Abstract:
A multidimensional spectrometer provides broadband white light pump and probe pulses to allow acquisition of multidimensional spectral information over a wide range commensurate with solar-related chemical processes.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application 62/026,949 filed Jul. 21, 2014, and hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under FA9550-12-1-0063 awarded by the National Science Foundation. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to multi-dimensional optical spectrometers and in particular to a multi-dimensional optical spectrometer suitable for measurements over large frequency ranges. 
         [0004]    Multi dimensional spectroscopy can reveal the interaction between coupled systems of atoms and/or molecules. For example, two-dimensional spectroscopy may provide an intuitively understandable two-dimensional spectrum in which the emission frequency of the system is plotted against the original excitation frequency. In a 2D spectrum, electromagnetic transitions of atoms or molecules matching the incident light give rise to signals that lie on the spectrum diagonal, but if there are interactions between transitions within or among molecules, then cross peaks will also appear in the spectrum. The diagonal and cross peaks can be used to deduce molecular structures or identify mixtures of compounds, etc. Multidimensional spectra of 3D or higher provide additional information. 
         [0005]    To collect multidimensional spectrum, one needs to generate multiple light pulses to interrogate the sample. Typically a set of probe pulses are used to excite the sample which is then interrogated by a probe pulse. Some examples of two-dimensional spectroscopy are described in US patent application 2006/0063188 filed Sep. 15, 2005; US patent application 2009/0161092 filed Dec. 21, 2007; and US patent application 2012/0236305 filed Sep. 20, 2012, all hereby incorporated by reference. 
         [0006]    Processes related to photosynthesis or solar energy, among others, could benefit from multidimensional spectrographic analysis; however, such analysis is challenging because of the large spectral range that must be analyzed, a range that can extend across the entire visible and near infrared wavelength region. Tuning the frequency of pump pulses over a large spectral bandwidth while preserving the necessary frequency and phase accuracy is difficult. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a multidimensional spectrograph that employs broadband “white light” for both pumping and probing pulses. By using broad spectrum pulses, center frequency tuning of the pulses is not required, eliminating the need for tuning mechanisms such as non-collinear optical parametric amplifiers or the like that be difficult to operate through a large range of frequencies. 
         [0008]    Specifically, the present invention provides a multi-dimensional spectrometer having a white light source providing source light pulses having a substantially continuous bandwidth of at least 500 nanometers. A first optical system directs the source light pulses through a sample volume as a series of probe pulses to be received by a light detector while a second optical system receives the source light pulses to break the light pulses into at least first and second probe pulses having a controllable time separation between the first and second probe pulses. These first and second probe pulses are then directed through the sample volume before the probe pulse. An electronic computer system communicating with the light detector receives electrical data therefrom and controls the second optical system to change the time separation between the first and second probe pulses over multiple light pulses to generate a multidimensional spectrograph. 
         [0009]    It is thus a feature of at least one embodiment of the invention to provide a multidimensional spectrometer that can accommodate measurements that require substantial spectral frequency range, for example, related to, but not necessarily limited to, chemical processes associated with photosynthesis or solar energy. 
         [0010]    The light pulses may have a substantially constant center frequency. 
         [0011]    It is thus a feature of at least one embodiment of the invention to provide a spectrometer that eliminates the need to tune the center frequency of narrowband pump pulses or probe pulses over a wide range, such tuning as may require expensive optical devices or be fundamentally impractical. 
         [0012]    The light pulses may be generated by interaction between a laser and a spectrum-broadening crystal material to provide white light without modulation. 
         [0013]    It is thus a feature of at least one embodiment of the invention to provide a simple method of generating a coherent white light source possible when frequency tuning is not required during the generation of the spectrograph. 
         [0014]    The probe pulses may have a spectrum substantially identical to the source light pulses. 
         [0015]    It is thus a feature of at least one embodiment of the invention to provide a simple generation of a probe pulse suitable for the entire measurement range. 
         [0016]    The white light source may be either a laser passing through a beam splitter to be received by separate static spectral broadening elements or a laser received by a single static spectral broadening element and then passed to a beam splitter to create two source light pulses. 
         [0017]    It is thus a feature of at least one embodiment of the invention to provide a flexible method of generating the necessary probe and pump pulses. 
         [0018]    The second optical path may provide a bifringent crystal separating the light pulse into first and second differently polarized light pulses, the bifringent crystal followed by a polarization-selective wedge delaying one of the first and second differently polarized light pulses and not the other, followed by a polarizer realigning the polarization of the first and second differently polarized light sources after delay of the one of the first and second differently polarized light pulses wherein the position-selective wedge is controlled by the electronic computer. 
         [0019]    It is thus a feature of at least one embodiment of the invention to provide a simple and robust method of controlling the separation between the pump pulses requiring only movement of the mechanical stage holding a wedge element. 
         [0020]    At least one of the first and second optical paths may include a controllable pulse delay element for changing a relative delay of the probe pulse after the pump pulses. 
         [0021]    It is thus a feature of at least one embodiment of the invention to permit adjustment of the data acquisition independently of control of the pump pulse separation. 
         [0022]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. The following description and figures illustrate a preferred embodiment of the invention. Such an embodiment does not necessarily represent the full scope of the invention, however. Furthermore, some embodiments may include only parts of a preferred embodiment. Therefore, reference must be made to the claims for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]      FIG. 1  is a simplified block diagram of the present invention showing the general principles using a white light pulse processed by a first optical system to produce a probe pulse and by a second optical system to produce pump pulses; 
           [0024]      FIG. 2  is a detailed block diagram of a portion of the second optical system generating two pump pulses from a single white light pulse; 
           [0025]      FIG. 3  is a more detailed diagram of the spectrometer of  FIG. 1  using the second optical system of  FIG. 2 ; and 
           [0026]      FIG. 4  is a fragmentary view of a portion of  FIG. 3  showing an alternative method of generating white light pulses for the first and second optical systems. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Referring now to  FIG. 1 , a laser  10  may provide a stream of pulses  12  directed to a beam splitter  14  directing part of the energy of each of the pulses  12  both to a first optical system  16  and second optical system  18  to develop probe and pump pulses respectively. 
         [0028]    Pulses  12  output from the beam splitter  14  are received by a first and second spectrum-broadening crystal  20  and  22  each producing a supercontinuum white light pulse  24 . A wavelength bandwidth of the white light pulses  24 , for example, may range from wavelength between 400-1400 nanometers (and hence having a bandwidth of no less than 1000 nanometers). The invention contemplates a bandwidth of no less than 900 nanometers or no less than 700 nanometers. Generally the bandwidth will exceed 1½ octaves and will include the wavelength of 1000 nanometers. 
         [0029]    In the first optical system  16 , the white light pulse  24  may be received by a pulse splitter  28  which controllably splits the white light pulse  24  into first and second pump pulses  30  and  32  of substantially equal energy and frequency profile but separated in time by a time value t The pump pulses  30  and  32  are directed through a sample volume  34  holding a sample to be analyzed (either by absorption or reflection). Pump pulses  30  and  32  leaving the sample Volume  34  may be absorbed by an absorber  36 . 
         [0030]    In the second optical system  18 , the white light pulse  24  is used as a probe pulse  24 ′ and may pass through the sample volume  34  to be received by a detector  38 , for example, a spectroscope, after stimulation of the material in the sample volume  34  by the pump pulses  30  and  32 . 
         [0031]    A signal from the detector  38  after receipt of the probe pulse  24 ′ is received by an electronic computer  40  which may also control the pulse splitter  28  to change the value oft. Generally electronic computer  40  will execute a stored, program  42  held in solid state memory or other non-transient memory structure to perform repeated “experiments” in which pump pulses  30  and  32  are used to excite material within the sample volume  34 , which material is then analyzed by a probe pulse  24 ′ (substantially identical in spectrum to white light pulse  24 ). Successive experiments may provide for different values oft so as to generate information necessary to produce a two-dimensional spectrogram  44  of a type generally understood in the art. Individual experiments with the same value of t may also be repeated and aggregated for the purpose of reducing measurement noise. 
         [0032]    Referring now to  FIG. 2 , in one embodiment the pulse splitter  28  may provide a translating, wedge-based, identical pulse encoding system (TWINS), for example, as described in D. Brida, C. Manzoni, G. Cerullo, “Phase-locked pulses for two-dimensional spectroscopy by a birefringent delay line”, Optics letters 37, 3027 (Aug. 1, 2012) hereby incorporated by reference. 
         [0033]    In this system, a white light pulse  24  having a first polarization of 45 degrees with respect to a surface such as an optical table  50  (indicated in the figure by an arrow) is generated by a wave plate  46 . The polarized white light pulse  24  is then received by an σ-BBO crystal  52  with an optical axis cut perpendicular to the surface of the table  50 . The crystal  52  splits the white light pulse  24  into vertically and horizontally polarized pulses  54  and  56  with some fixed time delay between them. 
         [0034]    Next a pair of a-BBO wedges  58  and  60  with optical axes cut parallel to the surface of the table  50  and perpendicular to the beam propagation axis are used to adjust the separation between pulses  54  and  56  by selectively delaying one pulse. This adjustment may be used to change value oft in the pump pulses  30  and  32 . Generally this adjustment is provided by physically moving one of the wedges (e.g.  58 ) by attachment of the wedge to the mechanical stage (not shown) controllable by the computer  40  (shown in  FIG. 1 ) to change a thickness of the wedge intersecting the path of the pulses  54  and  56 . 
         [0035]    A second pair of wedges  62  and  64  downstream from wedges  58  and  60 , with the optical axis cut parallel to both the surface of the table  50  and to the beam propagation, is used to fix the relative timing between the second pump pulse  32  and the probe pulse  24 ′, as will be discussed below, as well as partially correcting the frequency dispersion of the two pump pulses  30  and  32  that would otherwise he generated by changing of the amount of material in the beam path with the first two wedges  58  and  60 . 
         [0036]    A polarizer  70  is used after the wedges  58 ,  60 ,  62  and  64  to realign the polarization of the pump pulses  30  and  32  to a common polarization and to set the final polarization of the pump pulses  30  and  32 , for example, to be either parallel or perpendicular to the probe pulse  24 ′. 
         [0037]    Referring now to  FIG. 3 , the laser  10 , for example, may in one example produce nano w spectrum pulses  12  having a center frequency of 800 nanometers and a duration of 150 femtoseconds with a one kilohertz repetition rate and a per pulse energy of 300 μJ. A laser suitable for this purpose is commercially available from Spectra Physics of California, United States under the trade name Spitfire. 
         [0038]    After passing through the beam splitter  14 , narrow spectrum pulses  12  may he received along the first optical path through a polarizing wave plate  71 , collimating lens assembly  72  and spectrum-broadening crystal  20 . 
         [0039]    The white light pulse  24  is then received by a mirror array  74  providing an adjustable path length by means of a mechanically movable stage  76  controllable by the computer  40  as may be used to arbitrarily delay the white light pulse  24  with respect to the pump pulses  30  and  32  to produce the probe pulse  24 ′. The delay may be adjusted as necessary to capture the desired chemical phenomenon by the spectroscope. 
         [0040]    After the beam splitter  14 , narrow spectrum pulse  12  may also be received by a collimating optical assembly  80  on the second optical system  18  and by the second spectrum-broadening crystal  20  to produce white light pulse  24 . Both spectral broadening crystals  20  and  22  may, for example, be four millimeter thick yttrium aluminum garnet (YAG) crystal. 
         [0041]    A prism compressor  84  may then precompensate the white light pulse  24  against dispersion introduced by the pulse splitter  28 . The white light pulse  24  is then is split into two pump pulses  30  and  32  following the pulse splitter  28 . 
         [0042]    The pump pulses  30  and  32  and probe pulse  24  are then received by a mirror array  86  to be focused through the sample volume  34  with light from the probe pulse  24 ′ only being directed to the detector  38 . The detector, for example, may be a 150 mm focal length spectrometer  90 , for example, the Acton SP-2150 spectrograph commercially available from Princeton Instruments of New Jersey, United States, coupled with a light detector using an InGaAs photodiode array  92 , for example, the OMA-V:512-1.7 also commercially available from Princeton Instruments, 
         [0043]    Referring now to  FIG. 3 , it will be appreciated that an alternative method of generating the white light pulses  24  may direct the laser pulses  12  through collimating optical assembly  80  and a single spectrum-broadening crystal  20  before the beam splitter  14 . The beam splitter may then split a single white light pulse  24  into two white light pulses  24 . 
         [0044]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0045]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0046]    References in the claims and specification to a first and second modulator should not be interpreted as limited to two devices in separate housings or to two devices having separate components but may be realized with any device, including a single device, operating to provide independent modulation of two different light beams according to independent modulation signals. Likewise, each of the first and second modulators may be composed of multiple modulators. 
         [0047]    References to “a controller” and “a processor” can be understood to include one or more controllers or processors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
         [0048]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.