超快光学 第14章 超快光谱课件

1、超快光学 第14章 超快光谱,Ultrafast Laser Spectroscopy,How and why ultrafast laser spectroscopy? Generic ultrafast spectroscopy experiment The excite-probe experiment Lock-in detection Transient-grating spectroscopy Ultrafast polarization spectroscopy Optical heterodyne detection (OHD) Spectrally resolved exci

2、te-probe spectroscopy,超快光学 第14章 超快光谱,Ultrafast laser spectroscopy: Why?,Most events that occur in atoms and molecules occur on fs and ps time scales because the length scales are very small. Fluorescence occurs on a ns time scale, but competing non-radiative processes only speed things up because re

3、laxation rates add:,Biologically important processes utilize excitation energy for purposes other than fluorescence and hence must be very fast. Collisions in room-temperature liquids occur on a few-fs time scale, so nearly all processes in liquids are ultrafast. Semiconductor processes of technolog

4、ical interest are necessarily ultrafast or we wouldnt be interested.,1/tex = 1/tfl + 1/tnr,超快光学 第14章 超快光谱,Ultrafast laser spectroscopy: How?,Ultrafast laser spectroscopy involves studying ultrafast events that take place in a medium using ultrashort pulses and delays for time resolution. It usually

5、involves exciting the medium with one (or more) ultrashort laser pulse(s) and probing it a variable delay later with another.,The signal pulse energy (or change in energy) is plotted vs. delay. The experimental temporal resolution is the pulse length.,超快光学 第14章 超快光谱,Whats going on in spectroscopy me

6、asurements?,The excite pulse(s) excite(s) molecules into excited states, which changes the mediums absorption coefficient and refractive index.,The excited states only live for a finite time (this is the quantity wed like to find!), so the absorption and refractive index recover.,超快光学 第14章 超快光谱,The

7、simplest ultrafast spectroscopy method is the Excite-Probe Technique.,Excite the sample with one pulse; probe it with another a variable delay later; and measure the change in the transmitted probe pulse energy or average power vs. delay.,The excite and probe pulses can be different colors. This tec

8、hnique is also called the Pump-Probe Technique.,The excite pulse changes the sample absorption seen by the probe pulse.,超快光学 第14章 超快光谱,Modeling excite-probe measurements,Let the unexcited medium have an absorption coefficient, a0. Immediately after excitation, the absorption decreases by Da0. Excite

9、d states usually decay exponentially: Da(t) = Da0 exp(t /tex) for t 0 where t is the delay after excitation, and tex is the excited-state lifetime. So the transmitted probe-beam intensityand hence pulse energy and average powerwill depend on the delay, t, and the lifetime, tex: Itransmitted(t) = Iin

10、cident expa0 Da0exp(t /tex)L where L = sample length = Iincident expa0L expDa0exp(t /tex)L Iincident expa0L 1+Da0exp(t /tex)L assuming Da0 L 1 Itransmitted(-) 1+Da0exp(t /tex)L,超快光学 第14章 超快光谱,DT(t) / T0 = Itransmitted(t) - Itransmitted() / Itransmitted(),Modeling excite-probe measurements (contd),Th

11、e relative change in transmitted intensity vs. delay, t, is:,Itransmitted(t) Itransmitted(-) 1+Da0exp(t /tex)L,DT(t) / T0 Da0 exp(t /tex) L,超快光学 第14章 超快光谱,Modeling excite-probe measurements (contd),More complex decays occur if intermediate states are populated or if the motion is complex. Imagine pr

12、obing an intermediate transition, whose states temporarily fill with molecules on their way back down to the ground state:,超快光学 第14章 超快光谱,Lock-in Detection greatly increases the sensitivity in excite-probe experiments.,This involves chopping the excite pulse at a given frequency and detecting at tha

13、t frequency with a lock-in detector:,Chopped excite pulse train,Probe pulse train,Lock-in detection automatically subtracts off the transmitted power in the absence of the excite pulse. With high-rep-rate lasers, it increases sensitivity by several orders of magnitude!,The excite pulse periodically

14、changes the sample absorption seen by the probe pulse.,Chopper,Lock-in detector,The lock-in detects only one frequency component of the detector voltagechosen to be that of the chopper.,超快光学 第14章 超快光谱,Excite-probe studies of bacterio-rhodopsin,Rhodopsin is the main molecule involved in vision. After

15、 absorbing a photon, rhodopsin undergoes a many-step process, whose first three steps occur on fs or ps time scales and are poorly understood.,Excitation populates a new state, which absorbs at 460nm and emits at 860nm. It is thought that this state involves motion of the carbon atoms (12, 13, 14).

16、An artificial version of rhodopsin, with those atoms held in place, reveals this change on a much slower time scale, confirming this theory!,Zhong, et al., Ultrafast Phenomena X, p. 355 (1996).,超快光学 第14章 超快光谱,Excite-probe measurements in DNA,Pecourt, et al., Ultrafast Phenomena XII, p.566 (2000).,DN

17、A bases undergo photo-oxidative damage, which can yield mutations. Understanding the photo-physics of these important molecules may help to understand this process.,超快光学 第14章 超快光谱,Excite-probe measurements of Hypericin, an anti-viral substance,When excited by light, Hypericin deactivates HIV. So it

18、would be nice to understand how it works.,Relative change in absorbance,These curves (for two different solvents) show the rise time for a proton-transfer process important in its biological activity.,M.J. Fehr, et al., Ultrafast Phenomena IX, pg. 462 (1994).,超快光学 第14章 超快光谱,Excite-probe measurements

19、 of Terawatt femtosecond UV pulses in water,Pommeret, et al., Ultrafast Phenomena XII, p. 536 (2000).,High-intensity UV ultrashort pulses may someday be used in surgery. So understanding what these pulses do to water is important. Hydrated electrons are formed in very high concentrations (0.01 molar

20、).,The induced absorption seen here is very high.,超快光学 第14章 超快光谱,Excite-probe reflection spectroscopy,Exciting a surface and probing its reflectivity later reveals surface physics. Here, a quantum wire is studied using ultrashort pulses in a near-field scanning optical microscope to yield 200nm spat

21、ial resolution, too!,Emiliani, et al., Ultrafast Phenomena XII, p. 256 (2000).,DR/R vs. x and y for a delay of 10ps,超快光学 第14章 超快光谱,Excite-probe measurements can reveal quantum beats: Theory,Since ultrashort pulses have broad bandwidths, they can excite two or more nearby states simultaneously.,Probi

22、ng the 1-2 superposition of states can yield quantum beats in the excite-probe data.,超快光学 第14章 超快光谱,Excite-probe measurements can reveal quantum beats: Experiment,Here, two nearby vibrational states in molecular iodine interfere.,These beats also indicate the motion of the molecular wave packet on i

23、ts potential surface. A small fraction of the I2 molecules dissociate every period.,Zadoyan, et al., Ultrafast Phenomena X, p. 194 (1996).,超快光学 第14章 超快光谱,Quantum beats in polymers using 5-fs pulses,Excite-probe measurements in polydiacetylene show several different frequencies, implying several (vib

24、rational) states were excited.,Kobayashi, Ultrafast Phenomena XII, p. 575 (2000).,lpr,超快光学 第14章 超快光谱,The coherence spike in ultrafast spectroscopy,When the delay is zero, other nonlinear-optical processes occur, a involving coherent 4WM between the beams and generatingadditional signal not described

25、 by the simple Da model. As in autocorrelation, its called the coherence spike or coherent artifact. Sometimes you see it; sometimes you dont.,Alternate picture: the pulses induce a grating in the absorption and/or refractive index, which diffracts light from each beam into the other.,超快光学 第14章 超快光谱

26、,Taking advantage of the induced grating: the Transient-Grating Technique.,Two simultaneous excitation pulses induce a weak diffraction grating, followed, a variable delay later, by a probe pulse. Measure the diffracted pulse energy vs. delay:,This method is background-free, but the diffracted pulse

27、 energy goes as the square of the diffracted field and hence is weaker than that in excite-probe measurements.,Delay,Excite pulse #1,Sample,Excite pulse #2,Probe pulse,Diffracted pulse,Intensity fringes in sample due to excitation pulses,超快光学 第14章 超快光谱,A transient-grating measurement may still have

28、a coherence spike!,When all the pulses overlap in time, whos to say which are the excitation pulses and which is the probe pulse?,A transient-grating experiment with a coherence spike:,Delay,Excite pulse #1 (acting as the probe),Excite pulse #2,Probe pulse (acting as an excite pulse),Intensity fring

29、es in sample due to an excitation pulse and the probe acting as an excitation pulse,超快光学 第14章 超快光谱,What the transient-grating technique measures,It measures the Pythagorean sum of the changes in the absorption and refractive index. The diffraction efficiency, , is given by:,This is in contrast to th

30、e excite-probe technique, which is only sensitive to the change in absorption and depends on it linearly.,H. Eichler, Laser-Induced Dynamic Gratings, Springer-Verlag, 1986.,If the absorption grating dominates and the excite-probe decay is exp(- /ex), then the TG decay will be exp(-2 /ex):,超快光学 第14章

31、超快光谱,You might think that a grating can be induced only by a sinusoidal intensity pattern (caused by the interference of two parallel-polarized beams). But orthogonally polarized beams, which have a constant intensity vs. position, also induce a grating! An orientation grating. Variation of the elec

32、tric field vs. position:,Transient orientation gratings,Orientation gratings can also decay due to orientational relaxation.,超快光学 第14章 超快光谱,Induced gratings can also decay by diffusion.,Diffusion can wash out an induced grating. Sometimes diffusion is faster than excited-state decay.,Diffusion occur

33、s on a time scale that depends on the grating fringe spacing. If the fringes are closely spaced, diffusion is very fast; if the fringes are far apart, then its much slower. Varying the grating fringe spacing can determine the time scales for both decay mechanisms.,where D = diffusion coeff,超快光学 第14章

34、 超快光谱,Transient-grating measurements in multiple quantum wells,Both concentration (amplitude) and orientation (spin) gratings induced by excite beams with parallel and perpendicular polarizations. The orientation grating decays much faster.,超快光学 第14章 超快光谱,Time-resolved fluorescence is also useful.,E

35、xciting a sample with an ultrashort pulse and then observing the fluorescence vs. time also yields sample dynamics. This can be done by directly observing the fluorescence or, if its too fast, by time-gating it with a probe pulse in a SFG crystal.,超快光学 第14章 超快光谱,Time-resolved fluorescence decay,When

36、 different tissues look alike (i.e., have similar absorption spectra), looking at the time-resolved fluorescence can help distinguish them.,Here, a malignant tumor can be distinguished from normal tissue due to its longer decay time.,Normal tissue,Malignant tumor,Svanberg, Ultrafast Phenomena IX, p.

37、 34 (1994).,超快光学 第14章 超快光谱,Ultrafast Polarization Spectroscopy,Its also possible to change the absorption coefficient differently for the two polarizations. This is called induced dichroism. It also rotates the probe polarization and can also be used to study orientational relaxation.,A 45-polarized

38、 excite pulse induces birefringence in an ordinarily isotropic sample via the Kerr effect. A variably delayed probe pulse between crossed polarizers watches the birefringence decay, revealing the sample orientational relaxation.,超快光学 第14章 超快光谱,Nice features of ultrafast polarization spectroscopy,Its

39、 even easier to set up than excite-probe (just cross two beams in space and time, and cross the polarizers to more easily see the signal beam). Its almost background-free (crossed polarizers transmit as little as 10-6 of the incident light). Unlike excite-probe, it measures both absorption and phase

40、 effects. It can use lock-in detection. And simultaneously, it can use optical heterodyne detection, which optimizes the signal-to-noise ratio.,超快光学 第14章 超快光谱,Heterodyned Ultrafast Polarization Spectroscopy,This trivial (seemingly inappropriate!) change can actually improve the sensitivity by many o

41、rders of magnitude!,Optical Heterodyne Detection (OHD) polarization spectroscopy involves slightly uncrossing the polarizers. This allows some of the probe pulse to leak into the detector and combine coherently with the signal pulse.,超快光学 第14章 超快光谱,Heterodyned ultrafast polarization spectroscopy,As

42、long as the leaked probe intensity the signal intensity, we can neglect the latter:,Heterodyned polarization spectroscopy adds a small amount of the probe pulse, dEprobe(t), to the (even smaller) signal pulse. As a result, we now detect the squared magnitude of the sum of these two fields:,As long a

43、s the probe intensity is stable, this yields a huge improvement in sensitivityespecially when combined with lock-in detection.,This yields a signal term proportional to Esig(t), which is much larger than its squared magnitude. And it also yields its phase.,usual PS signal,超快光学 第14章 超快光谱,Heterodyned

44、ultrafast polarization spectroscopy of liquids,Castner and Chang, Ultrafast Phenomena X, p. 296 (1996).,Using different colors for the excite and probe pulses, this technique is called the Optical Heterodyne Detection-Raman-Induced Kerr Effect Spectroscopy (OHD-RIKES).,Sample media are various amide

45、s.,Notice how very clean the data are.,The excite pulse can induce a change in the refractive index seen by the probe pulse, which is enhanced when wex wpr = w10,超快光学 第14章 超快光谱,Heterodyned ultrafast polarization spectroscopy of CS2,Loughnane, et al., Ultrafast Phenomena X, p. 304 (1996).,OHD-RIKES s

46、tudy of CS2 at different temperatures.,超快光学 第14章 超快光谱,Temporally and spectrally resolving the fluorescence of an excited molecule,Exciting a molecule and watching its fluorescence reveals much about its potential surfaces. Ideally, one would measure the time-resolved spectrum, equivalent to its intensity and phase vs. time (or frequency).,Here, excitation occurs to a predissociative state, but other situations are just as interesting. Analogous studies can be performed in absorpt