This invention was made with government support under Grant Number CHE-9713388 awarded by the National Science Foundation. The government has certain rights in the invention.
The invention relates to systems and methods for making spectroscopy and metrology measurements using electromagnetic radiation, particularly, electromagnetic radiation in the terahertz frequency range.
Measurements with electromagnetic radiation are now conducted in essentially every range of the electromagnetic spectrum, ranging at least from x-rays through radio waves. Different regions of the spectrum are well suited for measurements of different materials and different material properties.
Not all regions of the electromagnetic spectrum are equally accessible for measurements. In some cases, generation of the electromagnetic radiation with the desired frequencies/wavelengths, tunability, intensity, pulse duration or other waveform characteristics may be difficult or costly. In other cases detection of the radiation with the desired sensitivity, time or frequency resolution, or other features may pose challenges. Finally the convenient use of the radiation for practical measurements may present additional difficulties, depending on the geometry of radiation generation and detection and of sample placement and on other factors.
The terahertz frequency range (e.g., 0.1-20 THz) or correspondingly the far-infrared (xe2x80x9cfar IRxe2x80x9d) wavelength range, is emerging as an important component in the spectroscopic study and optical imaging of a very wide range of samples extending from simple and complex liquids to biological materials to integrated circuits. For example, in liquids and partially ordered solids, the terahertz frequency regime probes properties on length and time scales intermediate between those characteristic of bulk low-frequency dielectric responses and of local intermolecular or interionic motions. Conventional dielectric spectroscopy rarely reaches frequencies over 100 GHz, and therefore components of the dynamics of complex materials such as supercooled liquids and mixed ferroelectric crystals remain elusive.
Unfortunately, there can be experimental challenges to generating and detecting terahertz radiation. Microwaves, which border terahertz-rays on the low frequency side, are typically generated using high frequency electronics while infrared waves, on the high frequency border, are generated by light sources, usually via blackbody radiation or lasers. Recently, femtosecond lasers producing optical pulses on the order of 100 fs or less have been used to illuminate photoconducting dipole antennas, which respond by generating broad bandwidth, coherent bursts of far-IR output with substantial spectral density from 100 GHz to 5 THz or higher. Such terahertz radiation, which propagates through free space, is focussed and/or collimated with far-IR optics, and can be detected using another, similar photoconductive dipole antenna, which is gated by a variably delayed femtosecond pulse.
The invention features a system and method for generating terahertz radiation by time-domain excitation of polaritons in a nonlinear optical crystal. One or more pulses of coherent optical radiation illuminate a non-centrosymmetric material to excite a polariton having a frequency less than or equal to the bandwidth of the excitation pulses. Spatial and temporal shaping of the optical excitation pulses can control the frequency, bandwidth, and propagation properties of the polariton. When the polariton propagates to the edge of the non-centrosymmetric material, its electromagnetic component can couple into an adjacent sample as terahertz radiation. The interaction between the sample and the terahertz radiation can be measured a number of ways.
For example, after its interaction with the sample, the terahertz radiation can couple into a second non-centrosymmetric material as a polariton, whose properties can be detected by a second set of one or more optical pulses. Alternatively, for example, the terahertz radiation can reflect back to the original non-centrosymmetric material for the optical detection measurement. In either case, the attenuation and delay of the detected terahertz radiation reveals the real and imaginary components of the dielectric properties of the sample in the terahertz regime. Moreover, the sample can be probed directly with, e.g., an optical beam, following its interaction with the terahertz radiation. Measurements can be repeated at additional polariton frequencies in an automated fashion to spectrally resolve the dielectric response of the sample.
In general, in one aspect, the invention features a spectroscopic method for characterizing a sample. The method includes: positioning the sample adjacent to a non-centrosymmetric material; directing at least one temporal pulse of coherent EM radiation into the non-centrosymmetric material to generate a polariton therein and cause EM radiation from the polariton to propagate into the sample, wherein the polariton has a frequency less than or equal to the bandwidth of the pulse; and measuring a response of the sample to the EM radiation from the polariton.
Embodiments of the method can include any of the following features.
The measuring step can include: positioning a second non-centrosymmetric material to receive EM radiation from the sample in response to the EM radiation from the polariton in the first-mentioned non-centrosymmetric material, wherein the EM radiation from the sample propagates into the second non-centrosymmetric material to form another polariton; and directing additional EM radiation to interact with the polariton in the second non-centrosymmetlic material; and measuring a response of the second non-centrosymmetric material to the interaction of the additional EM radiation and the polariton in the second non-centrosymmetric material. Furthermore, the response of the second non-centrosymmetric material can be measured by measuring at least one of transmission, reflection, polarization rotation, and diffraction of the additional EM radiation by the polariton in the second non-centrosymmetric material. Moreover, the at least one of the transmission, reflection, polarization rotation, and diffraction of the additional EM radiation can be spectrally resolved. The response of the second non-centrosymmetric material can also be one of sum-frequency generation and difference-frequency generation caused by the interaction of the additional EM radiation and the polariton in the second non-centrosymmetric material. Furthermore, the response of the second non-centrosymmetric material can also be measured by using the additional EM radiation to image the polariton in the second non-centrosymmetric material. The measured response of the second non-centrosymmetric material can be indicative of the amplitude and phase of the polariton in the second non-centrosymmetric material.
The measuring step can also include: directing additional EM radiation to interact with the EM radiation in the sample from the polariton; and measuring a response of the sample to the interaction of the additional EM radiation and the EM radiation in the sample from the polariton. The measuring step can include measuring at least one of diffraction, reflection, a change in absorption, and a change in polarization of the additional EM radiation by the sample caused by the presence of the EM radiation in the sample from the polariton. Moreover, the measured at least one of diffraction, reflection, the change in absorption, and the change in polarization can be spectrally resolved. Also, the response can be one of sum-frequency generation and difference-frequency generation caused by the interaction of the additional EM radiation and the EM radiation in the sample from the polariton.
The EM radiation from the polariton can also reflect from a reflecting surface and back into the non-centrosymmetric material to form a second polariton, in which case, the measuring step can include: directing additional EM radiation to interact with the second polariton; and measuring a response of the non-centrosymmetric material to the interaction of the additional EM radiation and the second polariton.
The measured response can be indicative of absorption by the sample, refractive index of the sample, and/or complex refractive index of the sample, at the frequency of the EM radiation from the polariton.
The method can further include repeating the positioning, directing, and measuring for a reference sample, and comparing the measurements of the first-mentioned sample and the reference sample. The method can further include temporally shaping EM radiation to form the at least one temporal pulse. The directing step can include directing a spatially periodic pattern of the at least one temporal pulse of EM radiation onto the non-centrosymmetric material. The method can further include selecting the period of the spatially periodic pattern to produce a selected frequency for the polariton. The directing step can further include crossing at least two beams of the at least one temporal pulse of EM radiation to form the spatially periodic pattern. Alternatively, the directing step can further include passing EM radiation through a mask to produce the spatially periodic pattern on the non-centrosymmetric material. The method can further include repeating the selecting, directing, and measuring steps for multiple frequencies of the polariton. The method can further include focusing the EM radiation from the polariton prior to it reaching the sample.
The sample can be one of a liquid, solid, and gas. It can separated from the non-centrosymmetric material. The frequency of the polariton can be the range of about 0.1 to 20 THz, e.g., in the range of about 1 to 10 THz.
In general, in another aspect, the invention features an apparatus for characterizing a sample including: a non-centrosymmetric material; a sample assembly configured to support the sample adjacent to the non-centrosymmetric material; a light source which during operation directs at least one temporal pulse of coherent EM radiation into the non-centrosymmetric material, the at least one temporal pulse having an intensity and bandwidth sufficient to generate a polariton in the non-centrosymmetric material and cause EM radiation from the polariton to propagate into the sample, wherein the polariton has a frequency less than or equal to the bandwidth of the pulse.
Embodiments of the apparatus can include any of the following features.
The apparatus can further include: a second non-centrosymmetric material positioned to receive EM radiation from the sample in response to the EM radiation from the polariton in the first-mentioned non-centrosymmetric material, wherein during operation the EM radiation from the sample propagates into the second non-centrosymmetric material to form another polariton, wherein during operation the light source directs a probe beam of EM radiation to interact with the polariton in the second non-centrosymmetric material; and a detector positioned to measure a response of the second non-centrosymmetric material to the interaction of the probe beam and the polariton in the second non-centrosymmetric material. The apparatus can further include a computer coupled to the light source and the detector, wherein during operation the computer analyzes the measured response to characterize the sample.
During operation of the apparatus, the light source can direct a probe beam of additional EM radiation to interact with the EM radiation in the sample from the polariton, and the apparatus can further include a detector positioned to measure a response of the second non-centrosymmetric material to the interaction of the probe beam and the polariton in the second non-centrosymmetric material. The apparatus can further include a computer coupled to the light source and the detector, wherein during operation the computer analyzes the measured response to characterize the sample.
The sample assembly can includes a reflecting surface configured to reflect the EM radiation from the polariton back into the non-centrosymmetric material to form a second polariton. During operation the light source can direct a probe beam of additional EM radiation to interact with the second polariton, and the apparatus can further include a detector positioned to measure a response of the non-centrosymmetric material to the interaction of the additional EM radiation and the second polariton.
During operation the light source can cause the at least one temporal pulse to form a periodic spatial pattern in the non-centrosymmetric material. The apparatus can further include a computer coupled to the light source to control the period of the periodic spatial pattern. To form the periodic spatial pattern, the light source can direct a pair of excitation beams to interfere in the non-centrosymmetric material, the pair of excitation beams including the at least one temporal pulse. Alternatively, the light source can include a laser source, a phase mask, and one or more imaging lenses, wherein during operation the laser source directs a beam to the phase mask, the phase mask diffracts the beam into multiple orders, and the one or more imaging lenses recombines the multiple orders in the non-centrosymmetric material to form the periodic spatial pattern.
Embodiments of the invention have many advantages.
For example, systems and methods described herein can be used to generate and/or detect terahertz frequency radiation without the use of a photoconductive dipole antenna. Such systems may be used as terahertz frequency spectrometers that involve only optical beams. Moreover, spatial shaping of the optical excitation pulses used to generate the polaritons can control the frequency of the polariton and the electromagnetic radiation derived there from. Thus, the systems can be automated to scan the sample with multiple, narrow-band pulses of terahertz radiation.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.