WO2006077476A1

WO2006077476A1 - Organic photodiode of poly(9,9-dioctyl)fluorene

Info

Publication number: WO2006077476A1
Application number: PCT/IB2006/000056
Authority: WO
Inventors: Guglielmo Lanzani; Alessio Gambetta; Tersilla Virgili
Original assignee: Politecnico di Milano
Current assignee: Politecnico di Milano
Priority date: 2005-01-18
Filing date: 2006-01-03
Publication date: 2006-07-27
Anticipated expiration: 2007-07-18
Also published as: ITMI20050051A1

Abstract

The invention relates to photodiode for the measurement of ultra-short pulses in the visible and ultraviolet field comprising at least two electrodes and a layer of poly(9,9-dioctyl)fluorene, in which one electrode preferably comprises a layer of indium tin oxide (ITO) and another electrode comprises layers of calcium and aluminium. In a preferred embodiment the layer or PFO in the photodiode according to the invention is sandwiched between the two electrodes.

Description

"Organic photodiode of poly (9, 9-dioctyl) fluorene"

DESCRIPTION

This invention relates to a photodiode for measuring ultra-short pulses in the visible/ultraviolet region .

In research laboratories it is often necessary to characterize lasers with ultra-short pulses , and these are currently defined as lasers having pulses of the order of 200 fs or less . Measuring instruments which are normally used for this purpose in the course of experiments with lasers are known. Among the properties investigated it is necessary to measure the duration of ultra-short pulses , but because of its nature this is difficult to do .

At the present time, in order to measure ultra-short pulses , especially in the wavelengths of visible light, it is necessary to build an optical self-correlator, making use of the strong non-linearity of crystals, which are in fact defined as being non-linear . In this technique two beams are caused to pass through a nonlinear crystal which has been previously configured so as to generate a frequency which is the sum of the two incident lasers . However self-correlators require the use of a special non-linear crystal having a high nonlinear coefficient for incident light beams and adequate transmission for the beams passing through . In addition to this, the non-linear crystals used in such applications are not only very expensive but are difficult to align because of the precise conditions required for the angles formed between the two incident beams and the crystal . In the ultraviolet region, in particular, the technique known as self-diffraction, i . e . a method of self-correlation which is based on the mixing of 4 waves ( "degenerate four wave mixing" ) , is used. Specifically, two laser beams are incident on a crystal and here spatially overlap in such a way that the pulses are also overlapped in time, forming a diffraction grating . In this way, at the output the two starting beams together with two additional beams are obtained with the same wavelength but having a different exit angle from the crystal . The intensity in the new directions depends on the temporal overlap between the pulses and, in particular, it is at its maximum when the delay between two pulses is zero . By drawing a graph of the intensities of the diffracted beams versus the delay between the pulses, it is then possible to obtain an self-correlation curve from which the temporal duration of the starting pulse can be determined. When this technique is applied in order to obtain the desired measurement, very high powers are involved in both the visible and the UV regions, with individual pulse energies of about 1 μj.

The use of inorganic photodiodes to avoid the disadvantages associated with the use of crystals is known from document US 6, 356, 381. Although the type of photodiode described involves still high costs , the method of measurement proposed is simpler and more economical than when crystals are used, because the sources of radiation only have to overlap on the photodiode, avoiding the problems relating to alignment of the incident laser beams . This patent describes the self-correlation of ultra-short pulses of wavelengths greater than 760 ran (preferably in the wavelength range from 775 nm to 1300 nm) , in which two radiation pulses of different intensity and wavelength have to be focused on the photodiode .

The obj ect of this invention is to provide a photodiode, which allows the measurement of the amplitude of an ultra-short laser pulse having a wavelength within the visible and ultraviolet range in a simple way and which is of low cost and quick to measure the pulse . The obj ects indicated above may be achieved through an organic photodiode having the characteristics indicated in claim 1.

The photodiode according to the invention comprises at least two electrodes and a layer of poly ( 9, 9- dioctyl) fluorene .

The invention will now be described in greater detail with reference to the appended figures in which:

Figure 1 is a diagram of an apparatus for measuring ultra-short pulses using a photodiode according to the invention,

Figure 2 is a graph showing photocurrent as a function of the time delay obtained using the photodiode according to the invention in a test measuring the pulse amplitude of a laser at 390 nm,

Figure 3 is a graph showing differential transmission as a function of time delay in a comparison experiment using the self-diffraction technique according to the prior art to measure the pulse amplitude of a laser at 390 nm, and

Figure 4 is a graph showing photocurrent as a function of time delay obtained using the photodiode according to the invention in a test to measure the pulse amplitude of a laser at 780 nm.

According to the invention, when the term "visible and ultraviolet" spectral range is used this means wavelengths from 320 nm to 780 nm and when lasers with ultra-short pulses are generically referred to these mean pulses of the order of 200 fs or less . In particular the photodiode according to the invention is capable of measuring pulse durations within the range of from 50 fs to 200 fs .

Poly ( 9, 9-dioctyl) fluorene, abbreviated PFO, which is used in the photodiode according to the invention, is a polycyclic aromatic hydrocarbon which is known for its optical absorption properties in the ultraviolet and the visible . In the solid state this polymer has long chains of carbon atoms with conjugated bonds and is characterized by strong intermolecular and intramolecular interactions which govern the absorption and emission capability of the final polymer material .

In accordance with the invention it has surprisingly been found that this material is a photoactive material which can be used as a component in a photodiode capable of working in the wavelengths of the visible and the ultraviolet .

According to the invention the photodiode preferably comprises a first electrode comprising a layer of indium tin oxide (ITO) and a second electrode comprising layers of calcium and aluminum.

Optionally the photodiode may also comprise a layer of polyethylene dioxythiophen ( PEDOT) , which has the function of facilitating charge transfer .

According to a preferred embodiment, in the photodiode according to the invention the polyfluorene layer is located between the two electrodes in a sandwich structure . In this embodiment one of the two electrodes must be transparent at the wavelength of interest .

According to the invention the photodiode for measuring the amplitude of ultra-short pulse lasers having a sandwich conformation can be prepared by the known technique of spin coating, that is coating by centrifuging. According to this technique a film of PFO is deposited from its solution in toluene on an anode and then the metal cathode is deposited as a coating of suitable metals by evaporation under vacuum. Preferably the solution used is a solution of 20 mg/ml of PFO in toluene . In a preferred embodiment the anode is a layer of indium tin oxide ( ITO) placed on a glass substrate . The thickness of this oxide layer is preferably within the range from 100 nm to 400 nm, and even more preferably it is approximately 200 nm. The metal cathode preferably comprises layers of calcium, having a preferred thickness of approximately 20 nm, and aluminum, having a thickness of approximately 150 nm. Even more preferably the PFO layer is a film of approximately 100 nm. When the layer of PEDOT is present, this preferably is between the layer of ITO and the layer of poly ( 9, 9-dioctyl) fluorene . This has a preferred thickness of approximately 20-30 nm and is preferably deposited using the spin coating technique .

The polyfluorene material has high polarisability, defined as being of third order, which involves strong non-linearity, which is useful in measurements with at least two photons .

According to the invention and as will be more apparent from the experimental part described below, when the material is excited at wavelengths in the visible/ultraviolet field by two branches of the same pump pulse spatially overlapped on the sample, the strongly non-linear response of the polyfluorene can be made use of, to measure the photocurrent exiting from the photodiode . Through the latter it is then possible to obtain the self-correlation curve for the pump pulse and determine the time duration of the pulse of the beam used.

Without wishing to be restricted to any theory, it is felt that the ultra-fast response of the photodiode in the wavelength range of the visible and ultraviolet according to the invention is due to the speed of the processes involved in the mechanism of charge generation, such as the sequential mechanism and vibrational release of the exited state . In the case of ultraviolet, the first pump pulse in fact reaches the vibrational state of the excited state of the singlet

(Si^*) and sequentially the second pulse populates a higher state of the ' singlet (S_n) , from which the charges are then generated. It is however felt that this process is in competition with relaxation of the first vibrational state to the singlet state ( Si) , which takes place in approximately 50 fs . Thus , if the second pulse arises after this relaxation, the pulse will populate a different singlet state ( S_m) which, unlike S_n, has a low charge generating efficiency. It is therefore felt that, for this reason, the charges generated optically follow the profile of the pump pulse and are a maximum when the two pulses arrive together and a minimum when they arrive separately . Specifically this process will involve an operation with two or three photons depending upon the wavelengths involved, the three photon mechanism being required for wavelengths greater than approximately 700 nm.

The invention therefore also relates to a self- correlator for measuring the duration of ultra-short laser pulses in the spectral region of the ultraviolet and the visible comprising the photodiode according to claim 17.

An embodiment of the invention in which the photodiode according to the invention is used to measure the amplitude of ultra-short pulses of a laser in the ultraviolet, an example of a comparison measurement using the self-diffraction technique according to the prior art and a further example in which the photodiode according to the invention is used to measure the amplitude of the ultra-short pulses of a laser in the visible will now follow.

Example 1

Measurement of the amplitude of laser pulses at 390 nm using the photodiode according to the invention.

A Ti : sapphire laser in mode locking regime, duplicated in frequency was used. This beam was characterized by a wavelength of 290 nm, a repetition frequency of 1 kHz , and a pulse energy of approximately 2 nJ.

With reference to Figure 1, the beam under investigation was separated into two parts , each with pulses of energy 1 nJ, through a beam splitter 1. One part of the beam was transmitted while the other part was reflected through an angle of 90 ° . Each of the two beams was then reflected by a pair of mirrors 2 in a cube edge configuration so that the beam reflected from the pair ran parallel to the incident beam on the pair .

One of the two pairs was mounted on a motorized slide

(not shown) which allowed to vary the position and therefore the optical path of the corresponding beam (and was as a consequence able to vary the time delay between the pulses in the two lines ) . The two beams were then realigned and rendered parallel by passing the transmitted beam through a further mirror 3. These beams were then caused to be incident upon a convex lens 4 in such a way that the spots were in a symmetrical position with respect to the centre of the lens . In this way ^'the two beams were focused simultaneously. These were spatially overlapped at a distance from the lens equal to its focal length and formed a propagation angle between the two beams which was associated with the ratio between their distance before the lens and the distance from the focus of the lens itself . Non-linear photodiode 5 comprising PFO was positioned at this point . Because the photodiode responded to the intensity of illumination providing a current proportional to the square of the incident intensity, the photogenerated current included a term which depended on the degree of temporal overlap between the pulses . By varying the delay between the pulses it was possible to obtain a graph of the photocurrent in relation to the time delay ( shown in Figure 2 ) with which the two beams were incident on the photodiode . To increase the sensitivity of measurement, the technique of "synchronous" amplification of the signal was used: in fact a mechanical modulator 6 capable of modulating the intensity of the beam at a fixed frequency ( 470 Hz in our case) , which thus allowed one pulse out of every two to pass, was positioned on the delay line . The non-linear component of the PFO photodiode current showed a modulation at the same frequency (given that it depends on the simultaneous occurrence of pulses on the material) : this signal was then passed to a "lock-in" electronic amplifier 7 to which the modulation frequency of the mechanical modulator was communicated. Amplifier 7 then amplified the signal only at the reference frequency, filtering out noise with a narrow band filter in such a way as to enormously increase the signal-to-noise ratio .

A bell-shaped curve illustrating the convolution between the pulse and the opto-electronic response of the material was obtained. With reference now to Figure 2 , which shows the photocurrent/time delay graph obtained, it was possible from the Gaussian shape of the graph to interpolate the curve, obtaining a measurement of the pulse as its "half-height width" . In order to obtain the pulse duration the curve was then deconvolved using the material response : in the case of

Gaussian pulses, knowing that the current response was proportional to a second order self-correlation

(absorption of two photons ) , it was sufficient to divide the "half-life width" by a factor of 1.41. As the amplitude of the pulse was 207 fs , the duration of the pulse was therefore calculated and found to be 148 fs .

Example 2

Measurement of the pulse amplitude of a laser at 390 nm using the self-diffraction technique ( IEEE Journal of Quantum Electronics , Vol . 25 , no . 12 , December 1989, page 2580 : Measurements of intense ultraviolet subecosecond pulses using degenerate four wave mixing, H . Schulz , H . Shchuler et al . ) .

In order to check whether the measurement made using the photodiode according to the invention is accurate the measurement was repeated applying the above technique and applying the self-diffraction technique . A Ti : sapphire laser having a wavelength of 390 nm, a repetition frequency of 1 kHz and a pulse energy of 4 μJ was used. The beam used in example 2 was therefore the same as in Example 1, but the pulse energy was greater so the amplitude could be measured using the self-diffraction technique . The laser was therefore separated into two parts using a beam splitter; one part of the beam was transmitted while the other was reflected through an angle of 90 ° . Each of the two beams were then reflected by a pair of mirrors in the "cube edge" configuration so that the beam reflected from the pair ran parallel to the beam incident upon the pair . One of the two pairs was mounted on a motorized slide, by which means its position and therefore the optical path of the corresponding beam could be varied (and as a consequence the time delay between the pulses in the two lines could be varied) . The two beams were then realigned through a mirror so that they ran parallel (at a distance of some 100 mm) and were incident upon a convex lens (focal length 400 mm) , being in symmetrical positions with respect to the centre of the lens . This had the result that the two beams were focused simultaneously, spatially overlapping at a distance from the lens equal to its focal length and formed an angle of propagation between the two beams associated with the ratio between their distance before the lens and the distance from the focal length of the lens . A sheet of transparent material, that is the crystal, and specifically a sheet of sapphire (Al₂O₃) , was positioned at that point . Because of the high power of the peak pulses within the sheet, a number of non-linear phenomena were initiated, including that known as self-diffraction . The interaction between the fields of the incident beams created a diffraction grating which brought about self- diffraction of the beams themselves . This phenomenon depended on the time delay between the two incident pulses on the sheet originating from the two lines : when the two pulses were perfectly overlapped, maximum interference occurred and therefore the maximum intensity of the "deviated" beam; vice versa if the two pulses were completely separated in time the intensity of the diffracted beam was zero . The intensity of the diffracted beam was then measured using a linear photodiode (of silicon) , which is capable of providing a current proportional to the incident optical intensity. The technique of "synchronous" amplification of the signal was used to increase the sensitivity of the measurement as in Example 1. By varying the delay between the pulses it was possible to obtain a graph of that intensity as a function of the time delay with which the two beams were incident upon the sheet, as illustrated in Figure 3. The curve obtained was bell- shaped and represented the convolution between the pulse and the optical response of the sheet . Assuming that the pulse was of Gaussian shape, it was possible to interpolate the curve obtaining the pulse amplitude as the "half-height width" . This was calculated and the value of 184 fs was obtained. In order to obtain the pulse duration it was necessary to deconvolve this curve using the response of the material : in the case of Gaussian pulses, knowing that the response was proportional to the third order self-correlation, the mechanism being one with three photons, in order to obtain the duration of the pulse it was sufficient to divide the pulse amplitude by a factor of 1.22. A value of 150 fs was obtained for the pulse duration .

The photodiode according to the invention is therefore capable of detecting the duration of ultra-short laser pulses in the ultraviolet with the same accuracy as the prior technique, but without being necessary to take precise angles of incidence into account, directly producing a current signal which reproduces the self- correlation between the pulses in relation to their time of delay.

Advantageously, the photodiode according to the invention works in the ultraviolet region with a power which is three orders of magnitude smaller than the conventional techniques as demonstrated above .

Example 3

Measurement of the amplitude of laser pulses at 780 nm using the photodiode according to the invention .

A Ti : sapphire laser in the "mode locking" regime was used at the fundamental wavelength of 780 nm; this beam was characterized by repetition frequency of 1 kHz , with a pulse energy of approximately 4 μJ.

With reference to Figure 1, the beam under investigation was separated into two parts, each having pulses of energy 2 μJ, through a beam splitter 1 : one part of the beam was transmitted, while the other part was reflected through an angle of 90 ° . Each of the two beams was then reflected by a pair of mirrors 2 in "cube edge" configuration in such a way that the beam reflected by the pair ran parallel to the beam incident upon the pair . One of the two pairs was mounted on a motorized slide (not shown) , through which its position and therefore the optical path of the corresponding beam could be varied (and as a consequence the time delay between the pulses in the two lines could be varied) . The two beams were then realigned and made parallel by passing the transmitted beam through a further mirror 3. These beams were then caused to be incident upon a convex lens 4 in such a way that the spots were in a symmetrical position with respect to the centre of the lens . In this way the two beams were focused at the same time . They were spatially overlapped at a distance from the lens equal to its focal length and formed an angle of propagation between the two beams associated with the ratio between their distance before the lens and the distance from the focal length of the lens . Non-linear photodiode 5 comprising the PFO was positioned at this point . Because the photodiode responded to the intensity of illumination providing a current proportional to the cube of the incident intensity, the photogenerated current incorporated a term which depended on the temporal degree overlap between the pulses . By varying the delay between the pulses it was possible to obtain a graph of the photocurrent as a function of the time delay (illustrated in Figure 4 ) with which the two beams were incident upon the photodiode . The technique of "synchronous" amplification of the signal was used to increase the sensitivity of measurement . In fact a mechanical modulator 6 capable of modulating the intensity of the beam at a fixed frequency ( 470 Hz in our case) , thus allowing one pulse out of every two to pass , was in fact positioned on the delay line . The non-linear component of the PFO photodiode current also showed modulation at the same frequency (given that it depends on the simultaneous presence of pulses on the material) . This signal was then sent to a "lock-in" electronic amplifier 7 to which the modulation frequency of the mechanical modulator was communicated. Amplifier 7 then amplified the signal only at the reference frequency, filtering out the noise with a very narrow band filter so as to increase the signal- to-noise ratio enormously.

A bell-shaped curve which represented the convolution between the pulse and the opto-electronic response of the material was obtained. Thus with reference to Figure 4 , which shows the photocurrent/time delay graph obtained, from the Gaussian shape of the graph it was therefore possible to interpolate the curve, obtaining a measurement of the pulse amplitude as the "half- height width" . In order to obtain the pulse duration this curve was then deconvolved using the response of the material . In the case of Gaussian pulses , knowing that the current response was proportional to a third order self-correlation (three photon absorption) it was sufficient to divide the "half-height width" by a factor of 1.22.

As the pulse amplitude was 227 fs , the pulse duration was therefore calculated to be 186 fs .

The photodiode according to the invention is therefore capable of detecting the duration of ultra-short pulses even in the visible range . For use in wavelengths from the visible to the ultraviolet it therefore proved to be easy to construct, economical and had dimensions such as to render its use convenient in research laboratories .

Claims

CIiAIMS

1. Photodiode comprising at least two electrodes and a layer of poly ( 9 , 9-dioctyl ) fluorene .

2. Photodiode according to claim 1, in which one electrode comprises a layer of indium tin oxide ( ITO) .

3. Photodiode according to claim 1 or 2 in which one electrode comprises layers of calcium and aluminum.

4. Photodiode according to any one of claims 1 to 3, further comprising a layer of polyethylene dioxythiophen ( PEDOT) .

5. Photodiode according to any one of claims 1 to 4 , in which the layer of PFO is sandwiched between the two electrodes .

6. Photodiode according to claim 5, in which the anode is a glass substrate adj acent to a layer of indium tin oxide ( ITO) , the thickness of the oxide layer being in the range from 100 to 400 nm.

7. Photodiode according to claim 5, in which the layer of indium tin oxide ( ITO) has a thickness of about 200 nm.

8. Photodiode according to any one of claims 5 to 7 , in which the metal cathode comprises layers of calcium of a thickness of approximately 20 nm and aluminum of approximately 150 nm.

9. Photodiode according to any one of claims 5 to 8 , in which the layer of PFO has a thickness of approximately

100 nm.

10. Photodiode according to any one of claims 5 to 9 , in which the layer of polyethylene dioxythiophen (PEDOT) , if present, overlaps an electrode .

11. Photodiode according to claim 10 , in which the layer of polyethylene dioxythiophen (PEDOT) is between the layer of indium tin oxide ( ITO) and the layer of PFO .

12. Photodiode according to claim 11, in which the layer of polyethylene dioxythiophen ( PEDOT) has a thickness of about 20-30 nm.

13. Process for the preparation of a photodiode according to any one of claims 5 to 12 comprising the steps of : a) depositing through spin coating poly ( 9, 9- dioctyl) fluorene from toluene solution on a first electrode constituting the anode, and b) depositing the metals which constitute a second electrode constituting the cathode by evaporation under vacuum.

14. Process according to claim 13 , in which the solution used is a solution of 20 mg/ml of poly ( 9, 9- dioctyl) fluorene in toluene .

15. Process according to claim 13 or claim 14 , in which the anode is a glass substrate adj acent to a layer of indium tin oxide ( ITO) and the metal cathode is obtained by depositing layers of calcium and aluminum.

16. Process according to any one of claims 13 to 15, in which before depositing the layer of poly ( 9, 9- dioctyl) fluorene a layer of polyethylene dioxythiophen ( PEDOT) is deposited on the anode by spin coating .

17. Self-correlator for measuring the duration of ultra-short laser pulses in the spectral region of the ultraviolet and the visible comprising the photodiode according to any one of claims 1 to 12.