the laser adopts a tunable femtosecond ultrafast laser as a light source; the pulsed light emitted by the light source passes through a beam splitter and is divided into two beams, one is pumping light and the other is probing light. The intensity of the pump light is generally taken as several times of the intensity of the probe light, for example, 4-20 times. After being reflected by the first reflector, the pump light passes through the time delay device, the second reflector, the third reflector and the first beam expander in sequence, and is vertically incident on the sample. The probe light is incident on the sample at a certain angle after passing through the second beam expander and the polarizer in sequence. After the detection light is reflected by the sample, the reflected light is detected by the CCD detector after passing through the multi-channel analyzer. The measurement system is controlled by a computer control system. The synchronization and delay between probe light and pump light are controlled by a computer control system through a stepping motor. The sample is mounted on a sample holder with vacuum negative pressure adsorption to realize non-destructive spectral detection.

The invention adopts the folded optical path to collimate the light.

The method of the present invention allows the laser to go back and forth between the two reflecting mirrors multiple times, and the optical path is extended several times in a limited space to improve the collimation precision of the sample. The light is reflected by the sample and passed through a multi-channel analyzer. The multi-channel polarizer is composed of m miniature analyzers, the azimuth angles of these m miniature analyzers are θ ₁ , θ ₂ , ... θ _m , and the azimuth angles θ ₁ ~ θ _m are at 0 Evenly distributed between ~π. m is generally an integer of 5-15. The reflected light passing through the multi-channel analyzer is detected by different areas of the CCD area array detector. The electrical signal received by the detector is input to the computer after AD conversion, and after computer analysis and processing, the corresponding optical constants are calculated.

In actual measurement, the polarizer is fixed at an optimal azimuth angle. For example, let P = 45 ^o , after performing fast Fourier transform on the analyzer angle A, the light intensity emitted from the analyzer varies with the azimuth angle of the analyzer as follows:

                                           (1)

(1)

Among them, I ₀ is the DC component of the light intensity I , and the rest are AC components. After calculation, it can be obtained:

(2)

为与系统光源相关的常数。根据(2)式中的三个系数可计算出椭偏参数r ₀和cosD，两者表示为： I _B is the background signal in the DC component I ₀ , which comes from the dark current of the detector, where

is a constant related to the system light source. According to the three coefficients in formula (2), the ellipsometric parameters r ₀ and cosD can be calculated, which are expressed as:

(3)

Through the numerical Fourier transform of the azimuth angle of the analyzer in formula (2) by the light intensity I , the value of each light intensity can be obtained as:

                                                (4)

(4)

In the formula, I _i is the light intensity signal received by the CCD detector when the azimuth angle of the polarizer is A _i , l is the corresponding wavelength, M is twice the number of polarizers in the unit of the polarizer group, and press The A _i value is the number of data acquisitions. After the ellipsometric parameters r ₀ and cosD are measured, the dielectric constant and other optical constants of the sample can be calculated. The optical complex permittivity e = e ₁ + i e ₂ can be calculated as:

                                     (5)

(5)

Then get other optical constants, such as complex refractive index N , absorption coefficient a and reflectivity R , etc.:

           (6)

(6)

where n is the refractive index and k is the extinction coefficient. The above calculations are done by computer.

Description of drawings

Figure 1 is a diagram of a time-resolved ellipsometry system.

Figure 2. Schematic diagram of multi-channel polarizer. The multi-channel polarizer is composed of m miniature analyzers, the azimuth angles of these m miniature analyzers are θ ₁ , θ ₂ , ..., θ _m , and the azimuth angles θ ₁ ~ θ _m are in Evenly distributed between 0~π.

Numbers in the figure: (1) laser, (2) beam splitter, (3) first reflector, (4) retarder, (5) second reflector, (6) third reflector, (7) first reflector One beam expander, (8) sample, (9) second beam expander, (10) polarizer, (11) multi-channel analyzer, (12) CCD detector, (13) computer control system, ( 14) Stepper motor.

Detailed ways

Describe the specific embodiment of the present invention by an example below:

1. The pulsed light emitted by the tunable femtosecond ultrafast laser source 1 is divided into two beams by the beam splitter 2, one is the pump light and the other is the probe light. The intensity of the pump light is generally taken as several times the intensity of the probe light, for example, the ratio of the two is 10:1-4:1.

2. After being reflected by the mirror 3, the pump light passes through the delayer 4, the mirror 5, the mirror 6 and the beam expander 7 in sequence, and then is vertically incident on the sample 8.

3. The probe light is incident on the sample 8 at a certain angle after passing through the beam expander 9 and the polarizer 10 in sequence.

4. After the detection light is reflected by the sample 8, the reflected light passes through a multi-channel analyzer 11 and is detected by a CCD detector 12.

5. The measuring system is controlled by the computer control system 13 . The synchronization and delay between probe light and pump light are controlled by a computer control system through a stepping motor 14 .

6. The sample is installed on the sample holder with vacuum adsorption function, realizing non-destructive spectral detection.

7. Use folded optical path for light collimation. The method of allowing the laser to go back and forth between the two reflecting mirrors multiple times extends the optical path several times in the limited optical space, so that the collimation accuracy of the sample can reach 0.01 degrees.

8. The light passes through the multi-channel polarizer 11 after being reflected by the sample. The multi-channel polarizer is composed of 12 micro-analyzers, and the polarization planes of these micro-analyzers are uniformly arranged in the range of 0~π. Twelve miniature analyzers of 1.5 mm×1.5 mm square are installed on a stainless steel bracket with a diameter of 9.5 mm and a thickness of about 2 mm. The azimuth angles of these 12 miniature analyzers are 0o, 16o, 33o, 49o , 66o, 82o, 90o, 98o, 114o, 131o, 147o, 164o.

9. The light rays passing through the 12 miniature analyzers are respectively detected by different areas of the 2-dimensional CCD area array 12 .

10. The electrical signal received by the detector is input to the computer after AD conversion, and the corresponding optical constants are calculated after computer analysis and processing.

In the design of the present invention, the incident angle is continuously variable, and data is automatically collected, analyzed and calculated. The system realizes time-resolved ellipsometry. The invention can be applied to many fields such as physics, chemistry, biomedicine, environmental science, etc., and has wide application prospect.

Claims (3)

1. time resolution elliptically polarized light spectral measurement system, it is characterized in that comprising connect successively as lower member: laser instrument, beam splitter, first catoptron, delayer, second catoptron, the 3rd catoptron, first beam expanding lens, sample, second beam expanding lens, the polarizer, hyperchannel analyzer, ccd detector, computer system, step motor; Wherein, laser instrument adopts tunable altra-fast fs laser as light source; The pulsed light of described light source outgoing is divided into two bundles through beam splitter, and a branch of is pump light, and is a branch of for surveying light; Pump light intensities is taken as 4-20 times that survey light intensity, pump light through first mirror reflects after successively by chronotron, second catoptron, the 3rd catoptron and first beam expanding lens, impinge perpendicularly on the sample; Described detection light is successively through inciding on the sample at a certain angle behind second beam expanding lens, the polarizer; After surveying the reflection of light process sample, reflected light is surveyed by ccd detector through the hyperchannel analyzer; Measuring system is by computer system control; Synchronous and the delay of surveying between light and the pump light is passed through a step motor control by computer system; Sample is installed in to have on the adsorbing specimen holder of negative pressure of vacuum, realizes harmless spectral detection.

2. time resolution elliptically polarized light spectral measurement system according to claim 1, it is characterized in that described hyperchannel analyzer by mIndividual miniature analyzer constitutes, and establishes this mThe position angle of individual miniature analyzer is θ ₁, θ ₂, θ _m , and the position angle θ ₁~ θ _m Between 0 ~ π, evenly distribute; mIt is 5-15 integer.

3. time resolution elliptically polarized light spectral measurement system according to claim 2 is characterized in that described computer system calculates relevant optical constant by following formula:

By analyzing angle A is done Fast Fourier Transform (FFT), close with the analyzer azimuthal variation from the light intensity of analyzer outgoing and to be:

(1)

Wherein I ₀It is light intensity IDC component, all the other are AC compounent; Relation is arranged:

(2)

I _BIt is DC component I ₀In background signal, come from the dark current of detector, Be the constant relevant with system source; Go out ellipsometric parameter according to three coefficient calculations in (2) formula r ₀And cosD, both are expressed as:

(3)

Pass through light intensity IThe numerical value Fourier transform is made at position angle to analyzer in (2) formula, and the value of trying to achieve each light intensity is:

(4)

In the formula I _i Be that the analyzer position angle is A _i The time ccd detector received light intensity signal, lBe corresponding wavelength, MBe the twice of analyzer group unit analyzer number, press in the one-period A _i Value is made the number of times that data are gathered; Ellipsometric parameter r ₀With cosD determined after, can calculate the specific inductive capacity of sample and other optical constant, wherein, the optics complex permittivity e= e ₁+ i e ₂Can be calculated, that is:

(5)

And then obtain other optical constant: complex index of refraction N, absorption coefficient aAnd reflectivity R:

(6)

Wherein nBe refractive index, kBe extinction coefficient, more than calculate and finish by computing machine.

Images