xicheng zhang - Academia.edu (original) (raw)

Papers by xicheng zhang

Frontiers of Optoelectronics, Dec 29, 2020

Pursuing a PhD degree in the United Stated was not an easy decision for me to make. The admiratio... more Pursuing a PhD degree in the United Stated was not an easy decision for me to make. The admiration for my advisor Professor Xi-Cheng Zhang was the main reason that drove me to spend years learning optics and terahertz technology at The Institute of Optics. Besides his success in the terahertz community, the care and the respect that he had shown to students deeply touched me. I am very grateful for the excellent academic environment and priceless advice he has offered to support my research. He inspired me to work smart, which helped me to make breakthroughs during my PhD study. I believe that the lessons I have learned from him will significantly impact on the rest of my life. I would like to thank other members of my thesis committee Professor Wayne Knox and Professor Roman Sobolewski, and the committee chair Professor Yongli Gao for reviewing my work. I appreciate the help from present and past group members in Professor Zhang's group. Yiwen E, our postdoc, helped to solve tremendous problems in my research. Kang Liu, Fabrizio Buccheri, and Betty Sun offered me many ACKNOWLEDGMENTS xi suggestions when I started the project for my thesis. Kaia Williams helped preparing early experiments. Kareem Garriga, Jiapeng Zhao, and Shenghan Gao provided insightful ideas in our discussions. Special thanks to Professor Jianming Dai for showing me how to professionally carry out an experiment, as well as offering me advice on my career and life.

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 2002

A multilevel microfabrication process has been developed to produce silicon Fresnel lenses for te... more A multilevel microfabrication process has been developed to produce silicon Fresnel lenses for terahertz waves. A repeated binary fabrication process was used to create lenses with up to eight levels in complexity and these lenses have been compared to both less complex structures and refractive optic lenses. The microfabrication required deep reactive ion etching and multilevel resist processing using SU8 photoresist. At the design frequency of 1 THz an eight-level lens showed significant improvement in intensity at the focus and had much reduced energy lost into sidelobes compared with refractive lenses.

SPIE Proceedings, 2002

A new bioaffinity sensor based on pulsed terahertz (THz) spectroscopy is able to sensitively dete... more A new bioaffinity sensor based on pulsed terahertz (THz) spectroscopy is able to sensitively detect the presence of ultra-thin bound biomolecular layers. The protein avidin and lipid biotin are noted for their very high binding affinity, and the ease for which they can be attached to residues with importance in many biosensing applications. We demonstrate the sensitivity of the pulsed THz spectrometer to thin avidin layers and to avidin amplified with micron-sized agarose beads. The experimental results can be simply modelled by considering transmission of the THz radiation at the thin film interfaces. We detect less than 10.3 ng/cm 2 avidin, giving the THz system a detection capability of sub-thin solid films better than ellipsometry and reflectometry techniques.

SPIE Proceedings, 2004

This paper describes a study conducted into the limit on spectral resolution due to the dynamic r... more This paper describes a study conducted into the limit on spectral resolution due to the dynamic range of a Tray spectrometer. The pulsed nature of terahertz time-domain spectroscopy (THz-TDS) sets a fundamental limit on its spectral resolution. The spectral resolution of THz-TDS can be improved by increasing the duration of the temporal measurement, but is limited by the dynamic range of the system in the time-domain. This paper presents calculations and experimental results relating the temporal dynamic range of a THz-TDS system to its spectral resolution. We discuss three typical pulsed terahertz sources in terms of their dynamic range and hence achievable spectral resolution.

SPIE Proceedings, 2003

The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz-70 T... more The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz-70 THz) radiation provide this emerging technology with the potential for reliable identification of buried objects such as non-metallic landmines. With a suitable integration of these attributes, one can envision a THz detection platform that provides: (1) accurate identification of buried objects, and (2) a source-to-sample working distance that is sufficient for remote sensing applications. In our preliminary laboratory studies, we have demonstrated the detection capabilities of THz radiation by imaging a small rubber object embedded in a moist, sand-like soil. Despite the significant attenuation of the THz radiation via water absorption and particle scattering, the initial transmission results showed that pulsed THz imaging could identify the non-metallic object when buried in a few inches of soil. The sub-millimeter resolution observed in our THz images illustrates the potential to discriminate landmines from other buried objects. Finally, THz calculations and measurements determined that our current THz source and detector has sufficient SNR to detect a buried object to a depth of 6 inches in moist sand.

Electronics and Structures for MEMS II, 2001

There is increasing interest among research groups around the world in the terahertz portion of t... more There is increasing interest among research groups around the world in the terahertz portion of the electromagnetic spectrum. Tray systems, driven by ultrafast THz pulses, offer a number of unique advantages over other techniques and are under investigation for a wide range of applications. Biomedical diagnostics is an area of particular emphasis. The sub-millimetre spectroscopic measurements obtained from Tray systems contain a wealth of information about the sample under test. A number of hurdles, however, hinder the application of Tray technology. One of the major hurdles to be overcome is the slow acquisition speed of modern THz systems. The chirped probe pulse technique offers a significant improvement in this context. We present results demonstrating the terahertz responses of biological samples measured using a chirped probe pulse, and discuss the problem of data processing and extracting sample characteristics. We show that different types of tissue can be classified based on their terahertz response measured with the chirped probe pulse technique. We consider chicken and beef samples and differentiate between bone and normal tissue. We demonstrate the performance of linear filter models for feature extraction and show that these models are significantly more accurate than a number of intuitive features.

SPIE Proceedings, 2003

Pulsed THz (T-ray) spectroscopy is an increasingly common tool for studying materials from physic... more Pulsed THz (T-ray) spectroscopy is an increasingly common tool for studying materials from physics to biology. Liquid transmission Tray studies are valuable for understanding solvation dynamics of salts, exploring longrange structure in mixtures and probing biomolecules in suspension. In this paper the uncertainty in parameter estimation of liquid samples is shown to be dependent on the thickness change of the sample, and on the noise of the Tray spectrometer. For many important liquids, such as water, with high THz absorption, we show that measurement uncertainty can be greatly decreased using a dynamically-modulated liquid sample, using differential Tray time-domain spectroscopy (DTDS). Preliminary experiments with a novel sample holder, based on a loud speaker, support these calculations. Amplitude and mean detection is used to simultaneously measure two waveforms in DTDS, potentially decreasing the noise by 10 2 .

, except for brief excerpts in connection with reviews or scholarly analysis. Use in connection w... more , except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.

SPIE Proceedings, 2003

Pulsed THz imaging systems have a number of potential advantages in inspection applications. They... more Pulsed THz imaging systems have a number of potential advantages in inspection applications. They provide amplitude and phase information across a broad spectral range in the far-infrared, and many common packaging materials are relatively transparent in this frequency range. We use Tray imaging to allow the identification of different powdered materials concealed inside envelopes. Using the terahertz spectral information we show that different powders may be uniquely identified. Different thicknesses of the powders are imaged to investigate the influence of scattering on the measured THz pulses and the classification model is extended to allow it to identify different materials independent of the material thickness.

Microelectronics Journal, 2000

The rapidly developing field of terahertz (or Tray) imaging promises to provide a non-invasive me... more The rapidly developing field of terahertz (or Tray) imaging promises to provide a non-invasive method of identifying the composition of various objects. Biomedical diagnostics, semiconductor device diagnostics, trace gas analysis and moisture analysis for agriculture are among the growing number of important Tray applications. We present results using an electro-optical sampling method and and discuss how this can be simplified by using a micro antenna array realised by MEMs technology. The challenges and advantages of both approaches are compared. In our setup , sub-picosecond pulses of terahertz radiation are produced via optical rectification, using an LT gallium arsenide wafer. A coherent method is used, whereby both amplitude and phase of the THz beam are detected, providing an exact signature of the material under observation.' From the analysis of the amplitude and phase it is possible to tomographically reconstruct a terahertz-domain image of an object.2 Current terahertz systems, however, remain too slow for many real-time applications and the need for high speed image processing is evident.35 Previous systems have relied on simple signal processing algorithms, based on Fourier transformations, for de-convolving the system response and extracting phase/amplitude information. We discuss enhanced processing based on wavelet transforms. Wavelet techniques for signal analysis provide near optimal thresholding for signal recovery and are computationally inexpensive for pulsed waveforms.6 The improved signal characterisation and speed of wavelet analysis over Fourier methods is demonstrated on sample Tray data.

Journal of Optics B: Quantum and Semiclassical Optics, 2004

Liquid transmission studies at terahertz frequencies (0.1-10 THz) are valuable for understanding ... more Liquid transmission studies at terahertz frequencies (0.1-10 THz) are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension. Tray (or THz) time-domain spectroscopy, based on terahertz pulse generation from ultrafast lasers, is a sensitive technique for measuring material parameters in this frequency range. This paper proposes and demonstrates a novel technique for increasing the sensitivity and repeatability of liquid studies with Tray time-domain spectroscopy (TDS), reducing relative parameter measurement errors below 0.0001. The proposed technique combines dual-thickness liquid measurement with rapid modulation (double-modulated differential TDS) to reduce the effect of both thickness-measurement errors and Tray noise errors below 0.0001. The possible reduction in error is calculated and a liquid differential TDS (DTDS) prototype is demonstrated, incorporating amplitude and mean detection for near-simultaneous measurement of two Tray waveforms.

IEEE Journal of Quantum Electronics, 2000

The authors report the use of an optoelectronic system for the measurement of terahertz (THz) pul... more The authors report the use of an optoelectronic system for the measurement of terahertz (THz) pulses by using chirped pulse technology. This system measures the spatio-temporal distribution of free-space pulsed radiation with an unprecedented data acquisition rate. Using a linearly chirped optical probe pulse with an electro-optic crystal, a temporal waveform of a copropagating THz field is linearly encoded onto the frequency spectrum of the optical probe pulse and then decoded by dispersing the probe beam from a grating to a detector array. Acquisition of picosecond THz field pulses without using mechanical time-delay devices have been achieved. A single-shot electro-optic measurement of the temporal waveform of a THz pulse has been demonstrated. Unparalleled by other THz sampling techniques, this single-shot method provides what is believed to be the highest possible data-acquisition rate. Temporal resolution, sensitivity, optimal optical bias point of electro-optic modulation, potential applications, and possible improvements are also discussed. In principle, this technique can also be used in magneto-optic measurements.

SPIE Proceedings, 2004

Pulsed THz (T-ray) spectroscopy is a sensitive, non-invasive technique for studying materials fro... more Pulsed THz (T-ray) spectroscopy is a sensitive, non-invasive technique for studying materials from material science to biology, but transmission measurements of liquid samples, especially water, are limited by noise. This paper shows that the accuracy of Tray material parameter measurements of liquid samples can be greatly increased, especially for highly-absorbing liquids, by using a rapid modulation of the liquid in the Tray beam path (Differential Time-Domain Spectroscopy), coupled with a novel implementation of mean and amplitude detection. The theoretical calculations are supported by proof-of-principle experiments. Liquid transmission Tray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension.

Physics Letters A, 2008

In modern terahertz (THz) sensing and imaging spectroscopy, water is considered a nemesis to be a... more In modern terahertz (THz) sensing and imaging spectroscopy, water is considered a nemesis to be avoided due to strong absorption in the THz frequency range. Here we report the first experimental demonstration and theoretical implications of using femtosecond laser pulses to generate intense broadband THz emission from water vapor. When we focused an intense laser pulse in water vapor contained in a gas cell or injected from a gas jet nozzle, an extraordinarily strong THz field from optically excited water vapor is observed. Water vapor has more than 50% greater THz generation efficiency than dry nitrogen. It had previously been assumed that the nonlinear generation of THz waves in this manner primarily involves a free-electron plasma, but we show that the molecular structure plays an essential role in the process. In particular, we found that THz wave generation from H 2 O vapor is significantly stronger than that from D 2 O vapor. Vibronic activities of water cluster ions, occurring naturally in water vapor, may possibly contribute to the observed isotope effect along with rovibrational contributions from the predominant monomers.

PhotoniX, 2021

Terahertz technology has broad application prospects in biomedical detection. However, the mixed ... more Terahertz technology has broad application prospects in biomedical detection. However, the mixed characteristics of actual samples make the terahertz spectrum complex and difficult to distinguish, and there is no practical terahertz detection method for clinical medicine. Here, we propose a three-step one-way terahertz model, presenting a detailed flow analysis of terahertz technology in the biomedical detection of renal fibrosis as an example: 1) biomarker determination: screening disease biomarkers and establishing the terahertz spectrum and concentration gradient; 2) mixture interference removal: clearing the interfering signals in the mixture for the biomarker in the animal model and evaluating and retaining the effective characteristic peaks; and 3) individual difference removal: excluding individual interference differences and confirming the final effective terahertz parameters in the human sample. The root mean square error of our model is three orders of magnitude lower tha...

Nonlinear Optics (NLO), 2019

We report the nonlinear optical generation of broadband terahertz (THz) waves from liquids by usi... more We report the nonlinear optical generation of broadband terahertz (THz) waves from liquids by using laser induced micro-plasma under the excitation of asymmetric optical fields. Additionally, we demonstrated that a water line is a better THz source than a water film under comparable experimental condition.

Biomedical Applications of Micro- and Nanoengineering, 2002

Optics and Photonics News, 2009

(a) The experimental setup. Inside the dashed line is the in-line phase compensator. β-BBO, beta-... more (a) The experimental setup. Inside the dashed line is the in-line phase compensator. β-BBO, beta-barium borate crystal; α-BBO, alpha-barium borate crystal; QW, quartz wedges; DWP, dual-wavelength waveplate; red and blue arrows indicate polarization of the ω and 2ω ...

The optical constants of a nematic liquid crystal, 4Ј-n-pentyl-4-cyanobiphenyl ͑5CB͒, in the freq... more The optical constants of a nematic liquid crystal, 4Ј-n-pentyl-4-cyanobiphenyl ͑5CB͒, in the frequency range 0.3-1.4 THz were determined by terahertz ͑THz͒ time-domain spectroscopy. The real parts of the extraordinary refractive index n e and the ordinary refractive index n o of 5CB varied from 1.74 to 2.04 and from 1.59 to 1.83, respectively. Liquid-crystal 5CB exhibits a relatively small absorption loss in this frequency range. The birefringence of 5CB was found to be as large as 0.21. The experimental results indicate that liquid-crystal 5CB is potentially useful for device applications in the THz frequency range.

Frontiers of Optoelectronics, Dec 29, 2020

Pursuing a PhD degree in the United Stated was not an easy decision for me to make. The admiratio... more Pursuing a PhD degree in the United Stated was not an easy decision for me to make. The admiration for my advisor Professor Xi-Cheng Zhang was the main reason that drove me to spend years learning optics and terahertz technology at The Institute of Optics. Besides his success in the terahertz community, the care and the respect that he had shown to students deeply touched me. I am very grateful for the excellent academic environment and priceless advice he has offered to support my research. He inspired me to work smart, which helped me to make breakthroughs during my PhD study. I believe that the lessons I have learned from him will significantly impact on the rest of my life. I would like to thank other members of my thesis committee Professor Wayne Knox and Professor Roman Sobolewski, and the committee chair Professor Yongli Gao for reviewing my work. I appreciate the help from present and past group members in Professor Zhang's group. Yiwen E, our postdoc, helped to solve tremendous problems in my research. Kang Liu, Fabrizio Buccheri, and Betty Sun offered me many ACKNOWLEDGMENTS xi suggestions when I started the project for my thesis. Kaia Williams helped preparing early experiments. Kareem Garriga, Jiapeng Zhao, and Shenghan Gao provided insightful ideas in our discussions. Special thanks to Professor Jianming Dai for showing me how to professionally carry out an experiment, as well as offering me advice on my career and life.

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 2002

A multilevel microfabrication process has been developed to produce silicon Fresnel lenses for te... more A multilevel microfabrication process has been developed to produce silicon Fresnel lenses for terahertz waves. A repeated binary fabrication process was used to create lenses with up to eight levels in complexity and these lenses have been compared to both less complex structures and refractive optic lenses. The microfabrication required deep reactive ion etching and multilevel resist processing using SU8 photoresist. At the design frequency of 1 THz an eight-level lens showed significant improvement in intensity at the focus and had much reduced energy lost into sidelobes compared with refractive lenses.

SPIE Proceedings, 2002

A new bioaffinity sensor based on pulsed terahertz (THz) spectroscopy is able to sensitively dete... more A new bioaffinity sensor based on pulsed terahertz (THz) spectroscopy is able to sensitively detect the presence of ultra-thin bound biomolecular layers. The protein avidin and lipid biotin are noted for their very high binding affinity, and the ease for which they can be attached to residues with importance in many biosensing applications. We demonstrate the sensitivity of the pulsed THz spectrometer to thin avidin layers and to avidin amplified with micron-sized agarose beads. The experimental results can be simply modelled by considering transmission of the THz radiation at the thin film interfaces. We detect less than 10.3 ng/cm 2 avidin, giving the THz system a detection capability of sub-thin solid films better than ellipsometry and reflectometry techniques.

SPIE Proceedings, 2004

This paper describes a study conducted into the limit on spectral resolution due to the dynamic r... more This paper describes a study conducted into the limit on spectral resolution due to the dynamic range of a Tray spectrometer. The pulsed nature of terahertz time-domain spectroscopy (THz-TDS) sets a fundamental limit on its spectral resolution. The spectral resolution of THz-TDS can be improved by increasing the duration of the temporal measurement, but is limited by the dynamic range of the system in the time-domain. This paper presents calculations and experimental results relating the temporal dynamic range of a THz-TDS system to its spectral resolution. We discuss three typical pulsed terahertz sources in terms of their dynamic range and hence achievable spectral resolution.

SPIE Proceedings, 2003

The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz-70 T... more The spatial, temporal, and spectroscopic characteristics associated with pulsed THz (100 GHz-70 THz) radiation provide this emerging technology with the potential for reliable identification of buried objects such as non-metallic landmines. With a suitable integration of these attributes, one can envision a THz detection platform that provides: (1) accurate identification of buried objects, and (2) a source-to-sample working distance that is sufficient for remote sensing applications. In our preliminary laboratory studies, we have demonstrated the detection capabilities of THz radiation by imaging a small rubber object embedded in a moist, sand-like soil. Despite the significant attenuation of the THz radiation via water absorption and particle scattering, the initial transmission results showed that pulsed THz imaging could identify the non-metallic object when buried in a few inches of soil. The sub-millimeter resolution observed in our THz images illustrates the potential to discriminate landmines from other buried objects. Finally, THz calculations and measurements determined that our current THz source and detector has sufficient SNR to detect a buried object to a depth of 6 inches in moist sand.

Electronics and Structures for MEMS II, 2001

There is increasing interest among research groups around the world in the terahertz portion of t... more There is increasing interest among research groups around the world in the terahertz portion of the electromagnetic spectrum. Tray systems, driven by ultrafast THz pulses, offer a number of unique advantages over other techniques and are under investigation for a wide range of applications. Biomedical diagnostics is an area of particular emphasis. The sub-millimetre spectroscopic measurements obtained from Tray systems contain a wealth of information about the sample under test. A number of hurdles, however, hinder the application of Tray technology. One of the major hurdles to be overcome is the slow acquisition speed of modern THz systems. The chirped probe pulse technique offers a significant improvement in this context. We present results demonstrating the terahertz responses of biological samples measured using a chirped probe pulse, and discuss the problem of data processing and extracting sample characteristics. We show that different types of tissue can be classified based on their terahertz response measured with the chirped probe pulse technique. We consider chicken and beef samples and differentiate between bone and normal tissue. We demonstrate the performance of linear filter models for feature extraction and show that these models are significantly more accurate than a number of intuitive features.

SPIE Proceedings, 2003

Pulsed THz (T-ray) spectroscopy is an increasingly common tool for studying materials from physic... more Pulsed THz (T-ray) spectroscopy is an increasingly common tool for studying materials from physics to biology. Liquid transmission Tray studies are valuable for understanding solvation dynamics of salts, exploring longrange structure in mixtures and probing biomolecules in suspension. In this paper the uncertainty in parameter estimation of liquid samples is shown to be dependent on the thickness change of the sample, and on the noise of the Tray spectrometer. For many important liquids, such as water, with high THz absorption, we show that measurement uncertainty can be greatly decreased using a dynamically-modulated liquid sample, using differential Tray time-domain spectroscopy (DTDS). Preliminary experiments with a novel sample holder, based on a loud speaker, support these calculations. Amplitude and mean detection is used to simultaneously measure two waveforms in DTDS, potentially decreasing the noise by 10 2 .

, except for brief excerpts in connection with reviews or scholarly analysis. Use in connection w... more , except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.

SPIE Proceedings, 2003

Pulsed THz imaging systems have a number of potential advantages in inspection applications. They... more Pulsed THz imaging systems have a number of potential advantages in inspection applications. They provide amplitude and phase information across a broad spectral range in the far-infrared, and many common packaging materials are relatively transparent in this frequency range. We use Tray imaging to allow the identification of different powdered materials concealed inside envelopes. Using the terahertz spectral information we show that different powders may be uniquely identified. Different thicknesses of the powders are imaged to investigate the influence of scattering on the measured THz pulses and the classification model is extended to allow it to identify different materials independent of the material thickness.

Microelectronics Journal, 2000

The rapidly developing field of terahertz (or Tray) imaging promises to provide a non-invasive me... more The rapidly developing field of terahertz (or Tray) imaging promises to provide a non-invasive method of identifying the composition of various objects. Biomedical diagnostics, semiconductor device diagnostics, trace gas analysis and moisture analysis for agriculture are among the growing number of important Tray applications. We present results using an electro-optical sampling method and and discuss how this can be simplified by using a micro antenna array realised by MEMs technology. The challenges and advantages of both approaches are compared. In our setup , sub-picosecond pulses of terahertz radiation are produced via optical rectification, using an LT gallium arsenide wafer. A coherent method is used, whereby both amplitude and phase of the THz beam are detected, providing an exact signature of the material under observation.' From the analysis of the amplitude and phase it is possible to tomographically reconstruct a terahertz-domain image of an object.2 Current terahertz systems, however, remain too slow for many real-time applications and the need for high speed image processing is evident.35 Previous systems have relied on simple signal processing algorithms, based on Fourier transformations, for de-convolving the system response and extracting phase/amplitude information. We discuss enhanced processing based on wavelet transforms. Wavelet techniques for signal analysis provide near optimal thresholding for signal recovery and are computationally inexpensive for pulsed waveforms.6 The improved signal characterisation and speed of wavelet analysis over Fourier methods is demonstrated on sample Tray data.

Journal of Optics B: Quantum and Semiclassical Optics, 2004

Liquid transmission studies at terahertz frequencies (0.1-10 THz) are valuable for understanding ... more Liquid transmission studies at terahertz frequencies (0.1-10 THz) are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension. Tray (or THz) time-domain spectroscopy, based on terahertz pulse generation from ultrafast lasers, is a sensitive technique for measuring material parameters in this frequency range. This paper proposes and demonstrates a novel technique for increasing the sensitivity and repeatability of liquid studies with Tray time-domain spectroscopy (TDS), reducing relative parameter measurement errors below 0.0001. The proposed technique combines dual-thickness liquid measurement with rapid modulation (double-modulated differential TDS) to reduce the effect of both thickness-measurement errors and Tray noise errors below 0.0001. The possible reduction in error is calculated and a liquid differential TDS (DTDS) prototype is demonstrated, incorporating amplitude and mean detection for near-simultaneous measurement of two Tray waveforms.

IEEE Journal of Quantum Electronics, 2000

The authors report the use of an optoelectronic system for the measurement of terahertz (THz) pul... more The authors report the use of an optoelectronic system for the measurement of terahertz (THz) pulses by using chirped pulse technology. This system measures the spatio-temporal distribution of free-space pulsed radiation with an unprecedented data acquisition rate. Using a linearly chirped optical probe pulse with an electro-optic crystal, a temporal waveform of a copropagating THz field is linearly encoded onto the frequency spectrum of the optical probe pulse and then decoded by dispersing the probe beam from a grating to a detector array. Acquisition of picosecond THz field pulses without using mechanical time-delay devices have been achieved. A single-shot electro-optic measurement of the temporal waveform of a THz pulse has been demonstrated. Unparalleled by other THz sampling techniques, this single-shot method provides what is believed to be the highest possible data-acquisition rate. Temporal resolution, sensitivity, optimal optical bias point of electro-optic modulation, potential applications, and possible improvements are also discussed. In principle, this technique can also be used in magneto-optic measurements.

SPIE Proceedings, 2004

Pulsed THz (T-ray) spectroscopy is a sensitive, non-invasive technique for studying materials fro... more Pulsed THz (T-ray) spectroscopy is a sensitive, non-invasive technique for studying materials from material science to biology, but transmission measurements of liquid samples, especially water, are limited by noise. This paper shows that the accuracy of Tray material parameter measurements of liquid samples can be greatly increased, especially for highly-absorbing liquids, by using a rapid modulation of the liquid in the Tray beam path (Differential Time-Domain Spectroscopy), coupled with a novel implementation of mean and amplitude detection. The theoretical calculations are supported by proof-of-principle experiments. Liquid transmission Tray studies are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension.

Physics Letters A, 2008

In modern terahertz (THz) sensing and imaging spectroscopy, water is considered a nemesis to be a... more In modern terahertz (THz) sensing and imaging spectroscopy, water is considered a nemesis to be avoided due to strong absorption in the THz frequency range. Here we report the first experimental demonstration and theoretical implications of using femtosecond laser pulses to generate intense broadband THz emission from water vapor. When we focused an intense laser pulse in water vapor contained in a gas cell or injected from a gas jet nozzle, an extraordinarily strong THz field from optically excited water vapor is observed. Water vapor has more than 50% greater THz generation efficiency than dry nitrogen. It had previously been assumed that the nonlinear generation of THz waves in this manner primarily involves a free-electron plasma, but we show that the molecular structure plays an essential role in the process. In particular, we found that THz wave generation from H 2 O vapor is significantly stronger than that from D 2 O vapor. Vibronic activities of water cluster ions, occurring naturally in water vapor, may possibly contribute to the observed isotope effect along with rovibrational contributions from the predominant monomers.

PhotoniX, 2021

Terahertz technology has broad application prospects in biomedical detection. However, the mixed ... more Terahertz technology has broad application prospects in biomedical detection. However, the mixed characteristics of actual samples make the terahertz spectrum complex and difficult to distinguish, and there is no practical terahertz detection method for clinical medicine. Here, we propose a three-step one-way terahertz model, presenting a detailed flow analysis of terahertz technology in the biomedical detection of renal fibrosis as an example: 1) biomarker determination: screening disease biomarkers and establishing the terahertz spectrum and concentration gradient; 2) mixture interference removal: clearing the interfering signals in the mixture for the biomarker in the animal model and evaluating and retaining the effective characteristic peaks; and 3) individual difference removal: excluding individual interference differences and confirming the final effective terahertz parameters in the human sample. The root mean square error of our model is three orders of magnitude lower tha...

Nonlinear Optics (NLO), 2019

We report the nonlinear optical generation of broadband terahertz (THz) waves from liquids by usi... more We report the nonlinear optical generation of broadband terahertz (THz) waves from liquids by using laser induced micro-plasma under the excitation of asymmetric optical fields. Additionally, we demonstrated that a water line is a better THz source than a water film under comparable experimental condition.

Biomedical Applications of Micro- and Nanoengineering, 2002

Optics and Photonics News, 2009

(a) The experimental setup. Inside the dashed line is the in-line phase compensator. β-BBO, beta-... more (a) The experimental setup. Inside the dashed line is the in-line phase compensator. β-BBO, beta-barium borate crystal; α-BBO, alpha-barium borate crystal; QW, quartz wedges; DWP, dual-wavelength waveplate; red and blue arrows indicate polarization of the ω and 2ω ...

The optical constants of a nematic liquid crystal, 4Ј-n-pentyl-4-cyanobiphenyl ͑5CB͒, in the freq... more The optical constants of a nematic liquid crystal, 4Ј-n-pentyl-4-cyanobiphenyl ͑5CB͒, in the frequency range 0.3-1.4 THz were determined by terahertz ͑THz͒ time-domain spectroscopy. The real parts of the extraordinary refractive index n e and the ordinary refractive index n o of 5CB varied from 1.74 to 2.04 and from 1.59 to 1.83, respectively. Liquid-crystal 5CB exhibits a relatively small absorption loss in this frequency range. The birefringence of 5CB was found to be as large as 0.21. The experimental results indicate that liquid-crystal 5CB is potentially useful for device applications in the THz frequency range.