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New photothermal deflection technique to discriminate between heating and cooling
Journal of Quantitative Spectroscopy & Radiative Transfer 204 (2018) 1–6
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
New photothermal deflection technique to discriminate between heating and cooling Ross S. Fontenot∗, Veerendra K. Mathur, John H. Barkyoumb Naval Surface Warfare Center, Carderock Division, 9500 MacArthur Blvd, West Bethesda, MD 20817, United States
a r t i c l e
i n f o
Article history: Received 5 January 2017 Revised 29 July 2017 Accepted 27 August 2017 Available online 30 August 2017 Keywords: Laser cooling of solids Optical refrigeration Photothermal beam deflection Photothermal spectroscopy Photothermal applications
a b s t r a c t Photothermal deflection spectroscopy (PDS) is a highly sensitive and precise technique that is used to measure the optical absorption and thermal characteristics of a sample. While most applications of PDS utilize a heating beam, laser cooling of solids, or optical refrigeration as it is sometimes called, use this technique to determine if a laser is cooling or heating a sample. Current PDS methods for laser cooling require multiple laser wavelengths in both the Stokes and anti-Stokes region to ensure that cooling is occurring. This can cause problems if lasers must be changed or no lasers in the desired wavelength are available. Herein, we present a photothermal deflection technique that uses the deflection of the probe laser to determine if microcooling is occurring inside a sample. Published by Elsevier Ltd.
Photothermal deflection spectroscopy (PDS) is one of the most sensitive methods of molecular absorption spectroscopy. It belongs to a class of photothermal spectroscopy that includes photoacoustic and thermal lensing. These methods rely on the absorption of electromagnetic radiation by a medium, which leads to some or all of this energy being converted to thermal energy through nonradiative processes. For PDS, the source of the electromagnetic radiation is supplied by a modulated laser beam (pump beam). This beam produces modulated thermal gradients, which is detected by a passing probe beam (usually a stable HeNe laser) located either inside the transparent media or adjacent to the medium. These thermal gradients produce changes of the index of refraction, which cause the probe beam to be spatially deflected. The amplitude of this deflection is determined using a position sensitive detector, which can be used to determine the optical absorption of the sample [1–4]. Typically, there are two types of PDS experiments: collinear or transverse. Transverse measurements are typically performed on opaque solids. Here, the probe beam typically propagates through a transparent medium, which is in contact with the surface of the solid that is being irradiated by the pump beam at the normal incidence [2]. For collinear PDS, the two beams are aligned such that they are nearly parallel and closely overlap within the sample. This geometry is advantageous because the beams have a larger interaction distance, which produces a larger deflection signal [2].
∗
Corresponding author. E-mail address: [email protected] (R.S. Fontenot).
http://dx.doi.org/10.1016/j.jqsrt.2017.08.019 0022-4073/Published by Elsevier Ltd.
Today, PDS is widely used owing to its high spectral, spatial, and temporal resolution, high sensitivity, as well as it being a noncontact, nondestructive method. As such, this technique is routinely applied to measure the optical, thermal, and thermo-elastic materials properties of materials and thin films [5–7]. PDS was first used by Boccara et al. to measure small optical absorptions in solids [8]. Since then there have been many PDS theoretical models and setups have been proposed to precisely determine the optical characteristics of materials. For example, Mandelis et al. proposed a 1D model to determine the optical absorption coefficient of opaque samples using the amplitude and phase of the photothermal deflection, Yacoubi et al. showed that the PDS measurements of the absorption coefficient and thermal conductivity of stacked heterostructures matched that of spectroscopic ellipsometry [9], and Skumanich used PDS to measure the optical absorption of C60 thin films down to 0.4 eV [10]. The results of these and many other researchers have shown that PDS is capable of measuring smaller optical absorptions more precisely than conventional optical transmission spectroscopy techniques [11]. Later, Saadallah et al. developed a model from carbon black film to determine the thermal diffusivity of paraffin oils using the PDS deflection angle [12]. Salazar et al. developed a complete theoretical model for PDS to determine the thermal diffusivity of solids [13]. Similarly, Fournier et al. developed a theoretical model to investigate optically thin and thick semiconductors [14]. Gharib et al. took it a step further and used PDS to measure the thermal diffusivity and conductivity simultaneously by depositing a layer of graphite on top of the film samples so that the measured signal is sensitive to both the thermal diffusivity and conductivity [15].
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One of the newer and more interesting applications of PDS is depth profiling. Since the thermal diffusion length is dependent upon the modulation frequency, the thermal wave penetration of the pump beam can be controlled by the modulation frequency. By simply changing the modulation of the pump beam, one can scan a thermally thin sample through its depth to measure its thermal properties [11]. Faubel et al. developed a photothermal double beam laser scanning system for scanning an artificial membrane that can be used for measuring thermal properties of a sample’s surface as well as the deeper layers without having to move the sample [16]. Similarly, Gaiduk et al. used PDS as a thermal imager to localize single viruses. In fact, they were able to determine that the optical absorption cross section for single chromophores was 4 A˚ 2 [17]. As one can see, PDS is a very powerful technique that has a wide range of applications especially in the thermal sciences. While most applications have been devoted to studying the heating effects, researchers looking at optical refrigeration or laser cooling of solids have begun using this technique to determine cooling. Laser cooling of solids uses anti-Stokes emission to annihilate phonons from materials, which in turn cools materials. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids [18–23]. In 2002, Epstein and his team was the first group to achieve laser cooling of solids using a ZBLANP glass sample [24]. Later in 2010, cryogenic temperature was first attained using a LiYF4 :Yb3+ crystal [20]. To date, laser cooling has been mostly limited to glasses and crystals doped with rare-earth elements [23]. However, research has expanded to semiconductors owing to their more efficient pump light absorption, potential for lower temperatures, and the ability to directly integrate the material into electronic and photonic devices. Recently, Zhang et al. demonstrated 40 K of cooling of CdS nanoribbons [25], while the authors measured a small 2.3 °C cooling of CdSe/ZnS QDs [26]. The transition from rare-earth crystals to semiconductors has meant going from millimeter sized samples to the nanoscale to reduce the probability of reabsorption of the emitted photons. As such, tedious noncontact temperature methods must be employed to accurately measure the temperature of the QD(s). As such, PDS is typically used to determine the optimum wavelength for cooling as well as determine if microscopic cooling is occurring within a sample. If microscopic cooling occurs, then there is the potential for bulk cooling of the sample. Unfortunately, current PDS methods for laser cooling of solids requires multiple pump laser wavelengths, i.e., wavelengths far in the Stokes region to guarantee heating as well as wavelengths in the anti-Stokes regions beyond the mean effective wavelength for possible cooling. The phase change produced by the cooling and heating beams is recorded on an oscilloscope to show cooling, i.e., the Stokes or heating beam is deflected one way, while the anti-Stokes or cooling beam is deflected in the opposite direction. Herein, we propose a new photothermal deflection method that does not require the use of multiple wavelengths to determine cooling. Instead, only the deflection of the probe beam with respect to the pump beam is required to determine if heating or cooling is occurring inside a sample. 1. Methods CdSe/ZnS QDs (QSP-630) were purchased from Ocean NanoTech. These QDs have an emission spectrum centered at 630 nm and an external quantum efficiency of 80% according to the manufacturer. A 3 mL solution containing 5 mg/mL of CdSe/ZnS to toluene (Fisher T324 ACS grade) was mixed inside a UV fused quartz cuvette with an airtight stopper (Thorlabs CV10Q3500FS). Coherent
Fig. 1. Diagram showing the photothermal deflection setup.
OBIS LX lasers were used for the anti-Stokes wavelengths, while a Thorlabs L5209120 laser diode was used for the Stokes wavelength. Photothermal deflection was employed to measure the local temperature gradients induced by the laser inside the colloid. Fig. 1 shows a diagram of the photothermal deflection setup. A mode stabilized Spectra Physics Model 117A HeNe laser probe beam was aligned such that the beam passes through the CdSe/ZnS colloid and is in the center of a position sensitive detector. The OBIS (637, 640, 647, and 660 nm) or Thorlabs (520 nm) pump beam was coaligned 100 μm to the right of the probe beam as it passes through the sample in a counter propagating position to minimize crosstalk with the probe beam in the detector. Each laser was set to their max power: 140, 100, 120, 100, 40, or 120 mW for the OBIS 637, 640, 647, 660, 685 nm or Thorlabs 520 nm lasers, respectively. The beam was aligned by moving a right angle prism with a Newport 850A linear actuator. Lenses with long focal lengths are used to focus the beams at the edge of the quartz cuvette containing the CdSe/ZnS colloid. Angular deflections of the HeNe probe beam, which are caused by thermally-induced refractive index gradients in the colloid, are measured using a position sensitive detector and recorded using an oscilloscope or Stanford Research SR530 dual phase lock-in amplifier. An optical chopper was set to 18.3 Hz to modulate the pump beam. 2. Results and discussion 2.1. Photothermal deflection results Photothermal results for the anti-Stokes wavelengths, i.e., 637, 640, and 647 nm, are shown in Fig. 2(a). Here, the black square wave signal shows when the laser is “on” and “off”. It should be noted that the 660 nm laser produced a very weak signal, while the 685 nm laser produced no detectable signal. As such, neither of these results is shown. Fig. 2(b) shows the photothermal deflection spectroscopy results for the 520 nm Stokes wavelength. If we compare the results in Fig. 2, one can see an unmistakable 180° phase difference between the anti-Stokes and Stokes waveforms. This indicates that the positive temperature gradient (heating) produced
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(a)
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(b)
Fig. 2. Photothermal deflection spectroscopy results for the (a) 637 (red), 640 (cyan), and 647 (blue) nm OBIS lasers and (b) 520 nm Thorlabs laser with the on time shown in black. All lasers were set to their max power for this measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Photothermal deflection results for the 637 nm pump laser showing the probe beam deflections when the probe beam is to the left and right of the pump beam.
by the Stokes laser becomes a negative temperature gradient (cooling) for the anti-Stokes wavelengths. While performing the PDS experiment, we found that phase reversal is not in and of itself a unique signature of cooling. We could obtain a phase reversal by interchanging the position of the pump and probe beam as shown in Fig. 3. Thus, a more reliable method is required to indicate cooling or heating. 2.2. New photothermal deflection method To prove if cooling or heating is occurring at the microscopic level, the position sensitive detector must be mapped to determine the position of the probe beam. This was done by first positioning the probe beam in the center of the position detector, which was determined by measuring zero voltage in both the horizontal and vertical channel. Then, a linear actuator was used to scan the HeNe probe beam in 10 μm increments. A lock in amplifier was used to
Fig. 4. Photothermal deflection spectroscopy phase results for the position sensitive detector. The x-axis is reversed to match the face of the detector.
determine the phase and amplitude at each of these points. The laser was pulsed using an optical chopper set to 18.3 Hz. It should be noted that the linear actuator was accurate to about 1 μm. Since the distance from the sample to the detector was 30 cm, the minimum observable deflection would be approximately 300 μrad or 0.017°. Using the basic laws of refraction, this would indicate a temperature sensitivity of approximately 25 μK. Fig. 4 shows the measured phase of the position sensitive detector. Notice that when the probe beam is to the right of center, the phase of the detector is about 100°. However, if it is to the left of the detector, then the phase is about −65°. As one expects, there is about a 180° phase change as the probe beam crosses the center of the detector. After the detector is mapped, the typical PDS procedures remain the same. Fig. 5 shows the phase of the PDS signal for the (a) 520 nm Stokes and (b) 637–660 nm anti-Stokes wavelengths. When the Stokes pump beam is moved 0.1 mm to the left of the probe beam (Fig. 5a), the probe beam phase is approximately 50°. Likewise,
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Fig. 5. Photothermal deflection spectroscopy phase results for the (a) Stokes and (b) anti-Stokes wavelengths. The solid black circles represent the phase of the position sensitive detector as one moves away from center. The open colored circles represent the deflection phase of the probe beam caused by the modulated thermal gradient being produced by the (a) Stokes or (b) anti-Stokes photons. (a) Shows that when the Stokes pump beam is 0.2 mm to the right of center, the probe beam phase is negative. Comparing this result to the position detector phase without the pump beam present, one can see this means the probe beam is deflection to the left or away from the probe beam. (b) Shows the complete opposite results. When the anti-Stokes pump beam is 0.2 mm to the right of center, the phase is positive. Comparing this result to the position detector phase without the pump beam being present, one can see that the deflection of the probe beam is towards the right, i.e., toward the pump beam. The x-axis was reversed in this figure to match the face of the detector.
(a)
(b)
Fig. 6. (a) Diagram showing the expanding heat/cold front produced by the pump beam. Here, ρ , n, and T represent the density, index of refraction, and temperature, respectively for (1) outside and (2) inside the hot/cold front. (b) Diagram showing the possible PDS scenarios for an electromagnetic beam interacting with a medium. First, the probe beam is aligned so that it passes through the material and aligns in the center of the position sensitive detector. When the pump laser is turned on, it produces a temperature gradient owing to the absorption of the electromagnetic radiation. This absorption can either lead to a cooling or heating of the medium, which will cause the probe laser to deflect toward or away from the normal, respectively.
when the Stokes pump beam is moved 0.1 mm to the right of the probe beam, the probe phase is approximately −140°. If we compare these results to the mapped detector phase, one can see that the probe beam deflects to the right when the pump beam is on the left as indicated by the positive phase. Likewise, the probe beam deflects toward the left when the pump beam is to the right as indicated by the negative phase. In other words, the probe beam is being deflected away from the pump beam. Fig. 5(b) shows that the phase of the probe beam deflection is reversed with the anti-Stokes wavelength pump beam. When the anti-Stokes pump beam is moved 0.1 mm to the left of the probe
beam, the probe phase is negative. If we compare these results to the detector phase, one can see that the probe beam is deflecting to the left. Similarly, the probe beam deflects toward the right when the pump beam is to the right of center as indicated by the positive phase. 2.3. New model for determining cooling or heating The physical phenomenon underlying these deflections can be explained using simple model in Fig. 6(a), which provides an exploded view of the interaction between the probe beam and ra-
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dially expanding heat/cold front. Assume S is a point source of heat/cold created by the absorption of pump beam by the colloid. The resulting T wave moves radially outward and interacts with the probe beam. The temperature inside this front, i.e., inside the circle of Fig. 6(a), is at a different temperature than outside the front. This temperature difference results in a change in the index of refraction. Now, if one draws a line that passes through the point source to the point of intersection with the front, this line will be normal to the surface of expanding heat/cold front. For simplicity, we assume that the front expands spherically in all directions. Due to the change in refractive index, the probe beam will deviate as it crosses this front. If it goes from a lower refractive index to a higher one, the probe beam will deflect toward the normal, i.e., toward the pump beam (source of heating or cooling), according to the laws of refraction. Similarly, if the probe beam goes from a higher index of refraction to a lower one, it will deflect away from the normal as shown in Fig. 6(b). This is governed by the laws of refraction, i.e.,
n1 sin θi = n2 sin θr
(1)
where θ i , θ r , n1 , and n2 are the angle of incidence, angle of refraction, index of refraction for 1 (outside), and 2 (inside) the heat/cold front, respectively. From Eq. (1), three cases are possible. Case 1: n2 > n1 T2 < T1 θ r < θ i Probe beam deflects toward the pump beam, i.e., the pump beam is cooling. Case 2: n2 < n1 T2 > T1 θ r > θ i Probe beam deflects away from the pump beam, i.e., the Pump beam is heating. Case 3: n2 = n1 T2 = T1 θ r = θ i No deflection occurs. Thus, by monitoring the deviation of the probe beam with reference to the pump beam, one can determine whether the pump beam is cooling or heating. Take the results shown in Fig. 5 as an example. For this experiment, the probe beam is first aligned so that it passes through the medium and is aligned such that it is in the center of the position sensitive detector as shown in Fig. 6(b). Once the pump beam is turned on, the medium will absorb some or all of the photons, which will create a temperature gradient that expands radially. For the Stokes case, i.e., heating beam, when the 520 nm pump beam interacts with the QD colloid, it produces a heat gradient inside the material that causes the material to become less dense. This lowers the index of refraction and causes the probe beam to move away from the normal, which in this case would be away from the pump beam. Similarly, when the anti-Stokes pump beam interacts with the QD colloid, it produces a cooling gradient (as long as the QDs can absorb wavelength of this light). This cooling gradient temporarily produces a denser medium, which will make the probe beam deflect toward the normal or pump beam in our case. Therefore, only the direction of the probe beam deflection with respect to the pump beam is needed to determine heating or cooling. Note that in this simple analysis, we are not trying to quantify the cooling or heating with an analytic solution to the heat diffusion equation, only trying to determine the direction of deflection from the sign of T. 3. Conclusions In conclusion, the PDS results show that CdSe/ZnS can be cooled at the microscopic level using laser wavelengths between 637 and 660 nm. In addition, a new photothermal deflection technique has been demonstrated that does not require the use of multiple pump lasers, i.e., a Stokes and anti-Stokes laser, to determine if heating or cooling is occurring inside a medium. Instead all that is required is the knowledge of how the probe beam interacts with the temperature gradient produced by the pump laser. In other
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words, if the probe beam is deflected toward the pump beam, cooling is occurring; however, if it is deflected away from the pump beam, then the laser is heating the sample. This new technique eliminates the need for multiple pump laser systems, i.e., only one anti-Stokes laser is required to show cooling. This not only simplifies the experiment, but also reduces the complexity of the experiment by eliminated the need to align multiple pump lasers at the same location. Moreover, since the new method uses a lock-in amplifier, it is capable of much higher sensitives compared to simply using an oscilloscope. Acknowledgments This work was supported by the NSWC Postdoctoral Research Fellowship program administered by the American Society for Engineering Education (ASEE) and funded by Carderock Division under the In-House Laboratory Independent Research (ILIR) program, Program Element 0601152N, managed by the NSWC Carderock Division Director of Research for the Office of Naval Research. References [1] Jackson WB, Amer NM, Boccara AC, Fournier D. Photothermal deflection spectroscopy and detection. Appl Opt 1981;20:1333–44. doi:10.1364/AO.20.001333. [2] Spear JD, Russo RE, Silva RJ. Collinear photothermal deflection spectroscopy with light-scattering samples. Appl Opt 1990;29:4225–34. doi:10.1364/AO.29. 004225. [3] Proskurnin MA, Volkov DS, Gor’kova TA, Bendrysheva SN, Smirnova AP, Nedosekin DA. Advances in thermal lens spectrometry. J Anal Chem 2015;70:249– 76. doi:10.1134/S1061934815030168. [4] Bialkowski SE. Photothermal spectroscopy methods for chemical analysis. New York, NY, USA: Wiley; 1996. [5] Castillo J, Goncalves S, Fernández A, Mujica V. Applications of photothermal displacement spectroscopy to the study of asphaltenes adsorption. Opt Commun 1998;145:69–75. doi:10.1016/S0030-4018(97)00425-2. [6] Olmstead MA, Amer NM, Kohn S, Fournier D, Boccara AC. Photothermal displacement spectroscopy: an optical probe for solids and surfaces,. Appl Phys A 1983;32:141–54. doi:10.10 07/BF0 0616610. [7] Welsch E, Ristau D. Photothermal measurements on optical thin films. Appl Opt 1995;34:7239–53. doi:10.1364/AO.34.007239. [8] Boccara AC, Fournier D, Badoz J. Thermo-optical spectroscopy: detection by the “mirage effect”. Appl Phys Lett 1980;36:130. doi:10.1063/1.91395. [9] Commandré M, Roche P. Characterization of optical coatings by photothermaldeflection. Appl Opt 1996;35:5021–34. doi:10.1364/AO.35.005021. [10] Skumanich A. Optical absorption spectra of carbon 60 thin films from 0.4 to 6.2 eV. Chem Phys Lett 1991;182:486–90. doi:10.1016/0 0 09- 2614(91)90112- M. [11] Ahmed MS. Photothermal deflection spectroscopy of Amorphous, nanostructured and nanocomposite thin films. University of Western Ontario; 2013. [12] Saadallah F, Attia L, Abroug S, Yacoubi N. Photothermal investigations of thermal and optical properties of liquids by mirage effect. Sens Actuators A Phys 2007;138:335–40. doi:10.1016/j.sna.2007.05.022. ´ ´ [13] Salazar A, Sanchez-Lavega A, Fernandez J. Theory of thermal diffusivity determination by the “‘mirage’” technique in solids. J Appl Phys 1989;65:4150. doi:10.1063/1.343320. [14] Fournier D, Boccara C, Skumanich A, Amer NM. Photothermal investigation of transport in semiconductors: theory and experiment. J Appl Phys 1986;59:787. doi:10.1063/1.336599. [15] Ghrib T, Yacoubi N, Saadallah F. Simultaneous determination of thermal conductivity and diffusivity of solid samples using the “mirage effect” method. Sens Actuators A Phys 2007;135:346–54. doi:10.1016/j.sna.2006.07.024. [16] Faubel W, Gotter B, Heißler S, Schlegel M, Neubert RHH. Development of a photothermal double beam laser scanning system in biopharmaceutical applications. J Phys Conf Ser 2010;214:12062. doi:10.1088/1742-6596/214/1/012062. [17] Gaiduk A, Yorulmaz M, Ruijgrok PV, Orrit M. Room-temperature detection of a single molecule’s absorption by photothermal contrast. Science (80.) 2010;330. 353 LP-356 http://science.sciencemag.org/content/330/6002/353. abstract. [18] Mungan CE. Thermodynamics of radiation-balanced lasing. J Opt Soc Am B 2003;20:1075–82. doi:10.1364/JOSAB.20.001075. [19] Sheik-Bahae M, Epstein RI. Optical refrigeration. Nat Photonics 2007;1:693–9. doi:10.1038/nphoton.2007.244. [20] Seletskiy DV, Melgaard SD, Bigotta S, Di Lieto A, Tonelli M, Sheik-Bahae M. Laser cooling of solids to cryogenic temperatures. Nat Photonics 2010;4:161–4. doi:10.1038/nphoton.2009.269. [21] Edwards BC, Anderson JE, Epstein RI, Mills GL, Mord AJ. Demonstration of a solid-state optical cooler: an approach to cryogenic refrigeration. J Appl Phys 1999;86:6489. doi:10.1063/1.371713. [22] Edwards BC, Buchwald MI, Epstein RI. Development of the Los Alamos solidstate optical refrigerator. Rev Sci Instrum 1998;69:2050. doi:10.1063/1.1148897.
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[23] Epstein R, Sheik-Bahae M. Optical refrigeration in solids: fundamentals and overview. In: Optical Refrigeration. Wiley-VCH Verlag GmbH & Co. KGaA; 2009. p. 1–32. doi:10.1002/9783527628049.ch1. [24] Epstein RI, Buchwald MI, Edwards BC, Gosnell TR, Mungan CE. Observation of laser-induced fluorescent cooling of a solid. Nature 1995;377:500–3. http://dx. doi.org/10.1038/377500a0.
[25] Zhang J, Li D, Chen R, Xiong Q. Laser cooling of a semiconductor by 40 K. Nature. 2013;493:504–8. doi:10.1038/nature11721. [26] Fontenot RS, Mathur VK, Barkyoumb JH, Mungan CE, Tran TN. Measuring the anti-Stokes luminescence of CdSe/ZnS quantum dots for laser cooling applications. In: Proceedings of SPIE tri-technology device refrigeration; 2016. p. 982103–7. http://dx.doi.org/10.1117/12.2222826.
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
New photothermal deflection technique to discriminate between heating and cooling Ross S. Fontenot∗, Veerendra K. Mathur, John H. Barkyoumb Naval Surface Warfare Center, Carderock Division, 9500 MacArthur Blvd, West Bethesda, MD 20817, United States
a r t i c l e
i n f o
Article history: Received 5 January 2017 Revised 29 July 2017 Accepted 27 August 2017 Available online 30 August 2017 Keywords: Laser cooling of solids Optical refrigeration Photothermal beam deflection Photothermal spectroscopy Photothermal applications
a b s t r a c t Photothermal deflection spectroscopy (PDS) is a highly sensitive and precise technique that is used to measure the optical absorption and thermal characteristics of a sample. While most applications of PDS utilize a heating beam, laser cooling of solids, or optical refrigeration as it is sometimes called, use this technique to determine if a laser is cooling or heating a sample. Current PDS methods for laser cooling require multiple laser wavelengths in both the Stokes and anti-Stokes region to ensure that cooling is occurring. This can cause problems if lasers must be changed or no lasers in the desired wavelength are available. Herein, we present a photothermal deflection technique that uses the deflection of the probe laser to determine if microcooling is occurring inside a sample. Published by Elsevier Ltd.
Photothermal deflection spectroscopy (PDS) is one of the most sensitive methods of molecular absorption spectroscopy. It belongs to a class of photothermal spectroscopy that includes photoacoustic and thermal lensing. These methods rely on the absorption of electromagnetic radiation by a medium, which leads to some or all of this energy being converted to thermal energy through nonradiative processes. For PDS, the source of the electromagnetic radiation is supplied by a modulated laser beam (pump beam). This beam produces modulated thermal gradients, which is detected by a passing probe beam (usually a stable HeNe laser) located either inside the transparent media or adjacent to the medium. These thermal gradients produce changes of the index of refraction, which cause the probe beam to be spatially deflected. The amplitude of this deflection is determined using a position sensitive detector, which can be used to determine the optical absorption of the sample [1–4]. Typically, there are two types of PDS experiments: collinear or transverse. Transverse measurements are typically performed on opaque solids. Here, the probe beam typically propagates through a transparent medium, which is in contact with the surface of the solid that is being irradiated by the pump beam at the normal incidence [2]. For collinear PDS, the two beams are aligned such that they are nearly parallel and closely overlap within the sample. This geometry is advantageous because the beams have a larger interaction distance, which produces a larger deflection signal [2].
∗
Corresponding author. E-mail address: [email protected] (R.S. Fontenot).
http://dx.doi.org/10.1016/j.jqsrt.2017.08.019 0022-4073/Published by Elsevier Ltd.
Today, PDS is widely used owing to its high spectral, spatial, and temporal resolution, high sensitivity, as well as it being a noncontact, nondestructive method. As such, this technique is routinely applied to measure the optical, thermal, and thermo-elastic materials properties of materials and thin films [5–7]. PDS was first used by Boccara et al. to measure small optical absorptions in solids [8]. Since then there have been many PDS theoretical models and setups have been proposed to precisely determine the optical characteristics of materials. For example, Mandelis et al. proposed a 1D model to determine the optical absorption coefficient of opaque samples using the amplitude and phase of the photothermal deflection, Yacoubi et al. showed that the PDS measurements of the absorption coefficient and thermal conductivity of stacked heterostructures matched that of spectroscopic ellipsometry [9], and Skumanich used PDS to measure the optical absorption of C60 thin films down to 0.4 eV [10]. The results of these and many other researchers have shown that PDS is capable of measuring smaller optical absorptions more precisely than conventional optical transmission spectroscopy techniques [11]. Later, Saadallah et al. developed a model from carbon black film to determine the thermal diffusivity of paraffin oils using the PDS deflection angle [12]. Salazar et al. developed a complete theoretical model for PDS to determine the thermal diffusivity of solids [13]. Similarly, Fournier et al. developed a theoretical model to investigate optically thin and thick semiconductors [14]. Gharib et al. took it a step further and used PDS to measure the thermal diffusivity and conductivity simultaneously by depositing a layer of graphite on top of the film samples so that the measured signal is sensitive to both the thermal diffusivity and conductivity [15].
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One of the newer and more interesting applications of PDS is depth profiling. Since the thermal diffusion length is dependent upon the modulation frequency, the thermal wave penetration of the pump beam can be controlled by the modulation frequency. By simply changing the modulation of the pump beam, one can scan a thermally thin sample through its depth to measure its thermal properties [11]. Faubel et al. developed a photothermal double beam laser scanning system for scanning an artificial membrane that can be used for measuring thermal properties of a sample’s surface as well as the deeper layers without having to move the sample [16]. Similarly, Gaiduk et al. used PDS as a thermal imager to localize single viruses. In fact, they were able to determine that the optical absorption cross section for single chromophores was 4 A˚ 2 [17]. As one can see, PDS is a very powerful technique that has a wide range of applications especially in the thermal sciences. While most applications have been devoted to studying the heating effects, researchers looking at optical refrigeration or laser cooling of solids have begun using this technique to determine cooling. Laser cooling of solids uses anti-Stokes emission to annihilate phonons from materials, which in turn cools materials. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids [18–23]. In 2002, Epstein and his team was the first group to achieve laser cooling of solids using a ZBLANP glass sample [24]. Later in 2010, cryogenic temperature was first attained using a LiYF4 :Yb3+ crystal [20]. To date, laser cooling has been mostly limited to glasses and crystals doped with rare-earth elements [23]. However, research has expanded to semiconductors owing to their more efficient pump light absorption, potential for lower temperatures, and the ability to directly integrate the material into electronic and photonic devices. Recently, Zhang et al. demonstrated 40 K of cooling of CdS nanoribbons [25], while the authors measured a small 2.3 °C cooling of CdSe/ZnS QDs [26]. The transition from rare-earth crystals to semiconductors has meant going from millimeter sized samples to the nanoscale to reduce the probability of reabsorption of the emitted photons. As such, tedious noncontact temperature methods must be employed to accurately measure the temperature of the QD(s). As such, PDS is typically used to determine the optimum wavelength for cooling as well as determine if microscopic cooling is occurring within a sample. If microscopic cooling occurs, then there is the potential for bulk cooling of the sample. Unfortunately, current PDS methods for laser cooling of solids requires multiple pump laser wavelengths, i.e., wavelengths far in the Stokes region to guarantee heating as well as wavelengths in the anti-Stokes regions beyond the mean effective wavelength for possible cooling. The phase change produced by the cooling and heating beams is recorded on an oscilloscope to show cooling, i.e., the Stokes or heating beam is deflected one way, while the anti-Stokes or cooling beam is deflected in the opposite direction. Herein, we propose a new photothermal deflection method that does not require the use of multiple wavelengths to determine cooling. Instead, only the deflection of the probe beam with respect to the pump beam is required to determine if heating or cooling is occurring inside a sample. 1. Methods CdSe/ZnS QDs (QSP-630) were purchased from Ocean NanoTech. These QDs have an emission spectrum centered at 630 nm and an external quantum efficiency of 80% according to the manufacturer. A 3 mL solution containing 5 mg/mL of CdSe/ZnS to toluene (Fisher T324 ACS grade) was mixed inside a UV fused quartz cuvette with an airtight stopper (Thorlabs CV10Q3500FS). Coherent
Fig. 1. Diagram showing the photothermal deflection setup.
OBIS LX lasers were used for the anti-Stokes wavelengths, while a Thorlabs L5209120 laser diode was used for the Stokes wavelength. Photothermal deflection was employed to measure the local temperature gradients induced by the laser inside the colloid. Fig. 1 shows a diagram of the photothermal deflection setup. A mode stabilized Spectra Physics Model 117A HeNe laser probe beam was aligned such that the beam passes through the CdSe/ZnS colloid and is in the center of a position sensitive detector. The OBIS (637, 640, 647, and 660 nm) or Thorlabs (520 nm) pump beam was coaligned 100 μm to the right of the probe beam as it passes through the sample in a counter propagating position to minimize crosstalk with the probe beam in the detector. Each laser was set to their max power: 140, 100, 120, 100, 40, or 120 mW for the OBIS 637, 640, 647, 660, 685 nm or Thorlabs 520 nm lasers, respectively. The beam was aligned by moving a right angle prism with a Newport 850A linear actuator. Lenses with long focal lengths are used to focus the beams at the edge of the quartz cuvette containing the CdSe/ZnS colloid. Angular deflections of the HeNe probe beam, which are caused by thermally-induced refractive index gradients in the colloid, are measured using a position sensitive detector and recorded using an oscilloscope or Stanford Research SR530 dual phase lock-in amplifier. An optical chopper was set to 18.3 Hz to modulate the pump beam. 2. Results and discussion 2.1. Photothermal deflection results Photothermal results for the anti-Stokes wavelengths, i.e., 637, 640, and 647 nm, are shown in Fig. 2(a). Here, the black square wave signal shows when the laser is “on” and “off”. It should be noted that the 660 nm laser produced a very weak signal, while the 685 nm laser produced no detectable signal. As such, neither of these results is shown. Fig. 2(b) shows the photothermal deflection spectroscopy results for the 520 nm Stokes wavelength. If we compare the results in Fig. 2, one can see an unmistakable 180° phase difference between the anti-Stokes and Stokes waveforms. This indicates that the positive temperature gradient (heating) produced
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(a)
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(b)
Fig. 2. Photothermal deflection spectroscopy results for the (a) 637 (red), 640 (cyan), and 647 (blue) nm OBIS lasers and (b) 520 nm Thorlabs laser with the on time shown in black. All lasers were set to their max power for this measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Photothermal deflection results for the 637 nm pump laser showing the probe beam deflections when the probe beam is to the left and right of the pump beam.
by the Stokes laser becomes a negative temperature gradient (cooling) for the anti-Stokes wavelengths. While performing the PDS experiment, we found that phase reversal is not in and of itself a unique signature of cooling. We could obtain a phase reversal by interchanging the position of the pump and probe beam as shown in Fig. 3. Thus, a more reliable method is required to indicate cooling or heating. 2.2. New photothermal deflection method To prove if cooling or heating is occurring at the microscopic level, the position sensitive detector must be mapped to determine the position of the probe beam. This was done by first positioning the probe beam in the center of the position detector, which was determined by measuring zero voltage in both the horizontal and vertical channel. Then, a linear actuator was used to scan the HeNe probe beam in 10 μm increments. A lock in amplifier was used to
Fig. 4. Photothermal deflection spectroscopy phase results for the position sensitive detector. The x-axis is reversed to match the face of the detector.
determine the phase and amplitude at each of these points. The laser was pulsed using an optical chopper set to 18.3 Hz. It should be noted that the linear actuator was accurate to about 1 μm. Since the distance from the sample to the detector was 30 cm, the minimum observable deflection would be approximately 300 μrad or 0.017°. Using the basic laws of refraction, this would indicate a temperature sensitivity of approximately 25 μK. Fig. 4 shows the measured phase of the position sensitive detector. Notice that when the probe beam is to the right of center, the phase of the detector is about 100°. However, if it is to the left of the detector, then the phase is about −65°. As one expects, there is about a 180° phase change as the probe beam crosses the center of the detector. After the detector is mapped, the typical PDS procedures remain the same. Fig. 5 shows the phase of the PDS signal for the (a) 520 nm Stokes and (b) 637–660 nm anti-Stokes wavelengths. When the Stokes pump beam is moved 0.1 mm to the left of the probe beam (Fig. 5a), the probe beam phase is approximately 50°. Likewise,
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Fig. 5. Photothermal deflection spectroscopy phase results for the (a) Stokes and (b) anti-Stokes wavelengths. The solid black circles represent the phase of the position sensitive detector as one moves away from center. The open colored circles represent the deflection phase of the probe beam caused by the modulated thermal gradient being produced by the (a) Stokes or (b) anti-Stokes photons. (a) Shows that when the Stokes pump beam is 0.2 mm to the right of center, the probe beam phase is negative. Comparing this result to the position detector phase without the pump beam present, one can see this means the probe beam is deflection to the left or away from the probe beam. (b) Shows the complete opposite results. When the anti-Stokes pump beam is 0.2 mm to the right of center, the phase is positive. Comparing this result to the position detector phase without the pump beam being present, one can see that the deflection of the probe beam is towards the right, i.e., toward the pump beam. The x-axis was reversed in this figure to match the face of the detector.
(a)
(b)
Fig. 6. (a) Diagram showing the expanding heat/cold front produced by the pump beam. Here, ρ , n, and T represent the density, index of refraction, and temperature, respectively for (1) outside and (2) inside the hot/cold front. (b) Diagram showing the possible PDS scenarios for an electromagnetic beam interacting with a medium. First, the probe beam is aligned so that it passes through the material and aligns in the center of the position sensitive detector. When the pump laser is turned on, it produces a temperature gradient owing to the absorption of the electromagnetic radiation. This absorption can either lead to a cooling or heating of the medium, which will cause the probe laser to deflect toward or away from the normal, respectively.
when the Stokes pump beam is moved 0.1 mm to the right of the probe beam, the probe phase is approximately −140°. If we compare these results to the mapped detector phase, one can see that the probe beam deflects to the right when the pump beam is on the left as indicated by the positive phase. Likewise, the probe beam deflects toward the left when the pump beam is to the right as indicated by the negative phase. In other words, the probe beam is being deflected away from the pump beam. Fig. 5(b) shows that the phase of the probe beam deflection is reversed with the anti-Stokes wavelength pump beam. When the anti-Stokes pump beam is moved 0.1 mm to the left of the probe
beam, the probe phase is negative. If we compare these results to the detector phase, one can see that the probe beam is deflecting to the left. Similarly, the probe beam deflects toward the right when the pump beam is to the right of center as indicated by the positive phase. 2.3. New model for determining cooling or heating The physical phenomenon underlying these deflections can be explained using simple model in Fig. 6(a), which provides an exploded view of the interaction between the probe beam and ra-
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dially expanding heat/cold front. Assume S is a point source of heat/cold created by the absorption of pump beam by the colloid. The resulting T wave moves radially outward and interacts with the probe beam. The temperature inside this front, i.e., inside the circle of Fig. 6(a), is at a different temperature than outside the front. This temperature difference results in a change in the index of refraction. Now, if one draws a line that passes through the point source to the point of intersection with the front, this line will be normal to the surface of expanding heat/cold front. For simplicity, we assume that the front expands spherically in all directions. Due to the change in refractive index, the probe beam will deviate as it crosses this front. If it goes from a lower refractive index to a higher one, the probe beam will deflect toward the normal, i.e., toward the pump beam (source of heating or cooling), according to the laws of refraction. Similarly, if the probe beam goes from a higher index of refraction to a lower one, it will deflect away from the normal as shown in Fig. 6(b). This is governed by the laws of refraction, i.e.,
n1 sin θi = n2 sin θr
(1)
where θ i , θ r , n1 , and n2 are the angle of incidence, angle of refraction, index of refraction for 1 (outside), and 2 (inside) the heat/cold front, respectively. From Eq. (1), three cases are possible. Case 1: n2 > n1 T2 < T1 θ r < θ i Probe beam deflects toward the pump beam, i.e., the pump beam is cooling. Case 2: n2 < n1 T2 > T1 θ r > θ i Probe beam deflects away from the pump beam, i.e., the Pump beam is heating. Case 3: n2 = n1 T2 = T1 θ r = θ i No deflection occurs. Thus, by monitoring the deviation of the probe beam with reference to the pump beam, one can determine whether the pump beam is cooling or heating. Take the results shown in Fig. 5 as an example. For this experiment, the probe beam is first aligned so that it passes through the medium and is aligned such that it is in the center of the position sensitive detector as shown in Fig. 6(b). Once the pump beam is turned on, the medium will absorb some or all of the photons, which will create a temperature gradient that expands radially. For the Stokes case, i.e., heating beam, when the 520 nm pump beam interacts with the QD colloid, it produces a heat gradient inside the material that causes the material to become less dense. This lowers the index of refraction and causes the probe beam to move away from the normal, which in this case would be away from the pump beam. Similarly, when the anti-Stokes pump beam interacts with the QD colloid, it produces a cooling gradient (as long as the QDs can absorb wavelength of this light). This cooling gradient temporarily produces a denser medium, which will make the probe beam deflect toward the normal or pump beam in our case. Therefore, only the direction of the probe beam deflection with respect to the pump beam is needed to determine heating or cooling. Note that in this simple analysis, we are not trying to quantify the cooling or heating with an analytic solution to the heat diffusion equation, only trying to determine the direction of deflection from the sign of T. 3. Conclusions In conclusion, the PDS results show that CdSe/ZnS can be cooled at the microscopic level using laser wavelengths between 637 and 660 nm. In addition, a new photothermal deflection technique has been demonstrated that does not require the use of multiple pump lasers, i.e., a Stokes and anti-Stokes laser, to determine if heating or cooling is occurring inside a medium. Instead all that is required is the knowledge of how the probe beam interacts with the temperature gradient produced by the pump laser. In other
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words, if the probe beam is deflected toward the pump beam, cooling is occurring; however, if it is deflected away from the pump beam, then the laser is heating the sample. This new technique eliminates the need for multiple pump laser systems, i.e., only one anti-Stokes laser is required to show cooling. This not only simplifies the experiment, but also reduces the complexity of the experiment by eliminated the need to align multiple pump lasers at the same location. Moreover, since the new method uses a lock-in amplifier, it is capable of much higher sensitives compared to simply using an oscilloscope. Acknowledgments This work was supported by the NSWC Postdoctoral Research Fellowship program administered by the American Society for Engineering Education (ASEE) and funded by Carderock Division under the In-House Laboratory Independent Research (ILIR) program, Program Element 0601152N, managed by the NSWC Carderock Division Director of Research for the Office of Naval Research. References [1] Jackson WB, Amer NM, Boccara AC, Fournier D. Photothermal deflection spectroscopy and detection. Appl Opt 1981;20:1333–44. doi:10.1364/AO.20.001333. [2] Spear JD, Russo RE, Silva RJ. Collinear photothermal deflection spectroscopy with light-scattering samples. Appl Opt 1990;29:4225–34. doi:10.1364/AO.29. 004225. [3] Proskurnin MA, Volkov DS, Gor’kova TA, Bendrysheva SN, Smirnova AP, Nedosekin DA. Advances in thermal lens spectrometry. J Anal Chem 2015;70:249– 76. doi:10.1134/S1061934815030168. [4] Bialkowski SE. Photothermal spectroscopy methods for chemical analysis. New York, NY, USA: Wiley; 1996. [5] Castillo J, Goncalves S, Fernández A, Mujica V. Applications of photothermal displacement spectroscopy to the study of asphaltenes adsorption. Opt Commun 1998;145:69–75. doi:10.1016/S0030-4018(97)00425-2. [6] Olmstead MA, Amer NM, Kohn S, Fournier D, Boccara AC. Photothermal displacement spectroscopy: an optical probe for solids and surfaces,. Appl Phys A 1983;32:141–54. doi:10.10 07/BF0 0616610. [7] Welsch E, Ristau D. Photothermal measurements on optical thin films. Appl Opt 1995;34:7239–53. doi:10.1364/AO.34.007239. [8] Boccara AC, Fournier D, Badoz J. Thermo-optical spectroscopy: detection by the “mirage effect”. Appl Phys Lett 1980;36:130. doi:10.1063/1.91395. [9] Commandré M, Roche P. Characterization of optical coatings by photothermaldeflection. Appl Opt 1996;35:5021–34. doi:10.1364/AO.35.005021. [10] Skumanich A. Optical absorption spectra of carbon 60 thin films from 0.4 to 6.2 eV. Chem Phys Lett 1991;182:486–90. doi:10.1016/0 0 09- 2614(91)90112- M. [11] Ahmed MS. Photothermal deflection spectroscopy of Amorphous, nanostructured and nanocomposite thin films. University of Western Ontario; 2013. [12] Saadallah F, Attia L, Abroug S, Yacoubi N. Photothermal investigations of thermal and optical properties of liquids by mirage effect. Sens Actuators A Phys 2007;138:335–40. doi:10.1016/j.sna.2007.05.022. ´ ´ [13] Salazar A, Sanchez-Lavega A, Fernandez J. Theory of thermal diffusivity determination by the “‘mirage’” technique in solids. J Appl Phys 1989;65:4150. doi:10.1063/1.343320. [14] Fournier D, Boccara C, Skumanich A, Amer NM. Photothermal investigation of transport in semiconductors: theory and experiment. J Appl Phys 1986;59:787. doi:10.1063/1.336599. [15] Ghrib T, Yacoubi N, Saadallah F. Simultaneous determination of thermal conductivity and diffusivity of solid samples using the “mirage effect” method. Sens Actuators A Phys 2007;135:346–54. doi:10.1016/j.sna.2006.07.024. [16] Faubel W, Gotter B, Heißler S, Schlegel M, Neubert RHH. Development of a photothermal double beam laser scanning system in biopharmaceutical applications. J Phys Conf Ser 2010;214:12062. doi:10.1088/1742-6596/214/1/012062. [17] Gaiduk A, Yorulmaz M, Ruijgrok PV, Orrit M. Room-temperature detection of a single molecule’s absorption by photothermal contrast. Science (80.) 2010;330. 353 LP-356 http://science.sciencemag.org/content/330/6002/353. abstract. [18] Mungan CE. Thermodynamics of radiation-balanced lasing. J Opt Soc Am B 2003;20:1075–82. doi:10.1364/JOSAB.20.001075. [19] Sheik-Bahae M, Epstein RI. Optical refrigeration. Nat Photonics 2007;1:693–9. doi:10.1038/nphoton.2007.244. [20] Seletskiy DV, Melgaard SD, Bigotta S, Di Lieto A, Tonelli M, Sheik-Bahae M. Laser cooling of solids to cryogenic temperatures. Nat Photonics 2010;4:161–4. doi:10.1038/nphoton.2009.269. [21] Edwards BC, Anderson JE, Epstein RI, Mills GL, Mord AJ. Demonstration of a solid-state optical cooler: an approach to cryogenic refrigeration. J Appl Phys 1999;86:6489. doi:10.1063/1.371713. [22] Edwards BC, Buchwald MI, Epstein RI. Development of the Los Alamos solidstate optical refrigerator. Rev Sci Instrum 1998;69:2050. doi:10.1063/1.1148897.
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